Fossils of prehistoric bivalves
The critters on this page belong to groups of "biota" (= living beings) that old-fashioned biologists described as "lower-ranking" than the so-called "higher animals" (mammals and birds). note
However, modern scientists do not accept such an unfairly "racistic" distinction. Furthermore, "lower animals" often show biological traits and behaviours traditionally considered typical of mammals and birds: social attitudes, parental care, intelligence, even some "warm-blood" abilities. Yet in popular media they may be treated as stupid/unfeeling brutes even to this day. Documentaries not excluded: within the Walking With series, for example, Walking with Dinosaurs and Walking with Beasts show dinosaurs and mammals as smart/social/caring creatures, while the meaningfully-named Walking With Monsters and Sea Monsters focus mainly on non-dino reptiles and all the animal groups listed below, with only animals vertebrates shown as being smart/social/caring (Artistic License – Biology).
We humans should be more grateful to extinct "lower beasts" however: several of them gave rise to modern animals, and some to Mankind itself (our ancestry goes back a lot further than the apes). And don't think they were boring: they were anything but, as you'll soon see.
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Amphibians
What is an Amphibian?
- In paleontology, the word "amphibian" has traditionally had a much broader meaning than how it's commonly used. Amphibians were all tetrapods
(four-limbed vertebrates) excluding amniotes (reptiles + mammals). Today, even scientists tend to restrict the word to indicate only modern frogs, salamanders, caecilians, and their common ancestors. If you want to use "amphibians" in its former, broader sense, you have to call modern groups "lissamphibians". We'll use here amphibian in the old, wider meaning because it's much handier to say this rather than "basal tetrapod" every time we refer to non-frog, non-salamander, and non-caecilian animals. Lissamphibians excluded, prehistoric amphibians are traditionally called "labyrinthodonts" ("labyrinth tooth") or "stegocephalians" ("roof-head"), but these terms shouldn't be used today, just like "thecodonts" for basal archosaurs or "pelycosaurs" for basal synapsids. They don't indicate any natural grouping of animals, but are instead catch-all words with little scientific significance in modern phylogenetic systematics. Labyrinthodont means "labyrinth teeth", because many of these animals had convoluted, labyrinth-like internal structures in their teeth, but this isn't a fundamental thing. Their importance was much, much greater than this and tied to a different aspect of their evolutionary history. They were, simply put, the links between fish and truly terrestrial vertebrates, a keystone group for mankind's evolution. And yet, just like synapsids and Mesozoic mammals, they have not gotten much attention in pop culture. If they appear at all in fictional works, they'll be simply described as "giant amphibians".
Hopping, crawling, and slithering: Triadobatrachus, Karaurus & Eocaecilia
- The only true amphibians in modern taxonomy, lissamphibians (frogs + salamanders + caecilians) appeared in the Triassic together with the first dinosaurs, but their deepest origins and their relationship with extinct amphibian groups are still unclear. The first frogs (Triadobatrachus, meaning "Triassic frog", is the most cited example) were already like modern ones, only with shorter hind legs less adapted for hopping. Frogs are possibly the most derived amphibians ever, with their complex social behaviours and the ability to produce loud vocal sounds (this ability is atypical for non-mammal/non-archosaur vertebrates). On the other hand, extinct salamanders retained the primitive crawling shape of earlier amphibians. In the curious case of Andrias scheuchzeri (a close relative of the modern Japanese Giant Salamander), the remains were initially believed to be a human who died during the Biblical Great Deluge! ("Andrias" simply means "man" in Greek.) The legless worm-like caecilians are as little-known in paleontology as they are in Real Life. Their fossils are extremely rare, but they are believed to have had small limbs at the start of their evolution (ex. Eocaecilia, "dawn caecilian").
Malagasy Giant Frog: Beelzebufo
- This guy deserves his own section. Beelzebufo was an enormous relative of modern horned frogs that lived in Madagascar during the Cretaceous period, alongside the herbivorous crocodile Simosuchus. It could grow up to sixteen inches in length, and weigh over nine pounds, making it larger than any modern frog, including the already gigantic goliath frog of Africa. It's also known as the "devil frog" (which is what its scientific name means). Given the eating habits of its modern relatives (which will devour mice) this amphibian could have easily eaten baby dinosaurs. With that said, it still would have lived in fear of large carnivorous dinosaurs such as Majungasaurus that also lived in the area.
Boomerang-heads, eel-bodies, and whip-tails: Urocordylus, Phlegetontia & the other Lepospondyls
- Lepospondyls were among the most numerous and diversified "amphibians" in the Carboniferous. They may have been the closest relatives of lissamphibians, and perhaps their ancestors. Usually having long bodies and weak limbs, they lived mainly in water or in soil. Some were like salamanders, ex. the whip-tailed Urocordylus; other were limbless and eel-like, ex. Phlegetontia. But the most interesting one is certainly Diplocaulus, who can be found on the Stock Dinosaurs (Non-Dinosaurs) page.
Amphibians of the Paleozoic: Prionosuchus, Cacops, Platyhystrix, & Crassigyrinus
- The most successful and diversified group of prehistoric amphibians, temnospondyls are often described in documentary media as "crocodile-like". Their jaws and teeth were smaller and weaker than a crocodile's, but they were nonetheless efficient predators. The most famous is the eight-foot-long, alligator-like Eryops from Early Permian North America (Dimetrodon's neighborhood), but there was also Prionosuchus ("saw crocodile") from Early Permian Brazil, which resembled an enormous gharial and of which one reported specimen is estimated to have been up to thirty feet, making it the largest-known temnospondyl. Other Permian relatives were more terrestrial: the dissorophids Cacops and Platyhystrix had an armor on their back to protect them from land predators. Cacops is remarkable for its large head relative to its body and stronger limbs than the more aquatic Eryops; Platyhystrix ("flat porcupine") was similar, but with striking dorsal spines very similar to a Dimetrodon's or an Edaphosaurus, possibly sustaining a "sail". Although not a temnospondyl, Crassigyrinus scoticus ("Scottish fat tadpole") is also worthy of note, as an example of how diverse paleo-amphibians were: living in the Carboniferous, it was a very specialized water-loving animal with tiny limbs, just as happened to some modern salamanders (the olm, the mudpuppy, the amphiumas, the sirens).
Amphibians of the Mesozoic: Trematosaurus, Gerrothorax, Metoposaurus & Koolasuchus
- Temnospondyls survived the huge Permian mass extinction and made their way in the Triassic: only competition with crocodilians at the end of the period caused their decline and near-total extinction before the Jurassic. The most famous Triassic amphibian is the massive Mastodonsaurus; also very large but much more slender was Trematosaurus, more similar to a gharial in shape. Aphaneramma looked like the latter, but not as big. It was a rare example of a marine amphibian, with no parallels in the modern world, where all amphibians are freshwater or terrestrial creatures. Gerrothorax ("wicker chest") was much smaller than the mastodonsaur, and a bit similar to the unrelated arrow-headed Diplocaulus. Interestingly, it shows neoteny: that is, adults retained the external gills of their larval stage, like the modern axolotl). Another known neotenic paleoamphibian was the salamander-like Branchiosaurus ("gill lizard" - nothing to do with the dinosaur Brachiosaurus, "arm lizard"). Few temnospondyls reached the Late Triassic: among them, the North American Metoposaurus had eyes located more frontally on its head than its earlier relatives, and was able to see Coelophysis in Real Life. In 1997 a new temnospondyl was unexpectedly discovered in Cretaceous terrain: Koolasuchus ("Koola's croc") was probably an isolated Australian late survivor which managed to resist the competition with freshwater reptiles. It shows up both in Walking with Dinosaurs and in Disney's Dinosaur (though rather inaccurately in the latter).
Close to becoming Reptiles: Proterogyrinus, Westlothiana & Diadectes
- The "amphibians" cited here, collectively called reptiliomorphs ("reptile-shaped"), were more related to amniotes (reptiles + mammals) than to any amphibian mentioned above; indeed, many of them were once variably classified as reptiles or amphibians depending on the scientist. More terrestrial than the latter, they progressively evolved the typical traits of amniotes, but we don't know when these traits appeared exactly: waterproof skin, water-storing lungs and kidneys, and eggshells. This last invention was crucial for vertebrate evolution: embryos inside shelled eggs were able to develop and hatch out of water, and the descendants of the reptiliomorphs were able to survive in arid environments. Traditionally the most known reptiliomorph has been Seymouria, but also well-known are the anthracosaurs or embolomeres, the biggest "amphibians" in the Carboniferous (some reached nine meters in length!): Eogyrinus ("dawn tadpole") and the crocodile-sized Proterogyrinus ("first tadpole") are among them. Also in the Carboniferous lived Westlothiana lizziae, a tiny lizard-like animal that was briefly considered "the first reptile" in the 1990s (and nicknamed "Lizzie"). One last example, the Early Permian Diadectes ("crosswise biter"), was similar to an iguana, and possibly one of the first land vertebrates that evolved (partial or total) herbivorousness.
Fish
What is a Fish?
- "Fish" is a catch-all word containing all non-tetrapod vertebrates; that is, all backboned animals which are not only fully-aquatic, but descend from fully-aquatic ancestors as well. Ichthyosaurs, plesiosaurs, mosasaurs and dolphins aren't fish, just because they did descend from land-living creatures. Cladistically-speaking all vertebrates could be considered "fish", because all the land-living vertebrates descend from the primitively aquatic ones. Similarly, all the amniotes (reptiles, mammals, birds) cladistically could be called "amphibians", for the same reason. But here we'll use the word fish in the non-cladistic sense, the commonly accepted one. There are only two groups of fish which are still successful today: sharks and ray-finned fish, while other groups have only few species and much lower variety (jawless fish, chimaeras, lungfish, coelacanths). The same definitively cannot be said within Prehistory, as you'll understand soon.
Fish with limblike Fins: Coelacanthus, Macropoma, Mawsonia, Dipterus & Ceratodus
- Sarcopterygians, aka "lobe-finned fish" or "lobefins". They could better be called proto-amphibians rather than fish: their blood circulation is double like land vertebrates, not simple like typical fish; their fins are bony and fleshy, and their nostrils are connected with their mouth (those of other fish aren't). And some can even breathe air like us, rather than absorbing oxygen from water with the usual gills. The latter are meaningfully called lungfish, they have one or two true lungs which originally were simple protrusions of the digestive tube. Lungfish are very few today: the most archaic is the Australian one, Neoceratodus, with robust paired "fins". Its name means "new Ceratodus": Ceratodus ("horned tooth") is the most abundant prehistoric lungfish, and was virtually identical to Neoceratodus. The other modern lungfish are the African Protopterus and South American Lepidosiren. Among the very first Paleozoic lungfishes were Dipterus ("two wings") and Scaumenacia; the former evolved in the Devonian. However, the most famous modern lobe-finned fish is Latimeria, the only surviving coelacanth. Prehistoric coelacanths, as well, were virtually identical to their modern descendants. One of the most common in fossil record is Macropoma from the Mesozoic. The namesake of the group Coelacanthus ("hollow spine") lived earlier in the Paleozoic. Mawsonia was Cretaceous and the biggest of the group, 12 feet long and one of the largest freshwater fish ever (and a possible prey of Spinosaurus). Unlike lungfish, coelacanths (also known as actinists) were and are exclusively aquatic: unable to breathe air, they are the most "fishy" among the lobe-finned fish. It's worth noting that during the Dinosaur Age lungfish and coelacanths were very abundant: coelacanths swam in great numbers in the sea, while lungfish lived in oxygen-poor swamps, sometimes obligated to walk out of water when the water pools dried out just like modern lungfish.
Fish that crawled onto Land: Osteolepis, Holoptychius, Panderichthys, Tiktaalik & Acanthostega
- Even though more closely-related to us than to true fish, lungfish and coelacanths were less close to terrestrial vertebrates than some fossil lobefins, the basal tetrapodomorphs (called "rhipidists" or "Osteolepidotes" in older sources). Among them, there was the common ancestor of all tetrapods (aka land vertebrates), but we don't know exactly which one it was. Traditionally the most-often cited is Eusthenopteron. Other relatives included Holoptychius and Osteolepis, both with a more normal-looking, non-"trident" tailfin. In the 1990s/2000s some new animals were discovered, which appear evolutionarily in the middle between a Eusthenopteron and an Ichthyostega: excellent examples are Panderichthys, Tiktaalik, and Acanthostega. The last was very similar to Ichthyostega, but had eight digits on each "hand" instead of seven.
The most common Fish Group today: Leedsichthys, Lepidotes, Leptolepis, Enchodus & Knightia
- Actinopterygians, aka "ray-finned fish" or "rayfins". Or, more simply, "fish". They are by far the most common fish today (95% of all fish species!), but in the Dinosaur Age they were only one of several fish groups roaming the seas and fresh waters. The most derived rayfins, the Teleostei, became widespread only in the Cretaceous. Almost all modern ray-finned fish are teleosts. However, most familiar teleosts appeared only after the K-Pg extinction, in the Age of Mammals. There were no goldfishes, swordfishes, seahorses, piranhas, ocean sunfishes or deep-sea anglers in the Cretaceous (only herrings, tarpons, and a few others). However, non-teleostan rayfins were already common in the Mesozoic, among them gars, sturgeons and their relatives. And then there were things such as Leedsichthys ("Leeds' fish"), a Jurassic fish with no modern relatives that may have as big as a humpback whale and the largest fish of all time. note Another non-teleostan fish was Lepidotes ("the scaly one"). It was one of the most common fish, with more than 100 species that lived over the course of the Mesozoic. Similar to a carp, it was actually not related with any modern fish: its primitiveness is revealed by its heavy armor-like scales. These scales are sometimes found inside the rib cages of other animals, e.g. the fishing dinosaur Baryonyx. Since the Cretaceous, all these early clades have been outcompeted by teleosts, which were more agile thanks to their lighter scales. One of the most common of them was Leptolepis ("thin scale"), an ancient herring-like fish. Another, Enchodus, is nicknamed the "saber-toothed herring" but was not a clupeiform (the herring group). Perhaps the most famous Cretaceous rayfin is Xiphactinus: also similar to a herring but with teeth, it was 5-6m long (comparable to a great white shark), and a voracious predator in competition with the giant sea reptiles of the time. Xiphactinus is most famous for a fossil in which a 4m-long specimen was preserved with a 2m-long fish in its gut. Among the Mammal-Age rayfins, the most common in the fossil record is Knightia, an ancient true herring.
The first Fish with Fishbones: Palaeoniscum & Cheirolepis
- Together, ray-finned fish and lobe-finned fish make up the traditional Osteichthyes, aka bony fish. Indeed, a bony skeleton was their main invention - all earlier fish groups had cartilaginous skeletons, while their bones were only outside their body in the form of scales and teeth. Bony fish (both lobefins and rayfins) appeared in the Paleozoic era, 200 million years before the first dinosaur.note Among the earliest ray-finned fish, Palaeoniscum (traditionally called "Palaeoniscus") and the Devonian Cheirolepis ("hand scale") resembled a cross between a shark and a modern bony fish: mouth placed downwards and asymmetrical caudal fin like a shark's; fins, opercula, and smooth scales like a bony fish's. Curiously some extant archaic rayfins like the sturgeon still preserve this mixed look. Others, like Polypterus (bichir) of modern freshwater Africa, deceptively resemble lungfish in shape & habits.
Spiny Sharks?: Acanthodes & Climatius
- In the Paleozoic there was also a unique fish group that went totally extinct before the Mesozoic: the acanthodians (so-called from their namesake Acanthodes, "the spiny one"). They were externally similar to the earliest ray-finned fishes above, but with a cartilaginous inner skeleton like a shark. Their name means "spiky" from their spiny fins, and have been nicknamed "spiny sharks", but they were actually closer to bony fish than to sharks. Among the other acanthodians we can mention Climatius. Together, sharks, acanthodians, bony fish and placoderms (see below) have one thing in common: jaws. See also "Tough Guys" below.
The earliest "sharks": Cladoselache, Stethacanthus, Xenacanthus & Helicoprion
- If you ask a paleontologist what are the most common vertebrate fossils, s/he'll answer "shark teeth". However, these teeth are usually found isolated - ironically, the remaining skeletons are among the rarest fossil finds (sharks' cartilage doesn't fossilize well, unlike bone). Even though sharks make up only a small percentage of modern fish species (about 5%), they were a very successful group in the past, even more diversified than they are today. But stop a moment. What is a shark, exactly? In common sense, sharks are things like the great white or the bull shark, but zoologists often use this word to indicate every cartilaginous fish, or more technically, every Chondrichthyian. Palaeontologists usually give the same meaning to "shark", too. The first-ever sharks appeared in the Devonian period 400 million years ago, and since then have changed very little: sharks are often cited as "living fossils". More precisely, the modern kinds of shark appeared in the Cretaceous: earlier sharks were only distantly related to them. Examples are Cladoselache, Stethacanthus, and Xenacanthus (once also called Pleuracanthus), all from the Devonian period, and also a little-known modern fish, the deep-sea chimaera (also called ratfish or rabbitfish). The latter has preserved to this day the mixed shark/bony fish anatomy of the acanthodians and the earliest rayfins. Cladoselache ("branch shark") was more like a true shark, with uncovered gills and tooth-like scales covered in enamel (its fossils have preserved prints of soft tissues), but its mouth was strangely placed at the front of its head like the modern, unrelated whale shark. Xenacanthus and Stethacanthus were more bizarre. The former had an eel-like body and a long filament protruding from its head (its name means "strange spine"); it was part of a broader group of eel-like fishes called the xenacanthids, which unlike modern cartilaginous fish almost exclusively lived in freshwater. The latter is nicknamed "ironing-board shark" from its flat head prominence (not a dorsal fin) with many dentines on the top. Possibly only males had these things, which resemble the much smaller tubercle of the male chimaera (if so, they would have been courtship devices). But the perhaps weirdest "shark" ever is Helicoprion ("helix saw"), the "whorl-toothed shark", whose teeth were arranged in a spiral coil inside its mouth! It now appears that both Helicoprion and Stethacanthus may actually belong to the chimaera lineage, thus not proper sharks. Other early chondrichthyans, Falcatus, Harpagofutator, and many others, also showed strange prominences above their heads.
Shark Tales: Hybodus, Edestus, Ptychodus, Cretoxyrhina & prehistoric rays
- Triassic and Jurassic sharks were still primitive, but with a more modern look. One of the most common was Hybodus ("humped tooth"), whose shape recalls a typical predatory shark but with small "horns" on its head, like the modern bullhead shark. This is the shark portrayed in the Walking With series in the two episodes in which Liopleurodon is the main character, acting in both as a minor predator. Other relatives became flatter and rather similar to a ray: Ptychodus is one example. Both Hybodus and Ptychodus belonged to the same group: the hybodonts, more derived than the ones above but more primitive than most our-day sharks. If we imagine swimming in Cretaceous waters, however, we'll see sharks virtually identical to modern ones; by that time, the hybodonts had almost entirely been displaced by the first true selachid sharks and survived mostly by retreating into freshwater environments, which selachids have difficult adapting to, before dying out during the end-Cretaceous mass extinction. Cretoxyrhina (nicknamed "the Ginsu Shark") was a notable early selachid and was very similar to a great white shark; it was in fact one of the first lamniforms, the open-ocean sharks that the great white is part of, which developed bullet-shaped bodies and crescent-shaped tails as adaptions for fast, energy-intensive swimming — other sharks and shark-like fishes had, and have, flattened bodies and tails with long upper lobes more useful for cruising along the sea bed. It was one of the top predators of the Late Cretaceous North American inland sea, in competition with mosasaurs, plesiosaurs, and the bony fish Xiphactinus above. But we might also encounter other kinds of fish which little resemble sharks, but technically are true sharks: rays and skates. They appeared in the Cretaceous, during the same time as those fish commonly called sharks, and have also little changed their anatomy since then.
Tough Guys: Bothriolepis, Pterichthyodes, Coccosteus, Lunaspis & the other Placoderms
- The Devonian is rightly called "the Fish Age". Almost every main fish group was present in Devonian waters: lobe-finned fish, ray-finned fish, cartilaginous fish, acanthodians, and several jawless fish (see further). But the perhaps most typical Devonian fish group was another one entirely: The Placoderms. Their name means "skin with plates", and they're nicknamed "armored fish"; their scales were notably thick and broad and covered their forebody like true armor. However, this armor was still flexible and didn't form a "shell", making these fish more agile than their armored predecessors, ostracoderms; unlike the latter, they were active predators. There were two main placoderm groups: antiarchs and arthrodires. The former had bizarre pectoral fins more similar to crustacean legs than the fins of other fish, and lived near the bottom of the seas: Bothriolepis and Pterichthyodes are the best-known examples. The latter had strange scissor-like teeth for cutting meat, and probably lived in more open waters. Ex. Coccosteus and Dunkleosteus. Another interesting placoderm is Lunaspis, with its crescent-shaped body. There is a curious thing about the evolutionary origin of jaws and teeth: the former arose from the first pair of gills of the earliest jawless fish, while teeth had the same origin as scales. After all, modern sharks still have enamel-covered scales on their body, the same shape as their teeth (only smaller). note In a sense, you could even say teeth are the only fish scales we human still preserve.
The Aspis Family: Hemicyclaspis, Arandaspis, Astraspis, Drepanaspis & the other "Ostracoderms"
- Welcome to the Aspis family. Most "ostracoderms" ("shelled skin") have this suffix. Despite this, they don't form a true taxonomic group: every armored fish devoid of jaws is traditionally called with this name, but they are actually distinct lineages, some closer to jawed fish than to other ostracoderms. Compared with placoderms, ostracoderms' armor was more complete, covering the whole body, and formed a true shell at the head level. Despite their badass look they were very harmless creatures; their size ranged from that of a thumbnail up to a human hand, and with their jawless mouths they could only have feed on tiny food items like algae, small invertebrates, etc. They often fell victim to predators like the eurypterids ("sea scorpions"), cephalopods and jawed fish that were strong enough to get through their thick armored skin. One successful ostracoderm lineage is the Osteostracans ("bony shell"), whose prototype is the flat-headed Cephalaspis ("head shield") but include also Hemicyclaspis ("semicircle shield") among the others. Another is the Anaspids ("with no shield"), more streamlined and with a lighter armor: Birkenia was a typical example. The Heterostracans ("different shell") had often long snouts; Pteraspis ("winged shield") is their prototype. Drepanaspis was a round-shaped example of this group; Arandaspis and Astraspis were related to the heterostracans. Finally, the Thelodonts (named after their namesake Thelodus), which were the most closely-related to jawed fish (and may have been their ancestors).
Our Earliest Origins: Haikouichthys, Yunnanozoon & Myllokunmingia
- However, ostracoderms were not the most primitive fish; they were already highly evolved animals, with complex brains, fins, and keen senses (Cephalaspis seemingly even shows electric sensors!). Their anatomy is unusually well-known because the inner portion of their head shows imprints of the brain, nerves, inner ear, and other soft tissues.note Actually, ostracoderms descended from unarmored fish, among them the very first fish that appeared. Sadly, as soft tissues don't usually fossilize, they are virtually unknown to science. Three exceptions are Haikouichthys ("Haikou's fish"), Yunnanozoon ("Yunnan's animal": Yunnan is one of the southernmost provinces of China), and Myllokunmingia (Kunming is the capital city of Yunnan); all from the Cambrian period and found in China, they were similar to the famous invertebrate lancelet, or to the larval stage of the modern lamprey (the so-called ammocoetes). Haikouichthys was a tiny animal the size of a human nail, and was probably a harmless filter feeder. Incredibly, Walking with Monsters managed to transform even this inconspicuous critter in a "terror": here, a shoal of Haikouichthys is seen feeding on the flesh of a wounded Anomalocaris like modern lampreys and hagfish would do. Actually, lampreys and hagfish (aka the cyclostomates) are highly evolved parasitic animals capable of feeding on large prey despite their lacking of jaws; their evolution is very poorly known.
Cone Teeth: the Conodonts
- Finally, we could not forget the enigmatic conodonts. These tiny fossils similar to toothed jaws (conodont means "cone tooth") have been a real headache for scientists. Invertebrates? Early vertebrates? Worms? Today we know they belonged to early jawless fishes that lived during the whole Paleozoic, but their lifestyle is still uncertain; they may have been the ancestors of the lampreys.
A 50-year old Mystery: Tullimonstrum, aka the "Tully Monster".
- But what was this thing? Found in 1966, and very common in the Carboniferous Mazon Creek Beds of Illinois, it looked like nothing paleontologists had ever seen before. It had a jaw like a crab claw, the body plan of a cuttlefish, and two weird stalked lumps on both sides of its body. The only thing anyone could classify this as was as an invertebrate, probably some kind of worm. Until 2016. A study by Yale University revealed the Tully Monster's true identity, and it was the last one anyone expected. They noticed the remains of a primitive notochord in the creature, meaning it was a vertebrate, and the weird lumps turned out to be eyestalks. Using these, and other parts of the fossil, they concluded that it was a jawless "fish", closely related to the lamprey. That having been said other studies that came out in 2017, 2019 and 2023 have cast doubt on this assessment for varying reasons, so it is still up in the air.
Arthropods
When thinking about animal fossils, we usually call to mind the petrified bones of dinosaurs. But dinosaurs in paleontology are extremely rare finds compared to other vertebrate groups, such as sea reptiles, Cenozoic mammals and fish. And vertebrates as a whole are in turn only a very small part of the total. Indeed, more than 90% of animal fossils that Earth left to us are from invertebrates. Some invertebrate groups like ammonites and trilobites are so common they're objects of collection by many paleo fans of modest means, while it's rare for dinosaur bones to be traded like this — some trade of dinosaur bones does exist, but it's highly debated if it's the right thing to do, since dino fossils are such a rarity.
Crabs and pseudo-Crabs: Prehistoric crustaceans & xiphosurans
- Today, crustaceans are by far the most abundant aquatic arthropods. Their prehistoric relatives were similar to modern ones, and have always been an important group. Examples of Mesozoic crustaceans include the shrimp Aeger, the crab Eryon, and the enigmatic Thylacocephalans. However, in the Paleozoic another group of aquatic arthropods was even more diversified: the early chelicerates, more related to spiders than to crabs. The most spectacular water chelicerates were the so-called sea scorpions (see the following paragraph), though the smaller xiphosurans were just as abundant. Like the coelacanth and the nautilus, they have classically been cited as "living fossils". Indeed, the modern Limulus (the horseshoe crab) is just the only surviving xiphosuran, and the only surviving aquatic chelicerate other than the little-known sea spiders or pantopods. All the other chelicerates became terrestrial, giving rise to the arachnids (true spiders, true scorpions, whip-scorpions, pseudoscorpions, soliphugans, harvestmen, ticks, mites and others). Xiphosura means "sword tail". Contrary to what is believed, the "sword" at the end of the horseshoe crab is not venomous or dangerous at all (it would probably lightly prick your foot if you stepped on it, but that's pretty much it), and is only a mechanical device to right the animal when capsized. Many extinct xiphosurans were identical to our modern Limulus, and probably behaved the same; our horseshoe crab lives mainly at the bottom of the sea like a trilobite, feeds only on small items, but comes ashore to lay its eggs. Curiously, its young look very similar to trilobites, although the two clades aren't closely related.
Scorpions of the Sea?: Eurypterus, Megarachne, & the other eurypterids
- When talking about evolution, the superpredators are usually presented as a sort of foe that tries to destroy our distant ancestors, almost as if their whole purpose was to delete modern Man's presence on Earth. Just see Walking with Monsters for an egregious example. When the portrayals are of Mesozoic life this treatment is typically reserved for dinosaurs (see The origin of Mammals); in Paleozoic settings, however, the classic choice has been the eurypterids (also called Gigantostracians, "giant shellfish"), better known as the sea scorpions. They were indeed related to modern scorpions (and may have been their ancestors), but technically they were not scorpions, and not all were marine (some became freshwater dwellers). They didn't have venomous stingers, and more resembled slender lobsters than scorpions. They had big compound eyes like insects, scorpion-like pincers, and their rear pair of legs were flattened and used to swim; they arguably lived both on the seafloor and in open waters. We don't know if they came on land to lay their eggs. Eurypterids were active predators, and the biggest ones were among the apex predators especially in the Silurian period; in the following Devonian they were outcompeted by jawed fish like Dunkleosteus, but they managed nonetheless to survive until the Permian. Eurypterus ("broad wing"), the namesake of the group, is the most common eurypterid in the fossil record; but the most famous is Pterygotus ("the winged one"). It was one of the biggest gigantostracians (the length of a human), and one of the biggest arthropods of all times along with the extinct millipede Arthropleura and the modern Giant Japanese Crab. Pterygotus is the sea scorpion most commonly portrayed in media, because of its size of course. Ironically, however, its even-larger cousin, Jaekelopterus was the largest-known eurypterid, and yet has still not appeared in broad-audience media.
Out of Water at last! Palaeophonus, Arthrolycosa, Euphoberia, Rhyniella & other non-insect land arthropods
- Within the evolution of animal life, it is universally agreed that arthropods and vertebrates have been the two animal groups which achieved the greatest success. Cephalopod molluscs, too, are very complex creatures; but they never managed to come on land. Arthropods and vertebrates did that, but it was the former which took the first step on dry land, in the Silurian period. Vertebrates joined them only later in the Devonian. Even when out of the liquid element arthropods and vertebrates have continued to coexist and co-evolve, and this competition has made both more and more perfected. It's actually unfair to declare arthropods have been the vertebrates' worst enemies, and that the latter had to fight a "war" against spiders-scorpions-insects (as said in the preface of Monsters); indeed, arthropods have unwillingly helped us vertebrates to became those successful beings we are. Among the very first land arthropods were the first true scorpions and the first myriapods (millipedes and centipedes). Critters such as Palaeophonus were already identical to modern scorpions; the same about the earliest myriapods like Euphoberia. Spiders appeared a bit later, in the Carboniferous (ex. Arthrolycosa); the first non-insect hexapods (hexapod = six legs) evolved in the Devonian (Rhyniella), but the first winged true insects took their first flight in the Carboniferous forests: they were the very first flying animals ever, and the only flyers until pterosaurs made their appearance in the Triassic, followed by birds and finally bats. In the Carboniferous, land arthropods became often huge; two in particular have become a staple in paleo-books and documentaries: Arthropleura and Meganeura. They have even made some appearances in fiction, too.note Walking With has also popularized other critters: the huge true scorpions Brontoscorpio from the Silurian and Pulmonoscorpius from the Carboniferous, the alleged giant spider Megarachne (it was actually a freshwater eurypterid that only looked spider-like, also from the Carboniferous) and the Early Cenozoic giant ants seen in Beasts. All, more or less, affected by Rule of Cool in the show.
Bugs Everywhere! The True Insects
An Extraordinary Success
- Insects. The most abundant, diversified, biomechanically efficient, and last but not least, by far the most studied animals by modern fauna zoologists. Sadly, in paleontology this is not the same. Their fossil record is extremely poor, for understandable reasons: their tiny bodies aren't precisely the best-suited to turn into stone, and their terrestrial habitat doesn't help either - most fossil animals discovered so far were aquatic, just because water helps a lot in the process of fossilization. However, for what we do know about the ancient relatives of modern insects, we have to thank pines, firs, spruces and larches. Remember Jurassic Park, and those fossilized mosquitoes in amber from which dinosaurian DNA was extracted? The DNA extraction thing is obviously fictional, but the amber thing itself is Truth in Television. Insects preserved in amber are perhaps the most marvelous fossils a paleontologist could wish for. Not only are they perfectly preserved in every detail, included external anatomy and even color (an extreme rarity among fossils), they even have their original tissues preserved, single cells included. And they can provide extraordinary clues to understanding the entire ecosystem in which they lived in indirect ways. Unfortunately, most insects fossilized in amber come from the Cenozoic era (the Mammal Age), a period by which insects were already similar to their modern relatives. We know little about their Mesozoic ancestors, still less about the Paleozoic ones - except for those living in the Carboniferous (such as the aforementioned Meganeura) because many of them did preserve well in coal. Anyway, we know some things with a good degree of certainty.
The First Flyers
- The first insects appeared in the Devonian. Technically these were the first hexapods, since springtails are no longer considered proper insects by modern entomologists; however, we follow the traditional view because it's more convenient. Anyway, they appeared later than scorpions and millipedes. They were still wingless, like modern springtails and silverfish, but then the Carboniferous saw their success: the very first flying animals appeared, among them the now-extinct Palaeodictiopterans with six wings. In this period they reached large sizes - the four-winged Meganeura had a wingspan of more than two feet - and started their radiations, which continue even today: not only dragonflies, but also cockroaches, grasshoppers and beetles appeared first in the Carboniferous. Other groups began their history in the Triassic: moths, wasps, flies and true bugs appeared at that time or a bit later, as well as parasitic kinds like fleas and lice. Insect evolution has always been intertwined with that of terrestrial plants. It's worth noting, however, that this relationship has always been far stricter than one may think: biologists talk about a veritable co-evolution between insects and seed-producing plants, especially the flowering ones (angiosperms).
Pollinators and Social Insects
- This partnership reached its climax in the Cretaceous, when flowering plants became the new dominant group, just because of the relationship with two new kinds of insects that had just appeared: pollinators and social insects. These categories aren't clades, but describe behavior and have some overlap - pollinators include butterflies, moths, bees, wasps, flies, mosquitoes, and even some beetles, while social insects include ants, bees, wasps, and termites. Both began to affect their ecosystems dramatically, indirectly conditioning the evolution of all the other terrestrial animals, dinosaurs included. Many paleontologists think that if modern birds and placental mammals are the most diversified land vertebrates today, they have to thank the insect-plant mutualism which has created suitable habitats for their (initial) small size and varied diets. Think about those birds and bats who feed only upon nectar, anteaters and pangolins which feed upon nothing but social insects, or the countless insectivorous/"angiospermivorous" modern animals. And think about all the plant-related products we humans utilize today. Keep this in mind, every time you crush a bug.
Molluscs & Other Invertebrates
No other animal group has had a greater importance in paleontology than molluscs. Their fossils are extremely abundant, to the point that many rocky formations are composed mostly of fossilized mollusc shells. Among molluscs cephalopods deserve a mention apart, being much more "evolved" than the others. Together with arthropods, cephalopods are the extinct invertebrates you're most likely to see in media - at least, documentary media; you'll rarely see a trilobite, ammonite, sea scorpion or nautiloid in Fictionland. If it happens, they'll be simple "ambient critters", and good luck if the animal is mentioned. There isn't much to say about the portrayal of extinct non-arthropod/non-cephalopod invertebrates in media: they rarely appear even in books and documentaries, much less in Fictionland, and when they do, they are almost never named (except sometimes for the names of each group, but only in popular science works). As an example, the original Disney's Fantasia showed several modern critters to symbolize the early evolution of invertebrates, but few or no truly prehistoric ones. The Walking With series did the same: only modern jellyfish, sponges and sea urchins appear, all live-action. Indeed, many modern invertebrate groups have populated our seas since the Cambrian Period, but many others are extinct today. Among modern land arthropods expect to see dragonflies, scorpions, centipedes, spiders, cockroaches, beetles, and sometimes crickets. They will be oversized: roaches the size of a rat, scorpions as big as cats, and so on. Finally, let's not forget a staple in every Paleozoic or Mesozoic sea: a jellyfish, used as the symbol of the most ancient forms of life.
Early Tentacles: Extinct Octopuses and Squid
- Extinct cephalopods have given fuel to many legends, even before paleontology itself was "invented" by Georges Cuvier at the end of the 18th century. Before that, those strange things today called fossils were believed to be Nature's jokes, the Earth's flowers, or other things (only Leonardo da Vinci correctly recognized their nature, but his discovery was long ignored). And then there are more specific legends about cephalopod fossils. Ammonites were believed to be stony horns ("ammonite" comes from Amun, an Ancient Egyptian deity who was often portrayed with rams' horns), or petrified snakes.note The lesser-known belemnites (technically belemnoids), with their straight pointed shape, were believed to be stony arrows, or even the Devil's fingers! Belemnites were cephalopods living in the Mesozoic era together with ammonites, and probably gave rise to squid. Like ammonites, only their shells are usually preserved. This shell was inside the animal's body and invisible in life; belemnites would resemble simple squid or cuttlefish if alive today. Their lifestyle was more active than ammonites, and they were probably able to do the same things modern squid do (spraying ink, swimming using the lateral "fins", catching prey with their suckers, seeing images with their eyes). True modern squid became widespread in the Cretaceous, and the first octopuses (Proteroctopus for example) were already around in the Middle Jurassic; these molluscs (called "coleoids") usually have no shell inside, or at least only a remnant of shell (ex. the famous cuttle-bone). Some Cretaceous squid were as large as a modern giant or colossal squid.
Finding Nemo: Orthoceras & the other "Nautiloids"
- Some fossil animals' names recall literary characters, rather than mythical things: nautiloids (literally "similar to the Nautilus") are named for the modern Nautilus, whose name (meaning "sailor" in Greek) incidentally recalls Nemo's submarine (we're talking about the human captain, NOT the cartoon fish!). Actually, nautiloids are an artificial assemblage of all cephalopods which were neither ammonoids (ammonites) nor coleoids (octopodes, squids, belemnites etc.). They include both the modern nautilus and the most ancient cephalopods, widespread in the Paleozoic. Their traits were probably like those of the nautilus: their shell was divided in sections which could be filled alternately with water or with air like in a submarine. This allowed them to regulate their buoyancy (ammonites had a similar inner-shell anatomy and probably did the same). As the nautilus' eyes are simple chambers lacking any lens and incapable of forming images, this was probably true for other nautiloids as well. They likely had dozens of sucker-less tentacles, and were probably slow swimmers. However, many Paleozoic nautiloids had straight conical shells, not curly like the nautilus; Orthoceras ("straight horn") is the classic example. Its shell even shows traces of its original colors. Rhizoceras ("root horn") had a slightly curved but still conical shell. Rayonnoceras was more similar to Orthoceras, but was longer than a human and lived in the Carboniferous. The Walking With series has popularized another straight-shelled nautiloid, the Ordovician Cameroceras (named the "giant orthocone"), chosen as the prototypical Paleozoic cephalopod only because it was 10m long and the biggest known nautiloid so far (and one of the largest Paleozoic animals in general).
Clams and pseudo-Clams: Rudists & prehistoric Lingula
- Gastropods, scaphopods, bivalves, brachiopods, chitons, monoplacophorans: who's the intruder? All these invertebrate groups were "shellfish", and all but one were molluscs: the exception is the brachiopods. They were only distant mollusc relatives, and more closely related to the coral-like bryozoans (see "sessile invertebrates" below). As a whole, molluscs and brachiopods are extremely abundant in fossil records of all ages, from the early Paleozoic up to the modern era. However, while molluscs are still a dominant group in modern seas, brachiopods are nearly extinct today, and thus cited as "living fossils". The most representative is probably Lingula ("small tongue"), a clam-like, filter-feeding animal that lives anchored to the sea floor with a fleshy protrusion (the "tongue"); prehistoric Lingula have been identical to modern ones since the Early Ordovician! Among extinct non-cephalopod molluscs worthy of note are the Rudists like Hippurites. Exclusively Cretaceous, these clams are notable both for their often-great size (some were as tall as a human), and for their unique shape. They can be described as giant cups with a lid: the lower valve (the one attached to the sea bed) was conical and much bigger than the flat upper valve. Like Lingula, rudists were probably almost motionless creatures that filter-fed on tiny organisms.
Geometrical Guys: Cystoids, blastoids & prehistoric crinoids
- Crinoids, echinoids, asteroidsnote , ophiuroids, holoturoids, blastoids, cystoids, carpoids: who's the intruder? To some extent the carpoids (see the following paragraph); all the others are confirmed to have been echinoderms. The asteroids ("star-like") are the starfish; ophiuroids are nicknamed "brittle stars" or "serpentine stars"; the echinoids ("husk-like") are the sea urchins and sand-dollars; while the holoturioids are the sea cucumbers or trepang. Thanks to their hard "skeleton", all these groups (except for the soft-bodied sea cucumbers) are common fossils, and have roamed the seas since the start of the Paleozoic. However, we're going to talk more about three other less-familiar groups of echinoderms: crinoids, blastoids and cystoids. Crinoids ("sea lilies") are the only ones still living, but are today a rarer sight than the four groups above; like brachiopods, they are considered "living fossils". They're not-very-movable filter feeders, and like all echinoderms, they more resemble plants than animals: symmetrically rayed and with several "arms" that make them look like ferns or flowers (hence "sea lily"). Adult echinoderms lack eyes, limbs and heads (but have a mouth nonetheless) and they usually lack distinct right and left sides to their bodies - however, their tiny planktic larvae do have bilateral symmetry. The astonishing thing is, echinoderms are among the closest relatives of vertebrates, being Deuterostomes like us and not Protostomes (like most invertebrates). Blastoids and cystoids are known exclusively from fossils, and were similar to crinoids but without the "arms"; their lifestyle was probably similar to the sea lilies'. Also worthy of mention are the (also extinct) edrioasteroids, which were perhaps similar to sand dollars or sea urchins.
Our Relatives?: Graptolites & Calcichordates
- In Prehistory, some odd-looking invertebrates were even closer relatives to us vertebrates than echinoderms were: graptolites and calcichordates (or homalozoans). Graptolites are among the most common fossils in the Paleozoic, and used as index fossils as an alternative to trilobites. They resembled floating aquatic plants, but the "plant" was not the whole organism. They were colonial animals like modern corals, and the individuals were actually inside the colony. Like brachiopods and crinoids, graptolites were passive filter feeders. Calcichordates (remember, they are the carpoids above) have been variably classified as primitive echinoderms, relatives of the graptolites, or even true chordates (that is, the group including sea-squirts and their relatives + lancelets + vertebrates). Calcichordates lived at the bottom of the sea, were movable animals (and bilaterally symmetrical unlike a typical echinoderm), with armor and a sort of "tail".note However, one of the most depicted calcichordates, Cothurnocystis elizae, lost its bilateral symmetry altogether for unknown reasons.
Survival of the Toughest: Prehistoric sessile invertebrates
- This is a merciless law even in palaeontology. With rare exception, only hard parts of organisms' bodies fossilize: bones, shells, armor, jaws, teeth, and whatnot. Many surviving invertebrate groups which don't have such tough elements have left little if any fossil record. For examples, most "worms" (annelids aka segmented worms, nematodes aka round worms, platyhelmintes aka flatworms, nemertheans aka ribbonworms, the microscopic rotifers aka "wheel-animals", the equally tiny tardigrades aka "water-bears", which thankfully have a few fossils in amber from the Americas, and many others) are virtually unknown in paleontology, and their evolution can only be guessed at. This is also true for coelenterates - that is, the cnidarians (jellyfish, sea anemones, hydrae, siphonophores) and the ctenophores (comb jellies). At least, the totally soft ones; fortunately, corals (which are also cnidarians) build tough external skeletons that fossilize well, and their extinct relatives are well-known from the start of the Paleozoic. The same is true of the unfamiliar bryozoans (moss animals), very similar to corals, but more closely related to the brachiopods. Finally, the enigmatic sponges (which many zoologists scarcely consider real animals) are also common fossils. They too have had an inner "skeleton" made of limestone (and sometimes glass-like silicon or a horny material, but these usually don't fossilize); one extinct group of possible sponge relatives were the archaeocyathans, which became the planet's first reef-building animals in the Early Cambrian.
A Treasure in the Rocks: Foraminifers, including nummulites
- Foraminifers ("forams" for friends) are really a treasure in the rock, in all senses. They have helped confirm the deep impact theory of dinosaur extinction, their shells have contributed to the formation of sedimentary rocks around the world, and they are cool in their own right, with their immense variety of shapes. The biggest forams, the nummulites, look often like literal coins emerging from the rocks (nummulus means "little coin" in Latin); they are used as index fossils for the Cenozoic era, and also created the material for the Egyptian pyramids! But wait: what are actually the foraminifers? Originally, scientists thought they were molluscs or mollusc-like critters; today we know they weren't even true animals. They were protozoans, aka single-celled organisms with animal-like traits. Most other one-celled "animals" have left very little if any fossil material; the ancestors of Amoeba, Paramecium, Noctiluca, Euglena, Vorticella, and so on are a mystery. Even the radiolarians: their shells are siliceous, and silicon usually dissolves before fossilizing. However, the Tintinnids (little-known ciliates today) have left noteworthy remains of their "shells" in the fossil record.
Once Upon a Time... The Cambrian Animals
An Unbelievable Luck
- The Cambrian. The first period of the Paleozoic era, in which the famous Cambrian explosion of life happened. The less famous fact is that we're unbelievably lucky to know anything about this remote event. At that time, animals just were starting to achieve hard parts in their bodies, and we already know that, usually, soft-bodied organisms don't fossilize at all. By astounding luck, one of the greatest exceptions to this rule are some deposits from the Cambrian period: it almost seems the natural world did this deliberately for us. The most famous and historically relevant is the Burgess Shale in British Columbia (first found in 1909 by Charles Walcott), but others are known (for example that of Chengjiang, China). For obvious reasons, our first question is: which were the first animals (not counting protozoans) that thrived in our oceans? Well, the answer isn't simple, but we can divide them into two groupings. One is made of those clades either still alive today or that went extinct several ages after the Cambrian: among the former, most non-arthropod/non-cephalopod invertebrate groups already mentioned; among the latter, the trilobites. We'll talk here about the second grouping: many Cambrian invertebrates were indeed exclusively of the Cambrian and didn't survive long, not even reaching the following period, the Ordovician - in which the most famous Paleozoic critters, sea scorpions, nautiloids, ostracoderms, etc. appeared. Thus, many of them are classically viewed as Real Life examples of Our Monsters Are Weird by paleo-fanatics. We still know very little about their lifestyles, but their appearance is extraordinarily well-known, because these Cambrian deposits have preserved soft bodies; not only that, they have preserved them very well! It would take too long to mention all the members of the Cambrian fauna: see the image here to get an idea. The large guy in the center is immediately recognizable, by far the biggest creature in this fauna: Anomalocaris. Of course it is the most portrayed Cambrian animal in documentaries and illustrations, often called "the first sea monster to appear on Earth". Actually, if alive today, the "terrible" anomalocaridid would look vaguely like a lobster, 3 ft long, shell-less and pincer-less... nothing dangerous for a tough-boned, tough-muscled, tough-skinned mammal such as a human.
An Ancient Sea Zoo
- Apart from Anomalocaris, the three probably most-portrayed Cambrian animals were very different from each other: Hallucigenia, Opabinia, and Pikaia. The first has a name that means "hallucination generator"; it was a sort of "worm" with long paired spikes on its back, of uncertain purpose, and long soft paired legs. The reconstruction of Hallucigenia was an astounding Science Marches On tangle for many years: it was first reconstructed upside-down, with the dorsal spikes believed to be legs, and the legs pointing upwards and thought to each end with a small mouth! Its taxonomy is uncertain, but it's tentatively classified as a lobopod, a distant relative of arthropods. Opabinia was related to Anomalocaris, but even weirder-looking; perhaps no other fossil animal more resembles a fictional space alien. It had five eyes placed in circular fashion on its head, and a pincer at the end of a long, flexible proboscis, often mistaken for the mouth, which was actually located behind the proboscis. When it was first described, many paleontologists didn't believe its describer was serious and openly laughed at it! But the most important find is the third guy, Pikaia: despite its rather insignificant slug-like appearance, it is the most well-known vertebrate ancestor, a sort of prehistoric relative of our lancelet (the closest extant relative of vertebrates). This relevance has made Pikaia one of the unofficial symbols of evolution, just like the ur-amphibian Ichthyostega, the ur-bird Archaeopteryx and the ur-horse Eohippus. Interestingly, while most of these organisms appearances in the media were only limited to documentaries, Hallucigenia particularly had gained a sudden fame when it revealed to be one of the main plot device in Attack on Titan. So much so that almost every YouTube video that makes mention of it would have its comment section filled with people quoting the series.
Missed Opportunity
- But wait... have you seen these critters on TV at least once? Unlikely, even if you watched Walking with Monsters. In this Rule of Cool-filled show, the only real Cambrian invertebrate to appear is... Guess what? Well, the superpredator Anomalocaris of course! The other two invertebrates that show up are... a modern jellyfish and an anachronistic phacopid trilobite - remember that phacopids first evolved in the Ordovician, while Cambrian trilobites looked very different from the classic image we have when thinking about these animals. The absence of such awesome animals like Opabinia and Hallucigenia - and still others, like the multi-tentacled Wiwaxia, the trilobite-like Marrella, the caterpillar-like Aysheaia, the slug-like Amiskwia, the shrimp-like Waptia, the lobster-like Sidneyia, the jellyfish-like Eldonia, the "hairy worm" Canadia, etc. - is another egregious example of a missed opportunity. Speaking of Pikaia, this time its absence is less of a problem: the aforementioned proto-vertebrate Haikouichthys filled its role. And another thing: if you read the list of creatures from the Burgess Shale, you'll note almost all animals (the main exception being Anomalocaris) have uncommonly short scientific names, most of them ending in -a. Rather amusing to read, and - let's face it - a true oasis of happiness among so many other unutterable, absurdly-difficult names.
Plants and Fungi
When thinking about fossils, we tend to think of animals. But plants have also left many remains, some of them just as spectacular as the animal ones (think about the petrified woods, the most famous being that in Arizona), and others less striking but even more significant, such as prints of leaves (very common in some deposits) and even fossilized pollen which has allowed us to understand not only the composition of ancient flora, but also the climate they lived in. And, naturally, the aforementioned amber which has often caught insects inside.
A Floral Aroma in the Cretaceous: Prehistoric magnolias, water lilies & true palms
- The most familiar plants today are the angiosperms aka flowering plants, including most modern trees, bushes, and herbs, but also seemingly non-flowering plants such as grass, palms, bamboos, and even some "seaweed" (like Posidonia). But as a group, they appeared only in the Cretaceous, or at least, became widespread only in that period. In modern taxonomy the angiosperms are called magnoliophytes (literally "magnolia plants"). Indeed, the magnolia was one of the earliest flowering plants to ever appear, in the Lower Cretaceous. This is the most commonly shown angiosperm in paleo works, which often portray herbivorous dinosaurs like Iguanodons and Triceratopses eating magnolia leavesnote . Among the other earliest angiosperms are water lilies. The first palm trees also appeared in the Cretaceous (but note that most Cretaceous palm-like trees were not palms, see further). However, most modern flowering greens - roses, apples, figs, oaks, etc. - appeared (or became a main component of the vegetation) only AFTER the dinosaurs' extinction. The angiosperms' success is tied to their relationship with pollinating animals, especially insects (see in the Arthropod section above), but also to those mammals/birds which still aid them in dispersing their seeds by eating their fruits, or by other means. However, some flowering plants returned again to traditional strategies, using the wind to disperse their pollen like the more archaic pines/firs do. Among them, ironically, are the most highly evolved and successful ones: the Poaceae, aka the grasses. See below.
Grasslands at last!: Prehistoric grasses
- Early in the Age of Mammals, landscapes were already similar to those surrounding us today... except for one thing: grasslands were still totally missing. Prairies and savannah appeared only 30-20 million years ago in the middle of the Cenozoic. Grasses were already living in the Cretaceous, but they still didn't form grasslands, they grew isolated within the undergrowth. Grasslands have had a crucial role in Earth's ecosystems: all modern herbivorous mammals living in the modern African savannah (just to take one example) were able to evolve only thanks to grass. They developed special teeth to cope with this especially tough food, and thanks to the scarcity of trees, they were able to become bigger and faster. We humans have to be grateful to the grasses, too. All cereals and most fodder belong to this family of plants, as well as bamboo and sugar cane, as well, naturally, as the common grass of lawns. However, their importance has been even greater than what you might think: if the grasses didn't evolve million years ago... you would not be here to read this. Simply put, without grasslands, the human race still would be small-brained, hairy, chimp-like apes. (see in the Mammal section).
Dinosaur Trees: Prehistoric ginkgos
- For the majority of the Age of Reptiles, however, most land plants had no flowers. Non-flowering plants belonged to two main groupings: those reproducing with seeds, and those reproducing with spores. The former are traditionally called "gymnosperms" ("naked seeds", due to the fact that their seeds aren't enclosed in fruits like flowering plants' are), but are actually several groups of plants not particularly closely related to each other. The most familiar gymnosperms are, obviously, the conifers (see below). In paleo-books, however, you'll often see mention of Ginkgo biloba. This is, indeed, the only surviving member of a whole group of seed plants (the Ginkgophyta) that were among the dominant greens in the Mesozoic. Its special status results in the ginkgo often being cited as a living fossil. However, it doesn't look like a pine or a fir: with its wide leaves and soft, fruit-like seeds, it resembles a typical flowering plant that you just never catch flowering. Indeed, ginkgophytes are probably the closest relatives of angiosperms. Even though the modern ginkgo is grown around the world as an ornamental tree, its wild ancestor lives only in East Asia.
A Resiny Aroma in the Jurassic: Prehistoric pines and firs, redwoods, yews, podocarps & monkey puzzles
- Fictional media would typically have us believe the trees from the Mesozoic were all palm-shaped. If non-palmlike trees appear, they usually have the shape of an angiosperm. Pines, firs, spruces, larches, cypresses and cedars are usually unseen in Fictionland... possibly because are usually associated with cold in the writers' minds (while Hollywood Prehistory and the Age of Reptiles are always in a very hot world filled with volcanoes). In Real Life things were very different. Conifers were among the dominant plant groups for the whole Mesozoic, and a common food source for camarasaurs, camptosaurs, centrosaurs, cetiosaurs, chasmosaurs, corythosaurs, and so on. However, there weren't just pines and firs note at the time: more common were some kinds of conifers which are rare or extinct today. For example, the araucarians (monkey puzzles), the podocarps, the yews, and the most spectacular of all, the sequoianote . If you think sauropods were the real titans of the Jurassic, think again: a Giraffatitan near a redwood would look like a house cat next to a full-grown man.
Palms, but Not: Seed ferns, pseudo-cycads & prehistoric cycads
- These are the Mesozoic seed plants that most resemble those seen in fictional portrayals of the Dinosaur Age. They did look like palm trees, but were not related to them at all. Cycads are the only ones still living, often used to embellish our cities. Bennettitales or cycadeoids ("pseudo-cycads") went extinct at the end of the Mesozoic. These two groups were very abundant in dinosaur times, but were already present before the Triassic (as well as the little-known Cordaitales, which may have been the ancestors of the conifers). Pteridosperms ("seed ferns") are so-called because of their external appearance, but were not true ferns, which reproduce with spores. Seed ferns were among the very first seed plants that ever appeared, in the Devonian, and survived until the Cretaceous. They actually predate the evolution of seed-bearing organs (cones and flowers), instead bearing seeds on their fronds, like true ferns do with spores. One of them, the Triassic Glossopteris, was widespread in the southern portion of the Pangaea supercontinent. This pteridosperm has been used to demonstrate the Pangaea hypothesis itself: its fossils have been found in every modern southern continent, showing landmasses were still united at the beginning of the Dinosaur Age (see also Lystrosaurus and Mesosaurus on other pages).
The Mesozoic Undergrowth: Prehistoric ferns & horsetails
- One of the worst errors in popular media (sometimes even documentary media) is to portray grasslands in the Mesozoic. Arguably, writers think grass is the simplest kind of plant ever... thus, the first ever to appear on Earth. As seen above, grass is actually some of the most derived plants and among the latest to become widespread on our planet. In the Dinosaur Age, the dominant small land plants were much, much more primitive: ferns, their close relatives, and horsetails. These are collectively called pteridophytes, and their modern descendants still make up a wide portion of the undergrowth in many forests. In the Mesozoic, ferns and horsetails already made up much forest undergrowth, but also formed true prairies where trees were absent. Only in the Age of Mammals were fern prairies definitively replaced by grasslands. But don't think ferns and horsetails were always small: in Prehistoria there were also giant horsetails and tree ferns, both deceptively similar to trees. In some places, they still live today: the 10m tall Equisetum giganteum is found in tropical landscapes. And tree ferns are still present in Australia and New Zealand - to the point that in the latter country they have become a national symbol, just like the kiwi bird. Indeed, the Land Down Under and its neighbor archipelago are a real mine of "living fossils", not limited to the platypus, the echidna, the kiwi, or the tuatara.
The Paleozoic Overgrowth: Lepidodendron & Sigillaria
- In some ways, Mesozoic vegetation wasn't so different from ours. True, most land plants in dinosaur times were pine-like, palm-like or fern-like, and flowering plants were still a minority... but conifer forests are still widespread today in northern territories. But if our time machine could bring us to the Carboniferous (aka the Coal Age, 100 million years before the Triassic), landscapes would really look like a sort of Otherworld. At the time, all the plant groups mentioned above were either a small portion of Earth's vegetation, or hadn't yet appeared. The dominant plants were the so-called lycopods - named after Lycopodium (lit. "wolf foot"), a tiny plant which is one of the few examples still surviving. Many of them would resemble extraterrestrial trees if alive today: Lepidodendron (lit. "scaly tree") and Sigillaria are two often-cited examples. Their trunks were indeed scaly, they sometimes had only two branches, and some grew up to 30 meters tall! Despite their massive aspect, they were quite fragile giants: giant lycopods were easily felled by the frequent Carboniferous storms. This was less a liability for them as a species than it would be for modern trees, because even giant lycopods grew rapidly, with a life cycle of no more than 10 or 15 years. The rotting logs then carbonized, becoming the fossil coal we burn today (hence "Carboniferous", meaning "coal-bearing"). It's worth noting that lycopods, ferns and horsetails have always been strictly tied with water; their spores can develop in adult individuals only in humid habitats, and this explains why they were so common in the Carboniferous swamp world. Then, in the following Permian period, the Earth mostly dried out, and giant lycopods weren't able to survive the change. Seed plants have become the more successful plant group since then: seeds are a bit like the reptiles' and insects' shelled eggs, well-adapted to survive in arid environments.
Paleo-Christmas Trees: Calamites
- These conifer-looking trees were a common sight in the Paleozoic era from the late Devonian period up to the early Permian period, with the Carboniferous period being the peak of their diversity. Despite their looks, however, Calamites were actually distant relatives of modern horsetails, and thus, like with other early plants like it, were strictly tied to swampy environments. Their trunks were usually hollow and resemble modern bamboo with vertical growth lines from the base to the canopy, and each segment contained rows of branches with 25 needle-shaped leaves each. They were known to reproduce by directly cloning themselves from underground root structures called rhizomes, the only plant of its age to even do so, which allowed them to remain anchored to loose wet ground and spread rapidly. Their resemblance to Christmas trees led to many paleontologists joking about if a time traveler were to spend the Christmas holidays in the Carboniferous, Calamites are the way to go.
The "First Tree": Archaeopteris
- Sometimes it seems paleontologists have fun making cryptic jokes with scientific names. While the traditional "first bird" is called Archaeopteryx, one of the first land plants to ever appear is the almost-homonymous Archaeopteris. But wait, Archaeopteryx means "ancient wing", Archaeopteris means "ancient fern". This plant lived in the Devonian (before the Carboniferous), just when the proto-amphibian Ichthyostega made the first step on dry land; it was one of the first terrestrial plants to develop to the size of a tree. Since at the time land animals were very few and mostly carnivorous or detritivorous, Archaeopteris and its relatives were able to spread worldwide, but never far from water, just like amphibians. Their appearance was like that of a tree fern: indeed, the fern's shape is considered one of the most primitive body plans among terrestrial plants. However, plants went on land before the amphibians. In the Silurian period (before the Devonian) there were already some aquatic plants (ex. the Psilophytes) emerging out of water; Cooksonia is an often-cited example. These tiny plants still kept their roots underwater, but their "branches" grew above the water's surface, capturing extra light. Terrestrial plants are actually very evolved organisms and have worked hard to develop adaptations for surviving outside the liquid element - among them, a vascular system (that is, tiny vessels for the flow of fluids), waterproof "skin", and fibers to make their stalks more robust against gravity.
Fungus Humongous: Prototaxites
- This species was first described in 1843, and for the longest time experts couldn't decide what it was. Many thought it was a partially rotten conifer tree (its name means "First yew"), others thought it was a giant aquatic alga. It wasn't until 2007 that scientists confirmed it was a giant terrestrial fungus! Since this thing grew up to 8 meters, it would've been by far the tallest organism of its time, from the Middle Ordovician to the Late Devonian. Many are still confused as to how such an organism could grow like that without photosynthesis: theories include the lack of predation and competition throughout most of its existence, due to land only being occupied by small invertebrates and short plants by then. The identification of molecules often associated with algae also suggests the fungus might have been a giant lichen - it is, a fungus in a symbiotic relationship with algae, in which the fungus provides resources and protection and the algae provides the carbohydrates it gains from photosynthesis.
Plants?: Prehistoric Mosses, Algae, Fungi & Lichens
- These beings, important in the modern world, are poorly known in paleontology. Their "bodies" fossilize very rarely, but they have been major components of both water and land ecosystems since the Paleozoic. On a side note, only bryophytes (mosses, hornworts and liverworts) can correctly be called "plants". Algae is an informal group of not very closely-related clades, some of which (cyanobacteria or blue-green algae) aren't even plants in modern systematics. Neither are fungi (mushrooms, toadstools, molds and yeasts), which can't even perform photosynthesis - they are actually more closely related to animals than to plants! And lichens are not even true organisms: they are the result of a symbiotic relationship between a fungus and an alga. Diatoms, single-celled algae, have a shell made of silicon (like the protozoan radiolarians) but silicon usually doesn't fossilize in rocks.
The Origin of Life
The creatures here are from the Precambrian, and lived/originated before the other organisms listed in all the other "Prehistoric Life" pages. Describing them is a difficult matter even for the most skilled biologists. Our minds have trouble imagining how the first living beings even looked, let alone how they lived. However, not everything is unknown to us.
The Greatest Paleontological Mystery: The Ediacara creatures
- Prior to 1956, the general consensus was that complex life didn't really start appearing until the Cambrian explosion, but a landmark discovery in England that year proved just how wrong that was when the 560-million-year-old organism Charnia was found. Since then, a myriad of different creatures has been discovered from the so-called Ediacaran period that existed between the Cryogenian Glaciation and the Cambrian explosion. Many experts think they were neither plants nor animals, but something else entirely. Notable groups include the uniquely tri-radial Trilobozoa, the ribbed oval "bags" knows as Proarticulata, the fern- or sea pen-like Rangeomorpha of which Charnia was a member, and the, um... "fleshy spiral" Eoandromeda. But despite all these Ediacaran Biota (often improperly called "Ediacaran Fauna") being wildly different in appearance, they do have things in common that define life from this era: they almost universally exhibit glide reflection in their body plan, they appear completely unrelated to modern animals/plants/fungi as mentioned above, they almost completely disappeared at the end of the Ediacaran period, and they were arguably completely sessile, probably either filtering nutrients out of the water or "growing" on top of microbial mats on which they fed on through "roots". This unique aspect has caused the group to sometimes be referred to as "the Garden of Ediacara". More of what we would call traditional animals also started to appear toward the end of the period, such as the slug-esque Kimberella (which might also have been one of the first creatures capable of locomotion), the arthropod/trilobite like Spriggina and the Cnidarian/jellyfish relative known as Haootia, which was rather similar to the modern freshwater Green Hydra.
The First Earthlings: Prehistoric Stromatolites and other bacteria
- One-celled organisms are usually not preserved in fossil records (the aforementioned foraminifers are a well-known exception). However, we're certain that the first Earthicans were bacterianote . They are the simplest forms of life, even lacking the nucleus which is the hallmark of the Eukarya - that is, animals + plants + fungi + algae + protozoans + other lesser-known organisms, all with nucleate cells. Some bacteria, the photosynthetic cyanobacteria (improperly called "green-blue algae"), have built strange rocky structures called stromatolites (they produce them still today); the most ancient stromatolites are from 2,000 million years ago, three times older than the first multi-celled organisms. They are among the most ancient forms of life that have left some fossils. It's worth noting that parasitic bacteria (those that carry diseases) could only have appeared after multi-celled organisms: otherwise what could have they infected? The same is true of the non-living virusesnote : they could have appeared only after true living things for the same reasons (some viruses infect bacteria, mind you). How life as a whole originated is one of the most fascinating fields within human knowledge, but here we're outside paleontology and science in general. It mostly remains a matter of speculation and philosophy, even though molecular biologists are undertaking great efforts to find the answer.
The RNA World
- The RNA world is about as far as we can look back. The RNA world is a well-founded hypothesis - it has enough evidence to be generally believed, but insufficient to be considered proven. In the modern world, DNA codes genes. Genes are copied to messenger RNA. Messenger RNA is read by ribosomes to create proteins. Almost all the functions of the cell are performed by proteins, including duplicating DNA and copying DNA to RNA, and most of the ribosome is protein. This works great, but how could it have started? DNA can't replicate without proteins, and proteins can't be made without the instructions in DNA. However, RNA can both store information and catalyze reactions. Although it would be grossly inefficient by modern standards, life based on RNA with no DNA and no proteins is quite plausible. The RNA world hypothesis is that such RNA life existed, and is ancestral to modern life. Evidence for this is functional RNA (RNA which does stuff directly, rather than simply being instructions to ribosomes), which performs just a few cellular functions, but very critical ones. Most central is that RNA forms the reactive core of the ribosomes.
