Dinosaur or Parrot?
Among the chosen examples you can tell the closest-to-ceratopsids apart from the most basal kinds by simply reading their names: the former usually have the suffix -ceratops (ex. Zuniceratops, Bagaceratops, Montanoceratops), the latter usually end in other ways (ex. Chaoyangsaurus, Yinlong, Stenopelix). The animal of the image is a very basal kind, the famed Psittacosaurus.
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Non-Stock Ceratopsid Predecessors
Horned Muzzle?: Montanoceratops
- The semi-bipedal Leptoceratops lived in Late Cretaceous North America together with the quadrupedal Montanoceratops cerorhynchos ("horned face from Montana with a horned beak"). The latter was originally classified in the 1940s as a new Leptoceratops species. Slightly bigger than Leptoceratops, Montanoceratops was once depicted with a small nasal horn like a miniature Centrosaurus: we now know it hadn't such a thing — the bone believed a horn was actually misplaced in the fossil. Protoceratops, Leptoceratops, Montanoceratops, and other animals made once one family, the Protoceratopsids; now Leptoceratops and Montanoceratops make their own family, Leptoceratopsids, together with other more recently-found animals such as Udanoceratops and Prenoceratops.
Tiny but Four-legged: Bagaceratops
- Another protoceratopsid, Asian Bagaceratops, was until recently put in its own family as well: Bagaceratopsids, together with other less-known genera such as Breviceratops and the unusually European Ajkaceratops. Found in Mongolia in the 1970s, Bagaceratops is notable because was one of the smallest quadrupedal dinosaurs that ever lived: only one meter long, shorter than a Compsognathus, yet still a bit bigger than the bipedal Microceratus, it had a particularly short frill and an incospicuous "hornlet" on its nose. Being so vulnerable, Bagaceratops might have lived in sheltered environments to hidden itself against gigantic predators like the contemporaneous Tarbosaurus. Many new basal ceratopsians have been described since the 1990s both in Asia and in North America; for example, Udanoceratops (one of the largest, twice longer than a Protoceratops) was close to Leptoceratops but living in Asia; Asiaceratops was also an Asian Leptoceratops-relative; Graciliceratops ("gracile horned face") was similar to Microceratus, and some alleged "microceratops" remains actually belonged to it; while Breviceratops ("short horned face") was originally considered a second Protoceratops species. Worthy of note is also Koreaceratops: found in the eponymous Asian peninsula in the first half of The New '10s, its describer said it was possibly a marine dinosaur, just like Spinosaurus.
The Earliest American Horns: Zuniceratops
- Differences between Proto-ceratopsids and Real-ceratopsids are considerable. There should have been at least one intermediate form between the two: how could it have looked? In 1998, the answer was found under the name Zuniceratops (which has detained the record of “the last member of the Dinosaur Alphabet” for some years). The most ancient North American ceratopsian, from Middle Cretaceous, it was only 4 m long (less than half a Triceratops), and had a mixed Triceratops / Protoceratops appearance: two long frontal horns like the former, and none on the nose like the latter. This Mix-and-Match Critter look surprised scientists, which used to think frontal horns were a very evolved trait of some advanced ceratopsids - while the nasal one was believed the most ancient horn in ceratopsid’s history. The ancestry of the frontal horns was confirmed in the 2000s, when some early centrosaurine true ceratopsids (the no-frontal-horns subfamily) showed long frontal horns like those of a chasmosaurine, ex. Albertaceratops. Now scientists think later centrosaurines (Centrosaurus, Styracosaurus, Pachyrhinosaurus, and so on) reduced secondarily the length of these horns. There was also a chasmosaurine which eliminated its nasal horn, resembling a Zuniceratops; this one is variably classified either as an odd-looking Triceratops species (Triceratops hatcheri, named after John Hatcher, one of Marsh's main collaborators) or a separate genus, Nedoceratops, originally called "Diceratops" ("two-horned face") or "Diceratus" — the name "Diceratops" was pre-occupied by an insect and had the same fate of "Microceratops".
Ceratopsian?: Yinlong
- Another, even more important missing-link was found as recently as the 2006: following the current trend about Chinese dinos’ naming, it was called Yinlong ("hidden dragon"). Living in Late Jurassic, it took the "most primitive ceratopsian" record away from Psittacosaurus. Its external appearance was the least Triceratops-like one can imagine: Yinlong was not only totally bipedal, but had neither any parrot-bill, nor spiky cheeks. Its only ceratopsian trait is a merely anatomical one: the “rostral bone” at the tip of its upper jaw, present in all ceratopsians and in no other dinosaur group. To compensate, Yinlong had small “canines”: this, together with its size (4 ft long) and shape, makes it quite similar to the basal ornithischian Heterodontosaurus. Indeed, this resemblance was once cited as the definitive proof that heterodontosaurids were not ornithopods but ancient relatives of ceratopsians and pachycephalosaurs, but now the latter is disputed. Among the other intermediate forms, Turanoceratops from Central Asia was perhaps close to Zuniceratops and maybe one of the ceratopsids' ancestors. But others guys have revealed to be more primitive than Protoceratops or Leptoceratops, if not at the same degree of Yinlong. Two of them have become the namesakes of their own family: the Early Cretaceous Archaeoceratops ("ancient horned face"), was a sort of middle-way between Psittacosaurus and Leptoceratops - the same about Auroraceratops "dawn horned face", which had an unusually-shaped skull; the second is the Late Jurassic Chaoyangsaurus (originally called "Chaoyoungosaurus" or "Chaoyangosaurus"). Described in 1983, Chaoyangsaurus (and its closest relative Xuanhuaceratops, originally named "Xuanhuasaurus" in 1986) initially were believed the earliest pachycephalosaurians, both from China; but they have then been revealed being very archaic ceratopsians between Psittacosaurus and Yinlong. Still mysterious is the identity of the poorly-known Early Cretaceous Stenopelix, whose pelvis was found in Europe in the XIX century. Originally believed an hypsilophodont and then a pachycephalosaur, today is regarded as a basal ceratopsian.
Cousins in the South?: Serendipaceratops & Notoceratops
- All confirmed ceratopsians have been found in the Northern Hemisphere, either in Asia - especially the earlier basal forms - or in North America (almost-all the horned ones). The two animals here were believed the only two exceptions of the rule. Notoceratops ("southern horned face" indeed) was found in South America in year 1918, and traditionally believed a Late Cretaceous protoceratopsian which migrated in South America from North America (like some contemporary ornithopods, see "Kritosaurus australis"). But its only remain, a piece of jaw, could be from an ornithopod instead. The Early Cretaceous Serendipaceratops, on the other hand, was found in Australia in 2003 by the two palaeontologists that described Leaellynasaura in 1989. Initially, the discoverers had not considered that its only remain (a forearm bone) might have been ceratopsian, as at this would have been the last group of dinosaurs one would have expected to find in the Land Down Under — to the point they thought it was from a theropod. Some months later, however, another colleague pointed out the similarity to this bone of the more-known Leptoceratops, and they decided to name it with the Meaningful Name Serendipaceratops. However, the remain is too incomplete to confidently be placed in any known ornithischian group, and some think it was an ankylosaur like Minmi.
