Among the chosen examples you can tell the closest-to-ceratopsids apart from the most basal kinds by simply reading their names: the former have usually the suffix -ceratops (ex. ''Leptoceratops'', ''Zuniceratops''), the latter usually end in other ways (ex. ''Psittacosaurus'', ''Yinlong'').
'''Porcuparrot:''' ''[[http://en.wikipedia.org/wiki/Psittacosaurus Psittacosaurus]]''
Together with ''[[StockDinosaurs Protoceratops]]'', ''Psittacosaurus'' is by far the most important and well-known ceratopsid predecessor. At least, if you ask paleontologists and paleo-fans. RuleOfCool always wins in pop-culture, with small-sized dinosaurs usually with very few chances to get consideration by writers or film-makers - points minus when they are plant-eaters.
Digression closed, here we have many things to say about ''Psittacosaurus'', definitively one of the most important dinosaurs. An Asian animal like ''Protoceratops'' it has classically been considered the most ancient ceratopsian ever (lived 100 million years ago, in the Early Cretaceous) and the forerunner of all the Late Cretaceous “neoceratopsians” (aka Proto+Horned). With its primitiveness, ''Psittacosaurus'' resembles anything but a ''Triceratops'': small (6 ft long), slender, with only hints of horns and frill. Once thought to be capable of walking on all fours, detailed study of its forelimbs shows it was entirely bipedal. The main trait revealing its relationship with ''Triceratops'' is the parrot-like bill (the hallmark of ''all'' ceratopsians) which gives it the name “Psittacosaurus” (“psittacos” is Greek for parrot). Another thing which ties ''Psittacosaurus'' with its horned descendants are the prominent bony “cheeks”.
''Psittacosaurus'' was discovered in the 1920s in Mongolia together with ''Protoceratops''. Its discovered was famed paleontologist Roy Chapman Andrews – a very adventure-loving guy, to the point he could have even been the inspiration for ''Franchise/IndianaJones''. Since then, psittacosaurs have been discovered everywhere in eastern Asia, but recognized a basal ceratopsian only in the 1970s (it was believed an ornithopod before). Its fossil record is extremely rich, just the same level of ''Protoceratops'' - individuals from all ages are known, and also several nests full of eggs. Our “parrot-dinosaur” also detains the record of the non-avian dinosaur with most species described, more than 10!
In the 2000s, many new discoveries have furtherly raised its importance, making it perhaps the most scientifically well-known member in the whole dinosaur world. The main discovery has been made in Liaoning, where one specimen has preserved integument which shows ''porcupine-like quills'' raising upwards from its tail, for uncertain purpose (Defense? Mating?). These were the ''very first'' filamentous skin-structures ever found in an ornithischian dinosaur; this has changed our perception of bird-hipped dinosaurs, which might be more similar to birds than previously thought. Indeed, a few scientists now argue those quills (or similar structures) could also be in all the other more evolved ceratopsians, ''Triceratops'' included. Another unexpected discover made in year 2000 in the same site, did debunk the classic “Mesozoic mammals were underdogs ruled by dinos”: the cat-sized carnivorous mammal ''[[http://en.wikipedia.org/wiki/Repenomamus Repenomamus]]'' has been found with ''[[EatsBabies baby]]'' [[EatsBabies Psittacosaurus]] ''[[EatsBabies remains]]'' [[EatsBabies in its ribcage]]!
'''The "sheep" of the Cretaceous:''' ''[[http://en.wikipedia.org/wiki/Leptoceratops Leptoceratops]]''
Try to tell everyone if ''[[StockDinosaurs Protoceratops]]'' was really sheep-like. If you manage to do it, then try with this: ''Leptoceratops'', the same length of ''Protoceratops'' but ''partially bipedal''. ''Leptoceratops'' has probably been the most common basal ceratopsid in docu-media after ''Protoceratops'' & ''Psittacosaurus''; like the former, it too was compared with a sheep in the past.
The first small-sized ceratopsian discovered (1910s), it was more primitive than ''Protoceratops'', being not only hornless but also bumpless, much slimmer-bodied, longer-legged, and with a much smaller frill. There is a surprising thing at this point: contrary to what one might expect, ''Leptoceratops'' lived ''later'' than ''Protoceratops'', at the very end of the Cretaceous. And roamed North-America, not Asia (where ceratopsians started their evolution), thus sharing the lands with ''Triceratops''. But for some reason, it had preserved the archaic bodyplan of its primitive ancestors.
Another relative which lived along ''Leptoceratops'' is the quadrupedal ''[[http://en.wikipedia.org/wiki/Montanoceratops Montanoceratops]]'', from [[ExactlyWhatItSaysOnTheTin Montana]]; once thought to have had a small nasal horn [[ScienceMarchesOn we now know]] it hadn't such a thing. ''Protoceratops'', ''Leptoceratops'' and other animals made once one family, the Protoceratopsids; now ''Leptoceratops'' and ''Montanoceratops'' make their own family, Leptoceratopsids. Another former protoceratopsid, Asian ''[[http://en.wikipedia.org/wiki/Bagaceratops Bagaceratops]]'', has been recently put in its own family as well. ''Bagaceratops'' is notable because is one of the smallest quadrupedal dinosaurs that ever lived: only one meter long, only a bit more than the bipedal ''Microceratus'' (see below).
'''A slimmer cousin:''' ''[[http://en.wikipedia.org/wiki/Microceratus Microceratus]]'' (once called "Microceratops")
Despite their partial bipedality, ''Psittacosaurus'' and ''Leptoceratops'' was still robust guys compared with, to say, the “gazelle dinosaur” Hypsilophodon. But they had also some slimmer relatives, which if they have had a normal-looking head, they’re surely be mistaken for ornithopods.
The most historically relevant was aptly called “Microceratops”. From Ancient China like the [[IncrediblyLamePun prototypical Protoceratops]], it’s one of the smallest dinosaurs ever, only the size of a rooster; and was a fast-running animal with slim body and agile legs, unlike the classic image of ceratopsians. Nonetheless, its head was unmistakeably ceratopsian, or rather, protoceratopsian. Very poorly-known, “Microceratops” has now fallen in disuse being preoccupied by an insect: [[ScienceMarchesOn we now need]] to call it ''Microceratus''. Still, the microceratops has appeared in some popular works, namely the first ''Jurassic Park'' novel and Disney's ''Dinosaur''.
'''The missing link:''' ''[[http://en.wikipedia.org/wiki/Zuniceratops 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 (Middle Cretaceous), it was only 4 m long and had a mixed ''Triceratops'' / ''Protoceratops'' appearance: two long frontal horns like the former, and none on the nose like the latter.
This MixAndMatchCritter 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: ''Albertaceratops'' is one example. Now scientists think later centrosaurines (''Centrosaurus'', ''Styracosaurus'' 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 a odd-looking ''Triceratops'' species, 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").
'''The other missing link:''' ''[[http://en.wikipedia.org/wiki/Yinlong 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''. Living in Late Jurassic, it now detains “the most primitive ceratopsian” record. Its external appearence was the least ''Triceratops''-like one can imagine. ''Yinlong'' 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 tiny size 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.
Many new basal ceratopsians have been described since the 1990s both in Asia and in North America. Some were related with the examples listed above: for example, ''Udanoceratops'' (one of the largest ones) was close to ''Leptoceratops''; ''Graciliceratops'' was similar to ''Microceratus''; while ''Turanoceratops'' was perhaps close to ''Zuniceratops'' and maybe one of the ceratopsids' ancestors. But others have revealed to be more primitive, if not at the same degree of ''Yinlong''. Two of them have become the namesakes of their own family: the Early Cretaceous ''[[http://en.wikipedia.org/wiki/Archaeoceratops Archaeoceratops]]'' ("ancient horned face") was a sort of middle-way between ''Psittacosaurus'' and ''Leptoceratops''; and the Late Jurassic ''[[http://en.wikipedia.org/wiki/Chaoyangsaurus Chaoyangsaurus]]'' (originally called "Chaoyoungosaurus" or "Chaoyangosaurus" and initially believed the earliest pachycephalosaurian) has been revealed being 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.