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Ceratopsian Month #09 – Diabloceratops eatoni

At this point in ceratopsian evolution we’ve reached the ceratopsids – the big, elaborately horned and frilled group that includes famous names like Triceratops and Styracosaurus. First evolving from their smaller North American ancestors around 90-80 million years ago, these dinosaurs rapidly diversified and developed a huge variety of different head ornamentations during the last 20 million years or so of the Cretaceous.

Here the family tree gets a little more complicated, with two major subdivisions of the ceratopsids splitting off from a common ancestor: the centrosaurs and the chasmosaurs. We’ll be focusing on the centrosaurs to start off, and moving on to chasmosaurs later in the month.

Ceratopsian Month #08 – Turanoceratops tardabilis

Turanoceratops (“Turan horned face”) was very slightly younger than Zuniceratops, living about 90 million years ago in the Late Cretaceous. At around 2m long (6′6″) it was another small transitional form very very closely related to the true ceratopsids, but there’s still disagreement about whether it was actually a proper member of that group or not.

Its fossil remains were discovered in Uzbekistan, making it the earliest ceratopsid-like ceratopsian known from Asia rather than North America – and showing that there were still populations of ceratopsians dispersing back and forth between the two continents.

Only fragmentary fossils have been found, so while we know it had large brow horns we don’t know whether it had a nose horn or what the shape of its frill was. So most of the rest of its appearance here is rather speculative.

Ceratopsian Month #07 – Zuniceratops christopheri

Zuniceratops (“Zuni horned face”) was discovered in New Mexico, USA, and dates to the Late Cretaceous (~91 mya). Estimated to have been about 3.5m long (11′5″), it was one of the earliest ceratopsians with well-developed horns over its eyes.

It had a transitional mix of features between basal ceratopsians and the later larger forms – having brow horns and a long narrow snout, but a frill lacking decorative scalloped edges – suggesting that the horned ceratopsids evolved from more “primitive” ancestors already within North America rather than migrating in later from Asia.

Bones from several different individuals have been found, showing that their teeth became more firmly rooted with age, going from single-rooted in juveniles to double-rooted in adults. The brow horns also appear to have grown continuously throughout Zuniceratops’ life, with those of older specimens being much larger.

Ceratopsian Month #06 – Protoceratops andrewsi

The protoceratopsids were a mainly Asian branch of the ceratopsians (with one species also known from Europe), splitting off shortly after the leptoceratopsids. Although still fairly small, they looked a lot more like a typical ceratopsian than their ancestors, with relatively big heads for their body size and large neck frills.

The best-known member of this group was the 1.8m long (6′) Protoceratops (“first horned face”), which lived in Mongolia during the Late Cretaceous (~75-71 mya). It was a very common animal, with hundreds of fossils representing all different stages of growth, and large numbers found together suggest it may have lived in herds.

The size and shape of its frill differed a lot between individuals, with some being short and compact and others being much longer or wider. Some paleontologists have considered this to be evidence of sexual dimorphism, but further studies show no clear distinctions between the proposed “male” and “female” morphs – it was probably just natural genetic variation among the population.

For many years a fossilized clutch of eggs was believed to belong to Protoceratops, with a nearby Oviraptor with a crushed skull thought to be an egg-eating thief killed while trying to raid the nest. The later discovery of an identical egg containing an Oviraptor embryo revealed that the story was the wrong way around – the Oviraptor was the one brooding and protecting its nest all along.

Another famous fossil preserves a fight between a Protoceratops and a Velociraptor, the pair dying together before being buried in sand. The predator’s sickle claw was lodged into the Protoceratops’ neck, while the ceratopsian was crushing the Velociraptor’s arm in its beak.

(Oh, and Protoceratops fossils also probably didn’t actually inspire legends about griffins.)

Ceratopsian Month #05 – Leptoceratops gracilis

During the mid Cretaceous, around 100-80 million years ago, the leptoceratopsids split off from the main ceratopsian lineage. Fairly small with low frills, they were a successful group lasting throughout the rest of the Mesozoic. Seeming to originate in western North America, a few species have been found in Asia that may represent a dispersal event back across Beringia, and partial remains are also known from Europe and Appalachia. There’s even a dubious identification from Australia.

Leptoceratops (“small horned face”) was one of the last of this branch of the ceratopsians, living during the very end of the Cretaceous (~66 mya). About 2m long (6′6″), its fossils have been found in Alberta, Canada, and Wyoming, USA, and it would have lived alongside some of its larger distant cousins like the much more famous Triceratops.

Its skeleton shows it was a stocky quadrupedal sort of pig-like animal. Short deep jaws with strong teeth gave it a powerful bite, adapted for slicing and crushing, and it was probably specialized for a diet of very tough plant matter.

Ceratopsian Month #04 – Auroraceratops rugosus

Auroraceratops (“dawn horned face”) lived during the Early Cretaceous of China (~130-100 mya). Around 2m long (6′6″), it had a shorter and wider snout than other early ceratopsians, with a more flattened broader head shape.

It was closely related to other small Asian species like Koreaceratops, and was probably primarily bipedal due to its proportionally short arms. But, since it falls at a transitional point in ceratopsian evolution between bipedal ancestors and quadrupedal later forms, it may have adopted a four-legged posture while feeding or moving at slow speeds.

Its skull had several patches of thickened bone, found directly in front of its eyes and on its lower jaw. These could have had some visible tough keratinous coverings in life – although I’ve not depicted any here – and could potentially have been used for pushing and butting during fights.

The back of its small frill shows evidence of attachment sites for its jaw muscles, suggesting that ceratopsian frills may have originally evolved to support large powerful chewing muscles before being secondarily modified into elaborate display structures.

Ceratopsian Month #03 – Aquilops americanus

Although most early ceratopsians are known from Asia, during the Early-to-Mid Cretaceous a few dispersed across to North America via a land connection in Beringia.

These migration events happened at least three times, and Aquilops (“eagle face”) was part of the first wave of immigrants, dating to 109-100 mya and representing one of the earliest ceratopsians discovered in North America. Known from a single tiny skull found in Montana, USA, its full size would have been only around 60cm long (2′) – about the size of a large rabbit.

The narrow triangular shape of its snout suggests it was probably a selective browser, feeding on leaves and soft shoots in low-level foliage. Its hooked beak also had a raised bump on the front, which may have supported a larger display structure such as a horn.

Ceratopsian Month #02 – Psittacosaurus sp.

Living during the Early Cretaceous (~126-101 mya) in Mongolia, Siberia, and China, Psittacosaurus (meaning “parrot lizard”) represents an early specialized offshoot of the ceratopsians – it wasn’t directly ancestral to any of the later forms, instead being more of an evolutionary cousin.

It’s one of the most well-studied of all non-avian dinosaurs, represented by hundreds of fossils at all ages from hatchlings to adults. A large number of species have been named within the genus, although there’s disagreement about how many are actually valid, and even some of the best-preserved specimens still haven’t been given an official species classification.

Growing up to about 2m long (6′6″), it had sideways-flaring cheek horns and little-to-no frill, and only four fingers on its hands instead of the five seen in all other ceratopsians. One fossil preserves an amazing level of soft-tissue detail, showing the outline of the body, a row of long bristles, skin membranes running from its ankles to the base of its tail, and even evidence of the countershaded color pattern the animal had in life.

While adults were fully bipedal, hatchlings and juveniles seem to have moved around on all fours – suggesting that some of their direct ancestors may have evolved to be quadrupedal before secondarily re-acquiring two-legged locomotion.

Ceratopsian Month #01 – Yinlong downsi

It’s time for another month of themed blog posts, and this August features one of the most iconic groups of dinosaurs: the “horn-faced” ceratopsians!

Existing for almost 100 million years, from the Late Jurassic all the way up to the K-Pg mass extinction, ceratopsians originated in Asia and were part of a group called marginocephalians, sharing a common ancestor with the closely related pachycephalosaurs. The earliest members barely resembled their more famous descendants, lacking showy headgear and looking more like fairly generic basal neornithischians – but by the time of the Late Cretaceous their descendants had migrated across to North America and evolved into large quadrupeds, with some forms like Triceratops being so incredibly common that they must have been the dominant herbivores in their environments.

So let’s start right at the beginning of the group with…

Unsolved Paleo Mysteries Month #23 – Puzzling Proto-Bats

Let’s finish off this month the same way we started: with flying vertebrates without any transitional forms!

Much like the pterosaurs, bats appear suddenly in the fossil record already fully flight-adapted. Despite being the second-largest group of mammals, bats’ small fragile bones and terrestrial habitats make fossils of them incredibly rare, and transitional forms are still entirely unknown. (Even the ancestral form illustrated above is a generic hypothetical mammal!)

The most “primitive” known bats come from the Early Eocene* (~55-52 mya) and various early representatives have been found as far apart as North America, Europe, India, and Australia – indicating they were already a widespread and diverse group by that time, and making it difficult to pin down just where and when they actually might have originated.

*I’ve seen mentions of a potential bat-like tooth from the Late Cretaceous of South America, but can’t find any actual references for it. So it’s possible bats may even have evolved before the K-Pg extinction.

Although bats were once thought to be related to archontans (treeshrews, colugos, and primates) based on morphological similarities, more recent genetic studies have shown them to instead be grouped with the laurasiatheres (eulipotyphylans, carnivorans, pangolins, ungulates, and whales). Based on this phylogenetic position the earliest ancestors of bats may have been small tree-climbing shrew-like animals who evolved flight while leaping in pursuit of insects. They might even be closely related to an obscure group called nyctitheriids – but without a lucky find of an exceptional fossil, we just don’t know.