What is the most abundant dinosaur?
If you guessed Psittacosaurus – a small horned dinosaur from across Asia – you are correct!
While most dinosaurs have just one species, Psittacosaurus has twelve. Twelve! Having eleven more species than the average dinosaur may seem insane, but when you consider that thousands of Psittacosaurus specimens have been discovered, it becomes easier to fathom.
Despite being so common, I bet you’ve never heard of Psittacosaurus. There are two understandable reasons for this. First, at under two meters long and weighing no more than 40 kilograms (88 lbs), Psittacosaurus was tiny compared to dinosaurs like Triceratops or Stegosaurus. Second, while it was a member of the ceratopsian order (horned dinosaurs), Psittacosaurus was an early genus and thus lacks the massive frills and horns that became widespread in later members.
I can understand why a small, nondescript dinosaur isn’t an icon of paleontology…to the public. While Psittacosaurus may not have been the most spectacular dinosaur, it is perhaps the most important. The sheer volume of specimens has allowed paleontologists to know more about Psittacosaurus than almost any other dinosaur. It’s fossils have also helped generate a greater understanding of the biology of all dinosaurs, making it crucial for our understanding of these enigmatic creatures.
Today’s article will focus on the research performed on Psittacosaurus and what it means for our understanding of dinosaurs. Before I get started, some facts about it are necessary. The name Psittacosaurus is derived from Greek and Latin. Psittacosaurus means “parrot lizard,” and is pronounced “si-tah-ko-sore-us.” Trust me, calling it “p-sit-ah-ko-sore-us” for years is not in your best interest.
As mentioned, Psittacosaurus fossils are found across Asia and are known from locations in China, Mongolia, and Russia. Psittacosaurus fossils encompass a vast amount of time within the Early Cretaceous period, with earlier species appearing around 125 million years ago and the last going extinct around 100 million years ago. For context, Homo sapiens have been around for a few hundred thousand years, so you can imagine how successful these dinosaurs must have been.
Psittacosaurus specimens are especially common in the Yixian Formation of Northeastern China. During the Early Cretaceous, Yixian was a subtropical conifer forest that underwent wet and dry seasons. The landscape was peppered with freshwater lakes and experienced regular volcanic eruptions. While these sporadic eruptions were devastating for wildlife, they ultimately produced one of the most productive fossil sites on Earth. The volcanic fossils of Yixian are famous for their excellent preservation, which partially explains how we have come to know so much about Psittacosaurus.
Growing up, Dinosaur Style!
Unlike most dinosaurs, fossils of juvenile and infant Psittacosaurus are common. In fact, juvenile Psittacosaurus fossils are almost as common as their parents, with hundreds of specimens stored in museums across the world. While the circumstances behind their fossilization are tragic, it has provided an excellent understanding of Psittacosaurus’ complete life cycle.
Research undertaken on the growth rings of Psittacosaurus bones has revealed how these animals developed through their life. Psittacosaurus grew substantially for its first four years of life, reaching sexual maturity around the age of seven and becoming full-sized by age nine[i]. The results of this study demonstrate that while Psittacosaurus grew much faster than reptiles, they were still slower than mammals, possibly hinting at a slower metabolism[ii].
What’s stranger is that Psittacosaurus appeared to have shifted from being a quadruped in infancy to a biped in adulthood. The legs of Psittacosaurus grew faster than their arms, eventually enabling Psittacosaurus to walk on its hind legs when it reached maturity.
Why Psittacosaurus adapted such behaviour is unclear, though access to more food and quicker movement from predators could provide an answer. It’s interesting that the juvenile’s quadrupedal behaviour mirrors later ceratopsians, who became too bulky for bipedal movement. Were the juvenile Psittacosaurus the first step towards quadrupedal ceratopsians? Perhaps, though this has not been found to be conclusive[iii].
The Highs and Lows of Dinosaur Parenting…
Did dinosaurs practise cooperative parenting? That was a question posed by paleontologists in 2004, when a subadult Psittacosaurus was discovered alongside over thirty infants. Studies on the bones of the subadult have revealed it was not sexually mature, meaning that the litter of infants was not its own children[iv].
Could it be that the dinosaur was…babysitting? It’s possible, but the specimen is controversial within the paleontological community. In China, where fossils are a prized commodity, alterations to make them more profitable occur frequently. The strange placement of a juvenile skull, but no skeleton, seems sketchy. Unfortunately, verifying these specimens is challenging, and any valuable information they could reveal is lost.
A counter to their parental abilities is the (unaltered) fossil of a small mammal from the Yixian Formation. Named Repenomamus, the fossilized stomach contents of this badger-sized mammal contain a baby Psittacosaurus. Partially digested and dismembered, it’s clear that the abundant young of Psittacosaurus would have made a good meal.
Does this mean that Psittacosaurus babies were on their own? While the specimen of a juvenile with infants is debated, other colonies of baby Psittacosaurus have been discovered. At the very least, infant Psittacosaurus stuck with each other for safety from predators like Repenomamus. Though, this strategy clearly wasn’t foolproof…
Feathers & Filaments: An Unexpected Revelation
For a long time, it was believed that the only feathered dinosaurs were theropods, animals like Velociraptor or Ornithomimus. Psittacosaurus, and the rest of the Ceratopsians, are Ornithischian dinosaurs. In theory, feathers were believed to be absent in Ornithischians. As you can imagine, it was quite a surprise when a Psittacosaurus specimen from Yixian was discovered with feathers on its tail.
Perhaps feathers are a bad word, and filaments are more appropriate. The hollow filaments ran atop top the tail and are branchless structures. Though feathered theropods developed more complex feathers over time, the filaments are still like some primitive theropod feathers[v].
The implications of these feathers – and those of the basal Ornithischians Tianyulong and Kulindadromeus – are immense. If these feathers developed alongside the theropods, it would mean that feathers evolved in basal dinosaur ancestors, almost 100 million years before initially believed. A development that may push the evolution of feathers even further back is the presence of feathers on Pterosaur fossils, which are the closest relatives to the dinosaurs. This may point to feathers being a basal trait in Archosauria, the reptilian order that includes dinosaurs, pterosaurs, crocodiles, and their extinct ancestors.
Contrarily, Psittacosaurus – and the others mentioned above – may have evolved feathers on their own. That would mean that feathers evolved multiple times in the archosaur lineage (at least five times; possibly more). While this may seem strange, evolution is never a straightforward process. If feathers provided some sort of competitive advantage, it’s logical they would develop repeatedly.
From a cultural standpoint, the feathers of Psittacosaurus has let paleoartists imaginations run wild. If Psittacosaurus had feathers, then perhaps later descendants like Triceratops did too! (Probably not, but it’s still awesome art, as you’ll see below).
Coloured in Dinosaurs: Can we Know their Appearance?
As technology develops, paleontologists have learned more about the dinosaurs than ever thought possible. Did you know that the colours of some dinosaurs have been revealed by analyzing molecules in their fossils? If not, surprise! While most dinosaurs with known colour patterns are small predators (also from China), Psittacosaurus is one exception to this trend.
A 2016 study revealed that Psittacosaurus utilized countershading, camouflage that is based on blending with shadows. The belly of Psittacosaurus was a light shade, while its back and head consisted of darker colours[vi]. This effect allows animals to blend into the background by making them seem two-dimensional, a tactic utilized by animals like Great White Sharks and another dinosaur, the nodosaurid Borealopelta. Camouflage would have been crucial for Psittacosaurus; in addition to the baby-eater Repenomamus, multiple raptor species and a few tyrannosaurids also roamed the forests of prehistoric Asia. Talk about necessary!
A Rather Private Discovery…
One specimen of Psittacosaurus – known as the Senckenberg Museum specimen based on its home in Germany – is famous for its exquisite preservation. Psittacosaurus’ feathers and camouflage are both known because of this specimen. However, a 2021 study on the fossil may be the icing on the cake.
Paleontologists identified it’s private parts. Seriously.
Reading headlines from when it was announced is absolutely wild.
No, dinosaurs did not have private parts as we do. The Senckenberg specimen revealed that dinosaurs had a cloaca, an opening used for all the essentials – reproduction, and passing waste[vii]. It wasn’t too much of a surprise, given that birds and crocodiles – the dinosaurs closest living relatives – have cloacal openings too. While I would prefer avoiding an intimate description of the Psittacosaurus cloaca, let’s just say it was excellently preserved and a thorough description was published…
Anyways, while the cloaca is…good (there’s no easy way to say it, is there?), it’s impossible to identify the dinosaur’s sex from the fossil. In fact, while the revelation of a cloaca has reaffirmed the dinosaur’s relation to birds and crocs, it doesn’t help with gender identification at all. If both males and females had the same part, then telling them apart using the eye test is nearly impossible.
Nonetheless, the discovery of a dinosaur butt is hilarious. Ok, fine, I couldn’t help myself.
To Infinity and Beyond!
What’s next for Psittacosaurus? A lot! On June 13th, paleontologists announced the discovery of a “dinosaur belly button” on a Psittacosaurus fossil. Though not an umbilical scar like those found on you or me, it is a scar from an embryonic attachment to a yolk sack within the egg[viii]. Like the cloaca, this trait has only been found in Psittacosaurus and no other dinosaur – for now. This discovery ties dinosaurs with archosaurian cousins, as this trait is found on modern crocodiles and alligators.
Once again, the fossil that led to this discovery was the Senckenberg Specimen. While some may perceive this as the species’ relevance hindering on a single good fossil, I see it otherwise. There are thousands of Psittacosaurus specimens; if one can provide three novel studies on dinosaur biology, then just imagine what 1,000 could do.
I hope you enjoyed today’s article! If you’re interested in fake fossils and the time National Geographic was fooled by one, then I recommend you read an article about it here!
Or, if you would like to know about the science behind dinosaur colours, read “The Colour of the Dinosaurs” here at Max’s Blogosaurus!
I do not take credit for any images found in this article.
Header image courtesy of Andrey Atuchin, found here
Profile image courtesy of Gabriel Ugueto, found here
Psittacosaurus herd courtesy of Andrey Atuchin, found here
Psittacosaurus adult and juveniles courtesy of Bob Nicholls, found here
Psittacosaurus babysitting courtesy of the University of Pennsylvania, found here
Repenomamus courtesy of James Gurney, found here
Psittacosaurus tail courtesy of archosaur musings, found at their WordPress here
Feathered Triceratops courtesy of John Conway, found here
Senckenberg Psittacosaurus courtesy of Wikimedia, found here
Psittacosaurus…privates courtesy of Bob Nicholls, found here
Final image courtesy of Phil Bell, found here
[i] Benton, M. J. The Dinosaurs Rediscovered: How A Scientific Revolution Is Rewriting History. Thames and Hudson, 2020.
[ii] Lucas, Spencer G. Dinosaurs: The Textbook. 6 ed., Columbia University Press, 2016.
[iii] Barrett, Paul M., and Susannah C. Maidment. “The Evolution of Ornithischian Quadrupedality.” Journal of Iberian Geology, vol. 43, no. 3, 12 Oct. 2017, pp. 363–377., https://doi.org/10.1007/s41513-017-0036-0.
[iv] Lomax, Dean R. Locked in Time: Animal Behavior Unearthed in 50 Extraordinary Fossils. Columbia University Press, 2022.
[v] Brusatte, Stephen. Dinosaur Paleobiology. Wiley-Blackwell, 2012.
[vi]Benton, M. J., and Bob Nicholls. Dinosaurs: New Visions of a Lost World. Thames & Hudson, 2021.