How many species of non-avian dinosaur were there? We will probably never know the definite total, but we can be sure that there were both more and less dinosaur species than have been named to date. The process of science is at the heart of this contradiction.

Skeletons are the key to distinguishing non-avian dinosaur species. Without preserved soft tissues, genes, or other clues used to separate species, bony clues are the primary means to examine how one dinosaur differs from another and where the boundaries of a species might be. But dinosaurs can be tricky critters. Many species are known from only a handful of partial skeletons, or less, that may have been distorted after death. Differences due to age at death, natural variation between individuals, and the way bones can be reshaped postmortem can sometimes make an individual of a known species look like something never before seen.

While the vast majority of dinosaurs that ever lived have yet to be found, paleontologists have nonetheless created a few too many names from the stock of dinosaur skeletons found so far. The little beaked herbivore Psittacosaurus is one such example of oversplitting.

View Images A cast of a Psittacosaurus skeleton. Photo by The Children’s Museum of Indianapolis, distributed under a Creative Commons Attribution-Share Alike 3.0 Unported license.

Since the time Henry Fairfield Osborn named Psittacosaurus mongoliensis in 1924, researchers have named fourteen other species of this bipedal ceratopsian from the 123 to 100 million year old strata of Asia. And that’s not counting very similar dinosaurs of different genera that lived at the same time. In fact, as reported by University of Pennsylvania researchers Brandon Hedrick and Peter Dodson, sites within the Lujiatun beds of northeastern China’s Yixian Formation have yielded specimens of three different psittacosaurs – P. lujiatunensis, P. major, and Hongshanosaurus houi. Are all these dinosaurs really distinct species that trotted their bristly-tailed selves around the same habitats, or might they all be the varied members of fewer species?

Other researchers had previously suggested that the number of psittacosaur species among the Lujiatun beds might be fewer than the three named, but Hedrick and Dodson decided to revisit the question by examining thirty psittacosaur skulls through a methodology called three-dimensional geometric morphometrics. Using specific landmarks found on each skull, the technique allowed the researchers to quantify differences in skull shapes and investigate how many distinct forms there were among the sample.

But distinct shapes don’t necessarily reflect distinct species. Disparate skull shapes, Hedrick and Dodson point out, can be a result of differences in age, sex, geographic location, or a variety of other factors. To determine how many species the psittacosaur skull shapes represent, Hedrick and Dodson paired the morphometric analysis with a reexamination of the specific skull characters said to uniquely distinguish P. lujiatunensis, P. major, and Hongshanosaurus houi from each other.

The dinosaur skulls were not so different. All the skull shapes fell into a single cluster, with the main differences owing to the ways the bones were smushed and altered after burial rather than in-life anatomy. The afterlife of the dinosaurs created variations that were mistaken for species differences. Combined with the reexamination of specific skull characters, Hedrick and Dodson found that the Lujiatun beds hold only only psittacosaur species – Psittacosaurus lujiatunensis.

(There is some taxonomic wrangling in this decision, as Hongshanosaurus houi was named before the other two, but Hedrick and Dodson make the case that P. lujiatunensis should be retained as the name for this dinosaur on the basis of better fossil material.)

Just like that, three dinosaur species sink into one. But there are other Psittacosaurus species that are up in the air, not to mention the many, many other dinosaurs with multiple species left to sort out. During Deep Time, there were probably many more dinosaur species than we’ll ever know – lineages distinguished by slight differences in color, size, or other characteristics that we can only detect when we have animals in the flesh to study. Nature is a squishy entity that does not fit into neat boxes of our devising. But, lacking the living creatures, paleontologists must continue to grapple with skeletons to understand the limits of dinosaur growth, variation, and the ways their remains were altered after death. The essentials of what the dinosaurs were remain locked in their bones.

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