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Allosaurus is one of the best-known dinosaurs, but it’s rare to find an extensive record of any single dinosaur genus. And there are some dinosaurs that we may never meet at all. (Photo taken at the Natural History Museum of Utah by the author)

During the past two centuries, paleontologists have discovered and named over 600 different non-avian dinosaur genera. At first glance, that might seem like a lot of dinosaur diversity (especially since only a handful of dinosaurs are well-known to the public). But it’s really just the tip of the Mesozoic iceberg. New dinosaurs are being described on a near-weekly basis, and, as estimated by paleontologists Steve Wang and Peter Dodson in 2006, there may have been over 1,800 different genera of dinosaur present on earth during their 160 million year reign between the Triassic and the end of the Cretaceous. Most dinosaurs remain undiscovered.

But will we ever find all the dinosaurs? I don’t think so.

The fossil record is a history biased by the circumstances required for preservation and discovery. Paleontologists and geologists have recognized this for over a century and a half.  As Charles Darwin, following the argument of his geologist friend and colleague Charles Lyell, pointed out in On the Origin of Species, the geological record is “a history of the world imperfectly kept and written in a changing dialect.” Consider the world’s strata to be like pages of a book that record the comings and goings of species over time, Darwin wrote. “Of this history we possess the last volume alone, relating only to two or three countries,” Darwin lamented, and “Of this volume, only here and there a short chapter has been preserved, and of each page, only here and there a few lines.”

Let’s apply this to dinosaurs. Of all the non-avian dinosaurs that ever existed, only a few died in circumstances amenable to fossil preservation. Dinosaurs bodies had to settle in a place where sediment was being laid down – a river, lake, dune-covered desert, floodplain, lagoon, or similar environment – to be preserved for the rock record. This means that we know a lot about lowland dinosaurs who lived near bodies of water, but dinosaurs that lived in upland habitats are not so well represented. These dinosaurs, who inhabited ancient mountains and similar habitats, were living in places where rock was being stripped away rather than new sediment laid down. In other words, upland dinosaurs didn’t live in the kind of habitats where they were likely to become preserved. There were undoubtedly entire populations, species, and even genera of dinosaurs that may have never entered the fossil record.

And preservation in the fossil record alone isn’t a guarantee that a particular dinosaur genus will be discovered. Of all the dinosaurs preserved in the rock, only a few are accessible in exposed portions of rock around the world. Fewer still are intact enough to identify and collect. The contingencies of fossilization, history, and our ability to search for fossils conspire to blur our picture of dinosaur diversity.

The picture isn’t entirely negative, though. There are swaths of dinosaur-bearing rock that are, as yet, little explored, and even extensively-searched areas can still yield surprises. I have no doubt whatsoever that paleontologists will continue to discover and describe previously-unknown dinosaurs for many decades to come. And, more than that, each new dinosaur tweaks our picture of dinosaur relationships and the details of when and where particular groups evolved. Using this knowledge, paleontologists can go back to the rock and target specific areas where new dinosaurs might be found. We probably won’t find every single dinosaur genus that ever existed, and we may not have an intricately-detailed record of every genus that we’re lucky enough to discover, but there is still an overwhelming array of dinosaurs out there waiting to be found.

Anchiceratops ornatus was a pretty successful dinosaur. The single known species of this elaborately horned herbivore survived for about two millions years during the Late Cretaceous—many thousands of years longer than the varieties of horned dinosaur which preceded it in prehistoric Canada. This is a recent realization. As I wrote last September, what were once thought to be two different species of Anchiceratops were actually one, and the idea that paleontologists have found both male and female forms of this dinosaur has also been struck down.

These changes stemmed from a better understanding of dinosaur variation. Often, small differences between dinosaur skeletons led paleontologists to establish new species or genera of dinosaur when those subtle variations were really just signs of individual disparity within a species. In the latest Royal Tyrrell Museum lecture, paleontologist Jordan Mallon, the lead author on the Anchiceratops paper, explains how he tracked variations among fossils to give us a better idea of dinosaur diversity and evolution.

[Hat-tip to ReBecca Hunt-Foster for sharing the video]

A restoration of Asilisaurus. Darker areas are missing portions of the skeleton. From the Nature paper.

What were the very first dinosaurs like? This is one of the most vexing questions in vertebrate paleontology. Even though paleontologists have found a number of early dinosaurs in recent years, details about the very first dinosaurs and their close relatives have been hard to come by, but in a new paper published this week in Nature paleontologists report an animal that helps put the origin of dinosaurs in context.

Dinosaurs did not just pop into existence out of nothing. All the numerous dinosaur lineages we know and love can be traced back to one common ancestor, and that animal was itself just part of another diverse group of creatures. To put it another way, all dinosaurs compose one group (the Dinosauria) which is nested within an even larger group called the Dinosauriformes, or dinosaurs plus their closest relatives. (We will stop here, but you could keep on going down the family tree all the way back to the first life on earth if you wanted to.)

The new creature described by paleontologists Sterling Nesbitt, Christian Sidor, Randall Irmis, Kenneth Angielczyk, Roger Smith and Linda Tsuji helps to better resolve these relationships. Discovered in 243-million-year-old rock in Tanzania, Asilisaurus kongwe was a close relative of earliest dinosaurs, but it was not ancestral to them. Instead the Asilisaurus illustrates that the group to which it belonged, the silesaurids, split from the earliest dinosaurs earlier than was previously thought and thus suggests that there is another 10 million to 15 million years of early dinosaur evolution yet to uncover.

The species’ bearing on questions about dinosaur origins is what has made headlines, but outside these considerations Asilisaurus is still a remarkable find. The paleontologists who discovered it found the remains of at least 14 individual animals, and altogether they have been able to piece together almost the entire skeleton. It was a slender animal, with a long neck and small hands, that moved about on all four limbs. What it ate is not definitely known, but its leaf-shaped teeth would have allowed it to be a herbivore or an omnivore. This latter point is especially significant because, like the earliest herbivorous dinosaurs, Asilisaurus evolved from a carnivorous ancestor, meaning that among the dinosauriformes, plant-eating forms independently evolved at least three times.

Compared alongside its close relatives and contemporaries, Asilisaurus suggests that by 245 million years ago there was a major radiation of archosaurs (an even more inclusive group of vertebrates which contains dinosaurs, pterosaurs, crocodiles, and their extinct relatives). At this time dinosaurs were not yet the dominant large vertebrates, but instead were just part of a greater diversity of types now extinct.

For more on this discovery, see this post as Chinleana.

Nesbitt, S., Sidor, C., Irmis, R., Angielczyk, K., Smith, R., & Tsuji, L. (2010). Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira Nature, 464 (7285), 95-98 DOI: 10.1038/nature08718