New Fossils Suggest High Diversity Among Close Dinosaur Relatives

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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

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