Wednesday, August 4, 2010

Cheerio, Chromogisaurus

Paleontologist Martin Ezcurra, who has been pretty prolific lately, has just published a description of a new Argentinian dinosaur, Chromogisaurus novasi, in the Journal of Systematic Paleontology. Additionally, Ezcurra takes on the issue of diversity among the earliest dinosaurs.

Cânion de la Peña
A canyon in the Ischigualasto region of Argentina. By Richard E. Ducker, via Flickr.

Chromogisaurus comes from the Ischigualasto geological formation of Argentina, one of the world's most important windows into the Triassic period. The Triassic is the first of the three periods that make up the Mesozoic era, a time when life on the supercontinent Pangaea was evolving in new directions in the wake of the devastating extinction event that closed the Permian period. One of those new directions was the dinosaurs, and Ischigualasto is one of the best areas to look into those early days of a biological dynasty that would last the better part of 200 million years.

We're talking about the lowest branches of the dinosaur family tree here. Chromogisaurus and its closest kin, including Saturnalia, Guaibasaurus, and Panphagia, were the very beginnings of the line that would lead to Diplodocus, Brachiosaurus, and the other titanic sauropods. Called sauropodomorphs, they display a mix of features between true sauropods and the theropods with whom they shared a common ancestor. As is suggested by the generic name Panphagia ("eater of everything"), these critters were so closely related to the earliest theropods that they likely were omnivorous. The skull of Chromogisaurus has not been found, but it's not too great a stretch to guess that it probably was, too. Omnivory wasn't a path the basal sauropods walked for long, and it would be their adaptation to a fully herbivorous diet that would lead to their exponential increase in size.

What's frustrating about the Triassic fossil record concerning dinosaurs is that when they first pop up - during a subdivision called the Carnian stage, about 230 million years ago - the three main branches of dinosaurs are present. We simply do not have many good places where the earlier stages of the Triassic are available for study, and none of them reveal earlier dinosaurs. There are plenty of near-dinosaur archosaurs, and it's a fool's errand to try to find "the first dinosaur," as it is to find the mythical "missing link" between man and ape. Life is a fuzzy thing. So we keep looking for new fossils that give a higher resolution picture of how life changed over time, when certain adaptations arose.

Ezcurra's analysis of the diversity of these earliest dinosaurs indicates that it was quite high, though these dinosaurs made up a small fraction of land vertebrates. This may mean that once the essential elements came together to form the most basic dinosaur body plan, they diversified rather quickly. Carnivorous theropods (Eoraptor and Herrerasaurus) and omnivorous ancestors of sauropods (Chromogisaurus and kin) and ornithischians (Pisanosaurus) took little time to evolve vital adaptations allowing them to live alongside lesser-known but still fascinating beasts like the crurotarsans and near-dinosaur archosaurs, and greatly diversify when another great extinction event cleared the stage for them at the beginning of the Jurassic.

The Triassic is one area I feel like I've neglected in the first year of this blog, owing in part to my loose grasp of it. I intend to write more on it. One of the best places for Triassic news is Bill Parker's Chinleana blog. I get such a large number of hits from Bill that I'd feel silly not mentioning it here. Triassic Life on Land, a book throughly documenting the era, was recently released; check out Brian Switek's review here.

3 comments:

  1. You said "Called sauropodomorphs, they display a mix of features between true sauropods and the theropod ancestors of the group"

    Theropods are not not ancestors of the group [Sauropodomorpha], although most people PROBABLY do envision the common ancestor of theropods & sauropodomorphs to have been theropod-like in form.

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  2. Thanks Jay, I reworded that bit. I did mean that it was more theropod-like, but that's not what I wrote!

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