September 17. Dinosaurs

A week ago we talked about the archosaurs, the reptilian ancestors of dinosaurs, and I mentioned the oldest known dinosaur, Eoraptor, dated to 231 million years ago, in the late middle Triassic. By late Triassic, dinosaurs were clearly expanding both in terms of diversity and geographically. Coelophysis was a 9-foot, 100-pound carnivore that lived in southwestern North America by about 225 million years ago. Fossils similar to Coelophysis have since been found in many parts of the world, indicating it was a successful and adaptable animal that took advantage of the more-or-less connected nature of the supercontinent of Pangaea.  

Coelophysis was a theropod, meaning “animal foot,” a branch of the dinosaurs that were bipedal, with an upright stance. This group also includes birds, which descended from a dinosaur lineage.

Herbivorous dinosaurs had also evolved by late Triassic time. Plateosaurus was an exclusively bipedal animal up to 30 or so feet long, much of that in its neck. This presumably made it able to reach higher into trees and other leafy vegetation.

Plateosaurus skeleton photo by FunkMonk, under creative commons license

Dinosaurs are defined taxonomically as archosaurs with distinctive bone structures that allowed them to carry their bodies more directly above their legs. This applies to both two-legged stances and four-legged stances. The non-dinosaur crocodilians continued a more sprawling stance, with the legs extended away from the body rather than beneath it. The upright stance allowed for a speedier gait. It’s possible that the earliest dinosaurs were bipedal, and that the later 4-legged varieties actually reverted to that anatomy somewhat later.

Dinosaurs competed with other varieties of reptile and with therapsids like cynodonts. Two extinction events, one at about 215 million years ago during the late Triassic and the other at the end of the Triassic affected the primitive reptiles and therapsids more drastically than the newly evolved dinosaurs, and this may have given them more ecological niches to expand into. We’ll talk more about the extinction events at the end of the month. But that end-Triassic extinction seems to have ultimately proved beneficial to the dinosaurs, because they really began to take off after it happened.

—Richard I. Gibson

Plateosaurus skeleton photo by FunkMonk, under creative commons license.