The "Dinosaurs" that Weren't: The [brief] Age of Pseudosuchians

Circa 250Ma (million years ago), the Paleozoic Era came to a dramatic close. The devastating Permian-Triassic Extinction, aptly nicknamed the "Great Dying," saw the extinction of over 90% of marine species and 70% of terrestrial species, and was the largest extinction in Earth's history. It may have taken the planet a whopping 10 million years to recover from the event. When the planet did recover, the landscape was not dominated by synapsids or amphibians anymore: this was the great Age of Reptiles, the Mesozoic. But the Mesozoic's most famous inhabitants, the dinosaurs, did not start out as the masters of the earth. In the Triassic, the world belonged to the false-crocodiles.

A psampling of pseudosuchians. Pictured are rauisuchians (top and bottom rows), an aetosaur (middle row, left), and a phytosaur (middle row, right). Illustrations by N. Tamura.

Enter the great clade of Pseudosuchia. One of the two major groups within Archosauria, which also includes dinosaurs and pterosaurs, the pseudosuchians are an incredibly diverse group of reptiles which had their 50 million years of fame at the very beginning of the Mesozoic. In the wake of such a large extinction, the pseudosuchians were the first to fill any and all empty niches. In fact, the pre-dinosaur world was very, well, dinosaurian in appearance: bipedal predatory juggernauts hunted low-lying, heavily-armored herbivores, while enormous crocodile-like hunters stalked the waterways. The shapes and sizes of these Triassic pseudosuchians can be thought of as a sort of biological foreshadowing for species yet to evolve, including theropods, ankylosaurs, and crocodiles.

The fauna of Triassic Arizona. All species pictured, except the ones with the big X's, are pseudosuchians, composing the majority of life on land, as well as major predators in the water.  Reconstruction by Jeff Martz. X's by yours truly.

The Triassic pseudosuchians experienced a period of extremely rapid, and greatly successful, evolution: the small, agile reptiles which had arisen in the beginning of the period had evolved into the dominant terrestrial predators by the middle, and by the end of the period they dominated all aspects of life. In fact, in this relatively short period of time, they had even more diverse habits and appearances than early dinosaurs. A comparison of Triassic archosaurs and phytosaurs revealed that early dinosaurs were half as diverse as other closely-related reptiles, including pseudosuchians, and displayed little difference in their anatomy and habits throughout the period (Brusatte et. al, 2008). 

Clearly, the extinction of the pseudosuchians of the Triassic does not mean they were inferior to their successors. Quite the opposite is true. Had the larger species not gone extinct at the end of the Triassic, it is likely that the pseudosuchians would have maintained their comfortable position as masters of the land. In fact, the body plans of several Triassic pseudosuchians were repeated by dinosaurs much later in history. Rather than competition or lack of resources, it is likely that large pseudosuchians went extinct due to a period of rapid global warming, leaving smaller species that lived in the shadows of increasingly abundant dinosaurs.

Although the dominant pseudosuchians went extinct at the end of the Triassic, suchians in general still thrived. Pictured are Paul Sereno and a variety of crocodylomorphs which once inhabited North Africa during the Cretaceous.

However, not all was lost for the pseudosuchians. One subgroup, Suchia, the "true crocodiles," contains some of the only surviving archosaurs: the crocodylians. The suchians, alongside the dinosaurs, reigned in the Mesozoic, evolving to fill a wide variety of niches, from insect-eaters to marine predators to species large enough to eat large dinosaurs. But even with their extreme diversity during the rest of the Mesozoic, the pseudosuchians would never again be the true masters of the planet in the way they were during the Triassic.

References

Brusatte, Stephen L., Michael J. Benton, Marcello Ruta, and Graeme T. Lloyd. 2008. “Superiority, Competition, and Opportunism in the Evolutionary Radiation of Dinosaurs.” Science 321 (5895) (September 12): 1485–1488. doi:10.1126/science.1161833.