The Crurotarsi were a group of Archosaurs that assumed primacy in the Triassic. They moved about on four legs and diversified quickly into both terrestrial and aquatic types. The crurotarsins were often covered in hardened plates affording them extra protection. Their snouts were usually long and their skulls large, thick and heavy. They moved about either in a sprawled position or on more erect limbs (see below).
During the Early
and Middle Triassic this group of reptiles was the most diverse and
the most numerous. While the crurotarsi included the herbivorous and
odd-looking Aeotosaurs, the majority were carnivorous and included
diverse groups such as the large predatory Rauisuchians which
reached lengths of 25 feet and the Phytosaurs who, although
they appear to be ancestors of our modern crocodiles, are not
An important difference between the Crurotarsi (“cross-ankles”) and the Ornithodira archosaurs (dinosaurs and pterosaurs) was in the structure of their ankle. The Crurotarsi ankle permitted some flexibility between the ankle and the foot, enabling the animal to either maintain its normal sprawled posture or raise its body high off the ground when it needs to move quickly as does a modern crocodile.
Some crurotarsi groups also underwent a change in their hip structure producing what’s termed a “pillar-erect” hip further allowing them to raise their bodies off the ground. The Rauisuchia (see image) possessed the pillar-erect hip construction and moved about on fully erect legs. They were a very common predator during the Triassic but died in the mass extinction that brought the period to a close.
By the Middle Triassic, the Crurotarsi were the top carnivores, a position they retained throughout the Triassic, winning the competition against the early dinosaurs. The herbivorous dicynodonts were probably a favorite meal.
In the last few years, a greater appreciation and understanding of the crurotarsans has emerged. Until 2004, it was the practice of paleontologists who found individual fossilized teeth with no other bones to attribute these finds to the ornithiscian dinosaurs. This was solely based on their appearance since no accompanying skeletons were found. But in 2004, a skeleton was found along with "ornithiscian" teeth and much to the surprise of paleontologists, it turned out to be a crurotarsan skeleton. Since then most of these teeth-only ornithiscian finds have been re-identified as crurotarsan1 , dramatically impacting the assumed population numbers of dinosaurs. Scientists now realizce that the crurotarsi and the early dinosaurs were so similar in physiology as to be distinguishable only by their ankle bones.2
A further surprise came when in 2009 researchers C.G. Farmer and Kent Sanders at the University of Utah discovered that modern alligators breathed in a unidirectional manner. This means that fresh air flows into the body and does not mix with carbon dioxide or low-oxygen air expelled by the body. (In humans, when we breathe in, the fresh, oxygen-laden air mixes with carbon dioxide and "old air" in our lungs). The unidirectional method is much more efficient at extracting oxygen from the air and thus enabled Triassic archosaurs (crurotarsi, pterosaurs and some dinsoaurs) to survive in a low-oxygen environment. (See the article on air sacs and unidirectional breathing.)
The significance of this new discovery is that not only were the crurotarsans fully equipped to be the dominant Triassic land animals, but they were as advanced as the dinosaurs. It also signifies that this feature occurred early on in archosaurian development, further indicating the reasons for archosaurian dominance of the Mesozoic Era.
But this also raises the question that if these crurotarasans were as physiologically similar as the dinosaurs, why did they fare worse than the dinosaurs during the Triassic Extinction? Only two groups of crurotarsans survived the mass extinction at the end of the Triassic, and since then they have remained in the waterways and aquatic environments of the world, never again to be successful on land. On the other hand, the dinosaurs survived the Triassic extinction in robust enough form that they were able to quickly expand during the Jurassic and soon ruled the land. It is a puzzle that is still being sorted out. Nevertheless the crurotarsans that survived the Triassic extinction went on to develop into our modern-day alligators and crocodiles while the dinosaurs are no more.
For a diagram of the dinosaur groups, go to the
1. Amy Barth, Battle of the Lizard Kings,
Discover Magazine, April, 2010, pages 11-12.