What Is a Crocodylomorph? Crocodiles Used to Be Far Stranger

Crocodylomorpha includes modern crocodilians and all their extinct relatives back to the Triassic, and the diversity within it is far greater than the surviving members suggest. Early forms were small, slender, long-legged land animals with an upright limb posture, quite unlike the sprawling semi-aquatic image, and several were fast terrestrial runners occupying roles that dinosaurs also filled. Later branches produced the notosuchians, a substantially land-dwelling and largely southern group including forms with differentiated teeth resembling those of mammals, some with grinding surfaces suggesting a plant or omnivorous diet, and some with short deep skulls and forward-facing eyes. Thalattosuchians went the other way entirely, becoming fully marine with paddle-like limbs, a tail fin and in some cases salt glands, and abandoning land completely. Alongside all of these ran forms resembling the modern body plan, which persisted while the others came and went.

Several extinct members are worth naming because they are so unlike anything living:

• •Terrestrial sprinters of the Triassic and Jurassic with long limbs held beneath the body, built for running rather than for ambush
• •Notosuchians with teeth differentiated into incisors, canines and multi-cusped cheek teeth, which is a mammalian arrangement arrived at independently
• •Armadillo-like forms with extensive body armour and a probable burrowing habit
• •Fully marine thalattosuchians with flippers and a vertical tail fin, which is the same solution ichthyosaurs and dolphins reached
• •Enormous river predators including Sarcosuchus and Deinosuchus, estimated at lengths well beyond any living species and capable of taking large prey
• •Small insectivorous and possibly arboreal forms, which is about as far from the modern image as the group gets

The uniformity of modern crocodilians is the exception rather than the baseline, and explaining it is more interesting than explaining the diversity. The semi-aquatic ambush design is extremely effective in warm shallow freshwater, and the surviving lineages occupy that niche almost exclusively. The extinctions that removed the others were not all the same event, with the marine forms declining through the Cretaceous and the diverse terrestrial groups persisting in some regions into the Cenozoic before disappearing, in several cases apparently as mammalian carnivores expanded. Climate is a strong constraint, since crocodilians depend on external heat and their distribution tracks temperature closely, which has restricted them to low latitudes as the world cooled, and the fossil record shows them far further north and south during warmer periods. The result is a group whose living members are a narrow and conservative sample of what it produced, which makes them a poor guide to their own history.

The surviving twenty-odd species are worth describing on their own terms, since they are more capable than the ambush-predator image allows. They are attentive parents, guarding nests, carrying hatchlings to water in their mouths and responding to calls from young in the egg, which is behaviour shared with birds and inherited from the common ancestor rather than evolved separately. They communicate vocally with a range of calls including territorial bellows that produce visible water displacement. Several species have been observed using sticks balanced on the snout to attract nesting birds, which is a claim of tool use that has been reported and debated. Their bite force is the highest measured in any living animal and their jaw-opening muscles are correspondingly weak, which is why a modest strap holds the mouth shut. They tolerate long fasting through extremely efficient digestion and low metabolic demand. And they are ectothermic, so their whole distribution tracks temperature, which is why none lives far from the tropics and why they occur as fossils in places that are now far too cold.

Crocodilians and birds are each other's closest living relatives, which surprises people who expect crocodiles to group with lizards and snakes. Both belong to the archosaurs, a group whose two main branches lead to crocodilians on one side and to dinosaurs and therefore birds on the other, and lizards and snakes sit outside it entirely on a separate lineage. That relationship makes crocodilians scientifically valuable as a bracket for inferring the biology of extinct dinosaurs, since a feature present in both crocodilians and birds was probably present in the animals between them. It also explains features that would otherwise be puzzling, including parental care of nests and young, vocal communication between hatchlings and adults, a four-chambered heart and a unidirectional lung airflow that was only demonstrated in the 2010s and which pushed the origin of that respiratory system back well before dinosaurs.

The group produced fast upright land runners, plant eaters with mammal-like grinding teeth, armoured burrowers and fully marine forms with flippers and a tail fin. Modern crocodilians are a narrow surviving sample confined to a single effective niche and limited by temperature. Their closest living relatives are birds rather than lizards, which is why they are used to bracket inferences about dinosaur biology.