Onto the Land and Back to the Sea: The Amniotes 261because it was one of the first early crocodilians to be described. This creature has a more
classic four-legged stance, but the limbs are still long and delicate, and the snout is still short
and slender.
How do we know that these delicate creatures are crocodilians? They sure don’t look
like it! But the superficial looks are deceiving. Crocodilians have a whole suite of distinctive
anatomical features, particularly in the skull and ankle, that are unique and unmistakable.
We don’t recognize something as a crocodilian fossil by its long snout with lots of teeth but
instead by these more subtle and more reliable features.
All of these animals have body proportions not much different from the other primitive
archosaurs of the Triassic, to which they are closely related. Thus we can trace the origin of
crocodilians from the great branching event of many archosaur lineages in the Early Triassic
through a variety of delicately built smaller forms to Protosuchus, and by the mid-Jurassic to
crocodilians that resemble something we might recognize as such (fig. 11.11).
Finally, you might be wondering why crocodilians remained so small and delicate until
the Jurassic. The answer probably lies in competition: during the Triassic, there were huge
armored semiaquatic archosaurs, known as phytosaurs, which filled the niche that croco-
dilians would eventually occupy. In most respects, phytosaurs looked like dead ringers for
crocs, from the large body size and armor, sprawling gait, and long tooth-filled snouts. But if
you look closely, you can tell in an instant that it’s not a crocodilian. All crocodilians have their
nostrils on the tips of their snouts. Phytosaurs have shifted the nostril openings to the top of
the skull, just in front of the eyes. Apparently, phytosaurs prevented crocodilians from occu-
pying that “croc niche” until they died out in the Late Triassic, and then the delicate Protosu-
chus-like crocodilians quickly evolved into large aquatic predators once the niche was open.
For Further Reading
Benton, M. J., ed. 1988. The Phylogeny and Classification of the Tetrapods. Vol. 1, Amphibians, Reptiles,
Birds. Oxford, U.K.: Clarendon.
Benton, M. J. 2014. Vertebrate Palaeontology. 4th ed. New York: Wiley-Blackwell.
Caldwell, M. W., and M. S. Y. Lee. 1997. A snake with legs from the marine Cretaceous of the Middle
East. Nature 386:705–709.
Callaway, J. M., and E. M. Nicholls. 1996. Ancient Marine Reptiles. San Diego, Calif.: Academic.
Carroll, R. L. 1988. Vertebrate Paleontology and Evolution. New York: Freeman.
Carroll, R. L. 1992. The primary radiation of terrestrial vertebrates. Annual Review of Earth and Planetary
Sciences 20:45–84.
Carroll, R. L. 1996. Mesozoic marine reptile as models of long-term large-scale evolutionary phenom-
ena. In Ancient Marine Reptiles, ed. J. M. Callaway and E. M. Nicholls. San Diego, Calif.: Academic,
467–487.
Carroll, R. L. 1997. Patterns and Processes of Vertebrate Evolution. New York: Cambridge University
Press.
DeBraga, M., and R. L. Carroll.. 1993. The origin of mosasaurs as a model of macroevolutionary patterns
and processes. Evolutionary Biology 27:245–322.
Gauthier, J. A., A. G. Kluge, and T. Rowe. 1988. The early evolution of the Amniota. In The Phylogeny
and Classification of the Tetrapods. Vol. 1, Amphibians, Reptiles, Birds. M. J. Benton, ed. Oxford, U.K.:
Clarendon, 103–155.
Laurin, M., and R. R. Reisz. 1996. A reevaluation of early amniote phylogeny. Zoological Journal of the
Linnean Society of London 113:165–223.