Crocodylo-Month Bonus: Order Choristodera
Crocodilians have long been the dominant aquatic reptiles on Earth, but they are by no means unique; they’ve been sharing their environments with other reptiles for hundreds of millions of years. Some of these other aquatic reptiles lived more recently than you might think...
Champsosaurus, the animal pictured above, might look like a crocodilian, but it actually belongs to an entirely different order of reptiles - the order Choristodera. Paleontologists are uncertain how to classify these animals, but the current leading theory is that the choristoderes are archosauromorphs - not true archosaurs, but more closely related to them than to other types of reptiles. (Image by Nobu Tamura.)
The earliest known choristoderes lived during the Early Jurassic (although fragmentary remains indicate that they might have lived in the Triassic as well). These early forms were small and lizard-like, and already well-adapted to life in the water. One such form, the Late Cretaceous Hyphalosaurus of China (pictured above), is one of the world’s most completely known fossil animals; thousands of specimens have been found, representing all growth stages from egg to adult. (Image by Matt Martyniuk.)
One Hyphalosaurus specimen, in a one-in-a-million stroke of paleontological luck, was preserved with two heads, making it the oldest known case of polycephaly.
In addition, Hyphalosaurus and its close relatives gave birth to live young. In that respect, they were unique among freshwater aquatic reptiles of the Mesozoic.
(Above: Champsosaurus, by R.J. Palmer.)
Later choristoderans became much larger - between five and ten feet in length - as well as more crocodilian in appearance. Their long snouts and strong jaws were adaptations for catching small, fast-moving prey underwater. These animals, called champsosaurs, were so specialized for aquatic existences that they were all but incapable of going on land. Only females could climb ashore to give birth; males could not leave the water.
The choristoderans survived the Cretaceous extinction and became widespread in North America, Europe, and Asia. Seven valid species of Champsosaurus are known from the United States, Canada, Belgium, and France.
Choristoderans were once believed to have gone extinct during the Eocene. However, the discovery of Lazarussuchus from the Miocene period proves that these animals survived even longer, until about 20 million years ago. In addition, Lazarussuchus is much more physiologically primitive than the champsosaurs, and bears a greater resemblance to more basal members of Choristodera. This implies that basal choristoderans survived for at least 100 million years longer than was once thought, creating a “ghost lineage” - a phylogenetic lineage that must be assumed to exist despite the absence of fossil evidence. (Image by Nobu Tamura.)
