Phytosaur

Phytosaurs are an extinct group of large, mostly semiaquatic Late Triassic archosauriform reptiles. Phytosaurs belong to the family Phytosauridae and the order Phytosauria. Phytosauria and Phytosauridae are often considered to be equivalent groupings containing the same species, but some studies have identified non-phytosaurid phytosaurians. Phytosaurs were long-snouted and heavily armoured, bearing a remarkable resemblance to modern crocodilians in size, appearance, and lifestyle, as an example of convergence or parallel evolution. The name 'phytosaur' means 'plant reptile', as the first fossils of phytosaurs were mistakenly thought to belong to plant eaters. The name is misleading because the sharp teeth in phytosaur jaws clearly show that they were predators. For many years, phytosaurs were considered to be the most basal group of Pseudosuchia (crocodile-line archosaurs), meaning that they were thought to be more closely related to the crocodilians than to birds (the other living group of archosaurs). Some recent studies of the evolutionary relationships of early archosauriforms suggest that phytosaurs evolved before the split between crocodile- and bird-line archosaurs and are the sister taxon of Archosauria. Others retain the older classification of phytosaurs as pseudosuchians. Phytosaurs had a nearly global distribution during the Triassic. Fossils have been recovered from Europe, North America, India, Morocco, Thailand, Brazil, Greenland and Madagascar. Fossils attributed to phytosaurs have been found in Early Jurassic rocks, possibly extending their temporal range beyond the Triassic-Jurassic boundary. Phytosaurs are known from many different morphologies, specifically with vastly different skull forms. These changes relate to the feeding and habits of the animals, not completely evolutionary modifications. Dolichorostral ('long snouted') phytosaurs have a long, slender snout with many conical teeth that are homodont (all the same). These taxa were most likely piscivores that were well adapted to capture fast aquatic prey, but not terrestrial animals. Paleorhinus, Rutiodon and Mystriosuchus are dolichorostral phytosaurs, but do not form a distinct group of taxa (named Mystriosuchinae of Friedrich von Huene) as other morphotypes such as Pseudopalatus are more closely related to Mystriosuchus than it is to the other long-snouted taxa. Brachyrostral ('short snouted') forms are the opposite, having a massive, broad snout, and very strong skulls and jaws. They are heterodont, as the front teeth are prominent fangs, and the rear teeth are blade-like for slicing food into chunks that can easily be swallowed. Taxa like this, such as Nicrosaurus and Smilosuchus, were powerful taxa that fed on stronger prey, such as terrestrial animals that came to the water to drink. Altirostral ('high snouted') animals are intermediate between the two distinct types. They had heterodont dentition but not as extremely developed as the brachyrostral type. Pseudopalatus is an altirostral phytosaur, and was most likely a generalist feeder. Modern crocodilians exhibit a similar morphological diversity, for example the broad snouted altirostral alligator and the long snouted dolichorostral gavial. Various phytosaurs have crests and similar ornamentions in their snouts. Nicrosaurus has a ridge along the snout that would have supported a keratinous crest in life, while Mystriosuchus westphali has several bony crests. Despite their great similarities in appearance and lifestyle, there are still a number of minor differences that distinguish phytosaurs from true crocodiles. For one thing, the phytosaur ankle structure is much more primitive than that of any crocodile. Also, phytosaurs lack the bony secondary palate that enables crocodiles to breathe even when the mouth has much water. However, it is possible that phytosaurs had a fleshy palate, as many Mesozoic crocodiles are presumed to have had. Phytosaurs were even better armoured than crocodiles, protected by heavy bony scutes (often found as fossils), and the belly reinforced with a dense arrangement of gastralia (abdominal ribs). Finally, and most noticeably, phytosaurs had nostrils placed near or above the level of the eyes, in contrast to crocodiles where the nostrils are near the end of the snout. This adaptation may have developed to allow them to breathe while the rest of the body was submerged. Unlike most crocodilians, phytosaurs have tooth serrations. This seems to suggest that they did not engage in the typical 'death rolls' associated with crocodiles and alligators, but probably relied on more typical meat-slicing techniques. Many species, such as Nicrosaurus and Mystriosuchus westphali, tend to have snout crests absent in crocodilians, which would perhaps conflict with the use of death rolls. In a 2001 study of the biomechanics of the dinosaur Albertosaurus's teeth, William L. Abler also examined a phytosaur's teeth, finding that it had had serrations so fine that they resembled a crack in the tooth. Albertosaurus had similarly crack-like serrations, but, at the base of each serration Abler discovered a round void, which would have functioned to distribute force over a larger surface area. This void, termed an ampulla, would hinder the ability of the 'crack' formed by the serration to propagate through the tooth. The phytosaur was found to lack adaptations for preventing its dental 'cracks' from propagating. Abler examined another sort of prehistoric predator, Dimetrodon, and found that it also lacked adaptations for guarding against crack propagation.