Phytosauria

Phytosaurs (Φυτόσαυροι in Greek, meaning 'plant lizard') are an extinct group of large, mostly semiaquatic Late Triassic archosauriform or basal archosaurian reptiles. Phytosaurs belong to the order Phytosauria and are sometimes referred to as parasuchians. Phytosauria, Parasuchia, Parasuchidae, and Phytosauridae have often been considered equivalent groupings containing the same species. Some recent studies have offered a more nuanced approach, defining Parasuchidae and Phytosauridae as nested clades within Phytosauria as a whole. The clade Phytosauria was defined by Paul Sereno in 2005 as Rutiodon carolinensis and all taxa more closely related to it than to Aetosaurus ferratus, Rauisuchus tiradentes, Prestosuchus chiniquensis, Ornithosuchus woodwardi, or Crocodylus niloticus (the Nile crocodile). 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 lizard', as the first fossils of phytosaurs were mistakenly thought to belong to plant-eaters.

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 studies of the evolutionary relationships of early archosauriforms have suggested that phytosaurs evolved before the split between crocodile- and bird-line archosaurs and are a sister taxon of Archosauria. The most recent study retains the former way of classifying 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. They may have also been present in rock layers dating to the Middle Triassic of China as evidenced by Diandongosuchus, however it is not known if this is truly a member of the clade.

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 be swallowed easily. 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 ornamentations 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 is full of 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.