Phytosauria
Phytosaurs 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. 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, 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, meaning that they were thought to be more closely related to the crocodilians than to birds. 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.
Description
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 phytosaurs have a long, slender snout with many conical teeth that are homodont. 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 as other morphotypes such as Pseudopalatus are more closely related to Mystriosuchus than it is to the other long-snouted taxa. Brachyrostral 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 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.
Differences from crocodiles
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 true bony secondary palate that enables crocodiles to strengthen their skulls and breathe even when their mouths are full of water. However, phytosaurs do possess a somewhat analogous structure that is derived from the ventral part of the extremely elongated premaxillae, which works to displace the internal nasal openings backwards, although the external nares being positioned far back on the top skull helps to bring the choanae back by itself in the first place, resulting in the premaxillary palate not being able to act as a partition between the nasal and buccal cavities in some genera such as Ebrachiosuchus, although still acting to strengthen the skull. In life, it is possible that phytosaurs had a fleshy palate in addition to the premaxillary palate, which would help complete the separation between the aforementioned nasal and oral cavities. Phytosaurs were even better armoured than crocodiles, protected by heavy bony scutes, and the belly reinforced with a dense arrangement of gastralia. 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.Teeth
Unlike most crocodilians, phytosaurs have tooth serrations.In a 2001 study of the biomechanics of the dinosaur Albertosaurus
A study on phytosaur microwear patterns has found Mystriosuchus to line with soft invertebrate consumers, Nicrosaurus with hard invertebrate consumers and Smilosuchus and Machaeroprosopus with carnivores and piscivores.
Locomotion and terrestriality
Phytosaurs have been traditionally held as rather "primitive" animals in regards to terrestrial locomotion, particularly in regards to archosaurs such as crocodilians, lacking the erect gait seen in these, other pseudosuchians, dinosaurs and pterosaurs. However, the Apatopus ichnofossil shows that the animals did in fact have an erect gait like their archosaur relatives.Most phytosaurs are thought to be aquatic animals, and indeed most do show adaptations for such a lifestyle; swim tracks attributed to phytosaurs, for example, are known. However, at least Nicrosaurus seems to have evolved towards a secondarily terrestrial lifestyle, developing longer limb bones, straighter femora and a deeper pelvis, and indeed occurs in terrestrial or marginal lacustrine settings. Combined with its deep upper jaw, it probably led a similar lifestyle to terrestrial predatory crocodylomorphs like sebecians.
Inversely, some dolichorostral forms like Mystriosuchus have become further specialised to life in the water, and occurred in marine environments. A skeleton of Mystriosuchus planirostris, found in a marine setting and with evidence of little post-mortem transportation – indicating that it died either at sea or in a freshwater environment nearby – shows that this animal had paddle-like limbs, less adapted for terrestrial locomotion than in most other phytosaurs. Furthermore, the tail of Mystriosuchus was laterally compressed and could have been used in propulsion.
Endocast studies
Scans on various phytosaur braincases suggest that these animals generally had long olfactory tracts, weakly demarcated cerebral regions, dorsoventrally short endosseous labyrinths and various sinuses, including large antorbital and dural venous ones; the general bauplan is vaguely similar to that of crocodilians, but differs significantly in the presence of multiple sinuses, smaller cerebral hemispheres and smaller endosseous labyrinths. The similarities are considered to be plesiomorphic in relation to the ancestral archosauriform design, lacking many features seen in avemetatarsalians, though convergence in terms of lifestyle might also play a role.Reproduction
No phytosaur eggs have been found so far. There are pits associated with footprints in the Chinle Formation, but these "nests" are apparently the result of sandstone weathering. A recent study suggests they might have had parental care.History
When the first phytosaur fossils were found, it was not immediately obvious what kind of animal/species they were. The first phytosaur species known to science was ambiguously referred to by G. Jaeger in 1828 as "the genus of Phytosaurus, which I call Cylindricodon." The collective group name, meaning "plant lizard with cylindrical teeth," was coined due to Jaeger's mistaken belief that petrified mud fillings in the jaw were herbivore teeth.Authorship of the genus name Phytosaurus is credited to Wagler, 1830, who was the first person to unambiguously use the binomen Phytosaurus cylindricodon when describing Jaeger's findings. This commonly used genus name Phytosaurus Wagler, 1830, is invalid according to ICZN code as a junior homonym of the collective-group name Phytosaurus Jaeger, 1928.
The name of the group – Phytosauria – was coined on the basis of Phytosaurus named by Jæger. However, while the name Phytosaurus is available as a collective-group name, it was originally not introduced simultaneously as a family-group name and genus-group name, with both categories being mutually exclusive, so it does not conform to the ICZN article 1.2.2. While the nominal genus Phytosaurus as named by Wagler is available, it represents a junior homonym. Its usage as a family-group name also does not conform to various ICZN articles ; it was not named in the nominative plural, so the authorship of Phytosauria purportedly should not be attributed Meyer but to Baur, and not subsequently used as valid in the family-group taxon. The 2022 study suggested that while the class-group names Parasuchia and Belodontia and the family-group name Belodontidae could be used instead of Phytosauria and Phytosauridae, the best solution to resolve this taxonomic issue would be to designate a neotype for Belodon plieningeri due to the taxonomic restrictions to the name Belodon.
The second species to be described was Belodon plieningeri by von Meyer in von Meyer and Plieninger 1844. The altogether more appropriate name Parasuchia was coined by Thomas Huxley in 1875 along with his discovery and naming of the Indian species Parasuchus hislopi, on the basis of a partial snout. The specimen is usually considered non-diagnostic, and the name Parasuchus is replaced by Paleorhinus.
The name phytosaur remains the standard vernacular for these animals, despite its invalid status by ICZN code and the fact that these animals have been clearly shown to be carnivorous. More valid names for the clade include Parasuchia and Belodontia, which are preferred for formal classification.