Phytosaurs are an extinct group of large semi-aquatic Late Triassic archosaurs. Phytosaurs belong to the family Phytosauridae and the order Phytosauria. They were long-snouted and heavily armoured, bearing a remarkable resemblance to modern crocodiles in size, appearance, and lifestyle, an example of convergence or parallel evolution. The name "phytosaur" (plant reptile) is very misleading, and their snapping jaws clearly show that phytosaurs were predators. The person who first described them mistakenly thought the specimens he was working with were plant-eaters (Jaeger 1828).
Although phytosaurs were not true crocodilians themselves, they were more closely related to the crocodilians than to other modern reptiles, as phytosaurs are basal members of the clade Crurotarsi. Crocodiles did not become superficially 'phytosaur'-like until the Early Jurassic.
Phytosaurs had a nearly global distribution during the Triassic. Fossils have been recovered from Europe, North America, India, Morocco, Thailand, and Madagascar. Fossils attributed to phytosaurs have been found in Early Jurassic rocks, possibly extending their temporal range beyond the Triassic-Jurassic boundary.
Early discoveries
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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 named
Phytosaurus cylindricodon - "plant lizard with cylindrical teeth" - by G. Jaeger in 1828 because he mistakenly believed that petrified mud fillings in the jaw were herbivore teeth. The specimen is too poor to be diagnostic, and this species name is no longer valid. The name of the group - Phytosauria - was coined by the German paleontologist
Hermann von Meyer in 1861, on the basis of this first species.
The next species to be described was Belodon plieningeri by von Meyer in von Meyer and Plieninger 1844. The altogether more appropriate name Parasuchia ("alongside the crocodiles", as they resembled crocodiles to a great degree) was coined by Thomas Huxley in 1875 along with his discovery and naming of the Indian species Parasuchus hislopi (Chatterjee, 1978), on the basis of a partial snout. The specimen also is usually considered non-diagnostic, and the name Parasuchus replaced by Paleorhinus. Although the names Parasuchidae and Phytosauridae are variously still used by different specialists, "phytosaur" is the standard generic name for these animals, despite the fact that these animals have been clearly shown to be carnivorous.
Anatomy
Three morphotypes
The phytosaur skull was characterized by three distinct morphotypes, which relate to feeding and habits and not (as was once thought) evolutionary relationships. These skull patterns are linked to characteristics of the dentition; specifically the differentiation or similarity of the teeth along the jaws.
Dolichorostral ("long snouted") types have a long, slender snout and a large number of conical teeth that are the same throughout. These were most likely piscivorous, able to capture fast slippery prey, but not so good at tackling a land animal. Some examples are Paleorhinus, Rutiodon carolinensis, and Mystriosuchus. At one time it was believed that Paleorhinus and Mystriosuchus belonged to a distinct group of phytosaurs (subfamily of family Mystriosuchinae/Mystriosuchidae Huene, 1915) characterised by this adaptation, but it is now known that Mystriosuchus is actually more closely related to Pseudopalatus, an "altirostral" form (Hungerbühler, 2002).
Brachyrostral ("short snouted") forms are the opposite, they have a massive, broad snout, and a very strong skull and jaws, with the front teeth like fangs for holding the prey, and the rear teeth blade-like for slicing the meat into chunks that can easily be swallowed (an animal with different types of teeth like this is called heterodont). These were powerful animals specialised for feeding on strong struggling prey, such as terrestrial animals that come to the water to drink. Examples of this type are Nicrosaurus and Smilosuchus
Altirostral ("high snouted") animals are intermediate between the two. They had heterodont dentition but not as extremely developed as the brachyrostral type. Angistorhinus and Pseudopalatus are typical examples here. These were most likely generalist feeders.
Modern crocodiles exhibit a similar morphological diversity, for example the broad snouted altirostral alligator and the long snouted dolichorostral gavial.
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).
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 bony
secondary palate that crocodiles have that enables them to breathe even when the mouth is full of water. It is possible however that phytosaurs had a fleshy palate, as many Mesozoic crocodiles are presumed to have had. 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
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. Phytosaurs are often excluded from a clade called
Suchia, which usually encompasses all other crurotarsans, including
aetosaurs,
rauisuchians, and
crocodylomorphs. In one of the earliest studies of crurotarsan phylogeny, Sereno and Arcucci (1990) found Crurotarsi to be a
monophyletic grouping consisting of phytosaurs, ornithosuchids, and the more
derived suchians, but produced a trichotomy between the three groups in their tree. In resolving this trichotomy, Parrish (1993) placed ornithosuchids, not phytosaurs, as the most basal crurotarsans. However, most other studies such as Sereno (1991) and Benton
et al. (2010) recover phytosaurs in a basalmost position among crurotarsans. Below is a cladogram modified from Benton
et al. (2010) showing the widely accepted phylogenetic relationships of phytosaurs: If this is the case, phytosaurs would be placed outside Pseudosuchia in a more basal position among archosauriforms. Phytosaurs would be considered closely related to the ancestors of both crocodilians and
dinosaurs. Furthermore, the definition of the clade Crurotarsi would change, as it is often defined by the inclusion of phytosaurs. Thus, Crurotarsi would include phytosaurs and all other archosaurs —including dinosaurs— under this phylogeny. Below is a cladogram showing the placement of phytosaurs from Nesbitt (2011):
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References
Abler, W.L. 2001. A kerf-and-drill model of tyrannosaur tooth serrations. p. 84-89. In: Mesozioc Vertebrate Life. Ed.s Tanke, D. H., Carpenter, K., Skrepnick, M. W. Indiana University Press.
Carroll, R.L. (1988). Vertebrate Paleontology and Evolution, WH Freeman & Co.
Chatterjee, S. (1978). A primitive parasuchid (phytosaur) reptile from the Upper Triassic Maleri Formation of India, Palaeontology 21: 83-127
Hungerbühler, A. (2002). The Late Triassic phytosaur Mystriosuchus Westphali, with a revision of the genus. Palaeontology 45 (2): 377-418
Jaeger, G.F. 1828. Über die fossilen Reptilien, welche in Würtemberg aufgefunden worden sind. Metzler, Stuttgart.
Kimmig, J. & Arp, G. (2010) Phytosaur remains from the Norian Arnstadt Formation (Leine Valley, Germany), with reference to European phytosaur habitats. Palaeodiversity 3: 215-224
Lucas, S.G. (1998). Global Triassic tetrapod biostratigraphy and biochronology. Paleogeog. Palaeoclimatol., Palaeoecol. 143: 347-384.
Olsen, P.E., Kent, D.V., H.-D.Sues,, Koeberl, C., Huber, H., Montanari, E.C.Rainforth, A., Fowell, S.J., Szajna, M.J., and Hartline, B.W., (2002). Ascent of dinosaurs linked to an iridium anomaly at the Triassic-Jurassic boundary. Science 296: 1305-1307.
Further reading
Ballew, K.L. (1989). A phylogenetic analysis of Phytosauria from the Late Triassic of the Western United States. Dawn of the age of dinosaurs in the American Southwest: Pp. 309-339.
Gregory, J.T. (1962). Genera of phytosaurs. American Journal of Science, 260: 652-690.
Long, R.A. & Murry, P.A. (1995). Late Triassic (Carnian and Norian) tetrapods from the southwestern United States. New Mexico Museum of Natural History and Science Bulletin, 4: 1-254.
External links
Translation and Pronunciation Guide
A Preliminary Biomechanical Analysis of Phytosaur Life Habits
Phytosauria - Palaeos
Mikko's Phylogeny - cladistic tree
Great Triassic Assemblages Pt 1 - The Chinle and Newark - some material on phytosaurs
