Phytosaur

Phytosaurs Temporal range: Carnian - Rhaetian, (possible Early Jurassic occurrence)
Restoration of Rutiodon carolinensis
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauromorpha
Clade:	Archosauriformes
Clade:	Crurotarsi
Order:	†Phytosauria von Meyer, 1861
Family:	†Parasuchidae Jaeger, 1828
Genera
See text
Synonyms
Parasuchia Huxley, 1875

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

Smilosuchus gregorii

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 The specimen is too poor to be diagnostic, and this species name is no longer valid.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

Illustration of a phytosaur, 1914.

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. 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 lacked adaptations for guarding against crack propagation as well.

Evolutionary history

Phytosaur skull

Phytosaurs first appear during the Carnian age, evolving from an unspecified crurotarsan ancestor. There are no clear intermediate forms, as the first phytosaurs found were already fully-formed and highly specialised.

The earliest phytosaurs belong to the primitive and comparatively unspecialised but very widely distributed genus Paleorhinus. A somewhat more advanced and larger form, Angistorhinus appears at the same time or soon after. Later in the Carnian, both these animals were replaced by more specialised forms like Rutiodon, Leptosuchus, and the huge Smilosuchus (Lucas 1998). The Carnian-Norian extinction meant that these animals died off, and the Early Norian sees new genera like Nicrosaurus and Pseudopalatus, both of which belong to the most derived clade of phytosaurs, the Pseudopalatinae. Later in the middle Norian the advanced and specialised fish-eater Mystriosuchus appears. Fossil remains of this widespread animal is known from Germany, northern Italy, and Thailand. Finally the large Redondasaurus in south-west North America and the long-snouted (altirostral) Angistorhinopsis ruetimeyeri in Europe continued the group into the Rhaetian. Phytosaur footprints (the ichnotaxon Apatopus) are also known from the latest Rhaetian of the East Coast of USA (the Newark Supergroup) (Olsen et al. 2002). This indicates that phytosaurs continued as successful animals until the very end of the Triassic, when, along with many other large crurotarsan reptiles, they were killed off by the end Triassic extinction event.

There have been reports of phytosaur remains found in lowermost Jurassic rocks. A fragment of a lower jaw from a longirostrine archosaur has been described from early Hettangian strata in the town of Watchet in Somerset, England. While teleosaurid thalattosuchians had similar longirostrine jaws to phytosaurs and were common in the Jurassic, they do not appear in earliest Jurassic rocks. The mandible is more similar to those of known phytosaurs than to thalattosuchians, and likely belongs to a phytosaur closely related to the genus Mystriosuchus. The presence of phytosaurs in the earliest Jurassic may have prevented thalattosuchians from occupying similar ecological niches until after their disappearance.

Classification

Phylogeny

Phytosaurs are generally regarded as the most basal group of Crurotarsi, a clade of archosaurs that includes crocodilians and their extinct relatives. Phytosaurs are often excluded from a clade called Suchia, which usually encompasses all other crurotarsans, including aetosaurs, rauisuchians, and crocodylomorphs. Some studies have found polytomies between phytosaurs and other groups like Ornithosuchidae and Suchia. In these cases, it is unclear whether phytosaurs are the most basal crurotarsans. 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:

Archosauria	Avemetatarsalia  Crurotarsi  Phytosauria  Suchia  Aetosauria Crocodylomorpha Ornithosuchidae Rauisuchia

A phylogenetic analysis of early archosaurs by Nesbitt (2011) found strong support for a sister taxon relationship between phytosaurs and Archosauria. 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):

Archosauriformes	Proterosuchus Erythrosuchus Vancleavea  Proterochampsia  Tropidosuchus Chanaresuchus Euparkeria  Phytosauria  Paleorhinus Leptosuchus Pseudopalatus  Archosauria  Pseudosuchia Avemetatarsalia

List of genera

Genus	Status	Age	Location	Notes	Images
†Angistorhinopsis	Junior synonym of Nicrosaurus.	Late Triassic.	Europe.	Named because von Huene believed it looked like the older phytosaur Angistorhinus. The name is now considered a Junior synonym or close relative of Nicrosaurus "Kimmig and Arp 2010".	Reconstruction of Angistorhinus grandis Illustration of "Belodon", 1894 Illustration of Nicrosaurus skull, 1913 Mounted skeleton of Redondasaurus bermani at the Carnegie Museum of Natural History Reconstruction of Rutiodon validus
†Angistorhinus	Valid.	Late Triassic.	Africa. North America.	Referred to both Rutiodon and Leptosuchus
†Arribasuchus	Junior synonym.	Late Triassic.	North America.	Junior synonym of Pseudopalatus.
†Belodon	Nomen dubium.	Late Triassic.	Europe.	A European pseudopalatine genus of dubious utility. Many of the remains attributed to Belodon have since been attributed to other animals or given their own genera. The name Belodon means "arrow tooth."
†Brachysuchus	Junior synonym of Angistorhinus	Late Triassic.	North America.
†Centemodon	"Nomen dubium"	Late Triassic.	North America.
†Coburgosuchus	Junior synonym of Nicrosaurus	Late Triassic.	Europe.
†Ebrachosuchus	Junior synonym of Paleorhinus	Late Triassic.	Europe.
†Francosuchus	Junior synonym of Paleorhinus	Late Triassic.	Europe.
†Leptosuchus	Valid.	Late Triassic.	North America.
†Mesorhinosaurus	Nomen dubium.	Late Triassic.
†Mystriosuchus	Valid.	Late Triassic.	Europe.	pseudopalatine
†Nicrosaurus	Valid.	Late Triassic.	Europe.	pseudopalatine
†Paleorhinus	Valid.	Late Triassic.	North America Europe Africa.
†Parasuchus	Valid.	Late Triassic.		Asia
†Pravusuchus	Valid.	Late Triassic: Norian	North America (Chinle Formation)
†Pseudopalatus	Valid.	Late Triassic: Norian	North America.	pseudopalatine
†Redondasaurus	Valid.	Late Triassic: Norian/Rhaetian	North America.	pseudopalatine
†Rutiodon	Valid.	Late Triassic.	North America.
†Smilosuchus	Valid.	Late Triassic.	North America.

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.
• Maisch, M.W. (2010). "A presumably marine phytosaur (Reptilia: Archosauria) from the pre-planorbis beds (Hettangian) of England". Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen. 257 (3): 373–379. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• 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.
• "A new taxon of phytosaur (Archosauria: Pseudosuchia) from the Late Triassic (Norian) Sonsela Member (Chinle Formation) in Arizona, and a critical reevaluation of Leptosuchus,Case, 1922". Palaeontology. 53 (5): 997–1022. 2010. {{cite journal}}: Cite uses deprecated parameter |authors= (help)

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