Phytosauridae

From Palaeos.org

o Archosauria 
|--Ornithodira
`--o CRUROTARSI
   |--Phytosauridae
   `--o--o?--Ornithosuchidae 
      |  `--+?--Prestosuchidae (grade?)
      |     `--+--Rauisuchidae
      |        `--+?--Ctenosauriscidae
      |           `?--Poposauridae
      `--o--Stagonolepididae
         `--+--Euscolosuchus  
            `--o--Gracilisuchus stipanicicorum
                |?--Poposauridae
                `--o--Erpetosuchus granti
                   `--Crocodylomorpha

Introduction

The phytosaurs (this unfortunate name means "plant lizards", because it was mistakenly believed that petrified mud fillings in the jaw of the first specimen found were herbivore teeth) were crocodile-like semi-aquatic thecodonts that suddenly appeared and became very abundant during the latter part of the Triassic period.

They are more appropriately known by the later (but less widely used) name Parasuchia ("alongside the crocodiles"), as they resembled crocodiles to a remarkable degree in size, appearance, and life-style. This is an amazing illustration of convergent or parallel evolution. Phytosaurs were in a sense "crocodile uncles", since phytosaurs and proto-crocodiles shared a common ancestor among the early Crurotarsi. But true crocodiles evolved only long after phytosaurs became extinct at the end of the Triassic.

In some ways, the differences between phytosaurs and crocodiles are minor. The most obvious difference between phytosaurs and crocodiles is in the position of the nostrils. Phytosaurs had nostrils placed near or above the level of the eyes. Crocodiles have nostrils placed far forward at the tip of the snout. Another difference is in the palate (roof of the mouth). True crocodiles have a secondary palate that enables them to breathe when partially submerged, even though the mouth is full of water. Phytosaurs lacked this adaptation, and used the nostrils on top of the head in order to swallow air while underwater. Phytosaur limbs were also somewhat more primitive in structure than those of crocodiles. Fossil footprints indicate however that phytosaurs could move in a semi-erect stance on land and did not drag their tails as modern crocodiles do.

Phytosaurs were if anything even better armoured than crocs, with the throat and back of the animal being are protected by heavy armoured scutes, and the belly reinforced with a dense arrangement of abdominal ribs. The armoured scutes are often found as fossils.

Phytosaur Morphotypes and Life-style

In fact, phytosaurs are characterized by three distinctive cranial morphotypes, and this seems to be independent of phylogenetic relations. Modern crocodilians exhibit a similar morphological diversity. 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, narrow snout; and teeth that are homodont, or similar in form. Good examples include Paleorhinus, Rutiodon carolinensis, and Mystriosuchus, the latter representing the highest development. They were most likely piscivorous, like the Gavialidae. The long, narrow snout is suited for capturing fast prey, while the weak teeth could hold and position the fish for swallowing, but could not produced the shear needed to cut up chunks of flesh.

Altirostral ("high snouted") animals have an intermediate condition and primarily heterodont (different lengths or size) dentition. Typical forms might be Angistorhinus and Pseudopalatus. They were probably generalist feeders like the Alligatoridae.

Brachyrostral ("short snouted") forms have a massive, relatively broad snout, skull and jaws; and tends to extreme heterodont dentition. These animals most likely fed on large tetrapods. The mediolaterally flattened posterior teeth would slice the carcass into chunks suitable for swallowing. In addition, they probably used their jaws in intraspecific combat. Nicrosaurus and Smilosuchus are good examples of this morphotype; the latter representing the most extreme adaptation along these lines.

Phytosaur Biozonation

Chatterjee, Lucas and others have shown that phytosaur genera can be used to identify the age of a particular formation. Phytosaurs are particularly useful for biozonation because these large river and swamp-dwelling animals were frequently fossilized. Thus Paleorhinus and the angistorhinines dominated the middle (or early part of the late) Carnian. Rutiodon-like forms such as Leptosuchus and Smilosuchus (some of these names perhaps do not deserve distinct generic status - this is a classic instance of "splitters vs lumpers") become a key index fossil for the Adamanian interval (the later part of the late Carnian).

In a number of papers (e.g. Lucas 1998), Dr Spencer Lucas and co-workers have proposed a series of biozones characterized by distinctive genera for the Triassic. In each of the four biozones that make up the late Triassic, different phytosaur types are found. Possibly these intervals were separated by periods of extensive climate change, which greatly stressed these large semi-aquatic reptiles, leading to rapid speciation.

The following table shows each of the four Late Triassic biozones and representative phytosaur genera and subgenera for each:

See also Triassic stratigraphy

Classification

The classification of phytosaurs has been somewhat debated. Gregory 1969 (cited in Westphal, 1976 p.117) has three separate lineages, the slender-snouted Paleorhinus-Mystriosuchus, the more heavily built Angistorhinus-Rutiodon/Belodon, and the most extreme (crested snouted) Nicrosaurus.

Chatterjee has only two families, the Parasuchinae, which includes not only primitive types such as Parasuchus and Paleorhinus, but also the typical forms like Rutiodon and Nicrosaurus and the Mystriosuchinae for Francosuchus and Mystriosuchus.

The traditional view since then has the phytosaurs divided into two subfamilies (recently considered distinct families): the slender-snouted gavial-like Mystriosuchinae (including Paleorhinus) and the heavy crocodile or alligator-snouted Angistorhininae (= Rutiodontidae). The former would seem to have fed largely on fish and perhaps other small animals, the latter on larger prey as well as fish. Both appear suddenly in the Middle Carnian and continue through to the very end of the Triassic period.

Advocates of this approach to phytosaur classification (e.g. Long & Murry 1995) tend to divide the group into two distinct (but related) families, as follows:

Parasuchia Huxley 1875
   Mystriosuchidae Huene, 1915 
      Palaeorhininae Long & Murry, 1995 
      Mystriosuchinae Nopcsa, 1928 
   Angistorhinidae Huene, 1936 (= Rutiodontidae Long & Murry, 1995)
      Angistorhininae Camp, 1930 
      Rutiodontinae Long & Murry, 1995 
      Pseudopalatinae Long & Murry, 1995 

Of the above taxa, the Palaeorhininae and the Angistorhininae are the two oldest groups; these are replaced in the Later Carnian by the Rutiodontinae. The Norian then saw these in turn replaced by the Pseudopalatinae and the Mystriosuchinae. The Pseudopalatinae survived till the end of the Rhaetian.

In an alternative classification [Doyle & Sues, 1995, cited in Mikko's Phylogeny - Phytosauria (Parasuchia)], the palaeorhinines (as represented by the genus Paleorhinus) are considered the primitive sister group of all other phytosaurs. The similarity between Paleorhinus and Mystriosuchus are thus the result of parallel evolution, not direct ancestry and descent. This is quite likely, considering that no intermediate forms between Paleorhinus and Mystriosuchus are known in the latest Carnian or the early Norian; such a long gap in animals so well represented in the fossil record is unusual.

A third approach, indicated by biozonation in association with plesiomorphic or highly derived traits, has the early, unspecialized, forms like Parasuchus giving rise to the Angistorhinidae, with the paraphyletic/grade-level Angistorhininae in turn being replaced by their descendents the rutiodontine-grade phytosaurs in the Later Carnian, and these in turn giving rise to the most advanced clade, the Pseudopalatinae, which unlike the earlier taxa is a proper monophyletic grouping, in the Early Norian. This succession of early primitive types with later more advanced forms, and those with their still more advanced descendents, is not unlike the evolution of equids, titanotheres, and other fast-evolving taxa. Of course real-world evolution is more an unruly bush than a tidy ladder, but the generic pattern could still hold to some extent.

In view of no agreed-upon model of phytosaur evolution, the following phylogeny is proposed, based on a synthesis of a number of approaches, along with the author's rampant speculation, and thus should not be considered definitive.

Phylogeny

Phytosauridae
   |--Parasuchus
   |--Paleorhinus 
   `--Euphytosauridae 
        |--Angistorhinus  
        |--Brachysuchus  
        `--Rutiodontinae  
             |--Rutiodon   
             |--Smilosuchus 
             `--Pseudopalatinae 
                  |--Pseudopalatus  
                  |--Nicrosaurus   
                  `--Mystriosuchus

References

• Chatterjee, S (1986), Late Triassic Dockum vertebrates: their stratigraphic and paleobiolographic significance, in K Padian (ed.), The Beginning of the Age of Dinosaurs. Cambridge Univ. Press.
• Doyle, KD & H-D Sues (1995) Phytosaurs (Reptilia: Archosauria) from the Upper Triassic New Oxford Formation of York County, Pennsylvania, J. Vert. Paleontol. 15: 545-553.
• Gregory, JT (1969), Evolution und interkontinentale beziehungen der Phytosauria (Reptilia). Paläontol. Zeit. 43: 37-51
• Long, RA & PA Murry (1995), Late Triassic (Carnian and Norian) tetrapods from the Southwestern United States. N. Mex. Mus. Nat. Hist. & Sci. Bull. 4: 1-254.
• Lucas, SG (1998), Global Triassic tetrapod biostratigraphy and biochronology. Paleogeog. Palaeoclimatol., Palaeoecol. 143: 347-384.
• Westphal, F (1976) Phytosauria, in O Kuhn (ed.), Handbuch der Palaeoherpetologie Teil 13: Thecondontia, Gustav Fisher-Verlag, Stuttgart.

Credits

text MAK030727, wikified HAJ100321