Phytosauria Translation and Pronunciation Guide

Introduction

Ben Creisler

The carnivorous, and glaringly misnamed, phytosaurs ("plant lizards") represent one of the most striking examples of convergent evolution in the fossil record. Phytosaurs (sometimes referred to as parasuchians "near crocodiles") and modern crocodilians share a nearly identical body plan, with an elongated narrow head, semi-sprawling limbs and a deep, heavy tail. Such similarities are superficial, however, and careful research has confirmed that phytosaurs are not the ancestors of crocodiles, contrary to some earlier classifications. The two groups of archosaurs are only distantly related--the surviving branch of crocodiles merely adapted to the semi-aquatic predator niche pioneered by the large phytosaurs that lurked along the shores of freshwater lakes and rivers during the Late Triassic.

Chatterjee (1978) provides a good description of a typical phytosaur: "The limbs ... are well built for terrestrial locomotion. The forelimbs being slightly shorter than the hind, are well adapted for grasping and holding the prey with sharp claws. The animal was probably capable of a rapid chase for a short distance. The relatively large and spreading manus and pes may have helped to prevent the feet sinking into the swampy land which was the probable habitat. The throat and back of the animals are protected by a formidable shield of scutes, making it almost impregnable. The abdominal ribs provide reinforcement for the belly. During swimming, the limbs would probably be folded against the body and the powerful compressed tail used for sculling through the water. The latter could strike as assailant a severe blow by a sidewise sweep."

The shape of the bones and muscle attachments make phytosaur limbs somewhat more primitive than those of crocodiles. Nonetheless, trackway evidence suggests that phytosaurs could move in a semi-erect stance on land and did not drag their tails as modern crocodiles do. The heavily armored throat and the especially dense arrangement of abdominal ribs also distinguish phytosaurs from typical crocodiles. It is not clear if the tail region in most phytosaurs was armored, although many restorations have added such features.

The most obvious differences between phytosaurs and crocodiles are found in the skull. Phytosaurs had nostrils placed toward the back of the head near or above the level of the eyes--crocodiles, by constrast, have nostrils placed far forward at the tip of the snout. Evolution remodeled different bones to form the elongated snout in each group--in phytosaurs the premaxillary bones that lie forward of the nasal bones extended, allowing the nostrils to shift toward the top of the skull, whereas in crocodiles the maxillary bones that lie behind the premaxillaries extended, pushing the nasal bones to the tip of the snout. Phytosaur-type nostrils would seem a clear advantage for an animal that needs to breath when submerged. However, the great success of crocodiles, including many highly derived marine forms, shows that the advantage may be only a minor one, at least in an ectotherm.

Unlike crocodiles, most phytosaurs had special tusk-like teeth jutting from the curved and somewhat expanded tip of both the upper and lower jaws. The arrangement was probably a feeding specialization, and some early researchers speculated, rather fancifully, that the narrow snout and backset nostrils indicated that phytosaurs probed deep into mud for food in the manner of long-beaked wading birds. More plausibly, the enlarged teeth at the snout tip were used to snatch fish, or other fast moving and slippery prey--or they perhaps they were used in some form of intraspecific combat.

The classification of phytosaurs continues to bedevil paleontologists, and experts have not reached a firm consensus about which taxa are distinct and definable, which names should be valid, or which forms belong in which families or subfamilies. The key taxonomic characters that appear to separate phytosaur species occur in the skull, but such characters often are obscured by wide variations in fossil specimens, especially in the shape of the crests along the snout--differences that may reflect growth stages and sexual dimorphism, or purely individual distinctions, further modified by pathologies from injury, or distortions from fossilization. The fragmentary type specimens of Phytosaurus, Parasuchus, Belodon and Rutiodon have added to the confusion, since some authors continue to use these old names as valid senior synonyms for a range of described genera.

Phytosaur skulls tend to fall into two major morphotypes: (1) gavial-like skulls (typified by Mystriosuchus) that have a long, low, slender snout tipped with a pronounced tusked "hook," and jaws lined with uniform conical teeth typical of fish-eating predators, and (2) more robust somewhat crocodile-like skulls (typified by Nicrosaurus) that have a shorter, wider, deeper snout, often with a raised crest, and jaws containing sets of serrated blade-like fangs (indicating a diet that included large animals) and more cylindrical crushing-type teeth, in addition to pointed conical teeth and the tusked "hook." Both types of skulls may have evolved in independent lines of phytosaur evolution in response to similar diets. Perhaps the most radical explanation was offered by the Austrian paleontologist O. Abel, who proposed in 1922 that slender Mystriosuchus-type skulls represented females, and heavier Nicrosaurus-type skulls represented males of the same species. He also suggested that the roughened, tumorous appearance of some phytosaur crests resulted from bites delivered by competing males. Camp (1930) supported the bite-origin for the rugged crests, but rejected sexual dimorphism as the distinction between the two skull types--both types were not always found in same geological horizons.

Many researchers now separate Mystriosuchus and Nicrosaurus in different families or subfamilies: the Mystriosuchidae and the Angistorhinidae (= Rutiodontidae Long and Murry 1995). However, the picture remains complex. Paleorhinus (classified in the Mystriosuchidae) had heterodont teeth and could prey on land vertebrates (confirmed by remains of small archosaurs and other reptiles in the stomach region of two specimens from India (Chatterjee 1978)), while Angistorhinus (classified in the same family as Nicrosaurus) had a slender snout without a raised crest and jaws with mainly conical teeth, though some at the back were rather blade-like.

Most known phytosaurs were big (from 3 to 8 meters), and a few species may have reached 12 meters (35 feet) or more in length, making them the largest land predators of the Triassic, though like modern crocodiles, they probably did not venture far from bodies of water. To date, phytosaurs have been found only in freshwater deposits, and members of the group do not appear to have adapted to saltwater or estuarine environments the way that their ecological successors the crocodiles would. Remains are known from Europe, India, North Africa and North America, with possible fossils from Madagascar and China (now disputed), suggesting the group did not reach the areas comprising modern sub-Saharan Africa, South America, Australia or Antarctica. Phytosaurs disappear from the fossil record at the end of the Triassic.

(A special thanks to George Olshevsky for sharing his updated classification of phytosaurs.)

1. Ballew, K. L. 1989. A phylogenetic analysis of the Phytosauria from the late Triassic of western United States: pp. 309-339, in S. G. Lucas nd A. P. Hunt, (eds.), Dawn of the age of dinosaurs in the American Southwest. New Mexico Museum of Natural History. Albuquerque.
2. Camp, C. L. 1930. A study of phytosaurs, with description of new material from western North America. Memoirs. University of California. 10: 161 pp. Berkeley.
3. Chatterjee, S. 1978. A primitive parasuchid (phytosaur) reptile from the Upper Triassic Maleri Formation of India. Palaeontology. 21 (1): 83-127.
4. Long, R. A. & P. A. Murry, 1995. Late Triassic (Carnian and Norian) Tetrapods from the Southwestern United States. New Mexico Museum of Natural History & Science. Bull. 4. 254pp.
5. Williston, S. W. 1914. Water Reptiles of the Past and Present. Chicago. 251 pp.