The phytosaurs: great, big, croc-like land predators of the Triassic

Long before modern crocodiles made an appearance, other animals with a similar lifestyle occupied the same niche: the phytosaurs, which inaccurately mean "plant reptiles", because they were incorrectly assumed to be herbivores at first, from inclusions of petrified mud in the jaws of the first fossil specimens being interpreted as herbivore teeth. They are an order of long-snouted, semi-aquatic thecodont archosaurs that bore a strong resemblance to crocodiles, their armour scutes being most commonly found as fossils. Their abdomens were also toughened with densely-arranged ribs. The phytosaurs appeared abruptly in the mid Triassic, becoming especially abundant during the Upper Triassic and finally becoming extinct during the Triassic-Jurassic extinction event as a result of climate change broughtabout by an increase in carbon dioxide levels.

This group is also known more suitably as the Parasuchia (‘near the crocodiles’) because they looked very crocodile-like in their size, lifestyle and morphology, which serves as a prime example of convergent evolution. The phytosaurs were distantly related to crocodiles as both proto-crocodiles and phytosaurs shared common ancestry among the early pseudosuchia. However, true crocodiles (Crocodylia) wouldn’t appear until the late Cretaceous, long after the phytosaurs disappeared. The differences between crocodiles and phytosaurs are somewhat insignificant, but the most notable difference of all is that the nostrils of phytosaurs are situated above and between the eyes, forming blowhole-like nostrils. 

On the other hand, crocodiles have their nostrils situated far at the front of the snout. The limb position of phytosaurs were also more primitive in appearance than modern crocodiles, which were held in a semi-erect gait with their tails not dragging along the ground as determined by fossilised footprints. One other difference is that crocodiles have a secondary palate helping them to breathe whilst partly submerged in water, despite the mouth being packed chock-full of water. However, the phytosaurs lacked this feature, instead using the nostrils high on their heads to help them breathe air whilst partly submerged.

The phytosaurs can be distinguished by three different types of cranial morphology, an identification separate from phylogenetic classification: altirostral, brachyrostral and dolichorostral. These skull types are related to dental features, in particular the similarities and differentiation of teeth embedded across the jawline. I shall be briefly describing one phytosaur genus of each of these three skull morphotypes:

Altirostral (‘ high snouted’) - a skull condition intermediate between the two other types, consisting of a set of teeth that are heterodont (meaning different sized). One such taxon with this skull type is Angistorhinus grandis, known from the Popo Agie Formation of Wyoming, United States. It has an average skull length of about 90-120 cm, with an elongated, reinforced rostrum and a downturned premaxilla. The teeth were slightly larger and more compressed towards the back of the jaws (Chatterjee, 1978).

Figure 1: My reconstruction of Angistorhinus grandis basking on a river bank while ‘mouth gaping’ like modern crocodiles in order to keep cool in the very hot Triassic climate.

Brachyrostral (‘short snouted’) - broad, enormous skull, jaws and snout, with strongly heterodont teeth - the jaws consisted of distinct fang-like teeth at the front of the snout and blade-like posterior teeth at the back of the jaws which were mediolaterally-flattened, ideal for slicing up the carcasses of large tetrapods into bite-sized chunks of flesh that could be swallowed easily. The genus where these adaptations are taken to the most extreme is the huge Smilosuchus, known from the Chinle Formation of Arizona, United States. It is one of the largest known phytosaurs with an estimated length of 7-12 metres as determined by the 1.5 metre long skull (Chatterjee, 1978). 

 

Figure 2: My reconstruction of Smilosuchus gregorii, with the WIP stages included. Height of human figure = approx. 1.8 m.

Dolichorostral (‘long snouted’) - a skull condition with a relatively long, slim snout, full of medium-sized, conical, homodont teeth (meaning that they are of the same size), with distinct vertical striations. Some genera, such as the 3-8 m long Rutiodon carolinensis from the Cumnock Formation of North Carolina, were likely fish-eaters, analogous to modern day gharials of the family Gavialidae . Their long, slim snouts may have been adapted for catching fast prey such as slippery fish, whereas the weaker back teeth could secure the prey for swallowing whole, as shear could not be implemented to slice up the prey into chunks (Chatterjee, 1978).

Figure 3: My reconstruction of Rutiodon carolinensis in a late Triassic swamp.  

Certain phytosaur genera may serve as useful index fossils in determining the age of various formations because they were large water-dwelling animals, which gives them a better chance of fossilisation. For example, the fossils of genera like Angistorhinus and Paleorhinus have been used to identify the Otischalkian interval of the early Upper Carnian, whereas some like Smilosuchus served as key index fossils for identifying the Adamanian interval of the late Upper Carnian.

In a study published by Lucas (1998), a sequence of four biozones were identified, each of them distinguished by distinct phytosaur genera from the late Triassic. These intervals may have been divided by episodes of prolonged climate change, putting pressure on these large semi-aquatic archosaurs to quickly undergo speciation (when populations become isolated to evolve into separate species).

References:

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.

Lucas, S. G. 1998. Global Triassic tetrapod biostratigraphy and biochronology. Paleogeography, Palaeoclimatology, Palaeoecology, 143, 347-384.

Stocker, M. R., Butler, R. J. 2013. Phytosauria. Geological Society, London, Special Publications, 379, 91-117.