Phytosaurs, (mostly) gharial-snouted reptiles of the Triassic, part I

As I hope I’ve said several or many times, there are many, many, many tetrapod groups that have never, ever received coverage on Tet Zoo. I know, it’s shocking. Today I’m extracting a section of text from a major in-progress book project. It’s on phytosaurs because they are among the neglected – readers with good memories might recall me hinting back in 2007 that I really want to get through all those Triassic archosauriform groups. Anyway…

Some representative phytosaur portraits. Top to bottom: Smilosuchus gregorii, Pravusuchus hortus, Mystriosuchus westphali, Paleorhinus bransoni, and Pseudopalatus pristinus. Image by Darren Naish: available on merchandise at the Tet Zoo Redbubble shop!

The long-snouted body shape that we associate with crocodiles (and the other amphibious crocodylomorphs) arose independently several times in reptile history. The Late (and perhaps Middle, and perhaps Early) Triassic waterways of the USA, Brazil, Europe, northern Africa, Madagascar, India, Thailand and elsewhere were inhabited by phytosaurs, an archosauriform reptile clade characterised by long, slender jaws, a laterally compressed tail and an overall crocodile-like body shape [image of reconstructed skeleton below by Piotrus]. While phytosaurs seem to have been cosmopolitan, the greatest number of specimens come from the western USA. Hungerbühler et al. (2013) noted that over 75 skulls belonging to the two genera Pseudopalatus and Redondasaurus alone have been collected from Arizona, Colorado, New Mexico, Texas and Utah over the last 100 years. Both of those names, incidentally, are probably best regarded as junior synonyms of Machaeroprosopus (Parker et al. 2013, Hungerbühler et al. 2013).

Spectacular reconstructed skeleton of the phytosaur Redondasaurus bermani, photographed a the Carnegie Museum of Natural History by Piotrus. Image CC BY-SA 3.0.

Phytosauria is also known as Parasuchia; the less familiar name Belodontia has also been used on occasion. While I’ve just said that phytosaurs were ‘crocodile-like’, many can, more specifically, be regarded as gharial-like in shape. Unsurprisingly, they’ve generally been imagined as amphibious predators of rivers, lakes and swamps. However, they might not all have been like this…. (see part II!).

Classic phytosaur reconstructions of old. At left, Samuel Williston's image of Mystriosuchus from 1914. At right, Joseph Smit's 1894 reconstruction of 'Belodon': actually a composite of the skull of the phytosaur Nicrosaurus and the aetosaur Paratypothorax. Both images in public domain.

Like so many fossil animal groups, phytosaurs are – while often mentioned in books and articles – rarely discussed in depth (individual specimens and species do, of course, receive detailed coverage in an extensive technical literature). Books tend to say the same stuff about them over and over – a ‘favourite fact’ being that they differ notably from crocodylomorphs in having their nostrils located well back from the tip of the snout and situated atop a bony mound. Books also tend to comment on the fact that the name is etymologically inappropriate, since phytosaur means ‘plant lizard’ and yet they clearly weren’t herbivores. The name was originally coined (in 1828) because Georg Friedrich Von Jaeger thought he saw blunt-tipped teeth suited for a leaf-based diet and hence decided to name these animals after their habit of eating leafy things, a truly distinctive feature that set them apart from the other animals. Imaginative name there, Jaeger. Anyway, about the only good published review of the group that I’m aware of is Stocker & Butler’s (2013) chapter in the prohibitively expensive Geological Society of London special volume Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin.

Outstanding life reconstructions of two Smilosuchus species; note the differences in size, skull shape and tooth configuration. Images by Jeff Martz, used with permission.

Shock-horror: phytosaurs in the Jurassic?

It’s generally thought that phytosaurs did not survive the end-Triassic extinction event, their youngest record being from the very top of the Rhaetian, the youngest part of the Upper Triassic. However, it has repeatedly been suggested that one or two species might have persisted beyond the Triassic and into the Jurassic. Several teeth from the Lower Jurassic of France and Germany seem to be those of phytosaurs, and it’s been said that they lack the signs of abrasion that might indicate reworking from older deposits. However, there are, of course, doubts about the phytosaurian identity of these teeth: they might be those of thalattosuchian crocodylomorphs (Stocker & Butler 2013) or sauropterygians (Maisch & Kapitzke (2010). Then there’s Pachysuchus imperfectus, named for a partial rostrum from the Lower Jurassic Lower Lufeng Formation of China and also suggested to be a late-surviving phytosaur. Barrett & Xu (2012) showed that the specimen (which is now lost) is actually from an indeterminate sauropodomorph dinosaur.

The Lower Jurassic (specifically, Hettangian) chunk of lower jaw suggested by Maisch & Kapitzke (2010) to be from a marine, Mystriosuchus-like phytosaur. Does it (and other scrappy specimens) show that phytosaurs really survived into the Jurassic?

More recently, a chunk of lower jaw from Lower Jurassic marine rocks of England was identified as belonging to a slender-jawed phytosaur, perhaps a representative of the gharial-like Mystriosuchus (Maisch & Kapitzke 2010). Given its Lower Jurassic age, the specimen would ordinarily be identified as belonging to a teleosaurid thalattosuchian. Maisch & Kapitzke (2010) argued that a phytosaurian identification was perhaps more likely because the specimen has two longitudinal parallel grooves that extend along its lateral surface. They regarded this twin-grooved specimen as characteristic of phytosaurs but absent in thalattosuchians. However, those twin grooves are apparently a normal feature of thalattosuchians (M. Young, pers. comm.). The specimen might be from a Jurassic phytosaur, but we need far better material before we can at all confident about this.

Some anatomy and some phylogeny

Pectoral girdle, partial forelimbs and throat region of Pseudopalatus pristinus, from Long & Murry (1995), seen in ventral view. At the top you can see an articulated, in-situ gular shield. Note that interlocking, polygonal osteoderms also cover part of the right forelimb. Scale bar = 10 cm.

The back of the phytosaur skull is wide and low, the eye sockets are high up on the sides of the skull and the external nostrils are… err, here’s that ‘favourite fact’ again… positioned well back from the tip of the snout and on top of a bony prominence that’s volcano-shaped in some species. A paired row of textured osteoderms are present along the top of the phytosaur neck and back. Exceptional articulated specimens also reveal how interlocking, polygonal osteoderms form a sort of basket-like arrangement across the gular region (forming the so-called gular shield). The forelimbs of at least some taxa were similarly encased in similar fashion (Long & Murry 1995). Distinctive skeletal features include a deeply notched anterior coracoid margin, large interclavicle, and a distinctive calcaneum where the tuber is proportionally huge and blunt-tipped. The biggest phytosaurs (certain Smilosuchus species) were 8 m long or perhaps even more, apparently (Long & Murry 1995).

Partial skeleton of Pseudopalatus pristinus showing position of paired midline osteoderms and osteoderms partially covering forelimb: from Long & Murry (1995). Scale bar = 10 cm.

The biggest known phytosaur skulls are about 80 cm long but they’re incomplete; in intact condition, total skull length was 1.2 m or more in such giants (Heckert et al. 2001). Check out the awesome Leptosuchus skull below [photo by Priscilla Jordão] to get an idea of how massively built and gnarly some phytosaurs must have been in life. A superficial similarity with the skull of the dinosaur Spinosaurus hints at some similarities in function, feeding behaviour and perhaps lifestyle. Phytosaurs are heterodont, with (typically) two pairs of elongate teeth at the slightly down-turned, sometimes slightly expanded, snout tip, recurved fangs along much of the jaws and short, subtriangular teeth in the posterior parts of the jaws.

Leptosuchus gregorii skull, photographed at the AMNH by Priscilla Jordao. Image CC BY-SA 3.0.

Tall caudal neural spines mean that the phytosaur tail is deep and laterally compressed and indicate that it was used as a sculling organ. Long, slender, strongly slanted and overlapping neural spines are present in the distal part of the tail of a probable Mystriosuchus species that appears from its overall anatomy to have been strongly aquatic (Renestro & Lombardo 1999) [this image is shown below: photo by Gyik Toma/Tommy]. This tail is somewhat paradoxical in view of the inferred lifestyle, since it’s shallower and stiffer than that of other phytosaurs. Either something unusual that we haven’t yet appreciated was going on here (example: maybe the animal had a giant soft tissue caudal fin), we’ve erred in interpreting this phytosaur as strongly aquatic, or it was pursuing an aquatic lifestyle that didn’t require all that much tail-paddling.

The articulated and near-complete Italian Mystriosuchus (or Mystriosuchus-like) specimen described by Renesto & Lombardo (1999). The specimen is about 4 m long in total. You might be able to see that the distal part of the tail is shallow and not really paddle-like. Photo taken at the Museo Civico di Scienze Naturali di Bergamo by Tommy, CC 2.0.

Numerous phytosaur taxa and a huge number of specimens are known. Indeed, the phytosaur fossil record is so good that gradual change in the anatomy of various skeletal features can be observed over time. Alas, phytosaur taxonomy is complicated and not yet resolved (some oft-used genera, including Paleorhinus and Leptosuchus, appear to be paraphyletic or polyphyletic). The majority have been included within the node-based clade Phytosauridae (Doyle & Sues 1995, Stocker 2010, 2012, 2013, Stocker & Butler 2013).

Morphological disparity within leptosuchomorph phytosaurs. From top to bottom: Angistorhinus grandis (image in public domain), Mystriosuchus westphali, and M. planirostris (both photos by Ghedoghedo, CC BY-SA 3.0).

Several clades have been recognised within Phytosauridae, including Leptosuchomorpha and Pseudopalatinae. It currently seems that Wannia and the several Paleorhinus species are outside a clade that includes all other taxa; this clade includes Angistorhinus, Rutiodon, Protome and Leptosuchomorpha. Leptosuchomorpha contains Leptosuchus as well as yet another clade, this one containing Smilosuchus, Pravasuchus and the pseudopalatines (Ballew 1989, Stocker 2010, 2012, 2013, Stocker & Butler 2013). [Adjacent Mystriosuchus images by Ghedoghedo.]

I mentioned Protome there. This is a recently named non-phytosaurid phytosaur – properly Protome batalaria Stocker, 2012 – from the Chinle Formation of Petrified Forest National Park in Arizona, the name of which means something like ‘warship animal-face’ (a protome is a decorative depiction, based on the shape of an animal’s head). Alas, the ‘warship’ bit doesn’t refer to the shape of the phytosaur’s face (Protome is actually another long-snouted, gharial-faced phytosaur), but to a locality in Petrified Forest National Park called Battleship NW (Stocker 2012). Whatever, it’s a cool name. And here’s an excellent life reconstruction of it, courtesy Smokeybjb (why don’t we know your real name?)…

We need more reconstructions of Triassic archosauriforms like this. Image of Protome batalaria, by Smokeybjb; CC BY-SA 3.0.

We’re not done yet – more phytosaurs next…

For previous Tet Zoo articles on other Triassic archosauriforms, see…

• Your Triassic armadillodiles
• The armadillodile diaries, a story of science ethics
• The surprising and hitherto undocumented late survival of non-dinosaurian dinosauromorphs
• Aetosaurs and whistle-blowing, the saga continues
• Aetogate: so where are we now?
• Yet more extreme Triassic weirdness: Vancleavea
• In support of Scientific Triassicism: Sues and Fraser’s Triassic Life on Land: the Great Transition

Refs – -

Ballew, K. L. 1989. A phylogenetic analysis of Phytosauria from the Late Triassic of the western United States. In Lucas, S. G. & Hunt, A. P. (eds) Dawn of Dinosaurs in the American Southwest. New Mexico Museum of Natural History (Albuquerque), pp. 309-339.

Barrett, P. M. & Xu, X. 2012. The enigmatic reptile Pachysuchus imperfectus Young 1951 from the lower Lufeng Formation (Lower Jurassic) of Yunnan, China. Vertebrata PalAsiatica 50, 151-159.

Doyle, K. D. & Sues, H.-D. 1995. Phytosaurs (Reptilia: Archosauria) from the Upper Triassic New Oxford Formation of York County, Pennsylvania. Journal of Vertebrate Paleontology 15, 545-553.

Heckert, A. B., Lucas, S. G., Hunt, A. P. & Harris, J. D. 2001. A giant phytosaur (Reptilia: Archosauria) skull from the Redonda Formaion (Upper Triassic: Apachean) of east-central New Mexico. In New Mexico Geological Society Guidebook, 52^nd Field Conference, Geology of the Llanl Estacado, pp. 169-176.

Hungerbühler, A., Mueller, B., Chatterjee, S. & Cunningham, D. P. 2013. Cranial anatomy of the Late Triassic phytosaur Machaeroprosopus, with the description of a new species from West Texas. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 103, 1-44.

Long, R. A. & Murry, P. A. 1995. Late Triassic (Carnian and Norian) tetrapod from the southwestern United States. Bulletin of the New Mexico Museum of Natural History and Science 4, 1-254.

Maisch, M. W. & Kapitzke, M. 2010. A presumably marine phytosaur (Reptilia: Archosauria) from the pre-planorbis beds (Hettangian) of England. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 257, 373-379.

Parker, W. G., Hungerbuhler, A. & Martz, J. W. 2013. The taxonomic status of the phytosaurs (Archosauriformes) Machaeroprosopus and Pseudopalatus from the Late Triassic of the western United States. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 103, 265-268.

Renesto, S. & Lombardo, C. 1999. Structure of the tail of a phytosaur (Reptilia, Archosauria) from the Norian (Late Triassic) of Lombardy (northern Italy). Rivista Italiana di Paleontologia e Stratigrafia 105, 135-144.

Stocker, M. R. 2010. 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, 997-1022.

- . 2012. A new phytosaur (Archosauriformes, Phytosauria) from the Lot’s Wife beds (Sonsela Member) within the Chinle Formation (Upper Triassic) of Petrified Forest National Park, Arizona. Journal of Vertebrate Paleontology 32, 573-586.

- . 2013. A new taxonomic arrangement for Paleorhinus scurriensis. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 103, 1-13.

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