Life-size model of the phytosaur Rutiodon at Dinosaur State Park, Connecticut. Image by Patrick Murphy, used with permission.

Time for more phytosaurs. The previous article is probably required reading.

Phytosaurs are (so far as we can tell) members of the great diapsid reptile clade Archosauriformes. After all, they have an antorbital fenestra and various other characteristic bony features of this group. Within this clade, they’ve usually been regarded as members of Archosauria – the croc-bird clade – and, specifically, as early members of the crocodile-line clade (variously termed Crurotarsi or Pseudosuchia). Mostly this is because they have a so-called crurotarsal or ‘crocodile-normal’ ankle (this is where a peg on the astragalus fits into a socket on the calcaneum, the two bones having a rotary joint that allows great flexibility in the foot).

Archosauriform cladogram from Sereno (1991): note that phytosaurs (D) are shown as part of Archosauria, as part of Crurotarsi/Pseudosuchia, and as the sister-group to other members of the crocodile lineage. The diagrams are mostly by Greg Paul; I-K are by Sereno and F is by the late great Alick Walker. (C) Society of Vertebrate Paleontology.

Close to Archosauria but no cigar

Archosauriform family tree from the cover of Nesbitt (2011). You'll note that crown-Archosauria consists of a croc branch (at left) and a bird branch. In Nesbitt's phylogeny, phytosaurs are positioned one step outside of this clade.

However, a major phylogenetic analysis of archosauriforms found phytosaurs to be outside both Crurotarsi/Pseudosuchia and Archosauria (Nesbitt 2011). This wasn’t wholly surprising to people who know phytosaurs and Triassic archosauriforms well, since phytosaurs have always looked weird and ‘archaic’ compared to archosaurs in several ways. They have a weakly constructed coracoid and simple ulna and femur relative to archosaurs, for example.

Rather than being unique to crocodile-line archosaurs, it now seems that the crurotarsal ankle (one of the main features used to place phytosaurs within the croc-line archosaur clade) is more widespread, apparently being present in early members of the bird-line archosaur clade as well as outside of Archosauria altogether (Nesbitt 2011). Several other croc-like features of the phytosaurian ankle also no longer seem unique to crurotarsans/pseudosuchians and hence don’t support inclusion of phytosaurs in Archosauria either. For now, the favoured hypothesis is that phytosaurs are the archosaur sister-group within Archosauriformes.

The projecting calcaneal tubers of selected archosauriforms (from Parrish 1993), once thought to evidence inclusion of these groups with the crocodile-like clade Crurotarsi or Pseudosuchia. You're seeing the bone in dorsal view, anterior at the top of the page. A. Rutiodon the phytosaur. B. Prestosuchus. C. Postosuchus. D. Alligator. Note that the phytosaur calcaneum is noticeably different from the others. 'T' stands for tuber calcis, 'G' for groove.

One thing worth noting here is that all the phytosaurs we have are ‘true phytosaurs’: that is, advanced and anatomically modified members of the group with all the key features of the clade. Based on the appearance of the animal that surround them in the cladogram (proterochampsids, euparkeriids, ornithosuchids and so on), we have to predict that phytosaurs evolved from terrestrial ancestors that were small (less than 1 m long), and shorter- and deeper-snouted than all known phytosaurs. Where are these ‘proto-phytosaurs’ in the fossil record? Or is it that we’ve found them already and haven’t yet recognised them?

Lives of the phytosaurs

Limited data is available on phytosaur palaeobiology. A difference between slender-snouted, mostly homodont taxa, massive-snouted, heterodont taxa and forms intermediate between the two suggest that some phytosaurs were gharial-like fish eaters, that others were predators of large terrestrial animals, and that yet others were generalists that did both (Hunt 1989). Indian specimens of Parasuchus preserve complete protorosaurs and the scattered remains of a rhynchosaur as stomach contents (Chatterjee 1978). These apparent prey animals are (assumed) terrestrial species, so their consumption by phytosaurs might show that phytosaurs grabbed animals from the water’s edge as some crocodile do. Alternatively, maybe the predation concerned occurred on land (read on). Or maybe they were scavenged and don’t tell us anything specific about phytosaur habits. It’s just about impossible to say either way.

Holotype tracks of Apatopus lineatus as figured by Klein & Lucas (2013): manus and pes prints are visible on these slabs, but note that the tracks on the right side of the slab in A are those of Brachychirotherium parvum, an ichnotaxon perhaps generated by an aetosaur. The white arrows show where the slabs were originally connected (they later became separated and sent to different institutions).

European fossils found in marine settings indicate that at least some gharial-snouted pseudopalatine phytosaurs frequented coastlines, including those around offshore islands (Renesto & Lombardo 1999). Swim traces produced by phytosaurs are known (Stocker & Butler 2013). North American trackways made by walking quadrupedal reptiles – the tracks have the ichnotaxonomic name Apatopus – have often been suggested to be those of phytosaurs. Most recently, Padian et al. (2010) and Klein & Lucas (2013) matched the tracks to phytosaurs on the basis of their proportions and anatomic details.

Not only do these tracks seemingly show for sure that at least some phytosaurs walked on land, their arrangement seems to show that the trackmakers were using a ‘high walk’: that is, walking with erect limbs that were held directly beneath the body. If this is valid, and if we combine it with what we saw above about the phylogenetic position of phytosaurs, we have to wonder whether the phytosaur erect gait evolved independently from that also present in several archosaur lineages, or is it that erect gaits were present in the phytosaur + archosaur common ancestor? Interesting stuff.

Many authors have suggested that the Apatopus prints might have been made by phytosaurs. Padian et al. (2010) showed how phytosaur skeletal proportions (the phytosaur shown here is Parasuchus hislopi) could be fitted quite tightly against Apatopus tracks. Scale bars = 50 mm (dark for skeleton, grey for tracks).

If phytosaurs were walking on land, was this an occasional bit of behaviour, or a routine one? Actually, the stereotypical view of phytosaurs as wholly amphibious or aquatic animals might not be accurate. Nicrosaurus has been discovered in sediments (including palaeosols and marginal lacustine sandstones) deposited in – or at the edges of – terrestrial settings. Furthermore, it has proportionally longer limb bones that other phytosaurs, a straighter femur and a deeper pelvis. On the basis of this combined set of features, Kimmig (2013) suggested that Nicrosaurus was predominantly or wholly terrestrial.

Reconstruction showing phytosaur scavenging from sauropodomorph carcass. Illustration by Darren Naish (and penned in 1998 to accompany an article discussing Hungerbühler's 1998 paper on phytosaur scavenging).

This idea is also supported by the marked heterodonty, ziphodont teeth and comparatively deep skull of this phytosaur, and perhaps by taphonomic data showing that heterodont, Nicrosaurus-like phytosaurs sometimes scavenged on dinosaur carcasses that were lying well inland (Hungerbühler 1998, 2000). I should say that the same pieces of evidence led me to come up with the idea of terrestrial heterodont phytosaurs some years ago. I certainly recall discussing the idea with Axel Hungerbühler on a trip to Bristol (UK) back in 1997 or so. Obviously I never did anything with it. It’s an intriguing idea but one that needs testing – one possibility is that isotopic data might show how terrestrial these animals were relative to other phytosaurs.

Variation within skulls attributed to Pseudopalatus buceros, from Zeigler et al. (2003). This variation might be sexual dimorphism, but other possible explanations exist too.

An oft-commented-upon feature of phytosaurs concerns the apparent intraspecific variation we see in their skulls. In the North American phytosaur Pseudopalatus, some specimens have gracile snouts and a short, triangular nostril mounds while others have a more robust snout and a long, low narial crest rather than a short mound. These sorts of differences have often been interpreted as representing sexual dimorphism (Zeigler et al. 2003, Hungerbühler et al. 2013). However, this idea is difficult to test due to obvious problems with sample size. Furthermore, the differences concerned might (in cases) be due more to ontogeny or taphonomic distortion than sexual variation.

Some especially awesome phytosaur skulls have giant, inflated, rugose crests that extend along much of the length of the rostrum. Nicrosaurus kapffi is the classic example. This rugose snout crest sure looks like a display structure of some sort, but note that gnarly and ‘swollen’ cranial structures can appear in modern crocodiles and assorted other animals for all sorts of reasons. In any case, a sexually dimorphic explanation for cranial variation is unlikely to be the case across all phytosaurs since robust and gracile forms in some taxa (like Smilosuchus) don’t occur at the same locality and, furthermore, differ enough in anatomical characters across the skull that they group in separate places in cladograms (Stocker 2010).

Nice life reconstruction of Nicrosaurus kapffi, drawn to deliberately accentuate the snout crest. I'm not sure who produced the illustration (the signature at lower left is too tiny to make out, even in the original), but it appears in Hungerbühler et al. (1989).

Do we know anything about social behaviour beyond this? It has been assumed that phytosaurs fought intraspecifically in crocodile-like fashion, their gular shields and the osteoderms on the limbs and body presumably helping to minimise injury. Pathologies on the snout and jaws have been reported and might show that phytosaurs gripped or bit one another in croc-like fashion, as we might predict. Pathologically fused vertebrae have recently been reported (Witzmann et al. 2014).

I've mentioned the phytosaur-themed merchandise at the Tet Zoo redbubble shop, right?

As might have been clear from this article and the preceding one, there’s an awful lot of work left to do on phytosaurs. There’s a confused and (in part) provisional taxonomy that needs resolving through detailed anatomical evaluation and phylogenetic analysis and there are intriguing anatomical problems that need further examination. What’s the deal with the septomaxilla? Did they have vomeronasal organs or not? (see Senter 2002). Were they really capable of high-walking, or of the proficient terrestriality suggested by some? And there are so many areas where we know nothing or next to knowing, including their early evolution, biomechanics, feeding behaviour, social and reproductive behaviour, and the mechanisms and timing of their extinction.

So, consider these two articles an introduction to Phytosauria. We will return to them in time. Tet Zoo phytosaur merchandise here.

A very rough illustration from way back (well, from 2006, I think), done to depict (in hastily sketched fashion) some of the cranial diversity within this group. I knew, in time, that I'd get to wrote about them eventually...

Next: TRIASSIC MARINE REPTILES, with special reference to a recently published 'proto-ichthyosaur'.

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

Refs - -

Chatterjee, S. 1978. A primitive parasuchid (phytosaur) reptile from the Upper Triassic Maleri Formation of India. Palaeontology 21, 83-127.

Hungerbühler, A. 1998. Taphonomy of the prosauropod dinosaur Sellosaurus, and its implications for carnivore faunas and feeding habits in the Late Triassic. Palaeogeography, Palaeoclimatology, Palaeoecology 143, 1-29.

- . 2000. Heterodonty in the European phytosaur Nicrosaurus kapffi and its implications for the taxonomic utility and functional morphology of phytosaur dentitions. Journal of Vertebrate Paleontology 20, 31-48.

- ., Lingham-Soliar, T., Maisch, M. W., Stein, F. & Westphal, F. 1989. Schwimmsaurier. Austellungskataloge der Universität Tübingen 24. Institut und Museum für Geologie und Paläontologie, Tübingen.

- ., 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.

Hunt, A. P. 1989. Cranial morphology and ecology among phytosaurs. In Lucas, S. G. & Hunt, A. P. (eds) Dawn of Dinosaurs in the American Southwest. New Mexico Museum of Natural History (Albuquerque), pp. 349-354.

Kimmig, J. 2013. Possible secondarily terrestrial lifestyle in the European phytosaur Nicrosaurus kapfii (Late Triassic, Norian): a preliminary study. Bulletin of the New Mexico Museum of Natural History and Science 61, 306-312.

Klein, H. & Lucas, S. G. 2013. The Late Triassic tetrapod ichnotaxon Apatopus lineatus (Bock, 1952) and its distribution. Bulletin of the New Mexico Museum of Natural History 61, 313-324.

Nesbitt, S. J. 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352, 1-292.

Padian, K., Li, C., & Pchelnikova, J. 2010. The trackmaker of (Late Triassic, North America): implications for the evolution of archosaur stance and gait. Palaeontology 53, 175-189.

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.

Sereno, P. C. 1991. Basal archosaurs: phylogenetic relationships and functional implications. Journal of Vertebrate Paleontology 11, Supplement to Number 4, Memoir 2, 1-49.

Senter, P. 2002. Lack of a pheromonal sense in phytosaurs and other archosaurs, and its implications for reproductive communication. Paleobiology 28, 544-550.

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.

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

Witzmann, F., Schwarz-Wings, D., Hampe, O., Fritsch, G. & Asbach, P. 2014. Evidence of spondyloarthropathy in the spine of a phytosaur (Reptilia: Archosauriformes) from the Late Triassic of Halberstadt, Germany. PLoS ONE 9(1): e85511.

Zeigler, K. E., Lucas, S. G. & Heckert, A. B. 2003. Variation in the Late Triassic Canjilon quarry (Upper Chinle Group, New Mexico) phytosaur skulls: a case for sexual dimorphism. Paläontologische Zeitschrift 77, 341-351