This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
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).
Close to Archosauria but no cigar
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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.
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.
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.
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.
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.
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).
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).
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.
Next: TRIASSIC MARINE REPTILES, with special reference to a recently published 'proto-ichthyosaur'.
For previous Tet Zoo articles on other Triassic archosauriforms, see...
The surprising and hitherto undocumented late survival of non-dinosaurian dinosauromorphs
In support of Scientific Triassicism: Sues and Fraser’s Triassic Life on Land: the Great Transition
Phytosaurs, (mostly) gharial-snouted reptiles of the Triassic, part I
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