Your Triassic armadillodiles

Having written articles lately on war rhinos, British big cats and rhinogradentians, I think it's time to come down to earth and cover some far more mundane, less speculative areas. Expect, then, a whole slew of articles on small lizards, brown passerines and mice. As regular readers will know, I find such animals just as interesting as the dinosaurs, pterosaurs, giant flightless birds, big cats and whales that I also sometimes write about. I need to get something off my chest, and to those interested in Mesozoic reptiles, you will be pleased to hear that it concerns aetosaurs, the omnivorous armoured crurotarsans of the Late Triassic...

Crurotarsans - the group of archosaurs that includes crocodilians and all their relatives* - remain ecologically significant today, just as they were throughout the Mesozoic and Cenozoic. And, indeed, we saw in a previous post (on sebecosuchians) that such animals even remained a significant presence during the so-called 'Age of Dinosaurs'. Arguably however, it was during the Triassic when crurotarsans were really at their peak. Crocodilians had only started to appear at this time and were small, predominantly terrestrial generalists. Freshwater environments were home to the amphibious, superficially crocodile-like phytosaurs, while a group of large and formidable carnivores, the rauisuchians, were the most significant terrestrial predators of the time. It's recently been realised that assorted fin-backed archosaurs, strange archosaurs with ornithischian-like teeth, and toothless, bipedal theropod-like archosaurs, were actually close relatives - or members - of the rauisuchian group. As exciting and blog-worthy as those animals are, it is their humble armour-plated cousins - the aetosaurs - that I'm going to be looking at here.

* Actually, the most inclusive (i.e., stem-based) name for this group is Pseudosuchia. I prefer not to use it where possible as it is so 'historically loaded' and, anyway, Crurotarsi can be used for the node-based pseudosuchian clade that includes the archosaurs we're looking at here (Senter 2005).

Restricted entirely to the Upper Triassic, aetosaurs appear to have occurred right across the Pangaean supercontinent, as their fossils are known from Europe, India, northern Africa, North and South America, Greenland and Madagascar. So far as we know, they were all quadrupedal, armoured crurotarsans, with both their dorsal and ventral surfaces encased in parallel rows of subrectangular osteoderms, some of which sported spikes in some species. These osteoderms are supposedly diagnostic for all (or most) of the different aetosaur taxa, and differ in their shape, surface ornamentation, and possession of bumps, spikes and ridges. A consequence of this ability to differentiate taxa based on osteoderms, and of osteoderm abundance, is that aetosaur taxa have been employed by some authors as index fossils. I will avoid discussing this topic here as there's a lot to say about it [adjacent image shows a remarkable death assemblage of 24 specimens of Aetosaurus found packed into a small area of just 2 m x 2 m, described in 1877].

Aetosaurs ranged in size from about 1 m in total length (for Aetosaurus and Coahomasuchus for example) to about 6 m in Desmatosuchus. This latter taxon, first described from Texas in 1892, is perhaps the most-figured of aetosaurs, not only because of its size, but also because it sported enormous curved spikes on its cervical osteoderms [see image at top of article, borrowed from here]. Some taxa (Typothorax and Paratypothorax) had particularly wide paramedian osteoderms and particularly wide bodies (paramedian osteoderms are the ones that run along either side of the dorsal midline).

Many features of the aetosaur skull are unusual [adjacent image, from palaeos.com, shows skull of Aetosaurus]. The bony nostril opening is huge in all aetosaur taxa and, at the back of the skull, the lower temporal opening is partially closed while the superior temporal opening faces laterally, rather than dorsally. The jaw joint is set below the level of the toothrow: a feature usually associated with herbivory. The premaxillary tips are moderately expanded, giving these animals a strange shovel-like snout tip. It's often been suggested that this structure was used in digging or rooting, and it's difficult to think of a good alternative function. Part or all of the premaxillary region is usually toothless but in Neoaetosauroides* from the Los Colorados Formation of Argentina, teeth were present right up the expanded premaxillary tip (Desojo & Baez 2007).

* Coined by Jose Bonaparte in 1969, this is one of those 'cumulative' names, constructed by the addition of new segments to an older name. Other examples include Propaleohoplophorus (a glyptodont), its associated subfamily Propaleohoplophorinae (!!), and the Eocene creodont Paracynohyaenodon.

Teeth are also absent from the rostral part of the lower jaw, and the entire lower jaw has a distinctive, unique shape where the rostral end curves upwards like the prow of a boat. Sawin (1947) and a few other authors have sometimes described the lower jaw as 'slipper shaped': one of those really annoying, culturally specific terms that requires you to know exactly what a slipper is in the first place. The toothless jaw tips have led some workers to suggest that aetosaurs might have had beaked jaw tips, or possibly just a beaked lower jaw tip. I used to consider this doubtful given that most aetosaur fossils seem to lack the features usually associated with the presence of beak tissue (such as a distinctive surface texture and sharp-edged jaw margins), but apparently these features are actually present.

Tooth morphology was rather variable among the group. In the forms conventionally regarded as among the most basal - such as Aetosaurus and Aetosauroides - the teeth are gently recurved and somewhat compressed. What is supposed to have been a carnivorous aetosaur has been reported from Texas, but to date an abstract is all that's been published on it (Murry & Long 1996), bar some comments in Long & Murry (1995). This taxon reportedly has sharp, strongly recurved teeth. In most other taxa, the teeth were subconical, and usually reported to be lacking in both denticles and wear facets. However, Small (2002) reported that both denticles and wear facets are present in Desmatosuchus and Stagonolepis, though it remains true that these features are lacking in other taxa. In Desmatosuchus, the teeth have bulbous crowns and a constriction at the crown-root junction.

While some aetosaurs are known from a substantial amount of disarticulated material, good, articulated specimens remain poorly known. The most complete published specimen is a 2.5 m long Typothorax coccinarum from the Bull Canyon Formation of New Mexico (Hunt et al. 1993). Unfortunately the paper that describes it is just four pages long, and of these, two are taken up by pictures and a third by the bibliography. Another remarkably complete, articulated specimen is currently under study in a German museum. I'm not allowed to talk about it, but I will say that, when it's published, it will reveal a fundamental feature that will make aetosaurs look rather different from the way in which they're conventionally portrayed.

How did aetosaurs live? Their stout legs, armoured bodies, expanded snout tip, (usually) toothless jaw tips and subconical teeth indicate that they were herbivorous or omnivorous. In the best known analysis of aetosaur lifestyle, Walker (1961) concluded that the specialized skull and stout forelimbs were used in digging and grubbing up roots and soft vegetation. The lack of tooth wear seen in some taxa suggested to Parrish (1994) that soft leaves formed most of the diet. What might be the most interesting idea has, to date, not been explored in full detail, and it is Small's (2002) proposal that aetosaurs may have been armadillo-like generalist feeders, eating social insects and other invertebrates, small vertebrates, carrion, tubers and other plant material (hence 'armadillodile': hybrid of armadillo and crocodile) [adjacent image shows Longosuchus].

While both the robust limbs, and some taphonomic data, indicates digging habits for some aetosaur taxa, it has yet to be determined whether the animals were eating objects that they found on or in the soil. This is (in theory) relatively easy to check, as the ingestion of tiny grit particles, whether accidental or deliberate, generally leaves microscopic pits and scars on a tooth's surface. So far as I know, no one has yet done any sort of tooth microwear analysis of aetosaur dentition: tell me if you know otherwise.

How much digging did aetosaurs indulge in? Did they actually dig burrows? This would presumably work for the small, narrow-bodied taxa (like Aetosaurus), but not the large, wide-bodied forms. Indeed Desojo & Baez (2005) found that aetosaur taxa differed quite considerably in limb and limb girdle anatomy and posture, indicating that diverse lifestyles were practised by the group. This is also backed up by the tooth diversity we looked at earlier: rather than all aetosaurs being alike, it seems instead that there are indications of varying amounts of carnivory and herbivory within the group.

How aetosaurs are related to other crurotarans remains a contested issue, as do the relationships between the different aetosaur taxa. But as usual I've gotten distracted. The main reason I wanted to write about aetosaurs concerns something quite specific: an unusual case of gazumping. Unless I get distracted again, I'll deal with that next.

PS - due to assorted small errors, I've made recent editorial changes to the rabbit-headed cats article. I also need to update war rhinos in view of the new data we now have on whether rhinos can or cannot be ridden (they can). Saw 300 last night. Was ok.

Refs - -

Desojo, J. B. & Baez, A. M. 2005. El esqueleto postcraneano de Neoaetosauroides (Archosauria: Aetosauria) del Triasico Superior del centro-oeste de Argentina. Ameghiniana 42, 115-126.

- . & Baez, A. M. 2007. Cranial morphology of the Late Triassic South American archosaur Neoaetosauroides engaeus: evidence for aetosaurian diversity. Palaeontology 50, 267-276.

Hunt, A. P., Lucas, S. G. & Reser, P. K. 1993. A complete skeleton of the stagonolepidid Typothorax coccinarum from the Upper Triassic Bull Canyon Formation of east-central New Mexico, USA. New Mexico Museum of Natural History and Science, Bulletin 3, 209-212.

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

Murry, P. A. & Long, R. A. 1996. A diminutive carnivorous aetosaur from the Upper Triassic of Howard County, Texas. Journal of Vertebrate Paleontology 16 (Supp. 3), 55.

Parrish, J. M. 1994. Cranial osteology of Longosuchus meadei and the phylogeny and distribution of the Aetosauria. Journal of Vertebrate Paleontology 14, 196-209.

Sawin, H. J. 1947. The pseudosuchian reptile Typothorax meadei. Journal of Paleontology 21, 201-238.

Senter, P. 2005. Phylogenetic taxonomy and the names of the major archosaurian (Reptilia) clades. PaleoBios 25, 1-7.

Small, B. J. 2002. Cranial anatomy of Desmatosuchus haplocerus (Reptilia: Archosauria: Stagonolepididae). Zoological Journal of the Linnean Society 136, 97-111.

Walker, A. D. 1961. Triassic reptiles from the Elgin area: Stagonolepis, Dasygnathus, and their allies. Philosophical Transactions of the Royal Society of London, Series B 248, 103-204.