Tracking basal sauropodomorphs

This is the long-delayed third instalment of my sauropodomorph trilogy. This discussion was sparked by a recent paper published in Acta Palaeontologia Polonica (Milàn et al. 2008). Laelaps has already blogged about this paper many weeks ago, but as I said it takes me a while to get around to things.
One feature of Late Triassic (Norian – Rhaetian) and Early Jurassic faunas is the usual dominance of basal sauropodomorph dinosaurs. I say usual because it wasn’t a global hegemony. Basal sauropodomorphs are strangely absent from the Triassic of North America. There are some reported scraps and a few teeth but these are not very convincing (see Nesbitt et al. 2007). Otherwise the sole Triassic records of Sauropodomorpha from the continent are the Eosauropus tracks, which may represent true sauropods but this remains contentious. Basal sauropodomorphs are likewise absent from the admittedly meagre Early Jurassic record of Europe (Ohmdenosaurus, a vulcanodontid-grade sauropod is an exception). However all other rich deposits are chock full of basal sauropodomorphs. Take the Elliot Formation for example: here it would be no exaggeration to suggest that more than 90% of finds in the upper part (Early Jurassic) are basal sauropodomorphs, while this proportion very nearly reaches 100% in the lower part (Late Triassic) of the formation. I mention all of this was just to establish the point that basal sauropodomorphs are very common in the body fossil record. So one might expect an equally rich footprint record. Not so, the footprint record from the early phase of dinosaur history is heavily biased in favour of theropods, which are the rarest of body fossils. All I can think of to explain this is that theropods, especially the coelophysoid-grade theropods from the Late Triassic and Early Jurassic loved to patrol lake margins and river side point bars where their tracks were more likely to be preserved.
The scarcity of non-theropod footprints is made all the more vexing by the inability of ichnologists to decide which of the remainder, if any at all, were made by basal sauropodomorphs. Perhaps the most commonly cited example is a single quadrupedal trackway in the Early Jurassic aeolian sandstone of the Navajo Formation. Called Navahopus falcipollex, interpretative drawings show a large four-toed hindfoot and a smaller, pronated (turned so the palm faces backward) forefoot with a large medially directed thumb claw (Baird 1980). The trackway was made in loose sand, as the animal travelled up a sand-dune, consequently details are not well recorded. Indeed many would suggest that the original reconstruction of the manus print goes too far. I failed to see an obvious thumb-claw print when I had a chance to look at a cast of the trackway. Indeed it seemed to me that partial infilling of the manus prints, where loose sand had spilled into the print from the leading (upslope) edge, had resulted in the transversely elongate, antero-posteriorly shortened manus prints. Hunt and Lucas (2006) also doubted that Navahopus was left by a sauropodomorph, noting that the supposed pollex print was not as robust as reconstructed by Baird and was rather thin and variable in its expression. Lockley and Hunt (1995) suggested that Navahopus was just a large, poorly preserved Brasilichnum (an ichnogenus believed to have been made by tritylodontid cynodonts). If Navahopus was left by a tritylodontid, then it would have had to have been a big one. That’s ok though, unusually large tritylodont body fossils are not unheard of, despite the common misconception that all synapsids in the age of the dinosaurs were small rat to shrew-sized creatures. Pictured here is a fox-sized tritylodont from the Early Jurassic Clarens formation of South Africa. It looks crappy because it is a cast made from a natural mould of the skeleton in coarse sandstone.
So if Navahopus isn’t a basal sauropodomorph track then what is? There are two main contenders: Tetrasauropus and Otozoum. I have little doubt that the Early Jurassic Otozoum prints fit the bill nicely. Otozoum are medium to large four toed tracks of a biped. One track records a moment when an individual got down on all fours but didn’t take any steps until it got back up onto its hindlegs. The diagram on the right shows an Otozoum footprint and the one known track way with hand prints (shown in grey). Both are redrawn from Rainforth (2003).
The digital proportions and apparent phalangeal formula of the feet (based on the admittedly dodgy method of using toe pads) match those of basal sauropodomorphs. Emily Rainforth’s 2003 paper sets out the evidence nicely and a convincing case is made. What really tickles me however is that the Otozoum matches very nicely the predictions made by Matt Bonnan and Phil Senter (2007) based on the skeletal anatomy of the shoulder girdle and forelimb of plateosaurian-grade basal sauropodomorphs like Plateosaurus and Massospondylus. They found that in these dinosaurs had very limited degrees of humeral abduction (that is sticking their forearms out laterally) and could not rotate their wrists either. Thus they were denied any means to bring the hand into a forward facing (pronated) position seen in the Navahopus tracks and were forced to keep their palms facing inwards, theropod style. This is exactly what is seen in the one Otozoum track where the animal briefly went down onto all fours. Some have argued that the lack of a print of a large recurved thumb-claw in this track argues against Otozoum being the track of a basal sauropodomorph. However modelling of the range of motions of the joints of the hand shows that it was perfectly possible for plateosaurian-grade sauropodomorphs to lift their large sharp thumb claw well clear of the ground when on all fours. The picture on the right is a reconstruction from Galton (1971) that shows the likely stance of the hand when it was placed on the ground.

Finally we now turn our attention to the featured paper. The Jesper Milàn and colleagues report on a new, better preserved Navahopus trackway (skipping over the theropod tracks entirely). Suprisingly (to me in any case) it more or less confirms Baird’s earlier interpretation of Navahopus. In particular the manus prints are much clearer and lo and behold there is a large medially directed thumb-claw. Sauropodomorphs are the only known four-toed tetrapods from that epoch with hyperenlarged thumb claws so I think we have to accept that Navahopus was indeed the spoor of a sauropodomorph. So what is Otozoum, and were basal sauropodomorph facultative quadrupeds after all? I think a plausible explanation is that we are dealing with two distinct kinds of basal sauropodomorph. Otozoum tracks were likely left by plateosaurian-grade basal sauropodomorphs (like Plateosaurus and Massospondylus), which probably were obligate bipeds (at least as adults). Navahopus, on the other hand was probably left by a basal sauropod, or near-sauropod sauropodomorph (like a smaller version of Antetonitrus or Melanorosaurus). These advanced near sauropods and early true sauropods do show the necessary modifications to their forelimbs, which would have allowed them to pronate their hands. Such a beast has yet to be found in the Navajo Formation but given the general paucity of vertebrate fossils in this unit I’m not unduly fussed about it. Is Navahopus unique or are there larger prints attributable to large Antetonitrus or Melanorosaurus-like creatures. Yes there are. The original Tetrasauropus unguiferus track from the Elliot Formation seems to be just such a track. It was produced by a large, quadruped with a four-toed hind foot and a smaller, pronated hand that bore a moderately large medially directed claw that made contact with the ground. Some have opined that this was left by a large crurotarsan (e.g. Rainforth 2003) largely on the grounds that it didn’t match Otozoum, which was taken to be a true basal sauropodomorph track. As I’ve suggested above it is possible that these tracks could both be sauropodomorph, and just represent different grades of basal sauropodomorph evolution.

References

Baird D (1980) A prosauropod dinosaur trackway from the Navajo Sandstone (Lower Jurassic). In: Jacobs LL, ed. Aspects of Vertebrate History. Essays in Honour of Edwin Harris Colbert. Flagstaff, Museum of Northern Arizona Press. pp. 219-230.

Bonnan MF, Senter P (2007) Were the basal sauropodomorph dinosaurs Plateosaurus and Massospondylus habitual quadrupeds? In: Barrett PM, Batten DJ, editors. Evolution and palaeobiology of early sauropodomorph dinosaurs, Special Papers in Palaeontology 77: 139-155.

Galton PM (1971) Manus movements of the coelurosaurian dinosaur Syntarsus and opposability of the theropod hallux. Arnoldia 5:1-8.

Hunt AP, Lucas SG (2006) The taxonomic status of Navahopus falcipollex and the ichnofauna and ichnofacies of the Navajo Lithosome (Lower Jurassic) of Western North America. New Mexico Museum of Natural History and Science Bulletin 37: 164-169.

Lockley MG, Hunt AP (1995) Dinosaur tracks and other fossil footprints of Western United States. New York, Columbia University Press. 338 pp.

Milàn J, Loope DB, Bromley RG (2008) Crouching theropod and Navahopus sauropodomorph tracks from the Early Jurassic Navajo Sandstone of USA. Acta Palaeontologia Polonica 53: 197-205.

Nesbitt SJ, Irmis RB, Parker WG (2007) A critical re-evaluation of the Late Triassic dinosaur taxa of North America. Journal of Systematic Palaeontology 5: 209-243.


Rainforth, EC (2003) Revision and re-evaluation of the early Jurassic dinosaurian ichnogenus Otozoum. Palaeontology 46: 803-838.

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image at top of page from wikipedia commons.