I’ve had no time to complete any new articles, and after the text you’re reading now – originally intended to be a comment appended to Cute, furry, has claws, bites – got to reasonable length, I thought I may as well turn it into a brief article. I’m still not sure how a long thread on identifying a rodent became subverted by a discussion on Cretaceous theropods, but anyway… [adjacent image from Andrea Cau’s Theropoda]
If you followed the comments, you’ll know that several Tet Zoo regulars (including Michael Ericson, Zach Miller and Thomas Holtz) have been discussing the possibility that the Madagascan abelisaurid theropod Majungasaurus might have been amphibious, or even aquatic. This idea has not been mentioned in the literature (so far as I know), but it will be familiar to you if you frequent Andrea Cau’s Theropoda: pointing to the large amount of space available for cranial adductor and mandibular depressor musculature, Andrea has wondered if Majungasaurus was opening its mouth underwater (he’s written a longer article on the subject here, but internet troubles are preventing me from currently viewing it). See below for comments on this.
The new debate that’s erupted in the Tet Zoo comments has mostly concerned the peculiar proportions of this theropod, as portrayed by Scott Hartman here (you’ll note that his reconstruction differs pretty substantially from the more familiar version portrayed elsewhere: see the image below for more ‘conventional’ renditions). Majungasaurus may indeed have very peculiar proportions, but I don’t see anything there that makes me think of amphibious or aquatic habits.
— The tail is not deep, nor laterally compressed: its distal part in particular is shallow and lightly muscled, and very different from what’s present in animals that scull with their tails.
— Animals that swim with their hindlimbs tend to have long metatarsi and/or toes, whereas Majungasaurus is evidently comparable with other, ‘normal’ theropods.
— The skull and vertebrae were very pneumatic (O’Connor & Claessens 2005, O’Connor 2007): amphibious or aquatic birds always – so far as I know – have strongly reduced pneumaticity compared to their terrestrial relatives.
— How did Majungasaurus balance its long thorax and neck, given that they ‘look’ so much weightier than the tail? I hypothesise that the large caudofemoral muscles would have made the tail relatively heavy compared to the long trunk (I am aware of unpublished data which shows that this was the case in some sauropods, and I think it’s reasonable to think that it was in ‘big-tailed’ theropods too).
— What about the evidence for large jaw-opening muscles alluded to above (and discussed by Andrea)? While it’s true that animals that gape underwater require special features to do this (in turtles and sauropterygians these include enlarged retroarticular processes, particularly big hyoids, and a relatively broad, shallow snout (Lemell et al. 2000, 2002, Rieppel 2002)) [hypothetical underwater gaper/suction-feeder shown below], it does not follow that features allowing rapid and/or powerful gaping necessarily correlate with underwater gaping. Special adaptations for powerful gaping are also present in terrestrial-feeding animals that stick their jaws into a substrate (such as soil, or wood) and then pry their jaws open. This is evident in many passerines: particularly good examples include starlings, and wattlebirds like the Huia. If Majungasaurus really was well suited for powerful gaping, was it using its jaws to pry something open: say, rotting carcasses or something? Its deep snout and small hyoids (compared to those of underwater-gaping turtles and so on) are not consistent with aquatic gaping.
So, at the moment, I don’t see any cause for interpreting Majungasaurus as aquatic or amphibious. You should feel free to discuss this among yourselves: I think you’ll agree that the discussion would be better placed here, rather than appended to a post about squirrels.
The caveat, by the way, is that animals can sometimes do things that we might not predict based on their anatomy. As I like to say (and as others have said before: see Smith & Redford (1990)), anatomy is not destiny. Of course, if I started writing about that, I might be here a long, long time…
Oh, and this’ll be the last Tet Zoo article on Mesozoic dinosaurs for a while, I promise. It’s all rodents, frogs, lizards and small, brown birds from hereon.
Refs – –
Lemell, P., Beisser, C. J. & Weisgram, J. 2000. Morphology and function of the feeding apparatus of Pelusios castaneus (Chelonia; Pleurodira). Journal of Morphology 244, 127-135.
– ., Lemell, C., Snelderwaard, P., Gumpenberger, M., Wochesländer, R. & Weisgram, J. 2002. Feeding patterns of Chelus fimbriatus (Pleurodira: Chelidae). The Journal of Evolutionary Biology 205, 1495-1506.
O’Connor, P. M. 2007. The postcranial axial skeleton of Majungasaurus crenatissimus (Theropoda: Abelisauridae) from the Late Cretaceous of Madagascar. Journal of Vertebrate Paleontology 27 (Supp 2), 127-162.
– . & Claessens, L. P. A. M. 2005. Basic avian pulmonary design and flow-through ventilation in non-avian theropod dinosaurs. Nature 436, 253-256.
Rieppel, O. 2002. Feeding mechanisms in Triassic stem-group sauropterygians: the anatomy of a successful invasion of Mesozoic seas. Zoological Journal of the Linnean Society 135, 33-63.
Smith, K. K. & Redford, K. H. 1990. The anatomy and function of the feeding apparatus in two armadillos (Dasypoda): anatomy is not destiny. Journal of Zoology 222, 27-47.