Upon the disclosure several days ago of the new and revamped Spinosaurus aegyptiacus (Ibraham 2015) there was something of a wild west shootout on social media - which I followed largely on facebook - but which spilled over on to many platforms. Oh and the debates, conjectures, criticisms, postulates, and endless musings were fun to watch. Like any shootout some just panicked and/or ran away in denial. Some were caught unaware. Some sprayed shots every which way and that. And some shot with pinpoint accuracy but were gunned down none the less.
As the dust settles on the Spinosaurus revelations I have seen no reason to discredit the chief findings of the paper - an unorthodox theropod morphology in a primarily aquatic animal. Some of the details, the P's and Q's, of this situation will undoubtedly change. Perhaps the hind limbs were not as dramatically reduced as some restoration suggest. But I think by and large the exceptionally elongated body, reduced hind limb length, retracted nares, supple tail, skull and tooth morphology, elongated neck, heavy bones, isotopic data, geologic setting and sedimentary setting of fluvial/deltaic/tidal river environment, inferred diet, adaptations of foot for spreading tetradactyl design become too large a body of evidence to ignore and gloss over. This was an animal that had crossed the tipping point of being partially amphibious to being largely aquatic to an extent no other known non-avian dinosaur has done so.
|Figure 2 Ibrahim et al 2015|
Not that the new morphology has created additional questions: chief among them was the use(s) of the sail and how it moved terrestrially. I will save my thoughts on the sail for a later post but here I want to address problems with the two dominant thoughts on how it moved - either bipedally or quadrupedally. The quadrupedal knuckle walking design that the authors postulated and featured in several of the animations surrounding the forthcoming NatGeo documentary has several serious flaws. Besides the constraints imposed by a relatively immobile shoulder girdle restricting limb movement and inability to pronate the hands, there is the even more obvious limitation of rather slender wrist and hand bones - they do not look weight bearing at all. Does this leave us then with a classic bipedal posture? There are some serious problems here as well, not the least of which is the center of gravity question.
|Figure S3 Ibrahim et al 2015. red dot denotes center of gravity|
Personally I think we have a bit of ontogenetic shift going on here. Younger individuals (i.e. the immature neotype FSAC-KK 11888) were likely relatively more longer in leg than adults and potentially more closely resembled the stance of other spinosauridae.
I like the pangolin type of locomotion proposed by Darren Naish as one of several possible locomotory possibilities. But as you notice in the pic/video below, pangolins will still occasionally touch down with their forelimbs for extra stability when covering rough terrain and I think the same would have held true for Spinosaurus (and spinosauridae as well) when covering rough terrain. They are, after all, many orders of magnitude larger and a fall could have been a lot more impactful. My gut feeling is that this form of bipedalism - with occasional forelimb stability touches - is an option that may have been common in the family as a whole and possibly Spinosaurus as younger/smaller individuals. All in all more work is needed on this family in terms of posture and locomotion. And correct me if I am wrong in the comments but do we even have preserved distal leg elements for any other members of the family besides Spinosaurus? Again, correct me if I am wrong, but if we only have foot remains for Spinosaurus does this not imply that the most parsimonious option is to infer spreading/tetradactyl feet for all of spinosauridae?
Of course I think these stability touchdowns would have not been with the claws and hands but the forelimb - and chiefly the massive ulna - taking the weight along with the ventral body surface. Let's just look at those ulnas again by the way (from Suchomimus via Dave Hone):
And finally why I think obligate bipedalism has its flaws is that traversing tidal mud flats as a large bipedal multi ton animal raises some serious issues of getting stuck in the mud!!! Fairly maladaptive for an animal that would have had to get across some fairly difficult terrain if you ask me. If you don't believe me try it yourself. Puny human bipeds run into trouble in thick mud - imagine the difficulties as a 10 ton Spinosaurus!!
|Hey I could have posted a pic of myself in the mud instead?|
So if there are significant problems with both obligate quadrupedalism and bipedalism then what are we left with? Who is the veritable Last Man Standing (or in this case waddling)? El Ultimo Hombre?
THE LOON WINS!!!
Belly Sliding/Combat Crawling/Mud Bank Sliding provides a nice and easy way to solve the problems of both the obligate biped/quadruped dichotomy.
As I first proposed here on August 16th Spinosaurus could simply fold up its arms, lower it's belly and scooch along propelled by the back legs. This is not redesigning the wheel - it is essentially just a slight modification of known theropod resting anatomy. Hey maybe not too classy but it will work nice in a muddy environment. Some variation of this theme is what crocs, pinnipeds, otters, loons, navy seals and maybe even Amphicoelias utilized when moving across such terrain.
Stay tuned I will be delving into my interpretation of the funtion of the 'sail' next and I promise my interpretation here will be even weirder!!