Anyway, back to the topic. If you’re part of the 1% who have not kept up with the Spino News, here’s what’s going on. On September 11th in Science, Nizar Ibrahim and Paul Sereno published their long-anticipated paper on Spinosaurus material recovered from Morocco and other parts of the Sahara, and the material doesn’t just conclude that Spinosaurus was the longest theropod dinosaur, but it also reveals it has anatomy which was, well, bizarre:
|OMG! What have they done to you Spiny!?|
“No T.rex, please don’t take my fish and try to make me reach for it…”
- Probably no Spinosaurus aegyptiacus ever. I mean seriously, Spino and T.rex never met. I'm sure all the bullying came from the Carcharodontosaurus.
I do find it interesting that spinosaurids have been suggested as quadrupeds so many times in literature. There have been mentions of Spinosaurus as a quadruped since (I believe) the 70s, and the discovery of Baryonyx further promoted this idea with its incredibly robust forearms. Furthermore, the idea of a quadrupedal theropod isn’t impossible. Quadrupedal locomotion evolved at least four other times in the dinosauria all independently, and even living avian dinosaurs occasionally use their wings to right themselves, climb, and perform other strange 4-limbed methods of locomotion. If anything it''s kind of weird that this group of dinosaurs never seemed to evolve even one semi-quadrupedal member. So is Spinosaurus the first? I wouldn’t count on it.
The proposed evidence that Ibrahim and Sereno present in the paper is not very convincing. The primary evidence that they propose is the shortness of the hindlimbs, which are positively tiny in the reconstructions. This apparently shifts Spinosaurus’ center of mass forwards towards the front, thus making it incapable of proper terrestrial movement. They also point to things such as the strange forelimbs which have processes which correspond to powerful flexion and extension muscles. (See Jaime Headden’s post to understand why this still doesn’t work.) This is such a weird arrangement that many have suggested that the new specimen might be a chimera of two separate individuals (maybe even of different species) which were stuck together. Cau pointed out earlier today why a chimeric origin is very unlikely, but it doesn’t matter, as this limb shortness might all be untrue anyways.
(Interestingly, the CGI Spinosaurus being used in the Nova documentary coming out next month looks like it has its leg proportions corrected. Although it still does the knuckle-walking thing…)
Despite this, the morphology of the arms and legs are still of great interest to me from a biological point of view. The large caudophemoralis muscle, the proportions of the femur and tibia, the flat and long digits and claws, the forward-facing hallux, and possible webbed feet all really suggest an aquatic lifestyle for this animal. What really seems to cement the idea, however, is the density of the long bones. The hind limbs are not hollow, are 40% more solid than other theropods, and incredibly dense. This is seen in aquatic animals to act as ballast and sink into the water easier while swimming. For a representation of this, go to 37:40 in the video below for similar limb density in the modern hippo.
What’s also weird in my opinion is the vertebral column. It’s pretty long, and in fact gives Spinosaurus a swan-like neck and dachshund-like body. It is actually kind of similar to the distantly related theropod Majungasaurus, who interestingly also has an elongated body, short hind limbs, and was suggested in passing a few times to be aquatic (Does this mean anything scientists?). However Spinosaurus has it even stranger, with the tail being highly flexible. The authors actually draw similarities with the tails of bony fish, and suggest that the tail was used for propulsion via undulations.
Er, wait… So is Spinosaurus a foot-propelled paddler, or did it use tail-powered undulations to move about? Normally animals only do one or the other, not both, and for good reason. Undulating the body around can mess up the pace of the legs (and vice versa), and provides no extra acceleration. This is why not many animals alive today use both methods while swimming. I think that a better explanation for the anatomy and swimming locomotion should be provided. I propose that most of its anatomy points to Spinosaurus being a foot-propelled paddler, with the flexible tail evolving as a rudder to help this giant turn its massive body while traversing underwater environments. We don’t need two swimming methods happening simultaneously when one is good enough and makes more sense.
Also, since Irritator and Oxalaia are close relatives to Spinosaurus, shouldn’t we regard them as having similar anatomy as well? Both of them are only known from skull material, and as Michael Mortimer showed, two caudals from the Alcantara Formation of Brazil were classified as Sigilmassasaurus. Since Sigilmassasaurus was just now found to be synonymous with Spinosaurus and spinosaurid in nature, it’s likely that they belong to Oxalaia. Thus, it would make much more sense for them all to have a similar anatomy and lifestyle rather than radically different anatomy, at least based on what we know. So rather than a one-off, maybe Spinosaurus was part of a much larger subfamily of semi-aquatic theropods which we’ve yet to discover?
Anyway, thanks for reading. As you can see, it has been about eight months since I last made a post on here, and I certainly regret not being able to. School has been extremely busy, and over the summer I was out doing tons of extra work to prepare for college next year. In an attempt to resurrect this blog and get back in the habit of posting (while at the same time juggling tons of other work), I’ll try posting shorter topics from now on, mixed in with occasional longer posts like this one.