Sunday, September 14, 2014

Spinosaurus is back!

So, if you’re a paleo-nerd like me, you’ve probably read all about the new Spinosaurus material announced by Nizar Ibrahim and Paul C. Sereno. If you’re also like me, you were probably way too ecstatic about the new material announcement to remember that the same date it was announced also happened to be the 13th Anniversary of World Trade Center Bombings. Looking back on it today, I’m seriously disappointed in myself that I forgot all about it and spent nearly the whole day running around like a giddy toddler on a sugar high about these remains. At the same time though, everybody else that I knew didn’t speak much about the anniversary either, even on Facebook posts, but I guess that shows that we’re all moving forward as a society.

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!?
After the initial shock of the new artistic restoration and all the reports in the media, I quickly hunted down the paper and started reading. The new remains show that Spinosaurus wasn’t only a fish-eating, aquatic-dinosaur like I have talked about previously, but that it had really weird anatomy to show for it: things like a more elongate vertebral column, a more flexible tail, longer and more-developed arms (even more so than other spinosaurids), and most shocking of all, the downright shortness of the hind legs and quadrupedal movement of the animal. This was quite interesting, to say the least, because not only did it challenge a lot of things we thought we know about Spinosaurus, but it changed a lot about what we thought we knew about theropod dinosaurs in general. Let’s look over this one-by-one: starting with the very short-legged elephant in the room…

“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.

Spiny! You're, well, still kinda short...
Scott Hartman was quick to point out on his blog that the skeletal given doesn’t appear to match the measurements provided by the authors in their own paper. When corrected, Spinosaurus gets a bit taller than before (though he still stays quite short), the center of mass moves back towards the legs, and the arms are no longer touching the ground. This simple change completely challenges a large number of the biological suggestions in the paper, and seems to move Spinosaurus back into bipedal territory. This also further convinces me that we should make a better effort so that discoveries like this are properly published, checked over, and peer-reviewed BEFORE major broadcasting stations latch onto them and turn them into major money-making exhibits and documentaries. (Predator X and Darwinius anyone?)

(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.

The forelimbs are also interesting in that they differ from other spinosaurids in the length of the hands, and their seemingly better-developed processes for forelimb muscles. Ibrahim and Sereno suggest that these longer hands would be better at ripping and dispatching of aquatic prey, but a few people online have also suggested to me that these adaptations are what you would see in a quadrupedal animal (they’re not, for the reasons listed above). What I wonder is, if the well-developed musculature of the forelimbs and seemingly longer digits correspond at all with paddling. The powerful flexion and extension muscles could be better used for pushing it along underwater, perhaps while traversing mangroves and river bottoms in tandem with the hind legs, and the longer digits, like the hind limbs, may have been webbed to assist in underwater movement.

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.

And then there’s the sail reconstruction, which I’m also skeptical about. Many recent sail reconstructions have the long spinuous processes continuing gradually along the spine and down the tail, but this reconstruction goes back to the early reconstructions seen throughout the 20th century and puts the sail directly over the torso and ending at the base of the hips. Like the paddling and the quadrupedal behavior, this is weird. Both Cau and Headden have stated prior that the tall backwards-sloping dorsal neural spine should instead be placed in the tail, as it’s much longer than any of the other dorsal neural spines, has a backwards slope to it similar to what we see in caudal vertebrae, and it better matches the sloping back of other spinosaurid ridges/sails. We’ll have to see in the long run who’s right about this conundrum.

Yummy, yummy, fishy, fishy...

Finally, and I just wanted to touch on this a bit, I’m surprised about how little the possibility of other semi-aquatic dinosaurs has been discussed in light of this discovery. Even Ibrahim and Sereno’s team have stated that they think that Spinosaurus was an “extreme evolutionary experiment” which went nowhere, and that dinosaurs were almost all landlubbers. While it is true that terrestrial dinosaurs represent the overwhelming majority, what about the proposed aquatic dinosaurs seen in literature occasionally as of recent? Thescelosaurus? CMN 8547? Lurdusaurus? Opisthocoelicaudia? Heck, and what about other spinosaurids? Did everyone forget about this paper?

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.

As always, I’d love to hear suggestions for future topics. I’m also now on Facebook and Deviantart, so subscribe on there if you want to discuss anything science-y with me or see my wonderfully amateur artwork and other projects that I post. As always, stay sharp until next time! Cheers!