The Saga Continues…
Today marks the publication of a new paper describing spinosaurine material from the Kem Kem of Morocco, specifically a set of quadrate bones, the upper portion of the original tetrapod jaw joint. It’s freely available in PLoS ONE, and you can follow that link to read it now if you wish. As normal, I have something to say about this in the general context of things, which is largely parroting what others have said. So this is really not going to be much of a unique post.
This one’s been in the works for a while, this time by a team including Christophe Hendrickx, Octávio Mateus, and Eric Buffetaut. This stems from Christophe’s work describing the anatomy (this is a preprint, pre-publication inviting commentary, it is not formally published yet), and terminology, of non-avian dinosaur quadrates (both of those papers are freely available for perusal; send PeerJ useful feedback!). And this paper remarks on a subset of them.
The story of Spinosaurus has been torrid, torn apart by war, the ignominious flow of time, and by the attempts of systematists to make sense of a slew of bones from across northern Africa. From the mid-1990s, it seemed clear to some that there were probably more than one morphology of spinosaur in the north of Africa, specifically in Morocco and surrounding nations. Others were far more skeptical, referring to variation as a symptom of our lack of knowledge; that the differences weren’t systematic, or taxonomically viable, but merely either regional, ontogenetic, or less significant than thought. Spinosaurs inflated from the eastern Spinosaurus aegyptiacus to the that eastern form but also the western forms Spinosaurus maroccanus and Sigilmassasaurus brevicollis. And while some works treated these variously, it would not be for another decade that they received new treatment, where it was deemed by Brad McFeeters amongst others that the last of these was distinct; then later, that the last of these included the second of these. That perhaps there were two distinct spinosaurs in the north of Africa. Speculation abounded to bones in Egypt as well, whether there were two taxa. Discussion of a skeleton uncovered in Morocco and acquired through various means, then described by Nizar Ibrahim and colleagues, followed by Serjoscha Evers’s discussion on the topic, raised the problems that became clear, asking ultimately the question:
How Many Spinosaurs?
All of this is covered here, and at posts linked to in those first paragraphs.
Two works now underscore something that Evers’ work argued. In the Moroccan Kem Kem, there were probably two different, distinct, spinosaurine spinosaurids. One of them might very well be Spinosaurus aegyptiacus, albeit with regional differences occluded by the fact that the skeleton has been claimed to be chimerical. The other is Sigilmassasaurus brevicollis. A number of other remains from the north of Africa include partial articular bones, snout fragments, vertebrae, and scores upon scores of teeth. Hendrickx et al. concerned themselves with quadrates, that portion of the jaw joint articulating with areas of the skull, and determined there were two distinct morphotypes. Through the use of taking geometric landmarks and a best-fit algorithm, they found that these quadrates separate into two distinct types, and referred one to Spinosaurus aegyptiacus (morphotype I, including five specimens) and Sigilmassasaurus brevicollis (morphotype II, including two specimens). The explicit reasons for which specimens make it to which taxa aren’t underscored; reasonably, one might not choose which taxa, leaving this to future discoveries. However, accepting Evers et al.’s claims on taxic identity, Hendrickx et al. opt to use the two taxa as the valid options, and thus sort the material into each of these containers.
[Edit: Hendrickx et al. also chose to accept that Ibrahim et al. have correctly identified FSAC-KK 11888 as the neotype, and a specimen otherwise, of Spinosaurus aegyptiacus; this, despite concerns raised by Evers et al. on the topic. Needless to say, two quadrate morphologies in the Kem Kem support two distinct spinosaurine taxa. It remains my personal opinion that the holotype of Spinosaurus aegyptiacus, formerly intact and not dust, is substantiated by its description and plates, and a specimen from 3000 miles away, from a different apparent stratigraphic layer, and without resort to collection in Egypt, is insufficient for erection of a neotype. Nevertheless, whatever FSAC-KK 11888 is – and there is reason to believe it is very similar to the Egyptian taxon – it is probably representative of the taxon that owned the morphotype 1 quadrates. This does not in any way substantiate Spinosaurus aegyptiacus in the Kem Kem.]
Their study is interesting in that it demonstrates these quadrates all occupy a distinct area of morphospace from other theropod dinosaurs in which the orientation of the distal condyles, which articulate to the lower jaw, are more eccentric, and angled further outward than in other theropods.
This meant many things, covered in their paper, such as how the jaw behaves when it is opened, increasing its width at the joint, and thus increasing the angle between the left and right mandibles. In some animals, this permits extreme depression (opening of the jaw) by allowing the attachment point of the mandibular depressor muscle at the retroarticular process to move lateral to the quadrate itself. This isn’t possible in spinosaurids due to articular shape, but it still permits extreme gape.
I was given the opportunity to illustrate this:
Before you ask, the skull is based on a generalized reconstruction, rather than a specific set of bones, so the shapes aren’t meant to be “perfect.” They capture what they are meant to show … artistically.
A mandibular symphysis without the extremely rugose texturing indicating a thick cartilaginous symphyseal tissue suggests a looser tissue would permit more mobility. The intramandibular joint, between dentary/splenial and angular/surangular, is poorly known, as no complete jaw of a spinosaurid is available for analysis.
Little further attention has been made of a further analysis, one by Ute Richter and colleagues, assessing teeth collected from the Kem Kem — one of the “low value” fossils Moroccan authorities permit locals to collect, sell, and export — in which they note that “spinosaurine” teeth found can be further clarified into three, rather than two, distinct morphotypes, dependent on fluting of the surface, which side of the crown if more heavily fluted, and wrinkling between the flutes. Work assessing dental variation in a single jaw is also further complicated by the absence of good, intact material, save for material referred to baryonychines. And so we continue to be left wondering.
As the story goes, more evidence piles up that there are not one north African, Cenomanian spinosaurid, but many, diverse and peculiar from one another. We move from knowing next to nothing and having nothing, to knowing much more, and having more, yet still end up with more questions. We don’t know the answer to the question. But we have better tool to answer it.
Hendrickx, C., Mateus, O. & Buffetaut, E. 2016. Morphofunctional analysis of the quadrate of Spinosauridae (Dinosauria: Theropoda) and the presence of Spinosaurus and a second spinosaurine taxon in the Cenomanian of north Africa. PLoS ONE 11 (1): e0144695.
Richter, U., Mudroch, A. & Buckley, L. G. 2012. Isolated theropod teeth from the Kem Kem Beds (early Cenomanian) near Taouz, Morocco. Paläontologische Zeitschrift 87 (2): 291-309.