Jaws, Jaws, and More Jaws!

Just a summary, without much in the way of commentary, until I have a chance to more thoroughly engage these papers:

1. The Beak Bites Back

Stress and strain relative to the edentulous, beak-less model, a partial beak, a larger beak, and a toothed model of Erlikosaurus andrewsi's skull. From Lautenschlager et al. (2013).

Stress and strain relative to the edentulous, beak-less model, a partial beak, a larger beak, and a toothed model of Erlikosaurus andrewsi’s skull. From Lautenschlager et al. (2013).

Stephan Lautenschlager’s work of late has been on the process of reconstructing the diet and biomechanics of Erlikosaurus andrewsi, and consequently that of therizinosaurids like it. Working with notable cranio-ologists like Larry Witmer and Emily Rayfield, as well as one of Erlikosaurus‘ original describers Perle Altangerel, Lautenschlager has produced a model reconstructing bite force as it might be influenced by the presence of a rhamphotheca, how large of one, without any, and without any but with teeth to the tips of the jaws. This paper is still in press, but has been published online and is broadly available. It concludes that a rhamphotheca, and one that is relatively large, likely reduced cranial stress during biting, and as such the edentulous rostrum was probably covered in a rhamphothecal beak, as has long been assumed. This is merely confirming the assumption through experimentation, both by removal of the target structure from the equation, and by modifying it. Stress is the jaws is highest at the rear, as is expected through a variety of jaw modelling: Any lever experiences higher load closer to the fulcrum than away from it, such that beams do not crack under the point of resistance, but proximal to it. This is also true of the jaws of animals. The tip experiences least stress, but is also where the bite initially occurs, or where a bolus is handled, and this can cause the tips of the jaws to be more efficient at handling resistance forces — but is also increases strain elsewhere in the jaw. A rhamphotheca is suitable to reducing this strain along the jaw, and as Farke et al. (2013) observed with placement of a rhamphotheca in Parasaurolophus, this increases efficiency at the tips of the jaw relative to muscle effort.

It is worth noting that the paper includes an odd tidbit whereupon it is implied that toothloss may have been important for the evolution of flight, but even their own data indicates that the relative weight (from their mass) of teeth is lighter than that of the beaks modeled. As such, in therizinosauroids, barring some fantastic basal discovery, may have had nothing to do with the development of flight, and really neither did the development of beaks. Even Archaeopteryx lithographica would have been beakless and close to avian ancestry, with enantiornithines being “confirmed birds” with fully dentate jaws. Beaks fit into the model of Zanno and Makovicky as an adaptation for herbivory, likely secondarily derived for predatory birds, but we’ll get into that at a later date.

2. Ichthyosaurs Don’t Suck

Comparison of "ram feeders" vs "suction feeders."

Comparison of “ram feeders” vs “suction feeders.” From Motani et al. (2013)

Ryosuke Motani and colleagues have produced a paper assessing the jaw angles of approach to the midline and the development of the hyoid apparatus across a select range of tetrapods (and some teleosts) and conclude that ichthyosaurs probably never exhibited suction feeding. The model is fairly simple, but requires a primer: “Suction feeding” is a behavior whereby the predator rapidly opens the jaw and, in this act, creates a vacuum into which prey are pulled. Some suction feeders can do this with a smaller gape, but require longer jaws, such as extant odontocetes, in which the tip of the jaw opens faster than the rear of the jaw, creating a moving “suck zone” that transports prey into the oral cavity. Predators such as the matamata (Chelus fimbriatus, which I discussed briefly here) and even walrus use a mechanism by which a short, broad, and rounded jaw is coupled with a large gular cavity or vaulted palate that, when the jaw opens, creates a short but very large vacuum within the mouth. Motani et al. categorizes this vacuum transport, rapid distension, and close-range suck feeding in the same category: predator remains relatively still, jaw opening transports prey into the mouth. Alternatively, “ram feeding” operates in the opposite manner: predator moves, opens mouth, and envelops the prey. However, both forms involve suction, and the broad distinction is based on relative RSI (or the “ram suction index” of Norton & Brainerd, 1993, who assessed the relative amount of movement between predator and prey in centrarchid and cichlid fish, which is somewhat of a continuum) and indeed without it, ram feeders would miss more than catch prey. This is why “ram feeding” is also called “ram-suction feeding,” and matamata specifically employ this method: They are not docile, jaw-opens only: the head and neck move forward as prey is pulled inward, even if the body is relatively inert. So the definitions are a little wonky.

But Motani et al. couple this with morphological factors: Most “true” suction feeders have larger lateral hyoid elements with robust rami for attachment of the glossopharyngeal musculature, which allows rapid opening of the gular cavity in connection with distension of the jaw. A large, ossified, medial hyal element is also found in many suction feeders, though it should be noted that amongst cetaceans, whether “suction” or “ram,” both these factors are true. Additionally, they note, the mandibular rami of “suction” feeders either approach one another in a broad arc or a narrow, parallel-sided prow, but not more gradually angled jaws. And it is this final point that they argue shows that ichthyosaurs are unlikely suction feeders: Virtually all ichthyosaur skeletons sampled shows jaws which approach at moderate angles, and when the hyoid apparatus is preserved, only ceratobranchials are present, which are slender, slightly bowed and rod-like. Amongst ichthyosaurs, only one was identified of a hyoid “corpus,” the medial, rostral hyoid element that usually supports or lies within the tongue. Problematically, failure to be preserved does not mean failure to exist: It is a logical fallacy to argue that absence of evidence equals evidence of absence [cite: paraphrasing Carl Sagan — sorry, JAH], as indeed most ichthyosaurs, plesiosaurs, and even crocodilian, and dinosaurs, typically never ossify this median element, even when it is expected to be large and well-ossified (for example, it is not always preserved in fossil parrot jaws, which have very, very LARGE epihyals).

Now much of this is forefront in my mind as I had completed a basic sketch of the skull of the eurhinosaur Eurhinosaurus longirsotris, one of a small clade of ichthyosaurs in which the mandible is fairly short, but the rostrum is very, very long. This inequal jaw length has been broadly compared to the swordfish, Xiphias (Xiphiidae), and the marlin, Makaira (Istiophoridae), both billfishes and both frankly likely in the same “singular” clade. And in these fishes, both are “ram suction” feeders: rapid distension of the jaw, with a narrow tip, with a large gular vacuum while traveling at high speeds. This definitely removes them from the suction side of the RSI, but it recalls that the distinction is a narrow one as the mandible approaches parallels … as it does in Eurhinosaurus longirostris. My preliminary reconstruction can be seen below:

Skull of Eurhinosaurus longirsotris in dorsal (top), left lateral (middle), ventral (bottom), and posterior (right) views. Mandible is in the “closed” position and articulated. Jaws roughly approach at a higher angle than in most ichthyosaurs.

It seems likely that most ichthyosaurs were not sedate, careful “waiters of food entering mouths” type, and that Motani et al.‘s conclusions are correct, for each of them. But the broad distinction on what counts as “suction” makes me wonder if they are missing the point: Even the RSI considers that “ram feeders” are suckers, too. No, the real question is: What the heck did Eurhinosaurus do with that nose!?

3. Head Full of Air

(Yes, that’s the title of a post of mine on pterosaur cranial pneumaticity. Trust me, it’s pertinent.)

Maxilla in multiple views of MPC-D 100/1844, Alioramus altai. From Leone Gold et al. (2013).

Maxilla in multiple views of MPC-D 100/1844, Alioramus altai. From Leone Gold et al. (2013). The sinus is shown isolated to the right.

Cranial pneumaticity in dinosaurs is not a novel thing; we’ve come a long way from the slow, dense, lumber behemoths that could barely trudge through their own extinction, they were so lazy! They were so bad at being extinct, in fact, that they survived — at least one lineage did, anyway. But non-avian theropod cranial pneumaticity hasn’t been extensively explored in most animals that are seen to exhibit it. Generally, only one or two elements are examined then broad conclusions made about the rest. The internal cranial anatomy of many extinct archosaur skulls fascinates some of us, however, including but not limited to Larry Witmer, Steve Brusatte, Amy Balanoff, and more. These people have been extensively exploring various taxa for cranial pneumaticity and its extent, and I must say it’s influenced my own observations for work I can’t talk about yet.

But let’s step back a bit and just talk about ONE animal, a real airhead of a dinosaur: Alioramus altai. Known from a nearly completely preserved skull which has been disarticulated meticulously, CT scanning of the braincase and cranium and jaw of this had revealed which bones, and how extensively, were pneumatized. And the numbers are large. That is, the volume of empty space in the head is extensive. And it pervades the mandible (There! a reason to include in this post!). As reported by Leone Gold, Brusatte, and Norell, much like other tyrannosaurs, Alioramus altai has a pneumatized maxilla (antorbital and promaxillary sinuses), nasal (nasal sinus), lacrimal (two different entries of the lacrimal sinus, and a possible medial lacrimal sinus, which arises directly from the antorbital one and is not confluent with the other lacrimal sinuses), frontal (frontal sinus, invaded from the lacrimal), parietal (parietal sinus), and squamosal (squamosal sinus, which has some homology issues about where it comes from); but also the quadrate (quadrate sinus), and palatine (palatine, which exibits very large, ontogenetically useful and taxonomically informative invasive fenestrae, and is invaginated from the antorbital, as is the ectopterygoid). What strikes me is not that the pterygoid, lying between a pneumatic quadrate and pneumatic palatine, isn’t pneumatic, but that the extopterygoid is so filled with sinus that there’s virtually no bone there; the entire element is a tetraradiate balloon.

The quadrate sinus is one of the larger of the paratympanic sinuses, and it pervades most of the main column as well as the pterygoid flange. But the paratympanic system also sends branches into the braincase, which aren’t discussed here, and into the mandible when relevant. And boy is it. Typically, the first element of the jaw that receives the paratympanic system is the articular, through an aperture medial or behind (or within) the articular fossa. But the articular is not present in the specimen. Instead, we have a surangular, and it is pneumatic. Large pneumatic features are present throughout, and this includes large fenestrae (the “surangular foramen”) which develop as the diverticulae that branch out from the surangular sinus resorb the bone around them. Unlike the articular, which can be inflated as much as the ectopterygoid in some taxa, the surangular is not a balloon or close to it; rather, the element is largely apneumatic internally, but diverticulae are closely associated with it, suggested that the medial surangular fossa possessed a suite of diverticulae and lacked its own sinus (which by definition should invade the bone).

4. No, there’s no fourth paper. Conclusions.

These new papers represent a rash of new data and new interpretations that increase the complexity of mandibular morphologies in sauropsidans, even if they aren’t all archosaurs. They all have implications to them. There’d be a fourth in here, as it’s even about jaws, on the pterosaur Istiodactylus latidens, but I’ve not had a chance to read the paper by this time. So you’ll have to wait for that summary and some perspective bit on it.

Farke, A. A., Chok, D. J., Herrero, A., Scolieri, B. & Werning, S. 2013. Ontogeny in the tube-crested dinosaur Parasaurolophus (Hadrosauridae) and heterochrony in hadrosaurids. PeerJ 1: e182. doi: 10.7717/peerj.182
Leone Gold, M. E., Brusatte, S. L. & Norell, M. A. 2013. The cranial pneumatic sinuses of the tyrannosaurid Alioramus (Dinosauria: Theropod) and the evolution of cranial pneumaticity in theropod dinosaurs. American Museum Novitates 3790: 1-46.
Motani R., Ji C., Tomita T., kelly, N., Maxwell, E., Jiang D.-y. & Sander, P. M. 2013. Absence of suction feeding ichthyosaurs and its implications for Triassic mesopelagic paleoecology. PLoS ONE 8 (12): e66075. doi:10.1371/journal.pone.0066075
Norton, S. F. & Brainerd, E. L. 1993. Convergence in the feeding mechanics of ecomorphologically similar species in the Centrarchidae and Cichlidae. Journal of Experimental Biology 176: 11-29.
Lautenschlager, S., Witmer, L. M., Perle A. & Rayfield, E. J. 2013. Edentulism, beaks, and biomechanical innovations in the evolution of theropod dinosaurs. Proceedings of the National Academy of Sciences, Philadelphia Published online ahead of print Dec 2, 2013 doi: 10.1073/pnas.1310711110

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6 Responses to Jaws, Jaws, and More Jaws!

  1. Well, the eurhinosaur rostrum doesn’t seem to be particularly analogous in function with the swordfish one, considering it doesn’t seem to be particularly blade-like, and the teeth would induce drag in the water.

    Given that it retains teeth, it probably suggests some form of foraging strategy unknown to living animals (I’ve seen one paper implying that it could had been used in scavenging, but I’m not seeing it), or it had some form of interspecific battle function.

    • I don’t imagine that the teeth would play a substantial role on impairing ram suction, as most acquatic predators employ it to feed (being unable to grab with any other limb). Most aquatic predators require jaw prehension to get prey, and I don’t think eurhinosaurs were any different. Having teeth shouldn’t matter much, as it doesn’t matter much to odontocetes. Exactly ho the teeth affect drag, though, that’s a question that did occur to me but I lack the means to answer this reasonably: How would the drago vortices look on a eurhinosaur mouth?How did it influence swimming? Could it improve, or disimprove, anything about swimming or feeding? Did eurhinosaurs have “lips,” or for that matter any ichthyosaur? These things all occur to me, too.

      • Well, Eurhinodelphis at least seems to have lost the teeth in the dystal area of the rostrum, where the “sword” begins, so lips probably wouldn’t prevent drag.

        • Lips in mammals at least streamlines the body form and would provide easier cutting through the water; it certainly helps reduce some drag, not that that’s always a bad thing: shark skin is developed to CREATE drag and form an enveloping “drag coat” that streamlines it. The issue is how teeth affect movement through the water, and certainly few extant animals are comparable; not even the sawfish may be comparable. It certainly hasn’t been done for Pristis that I can find.

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