Tuesday, August 9, 2016

A New Wrinkle in the Megaraptor Mega-Mystery

credit Coria & Currie. Murusraptor CC 4.0

Definitely shaping up to be the year of clan megaraptoridae with new species and information seeming to come down the pike on a weekly basis. When such a glut of data comes at us at such in such a breakneck pace and when - in the case of megaraptorans - such large questions remains such as "what the hell are they actually?" it does pay to sometimes take a breather and digest things a bit. By slowing down and picking things over details that might otherwise be glossed over might see the light of day. It is one such detail that I want to highlight in this post. It does take me into territory that I certainly don't specialize in nor have the inclination or background to really get into - cladistics.

Yup, in life you gotta know your strengths and weaknesses and, no disrespect to the hard work others put into this aspect of paleontology, it's simply not my bag folks. So no, don't expect any data matrix from me folks or some prolonged digression into some obscure processes or foramen. I like the "softer" aspects such as functional stuff, appearance, behavior, and ecology. That being said the one feature I want to highlight in this short post is a feature that bridges the adaptational approach which influences my thinking and the - no disrespect to practitioners - the bean counting of cladistics with its emphasis on character traits, data matrices, parsimony analysis... yawn I can almost feel myself getting sleepy just talking about it.

Megaraptorans have some pretty neat teeth to talk about, not so much for the features that they have, but for the features that they lack.

Megaraptoran teeth lack interdenticular sulci (White et al., 2015).

On lateral teeth:

"There are no interdenticular sulci between any of the denticles on the distal carina"

From the discussion:



As I keep saying (and feel free to say it along with me) : It really is all about the teeth.



Readers who have been following me for a while will no doubt recognize the importance of this dental feature in tracing my evolving line of thinking on the range of functional ability and carcass utilization in theropods that featured such adaptations. My first real exposure to this dental adaptation in theropods by a paper by Brink et al. (2015) Developmental and evolutionary novelty in the serrated teeth of theropods which I discussed in my post Death Comes Ripping: Bonesaw Theropods. Basically interdenticular sulci are recesses between the denticles that arise developmentally. They serve to alleviate stress and overall strengthen and prolong the life of the denticle and therefore cutting proficiency of the tooth over its lifespan and we see rough analogy in expansion slots built into bone saws and cutting blades. Such features are not found in sharks, monitors, and sabertooth cats therefore creating the argument that serrated theropod teeth are on a functional level superior to the the teeth of these animals in cutting longevity. Which makes perfect sense because it is theropods that had to carve up and butcher the largest, thickest skinned, bone plate armored, cartilaginous and tendinous food base the world has ever seen - their herbivorous brethren. Bone is just another tissue in this regard just as likely to be sawed through as armor plated skin or thick tendons and joints as enamel trumps all these tissues in hardness scale. It's not a mistake that a great many theropods had a head narrow side to side but thick and strong from top to bottom that bears some uncanny resemblance to a blade or hatchet. Bonesaw theropods are not likely right because they are "awesome-bro" but because from an adaptationist approach animals with such tough rinds were what they had to cut through on a day to day basis and we should expect their enamel covered (and therefore likely lip covered) teeth to do the task that was set out before them.

credit Brink et al. 2015 interdenticular sulci in theropods


An inference I am going to make is that megraptorans - as the most common large carnivore in their ecosystem in many places (but especially Australia) likely fed on titanosaurs (alive or dead it don't matter). Titanosaurs certainly had a tough rind and lots of evidence of osteoderms in that family.

From discussion White et al., 2015:


The logical question arises that if megaraptorans evolved from some putative tyrannosauroid or carcharodontosaurid why would they lose their interdenticular sulci with such a food base? The answer of course is that they would not lose such a feature that benefitted hypercarnivory and that they did not evolve from a hypercarnicorous carcharodontosaurid or tyrannosauroid. That still leaves open the potential of evolving from a tyrannosauroid that did not have interdenticular sulci and which was a small game scrounger - a possibilty White et al. allude to:


While evolving from a basal tyrannosauroid that lacked interdenticular sulci is still possible it might be more promising to look at even more basal common ancestors as a distinct possibility - a putative small game hunting coelurosaur. Something like Compsagnathus, Juravenator, or Scipionyx? Brink et al. (2015) suggested interdenticular sulci as a synapomorphy of theropods secondarily lost by troodontids and spinosaurids. However in their study they did not investigate basal coelurosaurs which might lack sulci due to their small prey diet. I don't know for certain if some coelurosaurs lack sulci? Anybody have any info on this question out there?

If some coelurosaurs lacked sulci a putative basal small prey coeulurosaur might just be the subject we are looking for. Such a culprit might produce the blending of features that have caused various analyses to suggest spinosaurid, carcharodontosaurid, and tyrannosauroid affinities. Such a culprit might make a good island hopper/rafter and colonizer (e.g. Japan/Australia) as several compsagnathid species do seem to have excelled at colonizing islands. Evolving from a small game hunter that lacked interdenticular sulci is consistent with the strange anomalous lack of interdenticular sulci in megraptorids given a likely "brontophagist" niche. Given enough time megaraptorids may have independently evolved interdenticual sulci but as they were possibly just recently patriated to brontophagy from (potentially) a small game hunting ancestor they only had simple serrations.

And if megaraptorids did indeed arise from a generalized, island hopping, small prey eating putative coelurosaur this sort of makes megaraptorids their own thang right? Not some obscure offshoot of carcharodontosaurids or tyrannosauroids but their own rightful clan of unique hypercarnivorous theropods. Not claiming this idea as unique to myself as I think several other bloggers/researchers have put forth the same idea of megaptorids being their own thing. But I think looking at the tooth adaptation adds another layer of evidence in favor of megaraptors being their own clan.

A final caveat is that just because megraptorids lacked interdenticula sulci does not suggest that they were inferior carcass renderers than theropods that had them. It merely means that their denticles did not last as long and something as simple as higher tooth replacement rates could have kept them equipped for efficient brontophagist shenanigans.

a very "coelurosaur looking" Megaraptor credit Tom Parker, 2015 CC 4.0


papers

Coria RA, Currie PJ (2016) A New Megaraptoran Dinosaur (Dinosauria, Theropoda, Megaraptoridae) from the Late Cretaceous of Patagonia. PLoS ONE 11(7): e0157973. doi:10.1371/journal.pone.0157973

Brink, K.S.  et. al. (2015)  Developmental and evolutionary novelty in the serrated teeth of theropod dinosaurs.
Scientific Reports 5, article no. 12338, July 2015


White, MA, Bell, PR, Cook, AG, Poropat, SF, Elliot, DA, (2015) The dentary of Australovenator wintonensis (Theropoda, Megaraptoridae); implications for megaraptorid dentition PeerJ Dec 2015 online


"A Long habit of not thinking a thing wrong, gives it a superficial appearance of being right, and raises at first a formidable outcry in defense of custom". Thomas Paine

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6 comments:

khalil beiting said...

Great points Duane. Megaraptorans are truely weird and very unique among Theropods. They may look "vanilla" (besides those massive claws) but they have a unique evolution. But I must say that although it's very likely they are descneded from basal Coelurosaurs similiar to Compsognathus and the ilk, they could still be basal Tyrannosauroids with the same points you give. When it comes down to the island hopping, it could easily be translated to basal Tyrannosauroids too. They were small afterall too, and all basal Tyrannosauroids are in the same size range as Compsognathids. Aviatyrannis, Santanaraptor, Timimus, Coelurus, Tanycolagreus, Dilong, Guanlong, Stokesosaurus, etc. are still in the same size range/niche as various Compsognathids. From tiny Juravenator to medium sized Sinocalliopteryx, they are in the same size range as Aviatyrannis to Tanycolagreus. I don't mean to say that Megaraptorans evovled from any of these exact taxa, I just mean that they could have evolved from the same style of small sized, basal Tyrannosauroids that are, metaphorically speaking, essentially just slightly more derived Compsognathids. Don't get me wrong though, I am leaning towards them being basal Coelurosaurs, and I can say without a doubt that they aren't Carnosaurs of some sort (except Neovenator). Megaraptorans really do just look like overgrown Compsognathids with big claws afterall, and I find them being Coelurosaurs of some sort the most likely. As a comparison, just look how Compsognathus like the most basal Tyrannosauroids Aviatyrannus and Coelurus are:
http://img06.deviantart.net/0b90/i/2015/191/4/0/aviatyrannis_jurassica_by_kingedmarka-d90sc6z.jpg
and
http://img04.deviantart.net/61e5/i/2015/071/a/b/coelurus_agilis_by_teratophoneus-d8lew1u.png

babehunter1324 said...

This actually seem like a very fair theory, probably the one that mades the most sense alongside them being very, very basal Tyrannosauroids.

It would be interesting to figure out how niche partitioning between Abelisaurids and Megaraptorids worked out, both clades seem quite cursorial on broad terms, though at least in South America there seem to be a tendency for larger size over time for Megaraptorids (Aerosteon is one of the youngest and largest ones known) vs an increase of cursoriality ib Carnotaurines (with the most recent members like Carnotaurus taking it very far).

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