August 25 , 2016

by Duane Nash

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"

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.

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.

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.

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