Last time I talked about Erlikosaurus andrewsi, it was Stephan Lautenschlager’s paper with Larry Witmer, Perle Altangerel, and Emily Rayfield discussing the biomechanical aspects of toothloss and beak formation in Erlikosaurus andrewsi. That work indicated a far likelier restoration of this almost-most-famous therizinosaur as having a beak at the tip of its jaws (a proposition that isn’t exactly perfectly obvious, nor has it been subject to much testing). This week, a new paper by the authors of that paper discusses the second of Erlikosaurus‘s cranial osteology papers, a follow-up to the older Perle et al. paper which examined the skull at hand. Lautenschlager et al. have CT scanned the holotype skull and thus have been able to look far deeper into regions of the skull than any previous worker (Lautenschlager et al., 2012; Lautenschlager, 2013) , examined the neural endocranial vault and cranial nerve pathways, and looked in detail at the distribution of cranial airsacs that invade the bones of the skull, previously only examined through breaks in the specimen. The influence of cranial pneumaticity and toothloss on the evolution of the therizinosaur cranium is, sadly, only approachable in limited scope: A complete skull is known only for one two taxa (Beipiaosaurus inexpectatus, Erlikosaurus andrewsi, while in the former the skull is preserved in 2.4D and hasn’t been examined in high detail), and partial skulls known for two additional taxa (Falcarius utahensis, Nothronychus mckinleyi), both of which were subjected to CT examination but are otherwise far less complete and prevent tracking the more extensive cranial evolution of therizinosauroids.
But knowing the detailed pneumatic expansions into the skull, and the deeper underlying anatomy that so fascinates with the peculiar skulls of therizinosaurs, tells us a lot about the mechanical stresses that may have permitted pneumatic invasion, limited it, and the conflicting or covariant mechanisms that affected bite performance. These things require more work like this, and deeper understanding of cranial anatomy without preconceptions about evolutionary relationships. This work precedes arguments of evolution, but at the same time influences and is influenced by it. Groundwork anatomical understanding helps provide the basis for phylogenetic information, but context of that anatomy requires a phylogenetic framework; and the use of the phylogenetic bracket, a tool of logical inference for muscle reconstruction and anatomical interpretation, informs both.
Lautenschlager, S. 2013. Cranial myology and bite force performance of Erlikosaurus andrewsi: a novel approach for digital muscle reconstructions. Journal of Anatomy 222: 260-272.
Lautenschlager, S., Rayfield, E. J., Perle A., Zanno, L. E. & Witmer, L. M. 2012. The endocranial anatomy of Therizinosauria and its implications for sensory and cognitive function. PLoS ONE 7: e52289.
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 110 (51): 20657-20662.
Lautenschlager, S., Witmer, L. M., Perle A., Zanno, L. E. & Rayfield, E. J. 2014. Cranial anatomy of Erlikosaurus andrewsi (Dinosauria, Therizinosauria): New insights based on digital reconstruction. Journal of Vertebrate Paleontology 34 (6): 1263-1291.