Stephan Lautenschlager, Pamela Gill, Zhe-Xi Luo, Michael J. Fagan and Emily J. Rayfield (2016)
Morphological evolution of the mammalian jaw adductor complex.
Biological Reviews (advance online publication)
The evolution of the mammalian jaw during the transition from non-mammalian synapsids to crown mammals is a key event in vertebrate history and characterised by the gradual reduction of its individual bones into a single element and the concomitant transformation of the jaw joint and its incorporation into the middle ear complex. This osteological transformation is accompanied by a rearrangement and modification of the jaw adductor musculature, which is thought to have allowed the evolution of a more-efficient masticatory system in comparison to the plesiomorphic synapsid condition. While osteological characters relating to this transition are well documented in the fossil record, the exact arrangement and modifications of the individual adductor muscles during the cynodont–mammaliaform transition have been debated for nearly a century.
We review the existing knowledge about the musculoskeletal evolution of the mammalian jaw adductor complex and evaluate previous hypotheses in the light of recently documented fossils that represent new specimens of existing species, which are of central importance to the mammalian origins debate. By employing computed tomography (CT) and digital reconstruction techniques to create three-dimensional models of the jaw adductor musculature in a number of representative non-mammalian cynodonts and mammaliaforms, we provide an updated perspective on mammalian jaw muscle evolution.
As an emerging consensus, current evidence suggests that the mammal-like division of the jaw adductor musculature (into deep and superficial components of the m. masseter, the m. temporalis and the m. pterygoideus) was completed in Eucynodontia. The arrangement of the jaw adductor musculature in a mammalian fashion, with the m. pterygoideus group inserting on the dentary was completed in basal Mammaliaformes as suggested by the muscle reconstruction of Morganucodon oehleri. Consequently, transformation of the jaw adductor musculature from the ancestral (‘reptilian’) to the mammalian condition must have preceded the emergence of Mammalia and the full formation of the mammalian jaw joint. This suggests that the modification of the jaw adductor system played a pivotal role in the functional morphology and biomechanical stability of the jaw joint.
Yann Heuzé, Kazuhiko Kawasaki, Tobias Schwarz, Jeffrey J. Schoenebeck and Joan T. Richtsmeier (2016)
Developmental and Evolutionary Significance of the Zygomatic Bone.
The Anatomical Record 299(12): 1616–1630
The zygomatic bone is derived evolutionarily from the orbital series. In most modern mammals the zygomatic bone forms a large part of the face and usually serves as a bridge that connects the facial skeleton to the neurocranium. Our aim is to provide information on the contribution of the zygomatic bone to variation in midfacial protrusion using three samples; humans, domesticated dogs, and monkeys. In each case, variation in midface protrusion is a heritable trait produced by one of three classes of transmission: localized dysmorphology associated with single gene dysfunction, selective breeding, or long-term evolution from a common ancestor. We hypothesize that the shape of the zygomatic bone reflects its role in stabilizing the connection between facial skeleton and neurocranium and consequently, changes in facial protrusion are more strongly reflected by the maxilla and premaxilla. Our geometric morphometric analyses support our hypothesis suggesting that the shape of the zygomatic bone has less to do with facial protrusion. By morphometrically dissecting the zygomatic bone we have determined a degree of modularity among parts of the midfacial skeleton suggesting that these components have the ability to vary independently and thus can evolve differentially. From these purely morphometric data, we propose that the neural crest cells that are fated to contribute to the zygomatic bone experience developmental cues that distinguish them from the maxilla and premaxilla. The spatiotemporal and molecular identity of the cues that impart zygoma progenitors with their identity remains an open question that will require alternative data sets.
Silvia Pineda-Munoz, Ignacio A. Lazagabaster, John Alroy and Alistair R. Evans (2016)
Inferring diet from dental morphology in terrestrial mammals
Methods in Ecology and Evolution (advance online publication)
Dietary inferences are a key foundation for paleoecological, ecomorphological and macroevolutionary studies because they inform us about the direct relationships between the components of an ecosystem. However, we need to consider the range of dietary variation we want to investigate and characterize before choosing a proxy. The goal of the present work is to evaluate the differences in dietary discrimination power between our new method, the multidimensional multi-proxy dental morphology analysis (MPDMA) and unidimensional dental morphology proxies such as orientation patch count (OPCR), relief index (RI) or slope.
In order to do that, we three-dimensionally scanned the dentitions of 134 extant mammals including 28 marsupials (order Diprotodontia) and 106 placentals (orders Carnivora, Primates and Rodentia) and classified their diets using a new classification scheme that emphasizes the primary resource in a given diet. Diet categories included herbivory, carnivory, frugivory, granivory, insectivory, fungivory, gumivory and generalist.
Unidimensional proxies significantly discriminate (P < 0·05) between one or two diet categories on the one hand and the rest on the other. For example, OPCR discriminates well between carnivorous and non-carnivorous species. However, none of the individual proxies discriminate all eight dietary categories. Multi-proxy dental morphology analysis demonstrates significant morphological differences across diets (MANOVA, d.f. = 7; F = 7·56; P < 0·05) and correctly discriminates diet for 67–82% of the specimens in the data set including and excluding rodents respectively.
Combining different morphological variables makes it possible to draw better dietary inferences and fully represent the multidimensional nature of dental morphology and dietary specializations. Our results have important applications in ecological, paleoecological and evolutionary research.