Jaw shape and mechanical advantage are indicative of diet in Mesozoic mammals

Jaw morphology is closely linked to both diet and biomechanical performance, and jaws are one of the most common Mesozoic mammal fossil elements. Knowledge of the dietary and functional diversity of early mammals informs on the ecological structure of palaeocommunities throughout the longest era of mammalian evolution: the Mesozoic. Here, we analyse how jaw shape and mechanical advantage of the masseter (MAM) and temporalis (MAT) muscles relate to diet in 70 extant and 45 extinct mammals spanning the Late Triassic-Late Cretaceous. In extant mammals, jaw shape discriminates well between dietary groups: insectivores have long jaws, carnivores intermediate to short jaws, and herbivores have short jaws. Insectivores have low MAM and MAT, carnivores have low MAM and high MAT, and herbivores have high MAM and MAT. These traits are also informative of diet among Mesozoic mammals (based on previous independent determinations of diet) and set the basis for future ecomorphological studies.

Postcranial elements of small mammals as indicators of locomotion and habitat

Many studies have shown a correlation between postcranial anatomy and locomotor behavior in mammals, but the postcrania of small mammals (<5 kg) is often considered to be uninformative of their mode of locomotion due to their more generalized overall anatomy. Such small body size was true of all mammals during the Mesozoic. Anatomical correlates of locomotor behavior are easier to determine in larger mammals, but useful information can be obtained from the smaller ones. Limb bone proportions (e.g., brachial index) can be useful locomotor indicators; but complete skeletons, or even complete long bones, are rare for Mesozoic mammals, although isolated articular surfaces are often preserved. Here we examine the correlation of the morphology of long bone joint anatomy (specifically articular surfaces) and locomotor behavior in extant small mammals and demonstrate that such anatomy may be useful for determining the locomotor mode of Mesozoic mammals, at least for the therian mammals.

Exploring ancestral phenotypes and evolutionary development of the mammalian middle ear based on Early Cretaceous Jehol mammals

We report a new Cretaceous multituberculate mammal with 3D auditory bones preserved. Along with other fossil and extant mammals, the unequivocal auditory bones display features potentially representing ancestral phenotypes of the mammalian middle ear. These phenotypes show that the ectotympanic and the malleus-incus complex changed notably during their retreating from the dentary at various evolutionary stages and suggest convergent evolution of some features to extant mammals. In contrast, the incudomalleolar joint was conservative in having a braced hinge configuration, which narrows the morphological gap between the quadroarticular jaw joint of non-mammalian cynodonts and the incudomalleolar articulations of extant mammals. The saddle-shaped and abutting malleus-incus complexes in therians and monotremes, respectively, could have evolved from the braced hinge joint independently. The evolutionary changes recorded in the Mesozoic mammals are largely consistent with the middle ear morphogenesis during the ontogeny of extant mammals, supporting the relation between evolution and development.

Two new localities of Mesozoic mammals in Russia (Krasnoyarsk territory, Lower Cretaceous)

Two new localities of Mesozoic mammals have been discovered: Bol’shoi Ilek and Berezovaya River (Russia, Krasnoyarsk Territory, Lower Cretaceous, Ilek Formation). Bol’shoi Ilek locality yields an edentulous fragment of the maxillary of Docodonta indet. A fragment of dentary without teeth attributed to Mammalia indet. (presumably eutriconodontan or symmetrodontan) is presented in the Berezovaya River locality. New localities fill the geographical gap between previously known mammalian localities of Ilek Formation in the basins of Kiya and Bol’shoi Kemchug rivers.

Bayesian Tip-dated Phylogenetics: Topological Effects, Stratigraphic Fit and the Early Evolution of Mammals

ABSTRACTThe incorporation of stratigraphic data into phylogenetic analysis has a long history of debate, but is not currently standard practice for palaeontologists. Bayesian tip-dating (or morphological clock) phylogenetic methods have returned these arguments to the spotlight, but how tip-dating affects the recovery of evolutionary relationships has yet to be fully explored. Here we show, through analysis of several datasets with multiple phylogenetic methods, that topologies produced by tip-dating are outliers when compared to topologies produced by parsimony and undated Bayesian methods, which retrieve broadly similar trees. Unsurprisingly, trees recovered by tip-dating have better fit to stratigraphy than trees recovered by other methods, due to trees with better stratigraphic fit being assigned a higher prior probability. Differences in stratigraphic fit and tree topology between tip-dating and other methods appear to be concentrated in parts of the tree with weaker character signal and a stronger influence of the prior, as shown by successive deletion of the most incomplete taxa from a sauropod dataset. Tip-dating applied to Mesozoic mammals firmly rejects a monophyletic Allotheria, and strongly supports diphyly of haramiyidans, with the late TriassicHaramiyaviaandThomasiaforming a clade with tritylodontids, which is distant from the middle Jurassic euharamiyidans. This result is not sensitive to the controversial age of the eutherianJuramaia. A Test of the age ofJuramaiausing a less restrictive prior reveals strong support from the data for an Early Cretaceous age. Our results suggest that tip-dating incorporates stratigraphic data in an intuitive way, with good stratigraphic fit a prior expectation that can be overturned by strong evidence from character data.