Patterns of mammalian jaw ecomorphological disparity during the Mesozoic/Cenozoic transition

The decline and fall of the mammalian stem

The mammalian crown originated during the Mesozoic and subsequently radiated into the substantial array of forms now extant. However, for about 100 million years before the crown's origin, a diverse array of stem mammalian lineages dominated terrestrial ecosystems. Several of these stem lineages overlapped temporally and geographically with the crown mammals during the Mesozoic, but by the end of the Cretaceous crown mammals make up the overwhelming majority of the fossil record. The progress of this transition between ecosystems dominated by stem mammals and those dominated by crown mammals is not entirely clear, in part due to a distinct separation of analyses and datasets. Analyses of macroevolutionary patterns tend to focus on either the Mammaliaformes or the non-mammalian cynodonts, with little overlap in the datasets, preventing direct comparison of the diversification trends. Here I analyse species richness and biogeography of Synapsida as a whole during the Mesozoic, allowing comparison of the patterns in the mammalian crown and stem within a single framework. The analysis reveals the decline of the stem mammals occurred in two discrete phases. The first phase occurred between the Triassic and Middle Jurassic, during which the stem mammals were more restricted in their geographic range than the crown mammals, although within localities their species richness remained at levels seen previously. The second phase was a decline in species richness, which occurred during the Lower Cretaceous. The results show the decline of stem mammals, including tritylodontids and several mammaliaform groups, was not tied to a specific event, nor a gradual decline, but was instead a multiphase transition.

A switch in jaw form-function coupling during the evolution of mammals

The evolutionary shift from a single-element ear, multi-element jaw to a multi-element ear, single-element jaw during the transition to crown mammals marks one of the most dramatic structural transformations in vertebrates. Research on this transformation has focused on mammalian middle-ear evolution, but a mandible comprising only the dentary is equally emblematic of this evolutionary radiation. Here, we show that the remarkably diverse jaw shapes of crown mammals are coupled with surprisingly stereotyped jaw stiffness. This strength-based morphofunctional regime has a genetic basis and allowed mammalian jaws to effectively resist deformation as they radiated into highly disparate forms with markedly distinct diets. The main functional consequences for the mandible of decoupling hearing and mastication were a trade-off between higher jaw stiffness versus decreased mechanical efficiency and speed compared with non-mammals. This fundamental and consequential shift in jaw form-function underpins the ecological and taxonomic diversification of crown mammals. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

Functional reorganisation of the cranial skeleton during the cynodont-mammaliaform transition

Skeletal simplification occurred in multiple vertebrate clades over the last 500 million years, including the evolution from premammalian cynodonts to mammals. This transition is characterised by the loss and reduction of cranial bones, the emergence of a novel jaw joint, and the rearrangement of the jaw musculature. These modifications have long been hypothesised to increase skull strength and efficiency during feeding. Here, we combine digital reconstruction and biomechanical modelling to show that there is no evidence for an increase in cranial strength and biomechanical performance. Our analyses demonstrate the selective functional reorganisation of the cranial skeleton, leading to reduced stresses in the braincase and the skull roof but increased stresses in the zygomatic region through this transition. This cranial functional reorganisation, reduction in mechanical advantage, and overall miniaturisation in body size are linked with a dietary specialisation to insectivory, permitting the subsequent morphological and ecological diversification of the mammalian lineage.

Ecological selectivity and the evolution of mammalian substrate preference across the K-Pg boundary

The Cretaceous-Paleogene (K-Pg) mass extinction 66 million years ago was characterized by a worldwide ecological catastrophe and rapid species turnover. Large-scale devastation of forested environments resulting from the Chicxulub asteroid impact likely influenced the evolutionary trajectories of multiple clades in terrestrial environments, and it has been hypothesized to have biased survivorship in favour of nonarboreal lineages across the K-Pg boundary. Here, we evaluate patterns of substrate preferences across the K-Pg boundary among crown group mammals, a group that underwent rapid diversification following the mass extinction. Using Bayesian, likelihood, and parsimony reconstructions, we identify patterns of mammalian ecological selectivity that are broadly similar to those previously hypothesized for birds. Models based on extant taxa indicate predominant K-Pg survivorship among semi- or nonarboreal taxa, followed by numerous independent transitions to arboreality in the early Cenozoic. However, contrary to the predominant signal, some or all members of total-clade Euarchonta (Primates + Dermoptera + Scandentia) appear to have maintained arboreal habits across the K-Pg boundary, suggesting ecological flexibility during an interval of global habitat instability. We further observe a pronounced shift in character state transitions away from plesiomorphic arboreality associated with the K-Pg transition. Our findings are consistent with the hypothesis that predominantly nonarboreal taxa preferentially survived the end-Cretaceous mass extinction, and emphasize the pivotal influence of the K-Pg transition in shaping the early evolutionary trajectories of extant terrestrial vertebrates.

Quantitative assessment of tarsal morphology illuminates locomotor behaviour in Palaeocene mammals following the end-Cretaceous mass extinction

Mammals exhibit vast ecological diversity, including a panoply of locomotor behaviours. The foundations of this diversity were established in the Mesozoic, but it was only after the end-Cretaceous mass extinction that mammals began to increase in body size, diversify into many new species and establish the extant orders. Little is known about the palaeobiology of the mammals that diversified immediately after the extinction during the Palaeocene, which are often perceived as ‘archaic’ precursors to extant orders. Here, we investigate the locomotor ecology of Palaeocene mammals using multivariate and disparity analyses. We show that tarsal measurements can be used to infer locomotor mode in extant mammals, and then demonstrate that Palaeocene mammals occupy distinctive regions of tarsal morphospace relative to Cretaceous and extant therian mammals, that is distinguished by their morphological robustness. We find that many Palaeocene species exhibit tarsal morphologies most comparable with morphologies of extant ground-dwelling mammals. Disparity analyses indicate that Palaeocene mammals attained similar morphospace diversity to the extant sample. Our results show that mammals underwent a post-extinction adaptive radiation in tarsal morphology relating to locomotor behaviour by combining a basic eutherian bauplan with anatomical specializations to attain considerable ecomorphological diversity.

Selectivity and the effect of mass extinctions on disparity and functional ecology

Selectivity of mass extinctions is thought to play a major role in coupling or decoupling of taxonomic, morphological, and ecological diversity, yet these measures have never been jointly evaluated within a single clade over multiple mass extinctions. We investigate extinction selectivity and changes in taxonomic diversity, morphological disparity, and functional ecology over the ~160-million-year evolutionary history of diplobathrid crinoids (Echinodermata), which spans two mass extinctions. Whereas previous studies documented extinction selectivity for crinoids during background extinction, we find no evidence for selectivity during mass extinctions. Despite no evidence for extinction selectivity, disparity remains strongly correlated with richness over extinction events, contradicting expected patterns of disparity given nonselective extinction. Results indicate that (i) disparity and richness can remain coupled across extinctions even when selective extinction does not occur, (ii) simultaneous decreases in taxonomic diversity and disparity are insufficient evidence for extinction selectivity, and (iii) selectivity differs between background and mass extinction regimes.

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.

Conflict Resolution for Mesozoic Mammals: Reconciling Phylogenetic Incongruence Among Anatomical Regions

The evolutionary history of Mesozoic mammaliaformes is well studied. Although the backbone of their phylogeny is well resolved, the placement of ecologically specialized groups has remained uncertain. Functional and developmental covariation has long been identified as an important source of phylogenetic error, yet combining incongruent morphological characters altogether is currently a common practice when reconstructing phylogenetic relationships. Ignoring incongruence may inflate the confidence in reconstructing relationships, particularly for the placement of highly derived and ecologically specialized taxa, such as among australosphenidans (particularly, crown monotremes), haramiyidans, and multituberculates. The alternative placement of these highly derived clades can alter the taxonomic constituency and temporal origin of the mammalian crown group. Based on prior hypotheses and correlated homoplasy analyses, we identified cheek teeth and shoulder girdle character complexes as having a high potential to introduce phylogenetic error. We showed that incongruence among mandibulodental, cranial, and postcranial anatomical partitions for the placement of the australosphenidans, haramiyids, and multituberculates could largely be explained by apparently non-phylogenetic covariance from cheek teeth and shoulder girdle characters. Excluding these character complexes brought agreement between anatomical regions and improved the confidence in tree topology. These results emphasize the importance of considering and ameliorating major sources of bias in morphological data, and we anticipate that these will be valuable for confidently integrating morphological and molecular data in phylogenetic and dating analyses.