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The relative contributions of trabecular (spongy) and cortical (compact) bone to bone strength and stiffness are poorly understood across mammalian body size. In mammals, some small species have notably reduced their vertebral trabecular bone structure, resulting in mostly hollow medullary cavities. To assess the importance of trabecular structure to the mechanical properties of small mammalian vertebrae, we conducted finite element analysis on the lumbar vertebrae of 25 species of shrews (Soricidae) weighing 2-100g. We analyzed two sets of models: vertebrae with the trabecular structure intact (full), and vertebrae with all trabeculae excised from the centrum (hollow). All models were scaled to the same ratio of load to surface area. The cranial end of the centrum was immobilized, and a 5N craniocaudally-oriented load was applied to the caudal end of the centrum. We measured mean von Mises stress (MVMS) to capture strength, and total strain energy to capture stiffness. MVMS and total strain energy both decrease as body size increases, and hollow models experience higher stresses and strains than full models. With increasing body size, the difference in total strain energy between full and hollow models decreases, but the difference in MVMS slightly increases. This suggests a difference in the functional advantage conferred by trabeculae among small mammals, as well as a possible selective pressure for different functional emphasis in very small and larger mammalian bones. 

The relative contributions of trabecular (spongy) and cortical (compact) bone to bone strength and stiffness, although investigated in humans, is mostly unclear. As a result, we do not understand how the skeleton of small animals, especially the axial skeleton, has evolved to deal with the particular challenges of life at tiny size. In mammals, some small species have notably reduced their vertebral trabecular bone structure, resulting in mostly hollow medullary cavities. To assess the importance of trabecular structure to the mechanical properties of small mammalian vertebrae, and incorporate the effects of both trabecular and cortical bone structure, we conducted finite element analysis on the lumbar vertebrae of 15 species of shrews (Mammalia: Soricidae). We analyzed two sets of models: vertebrae with the trabecular structure intact, and vertebrae with all trabeculae excised from the centrum. In all models, the cranial end of the centrum was immobilized, and a 5N load was applied to the caudal end of the centrum, parallel to the craniocaudal axis. Results indicate higher peak stresses and larger displacements in models lacking trabeculae. Although smaller body size constrains the number of trabeculae that small mammals develop, we expect that these trabeculae contribute disproportionately to bone strength and stiffness. Ongoing work will validate these analyses with empirical materials testing and assess how bone morphofunctional characteristics change as body size increases. 

Evolutionary radiations generate most of Earth’s biodiversity, but are there common ecomorphological traits among the progenitors of radiations? In Synapsida (the mammalian total group), ‘small-bodied faunivore’ has been hypothesized as the ancestral state of most major radiating clades, but this has not been quantitatively assessed across multiple radiations. To examine macroevolutionary patterns in a phylogenetic context, we generated a time-calibrated metaphylogeny (‘metatree’) comprising 1,888 synapsid species from the Carboniferous through the Eocene (305–34 Ma) based on 269 published character matrices. We used comparative methods to investigate body size and dietary evolution during successive synapsid radiations. Faunivory is the ancestral dietary regime of each major synapsid radiation, but relatively small body size is only established as the common ancestral state of radiations near the origin of Mammaliaformes in the Late Triassic. The faunivorous ancestors of synapsid radiations typically have numerous novel characters compared with their contemporaries, and these derived traits may have helped them to survive faunal turnover events and subsequently radiate. 

Abstract Trabecular bone is modelled throughout an animal’s life in response to its mechanical environment, but like other skeletal anatomy, it is also subject to evolutionary influences. Yet the relative strengths of factors that affect trabecular bone architecture are little studied. We investigated these influences across the Philippine endemic murine rodent clade Chrotomyini. These mammals have robustly established phylogenetic relationships, exhibit a range of well-documented substrate-use types, and have a body size range spanning several hundred grammes, making them ideal for a tractable study of extrinsic and intrinsic influences on trabecular bone morphology. We found slight differences in vertebral trabecular bone among different substrate-use categories, with more divergent characteristics in more ecologically specialized taxa. This suggests that the mechanical environment must be relatively extreme to affect trabecular bone morphology in small mammals. We also recovered allometric patterns that imply that selective pressures on bone may differ between small and large mammals. Finally, we found high intrataxonomic variation in trabecular bone morphology, but it is not clearly related to any variable we measured, and may represent a normal degree of variation in these animals rather than a functional trait. Future studies should address how this plasticity affects biomechanical properties and performance of the skeleton. 

A mammalian-wide pattern of allometry connects micro- to macro-evolution in the skulls of ruminant artiodactyls. 

The Junggar and Turpan basins of Xinjiang, northwest China, host a well-preserved terrestrial Permo-Triassic boundary sequence exposed on the flanks of the Bogda Mountains. During the Permo-Triassic, this region was located in mid-latitude northeast Pangaea (~45°N), making it an important comparison to the higher latitude record preserved in the South African Karoo Basin (~60°S). Broad similarities exist between the tetrapod records of both areas, such as the reported co-occurrence of Dicynodon-grade dicynodontoids and Lystrosaurus in the upper Permian and the high abundance of Lystrosaurus in the Lower Triassic. In the Bogda sections, the Permo-Triassic boundary falls within the upper Guodikeng Formation (= upper Wutonggou low order cycle), but several horizons have been proposed based on biostratigraphy, chemostratigraphy, and paleomagnetic data. A new Bayesian age model calibrated by multiple radiometric dates and tied to detailed litho- and cyclostratigraphic data offers new insight into the location of the Permo-Triassic boundary in Xinjiang and the opportunity to reconsider tetrapod occurrences in a highly resolved chronostratigraphic framework. We investigated the positions of new and historic tetrapod specimens relative to the revised Permo-Triassic boundary, including uncertainties about the locations of key historic specimens. The stratigraphic range of Dicynodon-grade dicynodontoids in Xinjiang is poorly constrained: most specimens, including the holotype of Jimusaria sinkianensis, cannot be precisely placed relative to the Permo-Triassic boundary. A new specimen of Turfanodon sp. for which we have reliable data occurs in the upper Permian. Despite their previous treatment as Permian in age, most Bogda chroniosuchians were collected in strata above the Permo- Triassic boundary and the therocephalian Dalongkoua fuae also may be Triassic. Some prior placements of the Permo- Triassic boundary in Xinjiang imply an upper Permian lowestoccurrence for Lystrosaurus, but all Lystrosaurus specimens that we can precisely locate fall above the Permo-Triassic boundary. The high abundance of Lystrosaurus in the Early Triassic of Xinjiang likely parallels an Early Triassic age for the interval of greatest Lystrosaurus abundance in the Karoo Basin, but additional research is needed to determine whether there was a single, globally synchronous time of highest Lystrosaurus abundance. 

Previous work has shown increased morphological variance within the forelimbs of the Permian synapsid group known as Therapsida over that of their Carboniferous and early Permian forerunners (“pelycosaurs”). Considering that disparity trends have been known to point to underlying macroevolutionary transitions, here we analyzed morphological variance alongside several additional macroevolutionary metrics to better isolate possible evolutionary mechanisms. Shape data was collected on a sample of 119 humeri and 99 ulnae comprising three major synapsid radiations with a temporal range from the Carboniferous into the Triassic. Taxonomic sample included all major groups of pelycosaur-grade synapsids, all five recognized non-cynodontian therapsid clades, and a sample of pre-prozostrodontian cynodonts. Procrustes variance - a multivariate quantification of morphospace occupation - was the chosen disparity metric for the study. Rate of phenotypic change, which considers the amount of shape change that would be necessary to achieve observed morphologies given the shape of the closely related taxa, was analyzed as the metric for evolutionary rate. Both metrics were considered through-time upon genera present in sequential 5 million year time bins. Our results expand upon previous findings that disparity increases throughout the earliest stages of the Permian, coincident with the diversification of pelycosaurs and the emergence of Therapsida. This expanded dataset further shows that disparity approaches an asymptote around 270 million years ago and only increases marginally through the late Permian, remaining between 0.018–0.021 from 275-245 mya. In contrast, evolutionary rate does not appear to asymptote during this same interval, starting at a low of 6.17e-6 (300 mya) and increasing to a peak of 1.78e-5 right before the End Permian Mass Extinction Event (252 mya). The continuing increase of evolutionary rate shows that morphological change continues across taxa, but the plateauing of morphological disparity suggests that morphospace is not expanding concurrent with this. The incongruence between these two metrics suggests a critical change in evolutionary mode, wherein morphological change continues rapidly but does not result in the evolution of novel morphologies. These results provide some of the strongest quantitative data yet of an evolutionary constraint acting upon the morphology of the synapsid forelimb through deep time. 

Evolutionary radiations generate most of Earth’s biodiversity, but are there common ecomorphological traits among the progenitors of radiations? In Synapsida (mammalian total group), ‘small-bodied faunivore’ has been hypothesized as the ancestral state of most major radiating clades. To quantitatively test this hypothesis across multiple radiations, we used a meta-phylogeny (‘metatree’) of Carboniferous through Eocene (305–34 Ma) species in conjunction with jaw lengths (as a proxy for body size) and diet reconstructions for 404 synapsid species. We focus primarily on five major radiations: (i) non-therapsid pelycosaurs, (ii) non-cynodont therapsids, (iii) non-mammaliaform cynodonts, (iv) non-therian mammaliaforms, and (v) therians. Contrary to our expectations, we did not find universal support for the hypothesis that ‘small-bodied faunivore’ is the ancestral state of radiating synapsid groups. Although faunivory was the typical ancestral diet of each major ecological radiation, the radiation forerunners were not relatively small-bodied in many non-mammaliaform synapsid groups. Instead, the small-to-large trend in body-size within radiations does not become common until the end-Triassic size bottleneck near the base of Mammaliaformes. We also find that ecomorphological diversification was often preceded by the extinction of contemporary clades. As a potential causal mechanism for the observed macroevolutionary patterns, it is tempting to assume that the forerunners of major radiations were relatively unspecialized faunivores with reduced extinction risk. However, ‘survival of the unspecialized’ does not fully explain our results. Many of the progenitors of major synapsid radiations may appear to be unspecialized faunivores, but this is likely due to observational bias: the early lineages of each radiation were ‘unspecialized’ relative to many of their later descendant lineages, but, compared to their contemporaries, they exhibit numerous novel characters. These characters were likely important in promoting their long-term survival and diversification, but it appears that mass extinctions and other faunal turnovers were necessary for the lineages that possessed these characters to reach their full evolutionary potential. Therefore, ‘survival of the novel’ appears to be a persistent macroevolutionary pattern throughout synapsid history. 

A large phylogenetic tree is a critical component of comparative analyses that examine broad macroevolutionary patterns, such as the tempo and mode of evolution or morphological disparity through time. However, the sample of species included in published phylogenies rarely aligns with the species that researchers wish to examine in comparative analyses. For instance, early synapsid phylogenies often focus on specific subclades, such as pelycosaurs or anomodonts, rather than broadly encompassing all known synapsid lineages, thus hindering analyses that require detailed sampling across synapsid lineages. To address this issue, we generated a time-calibrated meta-phylogeny (‘metatree’) of synapsid species from the Carboniferous through the Eocene (305–34 Ma). The metatree approach uses source character matrices (rather than source trees) and generates complete sets of most parsimonious trees, combining them rather than generating a single consensus tree. We incorporated 269 published morphological character matrices, which includes every non-mammaliaform synapsid character matrix that has ever been published (as of July 2021) and 57 mammaliaform-focused matrices. Due to evolving ideas of relationships and frequent matrix reuse, each of the matrices was weighted according to its publication year and its dependence on ‘parent’ matrices using an established metatree procedure. The metatree approach relies on XML metadata files that reconcile taxon names to valid Paleobiology Database taxa (PBDB). Because the metatree approach utilizes PBDB taxonomy, we vetted the PBDB information and made approximately 500 additions and corrections to taxon information. The resulting metatree includes 2,128 synapsid species, making it one of the largest fossil phylogenies ever produced. Approximately 1600 species are non-mammaliaform synapsids, and the remaining ~525 species are mammaliaforms, including many of the known Mesozoic and early Cenozoic mammaliaforms. The massive taxonomic and temporal breadth of the metatree make it broadly applicable to studies on synapsid macroevolution. The past decade has witnessed a resurgence of research on non-mammaliaform synapsids, and our new, comprehensive metatree provides a rigorous foundation for continuing work on macroevolutionary patterns and processes among the forerunners of mammals. 

Global temperatures significantly changed from the late Permian to the Early Triassic: the Earth transformed from a cool world to a hothouse climate. This transition undoubtedly had a strong impact on tetrapod physiology and distribution. During the global cooling, tetrapods generally increased their size; and the currently recognized late Permian tetrapod extinction, exemplified by the record preserved in the South African Karoo Basin, occurred in the late stage of cooling. Rapid warming in the Early Triassic is predicted to have resulted in extinctions and/or local extirpation of low latitude tetrapods, but the very limited fossil record from this region makes testing this hypothesis difficult. Warming is predicted to have had less negative impacts on the tetrapod diversity of mid-latitudes, and promoted the success of tetrapods in the high latitudes. Based on the known fossil record, a tetrapod gap could have existed in central Pangea between ~30◦ N and ~ 40◦S, and lasting from the Induan to the early Spathian. However, the exact boundaries of this gap likely varied over time, and it could have encompassed a larger area during the hottest phases (Griesbachian and near the Smithian–Spathain boundary).