Roger B. J. Benson’s research while affiliated with American Museum of Natural History and other places

Living reptiles include more than 20,000 species with disparate ecologies. Direct anatomical evidence from Neodiapsida, which includes the reptile crown-group Sauria and its closest extinct relatives, shows that this diversity originates from a single common reptilian ancestor that lived some 255 million years ago in the Paleozoic. However, the evolutionary assembly of crown reptile traits is poorly understood due to the lack of anatomically close outgroups to Neodiapsida. We present a substantially revised phylogenetic hypothesis, informed by new anatomical data derived from high-resolution synchrotron tomography of Paleozoic reptiles. We find strong evidence placing the clade Millerettidae as a close sister to Neodiapsida, which uniquely share a suite of derived features among Paleozoic stem reptiles. This grouping, for which we name the new clade Parapleurota, replaces previous phylogenetic paradigms by rendering the group Parareptilia as a polyphyletic assemblage of stem reptiles, of which millerettids are the most crownward. Our analysis presents hypotheses that resolve long-standing issues in Paleozoic reptile evolution, including the placement of captorhinids on the amniote stem lineage and firm support for varanopids as synapsids, which taken together provide a greatly improved fit to the observed stratigraphic record. Optimizations of character evolution on our phylogenetic hypothesis reveals gradual assembly of crown reptile anatomy, including a Permian origin of tympanic hearing, the presence of a lower temporal fenestra in the amniote common ancestor, with subsequent modifications on the reptile stem lineage, leading to the loss of the lower temporal bar. This evolutionary framework provides a platform for investigating the subsequent radiations of the reptile crown group in the Early Triassic, including the lines leading to dinosaurs (including birds), crocodilians, lepidosaurs, and extinct marine reptiles.

Here we describe a dataset of freely available, readily processed, whole-body μCT-scans of 56 species (116 specimens) of Lake Malawi cichlid fishes that captures a considerable majority of the morphological variation present in this remarkable adaptive radiation. We contextualise the scanned specimens within a discussion of their respective ecomorphological groupings and suggest possible macroevolutionary studies that could be conducted with these data. In addition, we describe a methodology to efficiently μCT-scan (on average) 23 specimens per hour, limiting scanning time and alleviating the financial cost whilst maintaining high resolution. We demonstrate the utility of this method by reconstructing 3D models of multiple bones from multiple specimens within the dataset. We hope this dataset will enable further morphological study of this fascinating system and permit wider-scale comparisons with other cichlid adaptive radiations.

The early diversification of tetrapods into terrestrial environments involved adaptations of their locomotor apparatus that allowed for weight support and propulsion on heterogeneous surfaces. Many lineages subsequently returned to the water, while others conquered the aerial environment, further diversifying under the physical constraints of locomoting through continuous fluid media. While many studies have explored the relationship between locomotion in continuous fluids and body mass, none have focused on how continuous fluid media have impacted the macroevolutionary patterns of limb shape diversity. We investigated whether mammals that left terrestrial environments to use air and water as their main locomotor environment experienced constraints on the morphological evolution of their forelimb, assessing their degree of morphological disparity and convergence. We gathered a comprehensive sample of more than 800 species that cover the extant family‐level diversity of mammals, using linear measurements of the forelimb skeleton to determine its shape and size. Among mammals, fully aquatic groups have the most disparate forelimb shapes, possibly due to the many different functional roles performed by flippers or the relaxation of constraints on within‐flipper bone proportions. Air‐based locomotion, in contrast, is linked to restricted forelimb shape diversity. Bats and gliding mammals exhibit similar morphological patterns that have resulted in partial phenotypic convergence, mostly involving the elongation of the proximal forelimb segments. Thus, whereas aquatic locomotion drives forelimb shape diversification, aerial locomotion constrains forelimb diversity. These results demonstrate that locomotion in continuous fluid media can either facilitate or limit morphological diversity and more broadly that locomotor environments have fostered the morphological and functional evolution of mammalian forelimbs. Read the free Plain Language Summary for this article on the Journal blog.

Living mammal groups exhibit rapid juvenile growth with a cessation of growth in adulthood¹. Understanding the emergence of this pattern in the earliest mammaliaforms (mammals and their closest extinct relatives) is hindered by a paucity of fossils representing juvenile individuals. We report exceptionally complete juvenile and adult specimens of the Middle Jurassic docodontan Krusatodon, providing anatomical data and insights into the life history of early diverging mammaliaforms. We used synchrotron X-ray micro-computed tomography imaging of cementum growth increments in the teeth2–4 to provide evidence of pace of life in a Mesozoic mammaliaform. The adult was about 7 years and the juvenile 7 to 24 months of age at death and in the process of replacing its deciduous dentition with its final, adult generation. When analysed against a dataset of life history parameters for extant mammals⁵, the relative sequence of adult tooth eruption was already established in Krusatodon and in the range observed in extant mammals but this development was prolonged, taking place during a longer period as part of a significantly longer maximum lifespan than extant mammals of comparable adult body mass (156 g or less). Our findings suggest that early diverging mammaliaforms did not experience the same life histories as extant small-bodied mammals and the fundamental shift to faster growth over a shorter lifespan may not have taken place in mammaliaforms until during or after the Middle Jurassic.

In birds, the quadrate connects the mandible and skull, and plays an important role in cranial kinesis. Avian quadrate morphology may therefore be assumed to have been influenced by selective pressures related to feeding ecology, yet large-scale variation in quadrate morphology and its potential relationship with ecology have never been quantitatively investigated. Here, we used geometric morphometrics and phylogenetic comparative methods to quantify morphological variation of the quadrate and its relationship with key ecological features across a wide phylogenetic sample. We found non-significant associations between quadrate shape and feeding ecology across different scales of phylogenetic comparison; indeed, allometry and phylogeny exhibit stronger relationships with quadrate shape than ecological features. We show that similar quadrate shapes are associated with widely varying dietary ecologies (one-to-many mapping), while divergent quadrate shapes are associated with similar dietary ecologies (many-to-one mapping). Moreover , we show that the avian quadrate evolves as an integrated unit and exhibits strong associations with the morphologies of neighbouring bones. Our results collectively illustrate that quadrate shape has evolved jointly with other elements of the avian kinetic system, with the major crown bird lineages exploring alternative quadrate morphologies, highlighting the potential diagnostic value of quadrate morphology in investigations of bird systematics.

The fossil record is spatiotemporally heterogeneous: taxon occurrence data have patchy spatial distributions, and this patchiness varies through time. Large-scale quantitative paleobiology studies that fail to account for heterogeneous sampling coverage will generate uninformative inferences at best and confidently draw wrong conclusions at worst. Explicitly spatial methods of standardization are necessary for analyses of large-scale fossil datasets, because nonspatial sample standardization, such as diversity rarefaction, is insufficient to reduce the signal of varying spatial coverage through time or between environments and clades. Spatial standardization should control both geographic area and dispersion (spread) of fossil localities. In addition to standardizing the spatial distribution of data, other factors may be standardized, including environmental heterogeneity or the number of publications or field collecting units that report taxon occurrences. Using a case study of published global Paleobiology Database occurrences, we demonstrate strong signals of sampling; without spatial standardization, these sampling signatures could be misattributed to biological processes. We discuss practical issues of implementing spatial standardization via subsampling and present the new R package divvy to improve the accessibility of spatial analysis. The software provides three spatial subsampling approaches, as well as related tools to quantify spatial coverage. After reviewing the theory, practice, and history of equalizing spatial coverage between data comparison groups, we outline priority areas to improve related data collection, analysis, and reporting practices in paleobiology.

Gomphodonts form a Triassic radiation of small to medium‐bodied (<0.5–2.5 m in length) quadrupedal cynodonts characterized by labiolingually expanded gomphodont postcanines. They were the dominant cynodont group in Middle and Late Triassic ecosystems from the Southern Hemisphere and the first predominantly herbivorous cynodonts to evolve. Gomphodonts were also the first therapsids to develop hypsodonty and a dentition with complex occlusal patterns, and their highly diagnostic upper and lower postcanines show many different morphologies. Here, we explored dental complexity in gomphodont cynodonts through time using geographic information system analysis and orientation patch count applied on 3D crown surfaces of upper and lower gomphodont postcanines belonging to 32 gomphodont taxa. This study reveals that the peak in postcanine complexity was reached early in the evolution of gomphodonts with the emergence in the Early Triassic of omnivorous or insectivorous forms with postcanines made of well‐separated cusps and cingular cuspules. Traversodontids evolved simpler postcanines via coalescence of cusps into crests and the development of large occlusal basins, and the Middle Triassic radiation of traversodontids led to a sharp decrease in mean postcanine complexity. Simplification of the postcanines in traversodontids is interpreted as being related to a gradual increase in the consumption of plant material. Interestingly, the trend of insectivory/omnivory high postcanine complexity and herbivory low dental complexity in gomphodonts is opposite to the trend of dental complexity reported in some extant mammals, with omnivorous having low dental complexity and herbivorous higher. Postcanine complexity remained relatively stable throughout the evolution of traversodontids and only slightly diminished in the Late Triassic due to the presence of minute forms with particularly simple postcanines in the Rhaetian. The major phylogenetic diversity and taxonomic richness of Gomphodontia are represented in two periods of time: at the end of the Anisian, an age in which the postcanine complexity is simplifying, and at the early Carnian when the postcanine complexity in traversodontids, the only Gomphodontia represented, is stable.

Here we describe a dataset of freely available, readily processed, whole-body μCT-scans of 56 species (116 specimens) of Lake Malawi cichlid fishes that captures a considerable majority of the morphological variation present in this remarkable adaptive radiation. We contextualise the scanned specimens within a discussion of their respective ecomorphological groupings and suggest possible macroevolutionary studies that could be conducted with these data. We also describe a methodology to efficiently μCT-scan (on average) 23 specimens per hour, limiting scanning time and alleviating the financial cost whilst maintaining high resolution. We demonstrate the utility of this method by reconstructing 3D models of multiple bones from multiple specimens within the dataset. We hope this dataset will enable further morphological study of this fascinating system and permit wider-scale comparisons with other cichlid adaptive radiations.

The fossil record is spatiotemporally heterogeneous: taxon occurrence data have patchy spatial distributions, and this patchiness varies through time. Inferences from large-scale quantitative paleobiology studies that fail to account for heterogeneous sampling coverage will be uninformative at best and confidently wrong at worst. Explicitly spatial methods of standardization are necessary for analyses of large-scale fossil datasets, because non-spatial sample standardization, such as diversity rarefaction, is insufficient to reduce the signal of varying spatial coverage through time or between environments and clades. Spatial standardization should control both geographic area and dispersion (spread) of fossil localities. In addition to spatial standardization, other factors may be standardized, including environmental heterogeneity or the number of publications or field collecting units that report taxon occurrences. Using a case study of published global Paleobiology Database occurrences, we demonstrate the strong signals of sampling that could be misinterpreted as biologically meaningful, and which spatial standardization accounts for successfully. We discuss practical issues of implementing spatial standardization via subsampling and present the new R package "divvy" to improve the accessibility of spatial analysis. The software provides three spatial subsampling approaches, as well as related tools to quantify spatial coverage. After reviewing the theory, practice, and history of equalizing spatial coverage between data comparison groups, we outline priority areas to improve related data collection, analysis, and reporting practices in paleobiology.

In birds, the quadrate bone acts as a hinge between the lower jaw and the skull, playing an important role in cranial kinesis. As such, the evolution of avian quadrate morphology may plausibly be assumed to have been influenced by selective pressures related to feeding ecology. However, variation in quadrate morphology across living birds and its potential relationship with ecology have never been quantitatively characterised. Here, we used three-dimensional geometric morphometrics and phylogenetic comparative methods to quantify morphological variation of the quadrate and its relationship with an array of key ecological features across 200 bird species covering all major extant lineages. We found non-significant associations between quadrate shape and several aspects of feeding and foraging ecology across different scales of phylogenetic comparison. By contrast, allometry and phylogeny exhibit stronger relationships with quadrate shape than other ecological features. We show that the avian quadrate evolves as an integrated unit and exhibits strong associations with the morphologies of neighbouring bones. Our results collectively suggest a macroevolutionary scenario in which the shape of the quadrate evolved jointly with other elements of the avian kinetic system, with the major lineages of birds exploring alternative quadrate morphologies-highlighting the potential diagnostic value of quadrate morphology in investigations of fossil bird systematics.