C. Nicolas Baird

The early Oligocene Tinguiririca Fauna from the Abanico Formation, central Chile, is the earliest mammal fauna dominated by high-crowned (hypsodont) herbivores known globally. The drivers of early hypsodonty in South America are thus of considerable interest. We reconstruct the palaeoenvironment for Tinguiririca based on: (1) generalised linear latent variable models (GLLVMs), which compare body mass distributions of Tinguirirican and modern South American herbivores; and (2) reconstructed diets of Tinguiririca fauna notoungulates inferred from dental mesowear. Based on GLLVMs, the Tinguiririca Fauna’s palaeoenvironment is interpreted as relatively warm (22˚C mean annual temperature), dry (1200 mm yr ^-1 mean annual precipitation) and open (23% tree cover). The Brazilian Caatinga is the closest modern South American analogue to the dry, low-vegetation-density palaeoenvironment inferred for Tinguiririca. We interpret the fauna’s notoungulates as browsers, based on their sharp ectoloph wear facets. To test this interpretation, we also analysed the mesowear angles and relative crown heights of a broad array of leontiniid and homalodotheriid notoungulates, two clades specialised for high browsing that never attained the levels of hypsodonty seen in other notoungulates. Both have sharp mesowear and greater relative crown heights than modern high browsers. Our data suggest that neither gradual habitat change nor increased levels of volcaniclastic sedimentation alone caused the early attainment of hypsodonty in notoungulates. Alternative explanations, such as one or more rapid pulses of environmental change driving the increase in hypsodonty or the unique taxonomic composition of South American faunas, must be invoked to explain the marked increase in crown height from preceding time periods into the Tinguirirican South American Land Mammal Age.

Synopsis: The taxonomic composition of the suborder Tylopoda is an ongoing debate. Recently, the internal otic region (petrosal and bony labyrinth) has been intensively studied as a source of additional morphological data, but the morphology of this region in extinct tylopods is not well documented. To remedy this, we used µCT scanning to image and describe the petrosal and bony labyrinth of two oromerycids (Protylopus, Eotylopus), four early camelids (Poebrotherium wilsoni, Poebrotherium eximium, Paratylopus primaevus, Stevenscamelus franki), and the living vicuña (Vicugna vicugna). Several early camelid specimens also preserved the ear ossicles (malleus, incus, stapes), enabling us to describe their morphology for the first time. Our sample allows us to not only compare among taxa, but to also examine variation within taxa and during ontogeny. We found that the morphology of the petrosal is far more variable than that of the bony labyrinth, both within and across taxa. There is no notable ontogenetic variation between the juveniles and adults in our sample. Protylopus has an unusual petrosal morphology, and its bony labyrinth is somewhat reminiscent of early dichobunoid artiodactyls. Conversely, Eotylopus has a transitional morphology that seemingly links it to camelids. Poebrotherium wilsoni and Po. eximium do not noticeably differ in their morphology, but there are identifiable differences in Pa. primaevus and S. franki, suggesting that the petrosal of camelids is diagnostic at a genus level. The early camelids were distinct from the vicuña in petrosal, bony labyrinth, and ossicular chain morphology, highlighting the importance of examining basally branching members to resolve evolutionary relationships.

The bushbuck (Tragelaphus scriptus) is an antelope widely distributed throughout sub-Saharan Africa that varies greatly in body size, horn length and coat patterning. Recent taxonomic work has suggested reclassification as two or more species. Molecular studies have shown a deep mitochondrial divergence between north-western (scriptus) and south-eastern (sylvaticus) bushbuck populations, and morphological assessments, based largely on coat patterning, have divided the bushbuck into as many as eight species and 32 subspecies. Molecular and morphological approaches have, to date, not been brought together in a single integrative study. Using 43 museum cranial specimens, we sequenced complete mitochondrial genomes and used three-dimensional geometric morphometrics to compare genetic and phenotypic divergence between the two main subclades of the T. scriptus species complex. We found a significant but weak relationship between phenotypic and genetic distances. Mean body size differed between males of each subclade, but morphological differences attributable to sexual dimorphism within subclades were greater than differences between subclades. Despite a mitochondrial divergence of nearly 3 Myr, cranial morphological differences between subclades were minor and could be accounted for largely by minor size differences between males (size–shape allometry). These results support the recognition of a single bushbuck species, as indicated by nuclear genetic data.