Taxonomic and ecomorphological diversification in horses

CANTALAPIEDRA, J. L.; ALBERDI, M. T.; SANISIDRO, O.; PRADO, J. L.

Alburquerque

Congreso; 78th ANNUAL MEETING SVP; 2018

SOCIETY OF VERTEBRATE PALEONTOLOGY

The fossil record of horses is rich and well-known. It also spans the last 60 million years, covering aperiod of marked ecological shifts in terrestrial ecosystems. Thus, horses are a perfect study group toassess the connections between changes in abiotic settings, phenotypic adaptations and diversity. We askwhether diversification in horses is related to shifts in phenotypic functional traits. We surveyed the NOWand the Paleobiology databases, using the literature to solve synonyms and maximize occurrence data.Finally, we gathered 3148 occurrences of 233 horse species, for which we also compiled information ofbody size, hypsodonty, number of functional toes, and seven traits that reflect molar function: presence ofacute lophs, occlusal morphology, presence of coronal cementum, overall tooth shape, number of lingualcusps, number of longitudinal lophs and number of transversal lophs. Using PyRate to analyze our fossiloccurrences, we estimated speciation and extinction through time while controlling for samplingheterogeneity. We found that horses diversification was constant through time, with extinction surpassingspeciation just in the last 2 myr. When the Neogene radiation of equine horses was analyzed separatelywe did find evidence for an early speciation pulse of the group beyond sampling effects. We constructedseveral time-calibrated trees for the 233 horse species using an informal super tree approach. We placed atotal of 213 species in a functional space using a Principal Component Analysis (PCA) based on theirfunctional traits, and used phylogeny-based techniques to model the first two components of the PCA(85% of the variance). We found large macroevolutionary shifts in optimal molar functionality (PC1)between 25 and 20 Ma, whereas evidence for shifts in optimal body size and functional toes (PC2) wasweak. In fact, PC2 evolved faster and under more relaxed selective pressures and constraints than molars.When analyzed separately, equine horses show an early Miocene speciation burst which postdates by 5myr the shifts in selection pressures. This suggest that, whereas shifting selection pressures shaped horsesecomorphology, diversity trends respond to other modulating factors.