Abstract
Horses became ubiquitous elements of Cenozoic communities and reached high diversity in Neogene times. Most accounts of their diversification history have so far focused on the Early Miocene cladogenesis of the subfamily Equinae (the so-called “grazing horses”), interpreting raw diversity counts at face value. In this chapter, we reconstruct speciation and extinction in horses, while accounting for sampling biases, and interpret these trends in the context of the evolution of body size and relative tooth crown height (hypsodonty). We found that fast species proliferation (speciation rate) is a shared feature in anchitehres and equines, likely stemming from the more changeable environments of the Neogene. The fast early-phase radiation in the subfamily Equinae was likely the result of reduced extinction rates and higher lineage survival rather than a substantial acceleration in speciation. The evolution of body size and hypsodonty was not faster in regions of the phylogeny with faster diversification, suggesting a broad-scale decoupling of ecomorphological and taxonomic diversification. Our models show that major phenotypic trends were not caused by phyletic progression, derived from sustained, ubiquitous directional selection, but more likely was the outcome of differential lineage-level survival and multiplication (species sorting).
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Acknowledgments
We thank Judith Galking for her hospitality and for making our stay in the American Natural History Museum at New York so enjoyable. We also thank Marielle Kaashoek for her helpful insights and comments on the manuscript.
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Tables S1 and S2
(S1) Body mass and hypsodonty estimates used in our phylogenetic analyses. New estimates are shown by representing the dental element from which they were obtained in an additional column. (S2) Parameter estimates of the phylogenetic models of phenotypic evolution run with mvMORPH. Models are ordered by decreasing fit to the data, based on their AICc weights (the probability of a given model being the best model given the data and the models compared). AICs is the score that measures the fit of the models after penalizing by model complexity. Note that rates of evolution are estimated separately for body size and hypsodonty, while controlling for the correlation among traits. Main families of models are: BM (Brownian motion, where rates can change among partitions); EB (a BM model with an acceleration/deceleration component); OU (Ornstein–Uhlenbeck models, where lineages follow a phyletic trend attracted toward optimal phenotypes, or constrained around an ancestral phenotype; optima can shift among predefined partitions). We use the term “Pan Equinae” in this table to refer to the partition defined by Equinae and the “stem equine anchitheres.” We use this short term for simplicity and not implying any true taxonomical meaning (XLSX 26 kb)
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Cantalapiedra, J.L., Sanisidro, O., Cantero, E., Prado, J.L., Alberdi, M.T. (2023). Evolutionary Radiation of Equids. In: Prins, H.H.T., Gordon, I.J. (eds) The Equids. Fascinating Life Sciences. Springer, Cham. https://doi.org/10.1007/978-3-031-27144-1_2
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