Ecomorphological adaptations to climate and substrate in late Middle Pleistocene caballoid horses

Due to our limited knowledge of the precise causes and mechanisms shaping mammalian adaptations, there are still many unknowns regarding the processes underlying ecomorphological variation in large mammals. Ambient temperature and humidity affect skeletal morphology both directly through thermoregulation and indirectly through their influence on the character of vegetation and substrate. Oscillations in these factors contribute, in conjunction with evolutionary and behavioural factors, to morphological variation. An examination of dietary and locomotor adaptations in the context of climatic information can give insight into the effects of specific environmental conditions on particular adaptive traits.During the late Middle Pleistocene of Europe, the caballoid horse lineage occurred over a large geographic range and throughout glacial–interglacial cycles, except for the most extreme glacial maxima. The ecomorphological signature of horse fossils from late Middle Pleistocene archaeological and palaeontological sites from northwest Europe was analysed with univariate and multivariate statistical methods, in order to assess the reliability of correlations between particular skeletal adaptations in horses and specific aspects of the environment.The analyses show that in these animals large body size and robust limb bones are correlated with mosaic steppe habitats. Glacial horses were generally small and robust, while horses that lived in temperate oceanic conditions were slender and sometimes also small. Locomotor adaptations indicate a less mobile lifestyle than in modern equids. Dietary adaptations reflect the character of the vegetation and the amount of grit present in the environment. Differences between late Middle Pleistocene horse fossils from the British Isles and those found in continental northwest Europe illustrate the relationships between these adaptations and environmental factors.