Supplementary information linked to the article, including: Word document containing: Laser Scanner details - Phylogenetic Reconstruction sources - Temporal ranges - Landmark Placements - Hypothetical ancestral fetlock generation - Iterative OPA Alignments - Rates of Trait Evolution - Covariates - Correlation Analyses - Species-level OSS Key - Supplementary References. (required software: MS Word or similar word processing software) List of specimens used in the analysis containing: genus and species - institutional location - catalogue number - higher taxonomy (required software: Adobe Reader or similar PDF reader) Excel spreadsheet / comma separated value (CSV) files containing: 3D landmark coordinates for all specimens - Procrustes coordinates averaged per species - Univariate covriates - Hypothetical Ancestor raw coordinates (required software: MS Excel) MorphoJ file containing: Procrustes alignment - PCA plots and scores - Phylogenetic Partial Least Squares analysis (required software: MorphoJ) R codes ZIP file including: phylogenetic tree (NEXUS file) - Disparity across turnover - Correlation Analyses - Ordinary Procrustes Alignment procedure - Rates on Branches of tree (required software: R version 3.6.3; unzipping software) 3D polygon files for all specimens. 3D models used in this study are also available via Morphosource.org, project "MacLaren (2021) Equoid metacarpal 3D models" (ID: 0000C1187) [link: https://www.morphosource.org/projects/0000C1187?locale=en]. These models will be available for download from Morphosource following reasonable request to the author / institution. (required software: 3D software capably of reading PLY files - examples include Meshlab, Geomagic, 3D viewer, Blender etc.)Funding provided by: Fonds Wetenschappelijk OnderzoekCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100003130Award Number: 11Y7615NFunding provided by: Fonds De La Recherche Scientifique - FNRSCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100002661Award Number: F.4511.19

Locomotion in terrestrial tetrapods is reliant on interactions between distal limb bones (e.g. metapodials and phalanges). The metapodial-phalangeal joint in horse (Equidae) limbs is highly specialised, facilitating vital functions (shock absorption; elastic recoil). While joint shape has changed throughout horse evolution, potential drivers of these modifications have not been quantitatively assessed. Here, I examine the morphology of the forelimb metacarpophalangeal (MCP) joint of horses and their extinct kin (palaeotheres) using geometric morphometrics and disparity analyses, within a phylogenetic context. I also develop a novel alignment protocol that explores magnitude of shape change through time, correlated against body mass and diet. MCP shape was poorly correlated with mass or diet proxies, although significant temporal correlations were detected at 0–1 Ma intervals. A clear division was recovered between New and Old World hipparionin MCP morphologies. Significant changes in MCP disparity and high rates of shape divergence were observed during the Great American Biotic Interchange, with the MCP joint becoming broad and robust in two separate monodactyl lineages, possibly exhibiting novel locomotor behaviour. This large scale study of MCP joint shape demonstrates the apparent capacity for horses to rapidly change their distal limb morphology to overcome discrete locomotor challenges in new habitats.