Shoulder-blade Runners: Utility of the Scapula Fossa Ratio to Investigate Locomotor Evolution in Equids (Mammalia: Perissodactyla)

The scapula fossa ratio (SFR) has been used in recent years to demonstrate functional locomotor differences in perissodactyls. The SFR describes the relative attachment areas of the lateral shoulder muscles: supraspinatus (extensor/stabilizer) and infraspinatus (lateral collateral stabilizer). Here, we use the SFR to investigate deep scapula muscle attachment sites of equids (Perissodactyla: Equidae), and whether any variation observed between extinct and extant species can be explained by habitat variation. 3D-surface models of scapulae were obtained via laser scanning and photogrammetry. Areas of the supraspinous and infraspinous fossae were calculated for 14 species of equids. We also included 9 cervid (Artiodactyla: Cervidae) and 3 alcelaphine (Artiodactyla: Bovidae) genera to test whether extinct equid SFRs are comparable to modern ungulates living in forested habitats (cervids) or open-grassland habitats (monodactyl equids; alcelaphines). Our results demonstrate that extinct tridactyl (three-toed) equid SFRs exhibit significant differences to modern monodactyl (one-toed) species. The infraspinous fossa is relatively larger in cervids and tridactyl equids compared to monodactyl equids; cervid and tridactyl equid SFRs could not be statistically separated. We interpret that the infraspinatus muscle is relatively more important for shoulder stability in tridactyl equids (and cervids) than in monodactyl equids. As monodactyl equids possess an advanced passive stay apparatus (muscle-tendon mechanism locking the shoulder to enable standing in open terrain with minimal energetic input), it is possible that the infraspinatus has become less integral for shoulder support during stance. Without such a mechanism, tridactyl equids still required shoulder support from the infraspinatus to exploit open habitats. This study demonstrates the utility of the SFR as a tool for investigating links between the musculoskeletal system and habitat use through equid evolution.