Modern Tapirs as Morphofunctional Analogues for Locomotion in Endemic Eocene European Perissodactyls

Abstract

Tapirs have historically been considered as ecologically analogous to several groups of extinct perissodactyls based on dental and locomotor morphology. Here, we investigate comparative functional morphology between living tapirs and endemic Eocene European perissodactyls to ascertain whether tapirs represent viable analogues for locomotion in palaeotheres and lophiodontids.Forelimb bones from 20 species of Eocene European perissodactyls were laser scanned and compared to a forelimb dataset of extant Tapirus. Bone shape was quantified using 3D geometric morphometrics; coordinates were Procrustes aligned and compared using Principal Component Analysis and neighbor-joining trees. Functional traits included lever-arm ratios (LARs; proxy for joint angular velocity), long-bone proportions (speed proxy), and estimated body mass. Results suggest that Paralophiodon and Palaeotherium magnum resemble Neotropical tapirs in humeral morphology and LARs. Palaeotheres demonstrate extensive forelimb shape disparity. Despite previous assessments, metacarpal shape analyzes do not support a strong morphological similarity between palaeotheres and tapirs, with Tapirus pinchaque representing the closest analogue for Eocene European equoid manus morphology. Our analyzes suggest lophiodontids were not capable of moving as swiftly as tapirs due to greater loading over the manus. We conclude that the variation within modern tapir forelimb morphology confounds the assignment of one living analogue within Tapirus for extinct European equoids, whereas tapirs adapted for greater loading over the manus (e.g., T. bairdii, T. indicus) represent viable locomotor analogues for lophiodontids. This study represents a valuable first step toward locomotor simulation and behavioral inference for both hippomorph and tapiromorph perissodactyls in Eocene faunal communities.

Biogeography a key influence on distal forelimb variation in horses through the Cenozoic

“…The results of the present study indicate that global equoid locomotor diversity across the Eocene-Oligocene boundary was dramatically reduced with the loss of multiple palaeothere taxa (figure 1c-e). The diversity of form in palaeothere locomotor anatomy has recently been quantitatively examined [8]. Equoid disparity prior to the 'Grande Coupure' supports these previous findings, and the loss of phenotypic diversity following the extinction of the morphologically diverse genus Palaeotherium (figure 1c-e) is, therefore, an unsurprising result from this study.…”

“…Equoid disparity prior to the 'Grande Coupure' supports these previous findings, and the loss of phenotypic diversity following the extinction of the morphologically diverse genus Palaeotherium (figure 1c-e) is, therefore, an unsurprising result from this study. Morphological examination of Plagiolophus minor (of one of the few palaeotheres to persist beyond the 'Grande Coupure') has highlighted potential locomotor advantages for small, cursorial ungulates in the drier, cooler habitats of post-Eocene Europe [8,43]. The placement of plagiolophine MCP joints in morphospace suggests that the sagittal keel of these species was less prominent than in contemporaneous true equids at this early stage of their evolution ( figure 1b; electronic supplementary material, figure S8).…”

“…The sister family to equids within Equoidea (the palaeotheres; Palaeotheriidae) also underwent digit reduction from tetradactyly to tridactyly (three-toes) during the Eocene and early Oligocene [7]. Eocene equids and palaeotheres were browsers with brachydont (low-crowned) dentition; species rarely exceeded 100 kg in body mass [8,9]. The last palaeotheres died out in the Oligocene of Eurasia, whereas equids proliferated in North America and began to evolve higher-crowned (hypsodont) cheek teeth [10,11].…”

“…By contrast, palaeotheres evolved large sizes in the Eocene (e.g. [8]) prior to their near-complete disappearance at the 'Grande Coupure' [14]. Turnover events, such as regional extinctions and biogeographic dispersals, have not been examined as impactors of equoid locomotor evolution, despite multiple such events occurring throughout the Cenozoic [15][16][17].…”

Hippariontracks and horses' toes: the evolution of the equid single hoof

“…The basal perissodactyl condition, retained today in tapirs, involves four digits in the manus and three in the pes, with small hooves on each digit and a posterior footpad. Rhinos have lost the fourth manual digit, but unlike most other perissodactyl lineages, equids show further digit reduction in evolution, from the three toes (digits II-IV) of earlier equids (four in the manus of the most basal ones, the hyracotheres) to the single main toe of the extant Equus, enclosed in a large single hoof [1][2][3][4][5][6][7][8][9] (see figures 1 and 2): i.e. a change from the tridactyl to the monodactyl condition.…”