Early Eocene lagomorph (Mammalia) from Western India and the early diversification of Lagomorpha

Abstract

We report the oldest known record of Lagomorpha, based on distinctive, small ankle bones (calcaneus and talus) from Early Eocene deposits (Middle Ypresian equivalent, ca 53 Myr ago) of Gujarat, west-central India. The fossils predate the oldest previously known crown lagomorphs by several million years and extend the record of lagomorphs on the Indian subcontinent by 35 Myr. The bones show a mosaic of derived cursorial adaptations found in gracile Leporidae (rabbits and hares) and primitive traits characteristic of extant Ochotonidae (pikas) and more robust leporids. Together with gracile and robust calcanei from the Middle Eocene of Shanghuang, China, also reported here, the Indian fossils suggest that diversification within crown Lagomorpha and possibly divergence of the family Leporidae were already underway in the Early Eocene.

Figure 1

Calcanei of Eocene lagomorphs in (a–d) dorsal and (e–h) lateral views. Garhwal University, Srinagar, India (GU), and Institute of Vertebrate Palaeontology and Palaeoanthropology, Beijing, China (IVPP). (a,e) Left calcaneus, GU/RSR/VAS 711 from Early Eocene Vastan mine, India; (b–d, f–h) right calcanei from Middle Eocene of Shanghuang, China, IVPP 15537, 15538, 15539, respectively. ff, fibular facet; sf, sustentacular facet; PCF, posterior calcaneal facet; cf, cuboid facet, cc, calcaneal canal.

Figure 2

Tali of Eocene lagomorphs in (a,b) dorsal, (c,d) ventral and (e,f) distal views. (a,c,e) Right talus from Vastan mine, India (GU/RSR/VAS 712); (b,d,f) left talus from Shanghuang, China (IVPP 15540).

Figure 3

Majority rule consensus tree of 1820 most parsimonious trees (MPTs) recovered by PAUP v. 4.0b10, rooted using three marsupial taxa as an outgroup. Numbers left of clades show the percentage of MPTs in which the clade occurs when this percentage is lower than 100%. Tree length, 1402 steps; CI, 0.2482; RI, 0.6941. Light grey shading indicates the position of Glires, intermediate grey shading shows Lagomorpha and dark grey shading shows Leporidae. Characters used in the analysis are listed in the electronic supplementary material, table 2.

Figure 4

Statistical analyses of scaled shape measures for Vastan and Shanghuang tarsals compared to extant lagomorphs (SPSS for Windows, v. 15.0.1). (a,d) PCA based on all measures of calcaneus and talus; graphs show first two principal components and the amount of variation explained by each component (filled circles, gracile leporids; open circles, robust leporids; filled squares, ochotonids; labelled crosses, fossil specimens). (b,c,e,f), Univariate distributions of selected scaled measurements by genus (filled boxes, Leporidae; open boxes, Ochotonidae; black boxes and crosses, fossil specimens). Boxplots indicate median and quartile values, with outliers (more than 1.5 times interquartile range) indicated by isolated dots. (b) Calcaneal width at the sustentaculum, (c) calcaneal distal elongation, (e) talar width, (f) trochlear asymmetry (lateral/medial keel lengths). See also electronic supplementary material, figures S1–S14.