Ecological Preferences and Behavior of Palaeologus Haydeni with Implications for Leporid Evolution

The ecological preferences and behavior of the early Tertiary leporid Palaeolagus haydeni are inferred from the natural history of extant taxa that are morphologically similar. Evidence suggests that P. haydeni functioned as an ochotonid. This determination is based on three lines of investigation: skull morphology, brain anatomy, and skull ontogeny. Similarity of skull morphology was determined by measuring 15 characters on two ochotonid species and 19 leporid species. Analysis of these measurements via base-10 log transformations and ratio diagrams place Palaeolagus haydeni into the rock rabbit ecomorph, based on similarities in skull proportions with extant pikas. In the investigation of the brain, the dimensions of the olfactory lobes and cerebral hemispheres were measured in extant Lepus californicus, Ochotona collaris, and Sylvilagus floridanus. Identical measurements were made on a cranial endocast of Palaeolagus haydeni. These data were analyzed by drafting regular geometric shapes based on an arrangement of measurements that approximated a frontal section through the brain. Areas of the sectioned olfactory lobes and cerebral hemispheres were calculated, and a ratio of cerebral hemisphere to olfactory lobe area was determined. A comparison of the taxa indicates that Palaeolagus haydeni has the largest olfactory lobes. The ontogenetic investigation compared the skulls of P. haydeni against a growth series of 102 skulls of Sylvilagus floridanus mearnsi. The ontogenetic ages of the extant series were determined by Hoffmeister and Zimmerman (1967) using the eye lens method that is based on the observation that the mammalian eye lens increases in mass with increasing age. Twelve characters of the skull were measured. These measurements also were analyzed by base-l0 log transformation and ratio diagrams. The resulting data show that P. haydeni skulls are similar to skulls of juvenile S. f. mearnsi. The implications of this study are twofold. First, the large olfactory lobes of Palaeolagus haydeni suggest that it was dependent on olfactory cues to a degree not now seen among extant lagomorphs. A possible use of this sense was to establish territories in a manner similar to extant pikas. Whether or not P. haydeni behaved like pikas in other ways is difficult to determine. The environment which P. haydeni preferred was probably valleys or meadows dominated by herbaceous cover and accessible water, apparently similar to the environment preferred by Pleistocene Ochotona princeps. Second, because the skull of Palaeolagus haydeni is proportioned unlike any extant adult leporid, but is similar to juvenile Sylvilagus floridanus mearnsi, it implies that leporid evolution progressed by extending the developmental process. The adult morphology of P. haydeni became the juvenile morphology for more advanced leporids