Lagomorpha is a clade of herbivorous mammals nested within Euarchontoglires, one
of the major placental groups represented today. It comprises two extant families
with markedly different body plans: the long-eared and long-limbed Leporidae (hares
and rabbits) and the short-eared and short-limbed Ochotonidae (pikas). These two
lagomorph lineages diverged probably during the latest Eocene/early Oligocene, but
it is unclear whether the last common ancestor of crown lagomorphs was more leporid-
or more ochotonid-like in morphology. Palaeolagus, an early lagomorph dominant in
western North America from the late Eocene to Oligocene is of particular importance
for addressing this controversy. Here, we present new and comprehensive data on the
cranial anatomy of Palaeolagus haydeni, the type species for the genus, based on micro-
computed tomography (μCT). Our μCT data allow us to confirm, revise and score for
the very first time the states of several leporid-like and ochotonid-like characters in the
skull of Palaeolagus. This mixed cranial architecture differentiates Palaeolagus from the
crown groups of Lagomorpha and supports its phylogenetic status as a stem taxon.
