Anatomy of the nasal and auditory regions of the fossil lagomorph Palaeolagus haydeni: systematic and evolutionary implications

Palaeolagus, a late Eocene to early Miocene North American lagomorph
genus, represented by numerous and well-preserved specimens, has been long
considered a basal leporid, although it is currently understood as a stem
lagomorph. Based on micro-computed tomography (μCT) data and 3D
reconstructions, here we present the first description of intracranial
structures of the nasal and auditory regions of a complete skull of
Palaeolagus haydeni from the early Oligocene of Nebraska. Although
Palaeolagus haydeni shows a puzzling mixture of extant leporid and
ochotonid characters, it helps to polarize and re-evaluate already known
lagomorph intracranial characters based on outgroup comparison with
Rodentia and Scandentia. Common derived features of Palaeolagus haydeni
and extant Lagomorpha are the dendritic maxilloturbinal and the excavated
nasoturbinal that contacts the lamina semicircularis. Generally,
Palaeolagus haydeni and Leporidae have several characters in common, some
of which are certainly plesiomorphic (e.g., thin wall of bulla tympani and
flat conic cochlea). Palaeolagus haydeni resembles Leporidae in having an
interturbinal between the two frontoturbinals, and three ethmoturbinals
plus one interturbinal between ethmoturbinal I and II. Now, this should
also be regarded as a plesiomorphic grundplan pattern for Leporidae
whereas ochotonids are derived from the lagomorph grundplan as concerns
the number of frontoturbinals. Concerning the middle ear, Palaeolagus
haydeni significantly contributes to the polarization of the anterior
anchoring of the malleus in extant lagomorphs. Palaeolagus haydeni
resembles the pattern observed in early ontogenetic stages of Ochotonidae,
i.e., the attachment of the malleus to the ectotympanic via a short
processus anterior. The patterns in adult ochotonids and leporids now can
be regarded as two different and apomorphic character states.
Autapomorphic characters of Palaeolagus haydeni are the reduced
frontoturbinal 2 and the additional anterolaterally oriented process of
the lamina semicircularis. Interestingly, among the investigated
intracranial structures the loss of the secondary crus commune is the only
apomorphic grundplan character of crown Lagomorpha.