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.

The study is based on µCT data obtained from the skull of an adult Palaeolagus haydeni (FMNH PM 9476) housed in the Field Museum of Natural History (Chicago, IL, USA). The skull was µCT-scanned using a high-resolution GE phoenix|x-ray v|tome|x L 240 scanner (GE Measurement & Control Solutions) at the American Museum of Natural History (AMNH, New York, NY, USA). The parameters of the scan of the entire skull were as follows: voltage 170 kV, current 170 mA, and 0.1 mm Cu filter. The total of 1401 images were acquired at a resolution of 50.69 μm (isotropic voxels). A second scan of the posterior part of the skull (orbitotemporal and otical regions) has the following scan parameters: voltage 170 kV, current 170 mA, and 0.1 mm Cu filter, resolution 36.49 µm. Raw data were further reconstructed with GE phoenix|x-ray datos|x 2.0 software resulting in 16-bit TIFFs (1900x1000 pixel in size). Based on the µCT data virtual 3D reconstructions were performed with Avizo 9.0.1 (Visualization Science Group). Turbinals were segmented with the manual segmentation tool; hereby, the entire turbinal structure up to the contact with the lateral wall of the nasal cavity, the lamina horizontalis, and the lamina cribrosa were considered. The bony labyrinth and the auditory ossicles were segmented with the manual and automatic (magic wand) segmentation tools. 

Files are in the .ply format for the 3D models and as image stacks for the nasal and ear regions.