DOI: 10.5061/dryad.kh189325r
title: Anatomy of the nasal and auditory regions of the fossil lagomorph Palaeolagus haydeni: systematic and evolutionary implications

description: 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.  
description: The study is based on µCT data
 obtained from the skull of an adult <i>Palaeolagus</i>
 <i>haydeni</i> (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. 
description: Files are in the .ply format for the 3D models and as image
 stacks for the nasal and ear regions.