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.

This study is based on CT-scan data from an almost complete cranium with mandible in occlusion of Palaeolagus haydeni (FMNH PM9476) from the early Oligocene of the Brule Formation, Nebraska, USA. The skull roof is partially missing in this specimen, the anterior nasals and the left zygomatic arch are also largely damaged.

FMNH PM9476 was CT-scanned in a high-resolution Phoenix v|tome|x L 240 scanner (GE Measurement & Control Solutions) at the AMNH. The specimen was scanned in a single scan with the following parameters: voltage 170kV, current 170 µA and a 0.1 mm Cu filter to minimize beam hardening. The CT-data were reconstructed with Phoenix datos|x 2.0 software obtaining a total of 1401 images at a resolution of 0.0365 mm (isometric voxels). The reconstruction of the raw data generated a 16-bit TIFF image stack comprising 980 slices (570 x 515 pixels). The original image stack was first uploaded into ImageJ 1.52a to remove beam hardening artefacts and increase the contrast between fossilized bone and rock matrix using the Brightness and Contrast tool (Image ->Adjust -> Brightness/Contrast). The adjusted images were then saved as an 8-bit TIFF image stack and uploaded into AvizoLite 2019.4 (Thermo Fischer Scientific 1995–2019) installed on a FUJITSU Celsius M740 workstation (processor: Intel Xeon CPU E5-2620 v4 2.10 GHz, RAM: 128 GB, graphics card: GeForce GTX 1050 Ti, hard drive: Samsung SSD 860 EVO 500 GB) operating on Windows 10 Pro (Microsoft 2019). Manual segmentation involved using the Lasso and Magic Wand tools. 3D models of individual bones and bone complexes were generated using the Unconstrained Smoothing option (smoothing extent = 2.5).

Institutional Abbreviation: FMNH, Field Museum of Natural History, Chicago, IL, USA

All files are in the PLY format.