Anatomy of the Nasal and Auditory Regions of the Fossil Lagomorph Palaeolagus haydeni: Systematic and Evolutionary Implications

Abstract

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.

Lagomorpha as a Model Morphological System

“…The middle ear morphology of extant Lagomorpha reveals unique family-specific patterns of the anterior attachment of the malleus by means of the processus anterior and its processus internus praearticularis; however, the phylogenetic polarization of this character still was pending (Maier et al, 2018). A first attempt to polarize the observed patterns could be achieved by the first high-resolution computed tomography (µCT) study on intracranial structures in a fossil lagomorph; Palaeolagus haydeni reveals that early ontogenetic stages of Ochotona may represent the plesiomorphic lagomorph pattern (Ruf et al, 2021). This clearly shows the potential of µCT investigations of fossil Leporidae and Ochotonidae for elucidating the evolution of intracranial characters.…”

Locomotor behavior and hearing sensitivity in an early lagomorph reconstructed from the bony labyrinth

“…The round window (fenestra cochleae) in Megalagus turgidus does not extend posteriorly beyond the PSC, similar to Palaeolagus (Ruf et al, 2021) and modern leporids. It is posterolateral to the oval window (fenestra vestibuli) and is directed posterolaterally with a dorsal inclination.…”

“…Its presence derives from the relative position of the LSC with respect to the PSC, where the inferior end of the latter reaches as far down as the plane defined by the LSC and meets the posterior end of the LSC, causing them to have a common course for a short distance and share also the hollow space containing the posterior ampulla (Figure 1). In modern lagomorphs, the inferior end of the PSC extends much lower than the plane defined by the LSC, which goes into the vestibule separately, thereby not forming a unified secondary common crus (Ekdale, 2013; Ruf et al, 2021).…”

“…Concerning the comprehensive anatomy of the bony labyrinth in extant lagomorphs, only the inner ear structures of Oryctolagus cuniculus have been studied in detail (Abd El‐Hameed et al (2023) for CT and MRI imaging; Wysocki et al (2007) for the topographical anatomy of the temporal). Recently, the first bony labyrinth for a fossil lagomorph ( Palaeolagus haydeni , an early Oligocene species) has been described (Ruf et al, 2021). However, Megalagus is a member of a more basal lineage of early lagomorphs (Fostowicz‐Frelik & Meng, 2013; see also López‐Torres et al, 2020) and the earliest lagomorph for which the structure in question is known, making it of arguably greater relevance to understanding primitive stages for the order.…”

The development of nasal turbinal morphology of moles and shrews

“…The turbinal bones are very thin and fragile. A few studies described the turbinals, based on fossils; however, there is a limit to how far we can understand the turbinal structure based on fossils (Maier 1983;Maier and Ruf 2014;Ruf et al 2014Ruf et al , 2021. Even if a fossil of a common ancestor of all eulipotyphlans species were to be found, the possibility of observing the turbinal structure would be quite low.…”