Tracing the Paleobiology of Paedotherium and Tremacyllus (Pachyrukhinae, Notoungulata), the Latest Sciuromorph South American Native Ungulates – Part II: Orbital, Auditory, and Occipito-Cervical Regions

Abstract

The caudal cranium and occipito-cervical region, although usually overlooked, are informative about the paleobiology of fossil mammals, allowing inquiry into vision and hearing abilities, as well as head and neck postures. Particularly for Pachyrukhinae, some related features remain unexplored. In this contribution, 22 specimens of Paedotherium and Tremacyllus were analyzed in a mainly qualitative and comparative framework. Pachyrukhines are characterized by having large orbits and hearing cavities, moderate to short necks with generalized morphologies, and S-shape postures. These features allow rejecting some types of specialized digging habits, and support the preference of open or low-humidity environments. Paedotherium typicum is distinguished by the presence of laterally oriented eyes, marked vaulted cranium and predominant short extensor and stabilizing neck muscles, and cervico-occipitalhyoid configurations suitable for ventro-flexed resting posture. These features indicate accentuated frontation and panoramic-vision, upward head postures enhancing substrate perception, and the resistance of impacts during leaping-cursorial locomotion. Conversely, P. bonaerense, and to lesser degree Tremacyllus spp., show less frontation and probably adopted more horizontal head postures. More particularly, stronger ventral and lateral neck and head flexors and extrinsic arm musculature are reconstructed for P. bonaerense, compatible with generalist or scratch-digging habits. Its smaller auditory cavities and stronger ear musculature, compared to the contemporary P. typicum and especially Tremacyllus spp., would indicate larger ears and microhabitat segregation. The integrative analysis proposed here and in the accompanying contribution aims to shed light on convergences with extant models, paleobiology, niche partitioning, and external appearance of the latest rodent-like ungulates.

Ecomorphological variation of the penguin wing

“…Penguins have a well‐preserved fossil record (Acosta Hospitaleche et al, 2019; Ksepka et al, 2008; Mayr et al, 2019) as well as living representatives closely related to extinct species (Giovanardi et al, 2021). Studying the morphology of closely related extant species, is a robust approach for quantitatively inferring the diet and other ecological traits of species that lived in the past (Ercoli et al, 2021). For instance, Madrynornis mirandus , a crown group penguin from the Miocene of Patagonia (Acosta Hospitaleche et al, 2007; Chávez‐Hoffmeister, 2014; Degrange et al, 2018; Ksepka et al, 2012), is a specimen that preserves an almost complete skeleton that has been studied from phylogenetic and palaeobiological perspectives (Chávez‐Hoffmeister, 2020; Degrange et al, 2018; N. Haidr & Acosta Hospitaleche, 2014; Tambussi & Acosta Hospitaleche, 2008).…”

“…Pursuit‐diving movements are performed mostly by wing muscles which leave a correlate in the osteological apparatus. Wing scars position and relative size, articulation shape and general bone morphology have the potential of showing differences that might correlate to animal ecology (Acosta Hospitaleche & Di Carlo, 2010; Bannasch, 1994; Ercoli et al, 2021; Livezey, 1989; Orkney et al, 2021; Serrano et al, 2017, 2020). For instance, the position of the muscle scars relative to the articulations can inform about the needed lever arm input forces (Serrano et al, 2017), the relative size of the scar can be used as a proxy of muscle cross‐sectional area (which is informative of the muscle power force) (Mayr, 2017; Serrano et al, 2017), the shape of a particular articulation, for example, between the coracoid and the sternum, is informative of the rotation capacity of the wing (Acosta Hospitaleche & Di Carlo, 2010; Bannasch, 1994); furthermore, the broadness and ossification of certain bones such as the scapula, coracoid, and sternum, where many diving muscle originates, can inform about the size and estimated transversal area of muscles involved in diving such as the m. pectoralis thoracica and m. supracoracoideus (Acosta Hospitaleche & Di Carlo, 2010; Mayr, 2017; Serrano et al, 2017).…”