Unravelling the distinctive craniomandibular morphology of the Plio‐PleistoceneEumysopsin the evolutionary setting of South American octodontoid rodents (Hystricomorpha)

Abstract

Echimyidae is a species-rich clade of Neotropical rodents, which diversified in association with forested biomes. Since the late Miocene, a few lineages from southern South America have been adapted to open environments. Eumysops is one of these southern echimyids, and its peculiar craniomandibular morphology has been assumed to be a result of adaptation to open environments. We performed a geometric morphometric analysis of craniomandibular shape variation to explore whether, as suspected, Eumysops is divergent from other echimyids and octodontoids. In addition, we explored whether deterministic factors driven by different ecological dimensions can explain the diversification of shape among octodontoids. We found that craniomandibular shape variation in octodontoids was related to ecological variables. Comparing competing evolutionary models suggested that the input of selective factors play a key role in octodontoid craniomandibular shape diversification; habitat and habits were found to be the most influential factors. In the analysed morphospaces, Eumysops was located distant from other echimyids due to its distinctive traits, especially wide and posteriorly displaced orbits, and related low craniomandibular joint. Divergent orbits and resulting wider panoramic vision support the interpretation of Eumysops as an open-habitat specialist echimyid. But what is more relevant, is that Eumysops occupied a sector of the octodontoid cranial morphospace not filled by living representatives; this highlights the contribution of fossils in providing key information on the specialization boundaries explored by a clade throughout its history.

Paralonchothrix gen. nov., the first record of Echimyini (Rodentia, Octodontoidea) in the late Miocene of Southern South America

“…Late Oligocene to middle Miocene representatives are related to living arboreal Echimyini (sensu Fabre et al 2017) from Amazonian and Atlantic forests, which agrees with the persistence of forests with tropical elements as far as southern Argentina in this period (Palazzesi & Barreda 2007). From the late Miocene on, southern fossil echimyids belong to lineages of terrestrial and fossorial Euryzygomatomyinae and Myocastorini (sensu Fabre et al 2017) that currently inhabit open, shrubby to grassland environments, and dry forests (Reig 1986;Verzi et al 1994Verzi et al , 1995Verzi et al , 2014Verzi et al , 2016Verzi et al , 2019Vucetich 1995;Vucetich et al 1997;Cartelle 1999;Hadler et al 2008;Olivares et al 2012bOlivares et al , 2017Olivares et al , 2020Sostillo et al 2015;Candela et al 2020). This taxonomic composition of the fossil record of echimyids is concurrent with Cenozoic palaeoclimatic changes (Pascual & Ortiz Jaureguizar 1990;Janis 1993;Denton 1999;Zachos et al 2001Zachos et al , 2008Tripati et al 2009;Arakaki et al 2011;Le Roux 2012;Palazzesi & Barreda 2012;Dunn et al 2015).…”

“…Nevertheless, some indicative features may be mentioned. Paralonchothrix differs from all arboreal echimyids in its less protruding mandibular symphysis (see Olivares et al 2020). In addition, the PVSJ 319 specimen preserves the posterior portion of the incisor, which is long, thick, and with its base located posterolateral to the m3.…”

“…The chinchillid rodent Lagostomus currently occupies grasslands and lowland deserts (Patton et al 2015). The other echimyid rodent recorded in the Loma de Las Tapias Formation, Ullumys, has a peculiar craniomandibular morphology, with wide and posterior orbits, reflecting specialisations to open environments (Olivares et al 2017(Olivares et al , 2020.…”

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

“…The morphology of the orbits and related structures (e.g., postorbital bar, position and shape of zygomatic arch) is very variable among clades and may be related to different masticatory models (e.g., less lateralized orbits when the mm. temporales show large development; see Noble et al 2000;Cox 2008;Hautier et al 2012), the development of other structures (e.g., bullae; Howell 1932), habitats exploited, and different daily activity patterns (e.g., Hautier et al 2012;Moyano et al 2018;Olivares et al 2020). Among the studied pachyrukhines, P. typicum possesses orbits with the more marked lateral orientation, enabling a wider angle of vision (enhanced panoramic visual field; Howell 1932;Heesy 2004).…”

“…As described by Kraatz et al (2015) for lagomorphs, all these features are part of a functional complex that would have allowed the largest frontation of orbits in P. typicum. The frontation of orbits enhances substrate perception and is related to fast-moving epigean species (e.g., DuBrul 1950;Kraatz et al 2015;Olivares et al 2020). The ventral orientation of the occipital plane and the set of above described associated features could also contribute to the positioning of the head in a higher and more posterior location above the body and neck, and a more vertical orientation of the anterior cervical region (Howell 1932;DuBrul 1950;see below).…”