Postcranial remains of basal typotherian notoungulates from the Eocene of northwestern Argentina

Abstract

Notoungulates represent the most taxonomically diverse and temporally and geographically widespread group among South American native ungulates. Here, we analyze anatomical and systematic aspects of proximal tarsal bones recovered from the Lower and Upper Lumbrera formations (middle and late middle Eocene) in northwestern Argentina. We provide detailed descriptions, comparisons, and infer foot stances and range of movements for the taxa implicated. Material studied includes astragali belonging to the oldfieldthomasiid Colbertia lumbrerense (Lower Lumbrera Formation), a set of proximal tarsals referred as Typotheria indet. (Lower Lumbrera Formation), and tarsals (also including navicular and cuboid) of the informal taxon "Campanorco inauguralis" (Upper Lumbrera Formation). The comparison of the tarsals of Colbertia lumbrerense (middle Eocene of Argentina) with Colbertia magellanica (early Eocene of Brazil) reveals several differences including variations on the development and arrangement of articular facets, and the size of the dorsal astragalar foramen in the Argentinean species. The specimen of Typotheria indet. shows morphological affinities with basal interatheriid taxa. However, its larger size contrasting with the overall small body sizes of Eocene interatheriids precludes an indisputable taxonomic assignment. Concerning "Campanorco inauguralis", our observations indicate that there is no morphological evidence for a close phylogenetic relationship with Mesotheriidae. It presents a "reversed alternating tarsus" condition, which is also observed in Leontiniidae, "Notohippidae", Toxodontidae, and some typotherians. However, the spectrum of singularities exhibited by this form precludes the assessment of its relationships in the context of the Paleogene radiation of Typotheria and it is necessary to extend the comparison to Eocene notoungulates. Finally, in a morphofunctional context a plantigrade foot posture is inferred for the specimens here reported. These observations have the potential to provide functional proxies for paleoecological reconstructions to be applied to the study of the early radiation of these notoungulate faunas.

Paleobiological inferences on middle Eocene native ungulates from South America: Functional morphological analysis of Notostylops and Notopithecus

“…The cuboid MPEF‐PV 1115 differs from that of “ Campanorco ” (Armella et al, 2020; Figure 5) in having a more stepped facet for the calcaneus, a relatively larger and kidney‐shaped navicular facet (vs. subcircular), and a shorter and wider facet for the metatarsus.…”

“…Notostylops shares with Notopithecus (Vera, 2012; Figure 5) a vertical crest on the shaft and a shorter tuber bearing a fossa, but differs from it in having a squared and laterally well‐expanded sustentaculum, which is more separated from the ectal facet, and a larger and more prominent peroneal tubercle. Notostylops also differs from “ Campanorco ” (Armella et al, 2020) in having a shorter body and better‐developed sustentaculum and peroneal tubercle, whereas Colbertia has a longer tuber and lack of vertical crest, among other differences (Cifelli, 1983; Figure 11).…”

“…Colbertia differs from Notostylops in having a more symmetrical and lower trochlea, a prominent tibial protuberance, a shorter neck, and a more developed dorsal crest (Bergqvist et al, 2007; Figure 5; Cifelli, 1983; Figure 11). With respect to “ C. Inauguralis ” (Armella et al, 2020; Figure 5), the astragalus of Notostylops is larger and has a deeper, longer, and more asymmetrical trochlea, a longer and more stylized neck, superior foramen more anteriorly located and narrower head, among other differences.…”

“…A deeply cupped proximal articular facet and a well‐developed medial process in N. murinus are shared with AMNH FM 28690 (Supporting Information: Figure 2l). Comparatively, the navicular of “ Campanorco ” (Armella et al, 2020) is rounded and flatter, and its distal facets for cuboid and cuneiforms face nearly at the same plane; it mainly differs from Notostylops in having a well‐visible facet for calcaneum (there is a navicular‐calcaneum contact).…”

“…Differences were also found when comparing with the tarsal bones of "Campanorco" from the late middle Eocene (Upper Lumbrera Formation; Armella et al, 2020), for example, its smaller size, presence of a navicular-calcaneus contact, and morphology of astragalus (e.g., lower and shallower trochlea, shorter neck, and wider head) and calcaneus (e.g., distally narrower and less-developed sustentaculum and peroneal tubercle).…”