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.
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