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A skull of Proargyrolagus, the oldest argyrolagid Late Oligocene Salla Beds, Bolivia, with brief comments concerning its paleobiology

Sanchez Villagra, M.R.; Kay, R.F.

Journal of Vertebrate Paleontology 17(4): 717-724

1997


ISSN/ISBN: 0272-4634
DOI: 10.1080/02724634.1997.10011019
Accession: 029791976

A skull of the oldest described argyrolagid, Proargyrolagus bolivianus (late Oligocene of the Salla Beds, Bolivia) allows an assessment of the phylogeny of this group and a reconstruction of its paleobiology. Several aspects of the cranial anatomy of the Monodelphis-sized Proargyrolagus serve to support the hypothesis that argyrolagids are marsupials: the palate has large vacuities, the angular process is medially inflected, the auditory bulla is composed of a wing of the alisphenoid, and the dental formula includes four molars. Like Argentine Plio-Pleistocene argyrolagids, Proargyrolagus has an unfused symphysis, and a phaneric and almost vertically oriented ectotympanic in the shape of a flattened ring. Proargyrolagus is more primitive in having a less globular braincase, smaller orbits, and a shallower mandible. The dental formula is 4/3.1/1.3/2.4/4 or 4/4.1/0.3/2.4/4 (vs. 2/2.0/0.1/1.4/4 for Argyrolagus). Large infraorbital foramina transmitted nerves and vessels of the snout. The snout is long with the nasals projecting well forward of the incisors and with a strong antorbital fossa for facial muscles. It can be inferred that this animal had well-developed vibrissae and a mobile proboscis, and relied heavily on its rostrum to gather tactile information. Food manipulation was assisted by an elongate, procumbent lower incisor that occluded with three sharply-edged upper incisors, as in living phalangeriform marsupials. Like Plio-Pleistocene argyrolagids and extant elephant shrews, Proargyrolagus has prismatic and high crowned cheek teeth suggesting an abrasive diet consisting perhaps of seeds.

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