Cranial Anatomy of Argyrolagus (Metatheria, Argyrolagidae): New Insights, New Interpretations

Argyrolagus and its closest relatives (Argyrolagidae; Late Eocene or Early Oligocene–Early Pleistocene) are usually regarded as outliers among marsupials, with many peculiar craniodental and postcranial features that differ considerably from those found in other recognized groups of metatherians. In the literature this factor has led to variant interpretations of basic anatomical features that usually present no difficulty in identification, like eye position and location of some neurovascular foramina. To shed light on these and other unresolved morphological issues, we report here on new and previously described cranial material of Argyrolagus using modern methods (µCT scanning, morphometrics). Argyrolagus was a small taxon (between 60 g to 150 g body mass) with an elongated, projecting rostrum convergently resembling that of the rodents Dipodomys and Sommeromys, or Rhynchocyon and Elephantulus among elephant shrews. Contrary to some reconstructions, but in line with G.G. Simpson's original interpretation, the orbit of Argyrolagus was most likely located in the caudal half of the cranium, and was large (∼9 mm) for its projected body size. The origin of the temporalis muscle, also disputed, appears to have been small and lain on the caudal part of the cranium above the external acoustic meatus. Orbital convergence was low while the angle of orbital verticality was high, indicating panoramic vision. Three-dimensional reconstruction and interpretation of the auditory region (including the tympanic membrane, middle ear ossicles, middle ear volume, inner ear) indicates that hearing in Argyrolagus favored low frequencies. This correlates with hearing capabilities of extant small mammals (mostly rodents) that live in open environments and/or inhabit underground tunnels. Both interpretations seem plausible considering the paleoenvironments in which Argyrolagus is thought to have lived. Other features, like the cranial circulatory system, appear to be a mosaic of primitive (e.g., small internal jugular vein, most cranial return through external jugular) and derived features (e.g., absence of proximal stapedial artery, absence of postglenoid vein, and a venous arrangement analogous to the sinus communicans found in some placentals). Although some authors have wondered whether argyrolagoids were metatherians or even therians, our reinterpretation of salient anatomical features in Argyrolagus is consistent with their closest affinities being metatherian, as exemplified by absence of an independent optic canal, lack of proximal stapedial artery, enhanced pericarotid system, and inflected angle of dentary, although their precise systematic placement remains elusive.