Craniomandibular Anatomy of the Subterranean Meridiolestidan Necrolestes patagonensis Ameghino, 1891 (Mammalia, Cladotheria) from the Early Miocene of Patagonia

ABSTRACT Among the collections made during the 1896–1899 Princeton Expeditions to Patagonia are three specimens with partial crania and postcrania of Necrolestes patagonensis Ameghino, 1891, from Santa Cruz Province, Argentina. Necrolestes has anatomical specializations found in extant subterranean mammals, and other features identifying Necrolestes as a late Miocene member of the South American Meridiolestida, a non-therian clade that otherwise is limited to the Cretaceous and Paleocene. Anatomical aspects of the three Princeton specimens have been reported previously but they are more fully described here after additional preparation. The most novel discoveries concern specializations of the snout that help identify Necrolestes as a head-lift digger, as are the extant African golden moles and Australian marsupial moles. As in other meridiolestidans and most non-therian mammals, Necrolestes has a septomaxilla at the external nasal aperture. However, the septomaxilla of Necrolestes has structures that buttress an internarial bar and what we interpret as ossified external nasal cartilages, a remarkable adaptation for digging not known in any extant subterranean therians. The upturned snout of Necrolestes produces a shovel effect and also is known in two lineages of subterranean placentals, namely Oligocene palaeanodonts and Miocene proscalopid soricomorphs. The braincase, ear region, and basicranial vasculature are built on a pattern like that in other non-therian mammals, but additional spaces in the squamosal and exoccipital bones increase the volume of the middle ear, which in extant therians is associated with enhancement of low-frequency hearing, another subterranean adaptation. Our findings reinforce the meridiolestidan affinities for Necrolestes and reconstruction of its subterranean habit.