New non-therian mammal from Marambio (Seymour) island in West Antartica: first Meridiolestid outside Patagonia.

MARTINELLI, A. G.; CHORNOGUBSKY, L.; ABELLO, M. A.; GOIN, F. J.; REGUERO, M.

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

The La Meseta Formation records ca. 25 ma of sedimentation (late Paleocene to ?early Oligocene) in the Marambio (Seymur) Island (Antarctic Peninsula). Its Cucullaea I Allomember contains most of the mammal record, including non-therian gondwanatheres, marsupials, and eutherians (Pilosa, Litopterna and Astrapotheria). This composition reflects a strong paleogeographical link between Paleogene faunas of Patagonia and West Antarctica. A small isolated tooth (MLP-91-11-4-3) was found in locality IAA 1/90 from Cucullaea I Allomember and was originally interpreted as a possible third upper molar from a bat or "insectivore" because of its zalambdodont appearance, and then more broadly assigned to Mammalia Incertae sedis. We present here an alternative interpretation, considering MLP-91-II-4-3 as a right lower molar of a non-therian Dryolestoidea, perhaps being a member of the clade Meridiolestida. The crown is dominated by three cusps and a distolingual talonid cusp. The protoconid is flanked mesially by the paracristid and distally by the metacristid, which reach the paraconid and metaconid, respectively. Both crests form an acute angle, without a clear notch at mid-way. The labial wall of the protoconid is convex whereas the lingual face is slightly concave. The paraconid is worn out and lower than the metaconid. The flexid is notorious and forms a ?v? shaped notch between paraconid and metaconid. The protoconid and metaconid are similar in height. The metaconid connects by means of a crest with the talonid. The talonid has a hook-like disto-lingual projection with a large cusp, slightly bent lingually, and an accesory more labially placed cuspule. The mesial cingulum descends from the level of the paraconid to the mesiolabial face of the paraconid, where reaches its maximum mesio-distally width. It is unknown if the cingulum continues on the labial slope of the protoconid. A portion of root preserved below the paraconoid-protoconid seems to be transversely wide. The crown morphology of MLP-91-II-4-3 closely resembles the meridiolestidans Barberenia and Brandonia from the Late Cretaceous of Patagonia (Los Alamitos and Allen formations). Meridiolestida is a successful group of dryolestoids recorded mostly from Patagonian Cretaceous outcrops, which survived the Cretaceous/Paleocene boundary with the younger representative from early Miocene age (Necrolestes). The new hypothesis of MLP-91-II-4-3 as a Meridiolestida, if proven correct, expands the distribution of the group to West Antarctica and constitutes the second non-therian mammal record from Antarctica. Thus agreeing with previous scenarios that showed Paleogene West Antarctica faunas being more closely related with the ones from Patagonia.