Additions to the knowledge of Notiolofos (Mammalia: Sparnotheriodontidae) early Eocene of West Antarctica.

GELFO, J.; DE LOS REYES, M.; ACOSTA HOSPITALECHE, C.; REGUERO, M.

Goa

Simposio; XII International Symposium on Antarctic Earth Science; 2015

One of the most importance significance of the fossil record of terrestrial vertebrates in Antarctica, is settled in the possibility of make inferences about past terrestrial connections with other continents. In this sense the Cretaceous and Paleogene vertebrate record is a major biological evidence of the Gondwana break up. In contrast to flying birds and marine vertebrates, the dispersal of other groups through or into Antarctica, are more constrained by fresh water supply and land connections. Their ancient presence in Antarctica could be used as an independent way of testing plate tectonics and paleogeographic hypotheses. In this sense, since no crocodiles, squamates or amphibians have been found, the information are restricted during Cretaceous to non-avian dinosaurs, and in the Paleogene, to a doubtful and isolated Ratite and, the mammalian record.Fossil mammals in Antarctica are by now limited to the West Antarctica record in the Eocene outcrops of La Meseta and Submeseta Formations in Seymour Island. The faunistic assemblage has a clear South American imprint and corresponds to endemic genera and species, of groups mostly represented in the Paleogene of Patagonia, Argentina. They include non-therian mammals as a probably Meridiolestida dryolestoid and a gondwanathere, closely related to Sudamerica ameghinoi from the Danian of Punta Peligro in Patagonia. Therians are represented by a wide diversity of ?opposum-like? metatherians, referable to ?Didelphimorphia?, Microbiotheria, Polydolopimorphia and Derorhynchidae; and, placental mammals, by Xenanrthra, Astrapotheria and sparnotheriodontid Litopterna. The Sparnotheriodontidae were a geographically widespread group of medium to large size herbivorous mammals during the Eocene, and part of the radiation of the so called, South American native ungulates. They were low-crowned browser characterized by a lophoselenodont and bicrescentic lower dentition, a strong lophoselenodont ectoloph and usually a bunoid lingual cusp in the upper molars. Known sparnotheriodontids include Victorlemoinea prototypica (Itaboraian SALMA - Itaboraí, Brazil), Victorlemoinea labyrinthica (Riochican SALMA - Patagonia), Sparnotheriodon epsilonoides (Casamayoran SALMA, Vaca subage - Patagonia), Phoradiadus divortiensis (Casamayoran? SALMA - Mendoza, Argentina), Heteroglyphis dewoletzky, assigned to the family with some doubts (Mustersan SALMA - Patagonia), and Notiolofos arquinotiensis, the most abundant terrestrial placental mammal in the Paleogene of Antarctica. The paleobiogeographic analysis of Sparnotheriodontidae, and also Astrapotheria, yields a widespread ancestral distribution, including South America and Antarctica, since the lower Paleocene up to middle-late Paleocene, when a vicariant event took place. It was recorded through most of the La Meseta Formation allomembers, including from base to top, Acantilados II, Campamento, Cucullaea I, and, Submeseta Formation.By now Notiolofos was only known by isolated teeth. Here we present the first associated jaw remains of Notiolofos. The material MLP 15-I-10-1 comes from a filling channel at the new locality IAA 2/15 in Acantilados II Allomember. The same unit but stratigraphically higher, than the bearing horizon of the oldest Antarctic mammal, which is the first record of Notiolofos. The jaw is broken in several pieces, due to diagenetic process, and most of them still in a rock matrix. The preliminary technical preparation allows the description of several important features. The largest portion of dentary is a transversal section of the jaw distal to the m3 but mesial to the coronoid fossa. The alveolar portion for the root of the talonid of the m3 is present but no mandibular canal is visible. The height of this section is around 65 mm, being a bit larger than the same portion in the Sparnotheriodon jaw. At least three teeth representing right m1-3 were recognized, and, other enamel portions left to be identified. The m1-2 were preserved in intimate association, and they are interpreted in this locus by the higher position of the labial cingulid and a small portion of their roots in the fore tooth, indicating the earlier eruption of the m1 respect to the m2. The first molar only preserves the labial side of the talonid, with part of the hypoconid and hypocristid enamel and some dentine exposure. In the m2 there is a small elevation and thickening of the labial enamel, in the position of the protocone. The paralophid has a wide dentine surface, the lingual side of the trigonid is not preserved as well as the internal flexid of the trigonid between the paraconid and the metaconid position. Even though the metaconid is not preserved, the labial enamel edge of the cristid obliqua and the protocristid, rise lingually to the position where the metaconid should be, indicating that this cusp was the highest one. The m3 is the best preserved tooth, with almost the complete labial side of the molar. The mesial part of the entoconid obliterated the talonid basin and almost contacts the massive crest which descends distally from the metaconid. At least the mesiodistal length between the protoconid and the hypoconid is longer than those of the other only known m3 of Notiolofos (MLP 01-I-1-1) which came from somewhat younger sediments (Cucullaea I Allomember).Notiolofos hypodigm comprises only isolated teeth from different locus found in several localities and more important, from distinct stratigraphic levels over a time span of at least 17.5 Ma. Despite their fragmentary preservation, the jaw and the m1-3 series here presented are the more complete finding of a single individual of Notiolofos, and, its second record from Acantilados II Allomember. MLP 15-I-10-1 will allow to test the alleged evolutionary stasis of Notiolofos while improves the faunistic knowledge of this allomember, the lowermost stratigraphic level (Ypresian) in Antarctica where mammals have been found.