An analysis of Antartic Paleogene terrestrial mammal biogeography: evidence for the existence of vicariance and dispersal patterns caused by geological events.

REGUERO, M.; ABELLO, M. A.; POSADAS, P.; ORTIZ JAUREGUIZAR, E.; GELFO, JAVIER N.; CHORNOGUBSKY, L.; SANTILLANA S. N.; CORIA, N.

Goa

Simposio; XII International Symposium on Antartic Earth Sciences; 2015

Background: West Antarctica terrestrial mammals were diverse, geographically widespread, stratigraphically long lived and largely terrestrial: they therefore provide an almost ideal? case study in Southern Paleogene biogeography. This mammal association includes eutherian (SAANUs Litopterna and Astrapotheria), metatherian (marsupials Polydolopidae, Derorhynchidae and Microbiotheria) and nontherian mammals (Gondwanatheria and Meridiolestida). A more complex picture of the origin of the Cenozoic Antarctic and South American mammalian faunas is starting to emerge, showing integration of faunal elements from different biogeographic events: surviving members of Mesozoic non-therian mammalian lineages, metatherian and eutherian lineages that presumably dispersed to Antarctica in the latest Cretaceous or early Paleocene. These biogeographic events are thought to be linked to geological events of connection and disconnection of these two land masses (Antarctica and South America). From an evolutionary perspective such likenesses and close relationships are difficult to accommodate because, by the end of the Late Cretaceous (~72 Ma), two of the major Gondwanan continental blocks (South America and West Antarctica) were well into the process of breakup and dispersion. Critically, South America (Patagonia) and Antarctic Peninsula (West Antarctica) are commonly thought to have been separated by vast marine barriers. To explain their paleontological linkages, it has thus been proposed that these isolated? land masses were connected to one another until as late as c. 58 Ma (Late Paleocene) via long causeways that routed through Antarctica. Methods: The current study has drawn together the most recent and detailed eutherian (Astrapotheria and Litopterna) and metatherian (Polydolopidae) phylogenies, and geographical and stratigraphical range data. This dataset was subjected to the cladistics biogeographical method known as Dispersal Vicariance Analyses and were performed using RASP software for therian mammals (i.e. ungulates and marsupials) in order to test for the presence of the repeated area relationships that are potentially indicative of a vicariance signal. The stratigraphic calibration of eutherian mammal phylogenies indicates the Late Paleocene as the minimum age at which their common ancestor was present on both areas, Antarctica and South America. Examination of ghost lineages of these eutherian mammals currently known to occur in Paleogene rocks of South America and Antarctica indicates that minimum divergence times for these taxa are in the Late Paleocene Astrapotheria (~58-59 m.y.) and Litopterna (~57-58 m.y.) or probably much earlier, thus substantially predating at least 5-6 m.y. earlier than their Early Eocene occurrences in both areas. The analysis of Astrapotheria and litoptern Sparnotheriodontidae 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 in ancestral area (West Antarctica Patagonia). Results: Unfortunately, the biogeographical history of some metatherian (marsupials, i.e., Derorhynchidae) groups present in the Paleogene of West Antarctica remains obscure and controversial, partly because of differences in the datasets studied and the methods applied. Many workers have proposed vicariance?, driven by continental fragmentation, as the dominant factor that determined metatherian distributions, especially during the Paleogene. By the way, the earliest record of marsupials in South America is the ameridelphian Cocatherium lefipanum from Paso del Sapo (42° 46?S; 69° 51?W) of Danian age, Chubut Province, Argentina. The analysis of the Polydolopidae marsupials show a distribution restricted to Patagonia during the lower and middle Paleocene and a dispersal event from Patagonia to Antarctica followed by a vicariance, both of them during the late Paleocene. The increasing number of Mesozoic mammal lineages now known to have survived into the Cenozoic of Antarctica and South America demonstrates the integration of the non-therian mammalian faunas of the Late Cretaceous into the eutherian and metatherian faunas that made their first appearance in the fossil record of South America during the Paleocene. The following clades will complete this study. Meridiolestida is a diverse group of dryolestoids recorded mostly from Patagonian Cretaceous outcrops, which survived the K/Pg boundary with the youngest representative of the clade coming from the early Miocene. The Antarctic Meridiolestida closely resembles to Barberenia and Brandonia from the Late Cretaceous of Patagonia (Los Alamitos and Allen formations). Gondwanatherians are an enigmatic group of extinct non-therian mammals apparently restricted to the Cretaceous and Paleogene of South America, Africa, India, Madagascar and the Antarctic Peninsula. Gondwanatherians are a strictly Gondwanan radiation, and the Antarctic taxon, cf. Sudamerica ameghinoi, belongs to the sudamericids that radiated in the Paleogene of Antarctica and South America (Sudamerica and Greniodon). Conclusion: From the results obtained we conclude that the biogeographic histories of eutherians (Astrapotheria and Litopterna) were different from those of metatherians (i.e. Polydolopidae), but all of them support the hypothesis of an early stage (late Paleocene) of the paleogeogeographic event leading to the development of a shallow epicontinetal sea which might be interpreted as the earliest tectonic phase (Paleocene) that eventually later led to the opening of the Drake Passage (Miocene). These analyses reveal biogeographical patterns that closely correlate with palaeogeography. The conclusions drawn from the biogeographical analyses of two eutherian and one metatherian clades reinforce the hypothesis that vicariance and dispersal patterns documented in the early Paleogene between West Antarctica and Patagonia are linked to geological events like an early stage of rifting produced stretching causing crustal thinning and widespread subsidence during the Paleocene leading to the development of a shallow but wide epicontinetal sea.