The South American complete extinction of Gondwanan mammals during the K/T mass extinction: some regional contributing causes

PASCUAL, ROSENDO; ORTIZ-JAUREGUIZAR, EDGARDO; GELFO, JAVIER N.; GOIN, FRANCISCO J.

Mendoza

Simposio; Gondwana 12 Conference "Geological and Biological Heritage of Gondwana"; 2005

Up to now the pattern of mammalian evolution across the K/T boundary in Gondwana is unknown because there are no records representing this time span. Two relatively well represented South American mammal-bearing localities of Campanian age, and one additional of Danian age are known from Patagonia. Comparisons suggest that the evolution of South American mammals across the K/T differ from those of the (still not well known) remaining Gondwanan continents, as well as from the much better known one of North America.  Three Gondwanan lineages (the dryolestan Peligrotheriidae, the monotreme Ornithorhynchidae, and the gondwanatherian Sudamericidae) survived only during the South American Danian, and another one (Sudamericidae) survived only up to the late Eocene in the Antarctic  Peninsula. No other lineage of Gondwanan mammals (e.g., Triconodonta, Docodonta,  Symmetrodonta, and other dryolestidans) have been  found in South America after the Late Cretaceous. Contrariwise, in the Laurasian North American and Eurasian continents the extinction was selective, and many lineages kept evolving long after the K-T span (e.g., multituberculates, plesiadapiform primates, marsupials). The objective of this contribution is to emphasize the total extinction in South America of the Gondwanan mammal lineages during the latest Cretaceous-early Paleocene span, and to analyze the long-term and short-term causal factors that quite probably contributed to   this complete extinction. This extinction marks the end of an episode, the Gondwanan Episode, unknown until quite recently, as well as the beginning of a new one, recognized as the South American Episode. Updated geologic and geophysic evidences, as well as the mammalian record, led us to conclude that the main long-term and short-term factors that contributed to the extinction of the Gondwanan mammals include: (1) separation of the South American Plate from the African one, although still connected to the Antarctic continent;  (2) drifting westward of the South American Plate, giving rise to the Southern Atlantic Ocean, to reach between 80 and 60 Ma a new geographical position, south  and near the Laurasian North American Plate;  (3) beginning of the eruption of the wide-ranged “flood basalt” on the proto-Caribbean region at about 125 m.y, with the inception of explosive island arc vulcanism, particularly close to 85 m.y.; (4) beginning of the final position of the Caribbean Plate, as well as the beginning of the differentiation of Central America by the formation of a  volcanic arc along the occidental margin of the Caribbean Plate; (5)  marine regression of the Late Cretaceous-Danian sea-way that separated  South America in a “nordgondwanien” region from a “sudgondwanien” region; (6) the short-term effects of the nearby impact of an extraterrestrial body had also to modify both the marine and terrestrial environments, and (7) the global cooling trend that happened after the Danian and before the (Thanetian) Late Paleocene Thermal Maximum. The geological phenomena that led to the completion of the Panamanian isthmus and  its connection with the South American Plate are related to the eastward displacement of the Caribbean Plate and the volcanic arc occurred during the Tertiary, hence without any effect on the already occurred extinction of South American Gondwanan mammals. However,  the arrival of marsupials and placentals to South America during the latest Cretaceous-earliest Paleocene interval suggests that by that time the volcanic phenomena associated to the “flood basalt” had built up some discontinuous connection, i.e., quite in advance to the final one occurred by 2,5 m.y.. It remains to be explained why the Laurasian mammals of the approximated and geographical related North American continent didn’t withstand total extinction but only differential survival and extinction, “compensated” with some recovery by dispersalism.