PATTERNS OF EXTINCTION AND SURVIVAL OF MARINE VERTEBRATES ACROSS THE K/PG BOUNDARY IN ANTARCTICA

The Maastrichtian to Danian marine sedimentary sequence on Seymour (Marambio) Island, Antarctic Peninsula is unique among marine sequences that exhibit the transition of organisms from the Cretaceous to the Paleogene. The siliciclastic sediments of the Lopez de Bertodano Fm. of the topmost unit of the Maastrichtian section, Unit 9, show continuous deposition without apparent erosional episodes and thus there are no time gaps. The horizon of iridium enrichment is at the base of the glauconite bed and the top of that bed marks the boundary between LB Units 9 and 10. This sequence of beds can be traced continuously over a linear distance of some five kilometers.

Elasmosaurid plesiosaur taxa have substantial and continuous record in the James Ross Basin from the Late Campanian Herbert Sound Mbr. (75 -73.5 Ma) to the Cape Lamb Mbr (73.4 - 69.7 Ma) of the Snow Hill Island Fm. and then to the Sandwich Bluff Mbr. of the lower portion of the Lopez de Bertodano Fm. (68.5 Ma) on Vega Island, with two different horizons in the upper portion of the Lopez de Bertodano Fm. (LB 5 70.9 Ma) and then close to the K/Pg boundary in LB9 (67 - 66.04 Ma) on Seymour Island. Mosasaurs are more spotty in their abundant occurrences, tylosaurines in the Late Campanian Herbert Sound Mbr. (75 -73.5 Ma), plioplatecarpines and mosasaurines from Lopez de Bertodano Fm. (68.5 Ma) on Vega Island, and then again close to the K/Pg boundary in LB9 (67 - 66.04 Ma) on Seymour Island.

Ten transects were sampled along the K/Pg exposure in Units 9 to 10, with the fossils recovered and recorded as to their position in one-meter intervals beginning at least 10 meters below the glauconite with the iridium to 6 meters above the boundary.

Three patterns were discerned concerning the biostratigraphic distribution of marine vertebrate taxa. First, the last occurrence datum (LAD) of various Cretaceous marine vertebrate taxa is below the level of the iridium horizon by 1.0 - 4.0 m. This pattern is exhibited by plesiosaurs (LAD =1.5 m below), mosasaurs (LAD 3-4 m), the teleost genus Enchodus (LAD 1-2 m) and the shark genus Cretolamna (LAD 2-3 m). Second, is the taxa which go through the boundary with no change in abundance, exhibited by the hexanchid shark Notidanodon and the chimaera Callorhinchus. Third, is where taxa that are rare below the K/Pg boundary have a dramatic increase in their abundance above the boundary, as seen in lamniform shark, Odontaspis.