Richly fossiliferous Upper Cretaceous-Paleocene beds exposed on Seymour Island provide an excellent opportunity for a combined sedimentological-paleoecological analysis of a high-latitude shallow marine sequence. The depositional paleoenvironments of this sequence were interpreted on the basis of the stratigraphy, sedimentological analysis (including grain-size analysis) of the sediments, and auto-and synecological evaluation of the fauna present.The sequence is composed of two formations. The underlying López de Bertodano Formation (upper Campanian to Paleocene) consists of 1,190 m of gray to tan, friable, sandy, muddy siltstone, and is subdivided into 10 informal units. The lower six units (informally named the Rotularía Units) contain a depauperate macrofauna and are dominated by the annelid Rotularía. They were deposited in a shallow marine environment, near a delta or estuary. Units 7 through 10 include an abundant macrofauna (Molluscan Units). Units 7 through 9 are interpreted as progressively deeper water deposits, with Units 7 and 8 representing middle shelf facies, and Unit 9, the outer shelf facies. Macrofauna in the most offshore portion of Unit 9 is characterized by an epifaunal suspension-feeding bivalve asssemblage dominated by Pycnodonte cf. P. vesiculosa, and an increased percentage of cosmopolitan ammonites. Regressive facies appear 30 m below the inferred Cretaceous/Tertiary boundary (contact between Units 9 and 10). Unit 10 was probably deposited in a middle shelf to inner shelf environment.The overlying Sobral Formation (Paleocene) follows unconformably, and is composed of as much as 255 m of maroon, well-laminated silts at the base, followed by cleaner sandstones that become more glauconitic and crossbedded toward the top. The Sobral is not very fossiliferous and represents the filling of the basin by the progradation of a deltaic system. Units 1 and 2 are interpreted as pro-delta facies followed by clean sands of a coastal barrier (Unit 3). The uppermost Sobral (Units 4 and 5) contains the delta top facies, mostly representing the lateral accretion of distributary channels.

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One of the richest and best preserved late Campanian-Maastrichtian ammonite faunas of the world occurs within the Lopez de Bertodano Formation on Seymour Island. The excellent exposure of this sequence has offered an opportunity for detailed stratigraphic study of the fauna, providing a stratigraphic control unavailable for most other Southern Hemisphere strata of similar age.Ammonites are restricted to the Cretaceous portion of the Lopez de Bertodano Formation, becoming more abundant and increasing in diversity within a 600-m interval below the Cretaceous-Tertiary boundary. The ammonite-rich levels are divided into three zones (from oldest to youngest: Pachydiscus ootacodensis, P. riccardi and P. ultimus zones), extending from the late Campanian-Maastrichtian to the latest Maastrichtian.The taxonomic study of this fauna shows a predominance of endemic taxa of the family Kossmaticeratidae, including the species Maorites tuberculatus Howarth, M. seymourianus (Kilian and Reboul), M. densicostatus (Kilian and Reboul), M. weddelliensis n. sp., Grossouvrites gemmatus (Huppe) and Gunnarites bhavaniformis (Kilian and Reboul). These taxa, together with the members of the family Desmoceratidae Kitchinites (Kitchinites) darwini (Steinmann) and K. (K.) laurae n. sp., are mostly restricted to the margins of the Late Cretaceous Weddellian Province that extended from southern South America to Australia.Cosmopolitan genera described in this work become more abundant up section and include the species Anagaudryceras seymouriense n. sp., Zelandites varuna (Forbes), Pseudophyllites loryi (Kilian and Reboul), Diplomoceras lambi Spath, Pachydiscus (Pachydiscus) ootacodensis (Stoliczka), P. (P.) riccardi n. sp. and P. (P.) ultimus n. sp.

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The Upper Cretaceous-lowermost Tertiary López de Bertodano and Sobral Formations Seymour Island, Antarctic Peninsula, contain one of the most important marine faunas known for this interval of the Earth's history. Faunal data from this sequence are providing important new understanding of the origin and biogeographic history of the marine biota of the Southern Hemisphere and insight into the faunal transition at the end of Cretaceous time.The bivalves described herein where collected during four expeditions (1975,1982,(1983)(1984)1985) to Seymour Island. In addition, the collections made by the Swedish South Polar Expedition, 1901-1903, housed in the Naturhistoriska Riksmuseet, Stockholm, were also examined during the course of this study. This chapter describes 13 new species and 2 genera of bivalves: Nucida (Leionucula) hunickeni n. sp., Australoneilo casei n. sp., Austrocucullaea n. gen. oliveroi n. sp., Cucullaea ellioti n. sp., Pinna freneixae n. sp., Phelopteria feldmanni n. sp., Entolium seymourensis n. sp., E. sadleri n. sp., Acesta shackletoni n. sp., A. webbi n. sp., Seymourtula n. gen. antarctica (Wilckens), Lahillia huberi n. sp., Marwickia woodburnei n. sp., Cyclorisma chaneyi n. sp., Surobula n. gen. nucleus (Wilckens), Thracia askinae n. sp. Twenty-one previously described species are redescribed and figured, and their taxonomy revised.