Abstract Gondwanatherians are an enigmatic group of extinct non-therian mammals apparently restricted to some of the western Gondwanan continents (Late Cretaceousearly Palaeocene of South America, and Late Cretaceous of Madagascar and India). They developed rodent-like incisors and the earliest known hypsodont cheek-teeth among mammals. Recently, a small rodent-like dentary fragment was recovered from middle Eocene beds on the Antarctic Peninsula, preserving part of the incisor; both the incisor enamel structure and the mandibular morphology suggest close affinities with Sudamerica ameghinoi from the early Palaeocene of Patagonia, up to now the youngest known Gondwanatheria. Thus, the new specimen becomes the youngest occurrence of a gondwanathere, adding significant direct and indirect evidence on: (1) the already documented cosmopolitanism of gondwanatheres among Gondwanan mammals; and (2) the crucial biogeographical role of Antarctica during the Cretaceous–Tertiary mammalian transition.

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Abstract The generally well developed and understood stratigraphic record associated with fossil mammals in North America is combined with independent chronological data sets that foster the development of high-resolution geochronology in nonmarine sequences. An updated chronology for all North American mammal ages (or subdivisions) is utilized to examine the tempo and mode of overland mammal immigration/emigration episodes during the Cenozoic Era. In addition to the thirty or more "background" dispersals involving only a few taxa, ten major immigration/emigration episodes are recorded during the Cenozoic Era in North America. All are important for evaluating the dispersal pattern, as well as for mammal age boundary definition. For the Paleogene interval, major immigration/emigration episodes define the following mammal ages (or intervals): Clarkforkian, Wasatchian, late Uintan, and Chadronian, with the Wasatchian and late Uintan being especially noteworthy. The interval that embraces the late Arikareean mammal age is the first immigration episode of the Neogene interval, but the events recognized in the early and late Hemingfordian mammal ages, respectively, are the most impressive. An important, "medial" Clarendonian emigration is reflected in the North American basis for the Old World "Hipparion" Datum. The events that define the beginning of the Hemphillian and late Blancan mammal ages also are founded on important immigrant first occurrences, but for the first time in the Neogene interval involve taxa from South America as well as from Asia. At other times, either there is no effective immigration or, if present, it involves only a few (four or less) taxa (= "background"). In certain intervals, apparently lowered sea level had no effect on dispersal, but in an even larger number of cases immigration took place in spite of what appears to have been times of relative sea-level highstand. Thus tectonic, climatic, and other factors must be considered to account for North America’s dispersal history during the Cenozoic Era.

During the latter part of the 19th century, the waters off the coast of the Antarctic Peninsula began to be visited by ships searching for new whaling grounds. During the austral summer of 1892–1893, two of these early whaling expeditions visited the Seymour Island region on the northeast tip of the peninsula. The Norwegian ship Jason, commanded by Captain C. A. Larsen, reached Antarctica in November 1892; after sailing south along the east side of the peninsula as far as the sea ice would allow, the crew returned to Seymour Island in mid-November and landed on the east side of the island in search of seals. While ashore, Captain Larsen collected the first fossils from the continent of Antarctica. Although he never published any account of his discovery, we know many of the particulars of Larsen’s discovery from Dr. Donald, one of two naturalists assigned to the Dundee Whaling Expedition that arrived in the area shortly after the discovery. In 1901, explorer/scientist Otto Nordenskjöld organized the Swedish South Polar Expedition (1901–1903) to explore the Seymour Island region. Nordenskjöld’s expedition was forced, due to the loss of his ship, the Antarctic, to spend 2½ years in Antarctica. During his forced stay, he explored Seymour Island and the surrounding area and made a number of important scientific discoveries. Although the Swedish South Polar Expedition was one of the most scientifically successful expeditions in the annals of Antarctic exploration, Nordenskjöld did not succeed, due to chance and circumstances beyond his control, in exploiting the truly remarkable fossil deposits that are present on Seymour Island.

A topographic map of Seymour Island was compiled at a scale of 1:20,000 with a 10-m contour interval. One-color Ozalid copies were produced for field use. Small-format reconnaissance aerial photographs were used for the compilation, scale control was obtained from a Landsat return beam vidicon image, and leveling was on shorelines. The limitations on precision, accuracy, and reliability arising from use of these unconventional methods are discussed. The map is published here in a three-color version.

Fossil vertebrates were collected on Seymour Island, Antarctica, using traditional field methods with minor modifications. It was found that freezing of plaster bandages prior to setting is not detrimental to their strength. The “cap” of the field jacket not only must cover the exposed surface but also must be secured to the jacket by taking several turns around the entire block to hold the two parts of the jacket securely together. Water-catalyzed polyurethane-impregnated fiberglass bandages were tested and found to protect the specimen in transit well, although they were difficult to apply to small- and medium-sized blocks. Polyethylene glycol is useful for consolidating fragile specimens in weather conditions typical of Antarctica.

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 Rotularia Units) contain a depauperate macrofauna and are dominated by the annelid Rotularia. 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. Three unconformity-bounded depositional sequences are recognized in the studied section. The lower sequence extends from Units 1 to 9 of the López de Bertodano Formation (upper Campanian to upper Maastrichtian). The intermediate sequence is restricted to Units 9 and 10 of the same formation (upper Maastrichtian to lower Paleocene); the upper sequence is represented by the Sobral Formation (Paleocene). Uncertainties in the dating of these sequences make their comparison with worldwide sea-level fluctuations premature.

Diverse, siliceous microfossil assemblages—including marine diatoms, silicoflagellates, ebridians, endoskeletal dinoflagellates, chrysophyte cysts (archaeomonads), radiolarians, and sponge spicules—were recovered from Seymour Island. Their stratigraphic occurrence is documented from the ~l,400-m-thick section of the López de Bertodano (upper Campanian into lower Paleocene) and Sobral (lower Paleocene) Formations. These units consist of detrital silt and fine sand deposited in a quiet shelf environment. The following new diatoms are proposed: Coscinodiscus sparsus, Gladius antarcticus, Gladius antarcticus f. alta, Hemiaulus huberi, Hemiaulus seymouriensis, Pterotheca minor, Pterotheca trojana, and Wittia macellarii. The microfossil assemblages compare well with floras of similar age from diatomites in the Ural Mountain region of the Soviet Union and from Deep Sea Drilling Project (DSDP) Hole 275 in the southwest Pacific. The present work documents the fourth occurrence of Campanian/Maastrichtian, and the first of Danian diatoms and silicoflagellates from the Southern Hemisphere. High sedimentation rates have preserved a detailed record of Late Cretaceous and early Paleocene siliceous microfossil evolution and extinction. Silicoflagellate assemblages show an abrupt composition change from Lyramula- dominated to Corbisema- dominated floras a few meters above a resistant glauconitic sandstone within the upper López de Bertodano Formation. The abundance of diatom resting spores increases from ~5 percent in the uppermost Maastrichtian to ~35 percent a short distance above this glauconite and continues at these high values through the lower Paleocene. These and other fossil data suggest that the Cretaceous/Tertiary (K-T) boundary is situated a few meters above the glauconitic sandstone. Unlike most other known K/T sections, clastic sedimentation was continuous across the boundary. Most Cretaceous diatom species (as much as 84 percent) continue into Tertiary beds. This suggests that the extinction event responsible for devastating the other major groups of Cretaceous plankton did not affect the diatoms; resting spore formation may have aided their survival.

Rich palynomorph assemblages occur throughout the Campanian to Eocene stratigraphic section of Seymour and surrounding islands, northeastern Antarctic Peninsula. The section comprises sediments referred to the López de Bertodano, Sobral, Cross Valley, and La Meseta Formations. Nearshore marine to coastal-deltaic sediments include marine palynomorphs (dinoflagellate cysts, acritarchs, other algae) and diverse land-derived palynomorphs (pollen, spores, fungal spores, fresh-water algae), plus a variety of other organic debris. Palynostratigraphic results are based on a survey of about 530 outcrop samples of Campanian through Paleocene age. Six palynomorph zones, informally designated 1 through 6 and based on dinocyst species, are recognized in the upper Campanian through Paleocene section on Seymour Island. López de Bertodano sediments include zones 1 to 4 of late Campanian to Maastrichtian age. These Cretaceous zones are characterized by an evolving complex of dinocyst species of Manumiella and related genera. Zone 5, of early Paleocene age, occurs in uppermost López de Bertodano Formation sediments and most of the Sobral Formation on Seymour Island. Zone 6, of probable late Paleocene age, occurs in the uppermost Sobral Formation. Paleocene zones 5 and 6 are characterized by dinocysts Spinidinium spp., Deflandrea and Ceratiopsis spp., Microdinium sp.; and Paleoperidinium pyrophorum in zone 5. Two distinct older assemblages of middle to late Campanian age are recognized from The Naze, situated on northeastern James Ross Island, and Cape Lamb, on Vega Island. Upper Cretaceous-Paleocene nonmarine palynomorphs reflect a cool, humid, podocarpaceous conifer vegetation with a varied understory of ferns and highly endemic angiosperms.

Unconsolidated, fine-grained (sandy silts), shallow marine sediments of the upper Lopez de Bertodano Formation include the Cretaceous/Tertiary boundary on Seymour Island, northeastern Antarctic Peninsula. These strata contain abundant palynomorphs and other micro- and macrofossils. The Cretaceous/Tertiary boundary is provisionally placed at a dinocyst zonal boundary that occurs within a laterally persistent glauconite-rich interval. This glauconite interval marks the highest, definitely in-place Maastrichtian macro- and microfossils. Strata above this interval are considered Danian, based on palynological evidence, in the absence of other age-diagnostic fossils. Association of Maastrichtian ammonites and microfossils with typically “Danian” dinocysts below the glauconite interval accentuates the transitional nature of the Cretaceous/Tertiary succession on Seymour Island. Changes in Cretaceous/Tertiary dinocyst assemblages may be related to local environmental change, including regression. Pollen and spore assemblages record little change and no evidence of an abrupt event for the land vegetation across the Cretaceous/Tertiary boundary. Instead, a longterm floral turnover is inferred from nonmarine species. This gradual change is consistent with climatic change, possibly cooling, through much of Maastrichtian and early Paleocene time.

Foraminiferal assemblages consisting of 76 genera and 145 species were recovered from Upper Cretaceous and Paleocene sediments in the James Ross Island region, northeastern Antarctic Peninsula. Open taxonomic nomenclature is used for 37 taxa and 10 new species are described, including Alveolophragmium macellarii, Spiroplectammina vegaensis, Dorothia paeminosa, Buliminella procera, Neobulimina digitata, Bolivina pustulata, Conorbina anderssoni, Cibicides nordenskjoldi, C. seymouriensis, and Anomalinoides larseni. Three biostratigraphic range zones are recognized for interregional and regional correlation: (1) the ?mid- to upper Campanian Gaudryina healyi Zone, used to correlate the Cape Lamb (Vega Island), The Naze (James Ross Island), Snow Hill, and lowermost Seymour Island beds; (2) the upper Campanian through Maastrichtian Hedbergella monmouthensis Zone, occurring only on Seymour Island; and the Danian Globastica daubjergensis Zone, also occurring only on Seymour Island. The foraminiferal transition from Upper Cretaceous to Lower Tertiary sediments on Seymour Island is characterized by extinction of all Upper Cretaceous planktonic species and numerous benthic taxa, a lowermost Tertiary dissolution facies that is barren of calcareous microfossils, and the appearance of the Danian marker species Globastica daubjergensis with low-diversity, high-dominance calcareous assemblages just above the dissolution facies. The stratigraphic distribution of foraminifera on Seymour Island attests to long-term environmental stability in the Upper Cretaceous sequence, in contrast to stressed environmental conditions in the lower Paleocene. Foraminiferal distributions indicate that the lowermost 250 m of the Seymour Island sequence was deposited in an inner neritic environment, and the overlying 950 m of Cretaceous and lower Danian sediments in an outer neritic setting. Foraminifera are absent from the younger Paleocene sequence, which was deposited in a relatively high-energy environment. The foraminiferal assemblages from the James Ross Island region include a mixture of cosmopolitan and provincial species. The Campanian-Maastrichtian fauna is characterized by: (1) the conspicuous absence of specialized benthic and keeled planktonic species, which are used as index fossils in low to middle latitude regions; (2) the predominance of long-ranging, cosmopolitan taxa; and (3) the occurrence of provincial species that were restricted to the southern, extratropical latitudes. These latter taxa are included in the foraminiferal Austral Province. The biogeographic distribution of Austral Province foraminifera supports the postulated trans-Antarctic seaways between East and West Antarctica and/or a marine communication through the Antarctic Peninsula during the Late Cretaceous. The Danian assemblages are characterized by opportunistic, cosmopolitan species and one new buliminellid species, which dominates the calcareous assemblages.

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: Nucula (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.

Dimitobelus (Dimitocamax) seymouriensis n. sp., a new subgenus and species of Dimitobelidae, a belemnite family restricted to the Cretaceous Southern Hemisphere, is described from the Campanian-Maastrichtian López de Bertodano Formation of Seymour Island, Antarctic Peninsula. The subgenus D. (Dimitocamax) n. subgen. is erected for those species of Dimitobelus possessing a regular hastate form and Actinocamax-like alveolar regions. D. (Dimitocamax) n. subgen. had a restricted stratigraphic and geographic distribution in contrast to the wider distribution of D. (Dimitobelus) Whitehouse.

Two species of macruran decapods are reported from Upper Cretaceous rocks of the López de Bertodano Formation on Seymour Island, Antarctica. Eighty-eight specimens of decapods were collected from the unit; all are from concretions. Hoploparia stokesi has been collected previously on Seymour Island, Cockburn Island, and James Ross Island, in Antarctica. A new species of palinurid lobster is described, Linuparus macellarii n. sp. The genus has not been reported previously from Antarctica, and its only published reports in the Southern Hemisphere have been from two localities in the Upper Cretaceous of Africa and Madagascar. Seven specimens of H. stokesi are encrusted with epibionts, including the oyster Pycnodonte cf. P. vericosa (Forbes) and the tube-forming worm Rotularía sp.