Francisco J. Goin

Late Cretaceous/Paleogene West Antarctica Terrestrial Biota and its Intercontinental Affinities

Introduction.- West Antarctica (WANT): tectonics and paleogeography.- -East Antarctica (EANT)/West Antarctica (WANT), gondwanic paleobiogeography.- Late Cretaceous/Paleogene stratigraphy of the James Ross Basin.- -Late Cretaceous.- -Paleogene.- South America/West Antarctica: Pacific affinities of the Paleogene Weddellian marine/coastal vertebrates.- -Late Cretaceous/Paleogene marine fossil vertebrates of the James Ross Basin.- -Weddellian Sphenisciformes: systematics, stratigraphy, biogeography and phylogeny.- The terrestrial biotic dimension of WANT.- -West Antarctica paleoflora.- -Late Cretaceous terrestrial vertebrates of the James Ross Basin.- -Paleogene terrestrial vertebrates of the James Ross Basin.- -Paleogene reconstruction of the Cucullaea I (Ypresian) and Submeseta (Priabonian) biotas.- -Correlation of the Cucullaea I terrestrial fauna with Early Paleogene Patagonian faunas.- -WANT terrestrial biota and its intercontinental relationships.

First gondwanatherian mammal from Antarctica

Abstract Gondwanatherians are an enigmatic group of extinct non-therian mammals apparently restricted to some of the western Gondwanan continents (Late Cretaceous-early 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.

The affinities of Roberthoffstetteria nationalgeographica (Marsupialia) and the origin of the polydolopine molar pattern

Abstract A reanalysis of the upper and lower molar morphology of the early Paleocene marsupial Roberthoffstetteria nationalgeographica led to an interpretation of the molar homologies of the Polydolopinae (Polydolopimorphia). Several derived features support the attribution of Roberthoffstetteria to the Polydolopimorphia: thick dentary, molars with thick enamel, upper molars with reduced preparacrista that points to stylar cusp A, open centrocrista (that is, postparacrista not connected to the premetacrista), and metaconule large to very large, forming a “hypocone.” Other derived features support the monophyly of Roberthoffstetteria + polydolopines: alignment of paraconule, protocone, and metaconule in a lingual row; well-expanded anterior and posterior cingula, which are level with the trigon basin; variable occurrence of accessory cuspules at the labial face of the upper molars; and the labially placed paraconid with respect to the metaconid in the lower molars. Both Roberthoffstetteria and the polydolopines are plesiomorphic in having stylar cusp E, which is absent in most other South American marsupials. Several character complexes may be involved in the evolution of the polydolopine molar pattern.

A NEW SPECIES OF HATHLIACYNIDAE (METATHERIA, SPARASSODONTA) FROM THE MIDDLE MIOCENE OF QUEBRADA HONDA, BOLIVIA

Abstract A new species of Hathliacynidae (Sparassodonta, Metatheria), Acyon myctoderos, from the middle Miocene of Quebrada Honda, Bolivia, is described. This new species is the largest known hathliacynid. Compared to the type species of the genus, A. tricuspidatus, Acyon myctoderos differs in having: (1) longer diastemata among premolars; (2) p2 comparatively more robust, with a better developed posterior cusp and with a sharp anterior crest; (3) lower molars with a more poorly developed anterobasal cingulum; (4) m1–m3 with hypoconulids less salient posteriorly and more vertically oriented; and (5) larger hypoconids at least on the m2. A phylogenetic analysis including nine taxa of Sparassodonta, with Mayulestes as the outgroup, showed that Acyon is more closely related to Cladosictis than to any other hathliacynid.

The Terrestrial Biotic Dimension of West Antarctica (WANT)

The Late Cretaceous terrestrial biota from Antarctica come from the marine sediments of the James Ross Basin and the western flank of the Antarctic Peninsula. A compilation of data for Cretaceous and Paleogene Antarctic floras from these areas provides different perspective on floristic and vegetation change when compared with those coeval floras from southern South America.The Paleogene sequence in southern South America (Patagonia) and the Antarctic Peninsula reveals floristically distinct periods (late Paleocene, early and middle Eocene and latest Eocene), based largely on leaf assemblages. The La Meseta paleoflora is distinctive in having a predominance of Antarctic taxa especially Nothofagus, podocarps, and araucarian conifers in the Eocene deciduous and evergreen forests. This suggests a cooling trend during the Eocene of Antarctica with mid- to late Eocene seasonal, cool-temperate, rainy climates and latitudinal and altitudinal gradients. In the Late Cretaceous of James Ross Basin at least nine taxa of non-avian dinosaurs (a megalosaur-like theropod, a nodosaurid ankylosaur, a dromaeosaurid theropod, an iguanodontid, a hypsilophodontid, and a large-bodied lithostrotian titanosaur) and at least four avian dinosaurs have been reported or described from the Campanian/Maastrichtian deposits of this basin. Additional non-avian dinosaur evidence from the same area is based upon the occurrence of Maastrichtian sauropod? footprints of Snow Hill Island. In the Paleogene, of the six allomembers of the La Meseta Formation, only three, Acantilados (Ypresian), Cucullaea I (Ypresian/Lutetian) and Submeseta (Priabonian), contain endemic terrestrial mammals (metatherians and meridiungulates) and birds. Seafloor spreading between Antarctic Peninsula (West Antarctica) and South America possibly opened an intermittent seaway by the end of the Paleocene (Thanetian, 55 Ma) but subsequent tectonic and sedimentary events may have delayed the final break up between the two continents until the end of the Late Eocene (Priabonian, 34 Ma) with the opening of the Drake Passage.

West Antarctica: Tectonics and Paleogeography

West Antarctica is an ensemble of blocks that have moved independently of each other and of cratonic East Antarctica. The onset of Terra Australis orogenesis was responsible for the termination of passive margin sedimentation along the greater part of the Pacific margin of Gondwana and began a long-lived process of accretion that added much of the crust that defines eastern Australia, West Antarctica (domain 5 of Boger 2011), and western South America. By the Late Cretaceous, the Antarctic Peninsula and the remainder of West Antarctica are believed to have been comprised of a number of discrete micro-continental blocks forming a single elongated landmass that extended southward from southern South America. Since at least that time the Antarctic Peninsula has been in its present position relative to South America, at almost the same paleolatitude (South 60–65°).

A Late Cretaceous mammal from Brazil and the first radioisotopic age for the Bauru Group

In the last three decades, records of tribosphenidan mammals from India, continental Africa, Madagascar and South America have challenged the notion of a strictly Laurasian distribution of the group during the Cretaceous. Here, we describe a lower premolar from the Late Cretaceous Adamantina Formation, São Paulo State, Brazil. It differs from all known fossil mammals, except for a putative eutherian from the same geologic unity and Deccanolestes hislopi, from the Maastrichtian of India. The incompleteness of the material precludes narrowing down its taxonomic attribution further than Tribosphenida, but it is larger than most coeval mammals and shows a thin layer of parallel crystallite enamel. The new taxon helps filling two major gaps in the fossil record: the paucity of Mesozoic mammals in more northern parts of South America and of tribosphenidans in the Cretaceous of that continent. In addition, high-precision U-Pb geochronology provided a post-Turonian maximal age (≤87.8 Ma) for the type stratum, which is overlain by the dinosaur-bearing Marília Formation, constraining the age of the Adamantina Formation at the site to late Coniacian–late Maastrichtian. This represents the first radioisotopic age for the Bauru Group, a key stratigraphic unit for the study of Cretaceous tetrapods in Gondwana.

Paleobiology and Adaptations of Paleogene Metatherians

Diversity, dietary, and body mass analyses suggest that the early Eocene represents the major radiation event in South America metatherian evolutionary history. During this period, representatives of all orders typical of the Paleogene reached their greatest diversity (i.e., “basal ameridelphians”; Polydolopimorphia Polydolopiformes, and Bonapartheriiformes Bonapartherioidea); frugivory was the dominant trophic niche. By the middle late Eocene occurs a functional and taxonomic turnover. Among the Polydolopimorphia, frugivore types declined and were replaced by larger-sized frugivores/folivores (Polydolopiformes) and smaller-sized granivores (Bonapartheriiformes). The Sparassodonta showed a diversity increase and occupied the large-sized hypercarnivore niches. The Eocene–Oligocene boundary constitutes another extinction and turnover event marked by the disappearance of “basal ameridelphians”, the Polydolopiformes and Bonapartheriiformes Bonapartherioidea. Lineages that survive into the Deseadan are the Sparassodonta, Paucituberculata, Microbiotheria, and Bonapartheriiformes Argyrolagoidea. Dominant trophic types were those of carnivores and granivores. Environmental factors probably modeled the Paleogene metatherian faunal dynamics in South America. Mean annual temperatures (MAT) and precipitations seem the main factors modeling the taxonomic and trophic diversity, respectively. The adaptive radiation of the early Eocene seems associated with the maximum thermal event of the late Paleocene-early Eocene. The turnover event of the late Eocene seems associated with a sharp drop in the rainfall regime. The extinction and turnover event of the Eocene–Oligocene boundary also seem associated with a strong drop in ambient temperatures. The diversity in evolution of Paleogene metatherians shows a pattern similar to that of living marsupials at the latitudinal level. For a given mean temperature, the number of species in extinct associations is very close to that of the living ones.

South America/West Antarctica: Pacific Affinities of the Weddellian Marine/Coastal Vertebrates

Late Cretaceous/Paleogene strata of the James Ross Basin, West Antarctica (63°S), yield the only association of marine and terrestrial vertebrates and plants known from anywhere in the continent. Fossil marine vertebrates from the extensive Cretaceous (Coniacian, Campanian and Maastrichtian) marine deposits of the James Ross Basin include marine reptiles (plesiosaurs and mosasaurs), turtles, and neoselachian and teleostean fishes. The ichthyofauna recovered from levels of the early-middle Eocene (Ypresian/Lutetian) Cucullaea I Allomember include 35 species referable to 26 families, thus making it one of the most abundant and diverse fossil marine vertebrate faunas yet recorded in southern latitudes. The diversity of the Seymour Island penguin fauna of the late Eocene (Priabonian) Submeseta Allomember indicates that this group was significantly more abundant and diverse in the Paleogene than it is today. The first significant radiation of the Weddellian penguins took place by early Eocene (Ypresian) times (basal Cucullaea I Allomember), with eight sympatric species.

Late Cretaceous/Paleogene Stratigraphy in the James Ross Basin

Late Cretaceous sedimentary rocks are only exposed around the northern part of West Antarctica, on the South Shetland Islands and James Ross Island Group, Weddell Sea. They were deposited in very different tectonic settings and environments. The South Shetland Island sequence represents a Cenozoic outer-arc (Birkenmajer 1995) or fore-arc (Elliot 1988) succession composed mainly of terrestrial volcanic and sedimentary deposits. It should also be noted that some very thin and intermittent sequences of terrestrial sedimentary rocks interbedded within extensive volcanic units on King George Island, South Shetland Islands may be of Maastrichtian age. The Paleogene back-arc deposits comprise more than 1000 m of shallow marine to coastal fossiliferous clastic sedimentary rocks mainly of Paleocene and Eocene ages. They are exposed on Seymour and Cockburn islands approximately 100 km SE of the northern tip of the Antarctic Peninsula representing the uppermost part of the James Ross Basin succession.