Judd A. Case

Dispersal, Vicariance, and the Late Cretaceous to Early Tertiary Land Mammal Biogeography from South America to Australia

A review of paleontological, phyletic, geophysical, and climatic evidence leads to a new scenario of land mammal dispersal among South America, Antarctica, and Australia in the Late Cretaceous to early Tertiary epochs. New fossil land vertebrate material has been recovered from all three continents in recent years. As regards Gondwana, the present evidence suggests that monotreme mammals and ratite birds are of Mesozoic origin, based on both geochronological and phyletic grounds. The occurrence of monotremes in the early Paleocene (ca. 62 Ma) faunas of Patagonia and of ratites in late Eocene (ca. 41-37 m.y.) faunas of Seymour Island (Antarctic Peninsula) probably is an artifact of a much older and widespread Gondwana distribution prior to the Late Cretaceous Epoch. Except for South American microbiotheres being australidelphians, marsupial faunas of South America and Australia still are fundamentally disjunct. New material from Seymour Island (Microbiotheriidae) indicates the presence there of a derived taxon that resides in a group that is the sister taxon of most Australian marsupials. There is no compelling evidence that dispersal between Antarctica and Australia was as recent as ca. 41 Ma or later. In fact, the derived marsupial and placental land mammal fauna of Seymour Island shows its greatest affinity with Patagonian forms of Casamayoran age (ca. 51–54 m.y.). This suggests an earlier dispersal of more plesiomorphic marsupials from Patagonia to Australia via Antarctica, and vicariant disjunction subsequently. This is consistent with geophysical evidence that the South Tasman Rise was submerged by 64 Ma and with geological evidence that a shallow water marine barrier was present from then onward. The scenario above is consistent with molecular evidence suggesting that australidelphian bandicoots, dasyurids, and diprotodontians were distinct and present in Australia at least as early as the 63-Ma-old australidelphian microbiotheres and the ancient but not basal australidelphian,Andinodelphys, in the Tiupampa Fauna of Bolivia. Land mammal dispersal to Australia typically has been considered to be at a low level of probability (e.g., by sweepstakes dispersal). This study suggests that the marsupial colonizers of Australia included already recognizable members of the Peramelina, Dasyuromorphia, and Diprotodontia, at least, and entered via a filter route rather than by a sweepstakes dispersal.

Land mammal biostratigraphy and magnetostratigraphy of the Etadunna Formation (late Oligocene) of South Australia

ABSTRACT Field work recently completed in the Lake Eyre Basin, South Australia, has resulted in the development of a land mammal (marsupial) biostratigraphy of the Etadunna Formation. Whereas traditional interpretations of the age of this sequence suggest it is about 15 m.y. old, new information indicates that the Etadunna likely is 24–26 m.y. old. In either case, it appears possible to document a four-fold fossil mammal zonation of this rock unit at lakes Palankarinna, Kanunka, Pitikanta, and Ngapakaldi, in a composite section of strata that spans at least 30 m. Magnetostratigraphic data for the same succession are generally consistent with the correlation of the Etadunna Formation sites at Lake Palankarinna with of those at lakes Kanunka, Pitikanta, and Ngapakaldi to the north, as based on paleontological information. The magnetic polarity zonation of these Etadunna Formation strata is consistent with a correlation to the world magnetic polarity time scale at about 24–26 m.y. This is the first fine-scal...

“South American” Marsupials from the Late Cretaceous of North America and the Origin of Marsupial Cohorts

Newly described marsupial specimens of Judithian (late Campanian) and Lancian (Maastrichtian) age in the western interior of North America (Wyoming to Alberta) have dental morphologies consistent with those expected in comparably aged sediments in South America (yet to be found). Three new Lancian species are referable to the didelphimorphian Herpetotheriidae, which suggests that the ameridelphian radiation was well under way by this time. The presence of a polydolopimorphian from Lancian deposits with a relatively plesiomorphic dental morphology and an additional polydolopimorphian taxon from Judithian deposits with a more derived molar form indicate that this lineage of typically South American marsupials was diversifying in the Late Cretaceous of North America. This study indicates that typical South American lineages (e.g. didelphimorphians and polydolopimorphians) are not the result of North American peradectian progenitors dispersing into South America at the end of the Cretaceous (Lancian), or at the beginning of the Paleocene (Puercan), and giving rise to the ameridelphian marsupials. Instead, these lineages, and predictably others as well, had their origins in North America (probably in more southerly latitudes) and then dispersed into South America by the end of the Cretaceous. Geophysical evidence concerning the connections between North and South America in the Late Cretaceous is summarized as to the potential for overland mammalian dispersal between these places at those times. Paleoclimatic reconstructions are considered, as is the dispersal history of hadrosaurine dinosaurs and boid snakes, as to their contribution to an appraisal of mammalian dispersals in the Late Cretaceous. In addition, we present a revision of the South American component of the Marsupialia. One major outcome of this process is that the Polydolopimorphia is placed as Supercohort Marsupialia incertae sedis because no characteristics currently known from this clade securely place it within one of the three named marsupial cohorts.

The first duck-billed dinosaur (Family Hadrosauridae) from Antarctica

Department of Earth Sciences, University of California, Riverside, California 92521Duck-billed dinosaurs or hadrosaurs are a very common family ofdinosaurs in the Late Cretaceous of North America and Eurasia withrare occurrences in South America. Here, we report the first hadrosaurrecovered in Antarctica from sandstones of late Maastrichtian age, VegaIsland, Antarctic Peninsula (Fig. 1) during a joint U.S.-Argentinian geo-logical and paleontological field expedition to the island. This discoverysupports the hypothesis of a dispersal route between southern SouthAmerica and Antarctica in the Maastrichtian.Dinosaurs are extremely rare in the dominantly marine deposits ofLate Cretaceous age in Antarctica, so the discovery of a single hadro-saur tooth (Fig. 2) by the third author (DSC) was unexpected. Hadro-saurs are not the first dinosaurs from Late Cretaceous deposits in theJames Ross Basin. Theropod, ornithopod, and ankylosaur specimenshad been previously recorded (Gasparini et al., 1987; Hooker et al.,1991; Molnar et al., 1996). However, these dinosaur groups representeither cosmopolitan taxa or native Gondwanan taxa, thus the duck-billeddinosaur represents a new immigrant family of dinosaurs into this south-ernmost continent.The fossil-bearing deposit occurs at 61 51 S and 53 33 W, at Sand-wich Bluff on Cape Lamb, Vega Island, Antarctic Peninsula. The ‘‘Rep-tile Horizon,’’ named for the numerous mosasaur and plesiosaur spec-imens recovered from this stratigraphic level, is in the upper third ofthe Sandwich Bluff Member (Crame et al., 1991; Pirrie et al., 1991) orUnit C (Olivero, 1992) of the Lopez de Bertodano Formation. Thismember is a nearshore marine deposit of composed of fine-grained,ferruginous, loosely consolidated sandstones that are latest Maastrich-tian age (approximately 66–68 million years ago), based on correlationsof ammonite and palynological taxa (Crame et al., 1991; Pirrie et al.,1991).In addition to the hadrosaur, at least four different bird species (whichcan be referred to modern avian orders, including charadriiform andgaviid birds, based on the morphology of their respective tarsometatar-sals), have been recovered from this same stratigraphic level. Addition-ally, numerous specimens of plesiosaurs and mosasaurs were collectedfrom this same horizon, as well as the 5-meter horizon immediatelybelow (Martin et al., 1999a, b). At least five taxa of marine reptiles arerepresented on Vega Island, and numerous specimens of juvenile indi-viduals were collected, concentrations of which are relatively rare inthe North American marine reptile record.SYSTEMATIC PALEONTOLOGYREPTILIAA

A new European marsupial indicates a Late Cretaceous high-latitude transatlantic dispersal route

The first record of an undoubted opossum-like marsupial from the Mesozoic of Europe indicates an invasion from North America at the end of Late Cretaceous (Maastrichtian). The new 66.1 million-year-old marsupial, Maastrichtidelphys meurismeti n. gen., n. sp., represented by a right upper molar, comes from the type Maastrichtian of The Netherlands. The Maastricht marsupial exhibits affinities with earlier (early Maastrichtian) North American herpetotheriids providing definitive evidence of a high-latitude North Atlantic dispersal route between North America and Europe during the latest Cretaceous. Previously, the first major interchange for marsupials was thought to have occurred nearly 10 million years later in the Eocene. The occurrence of this new marsupial in Europe implies that at some time during the latest Cretaceous, sea level and climatic conditions must have been sufficiently favorable to allow for such a high-latitude dispersal. The fragmentary remains of hadrosaurid and theropod dinosaurs, as well as boid snakes from northwestern Europe which have affinities with North American taxa help substantiate assumptions made by the occurrence of the herpetotheriid marsupial in Maastricht.

New Discoveries of “Opposum-Like” Marsupials from Antarctica (Seymour Island, Medial Eocene)

Five new species of marsupials are described from the middle Eocene La Meseta Formation of Seymour Island, Antarctic Peninsula. Three are derorhynchid didelphimorphians; one species is a prepidolopid polydolopimorphian, and the last is a microbiotheriid australidelphian. Additionally, fragmentary specimens representing an indetermined derorhynchid and a possible marsupial are also described. The prepidolopid and one of the derorhynchids are sufficiently derived as to preclude any close relationship to other members of that family, but the remaining taxa show the closest affinity with species otherwise known only from Itaboraian and older faunas in Patagonia. This differs from the affinity to early Eocene (Casamayoran) taxa shown by the polydolopid marsupials and placental mammals previously known from the La Meseta Formation. The newly described marsupials indicate that the relict La Meseta Fauna is composed of forms that must have dispersed to Antarctica no later than about early late Paleocene, whereas the previously known taxa apparently arrived in the early Eocene. Ecologically, the La Meseta Fauna is composed mostly of small-sized marsupials of likely insectivorous to frugivorous habits and larger-sized placental herbivores. Whereas the ratite bird of the La Meseta Fauna was probably also herbivorous, the phorusrhachoid and falconid birds comprised a large and smaller carnivorous to possibly scavenging component, respectively. Compared to contemporary faunas of Patagonia, the medium- to large-sized marsupial carnivores are lacking in the Antarctic Peninsula. Nevertheless, the La Meseta Fauna is Patagonian in origin and affinity. In conjunction with new faunas of Itaboraian age (early late Paleocene) in Patagonia, the evidence available indicates that from at least Itaboraian time onward the land mammal fauna of Patagonia and northern South America, as well, is a self-contained unit, developing the diversity characteristic of the Paleogene in that continent, including the australidelphian (but South American) microbiotheres. This, in combination with the apparent separation of Australia from Antarctica at ca. 64 Ma, reinforces interpretations that the precursors of the Australian marsupial fauna most likely dispersed from South America to Australia in the late Cretaceous–early Paleocene.

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.

Antarctica: the effect of high latitude heterochroneity on the origin of the Australian marsupials

Abstract The record of the Antarctic marine fauna during the Paleogene indicates the occurrence of taxa in high latitude regions (>60°) before these same taxa are known in midto low latitudes. It has been hypothesized that high latitude regions serve both as ‘holding tanks’ for taxa and as regions in which novel adaptations leading to new lineages within a taxon can arise. These features of high latitude regions appear to have had a major impact on the origin of the Australian marsupial fauna. The australidelphian marsupial clade (all Australian marsupials plus South American microbiotheriids) and the ameridelphian marsupial clade (all South American marsupials except microbiotheriids) diverged in the Late Cretaceous. The South American marsupial radiation occurred in mid- to low latitude regions during the Late Cretaceous to early Paleocene. The Australian marsupial radiation probably did not occur until the medial or late Eocene. If accurate, this represents a time differential of 20 to 25 Ma between the two marsupial radiations. The heterochroneity of the marsupial radiations results from the fact that during the Late Cretaceous to Eocene, Australia resided in a high latitude region, the cool temperate Weddellian Biogeographical Province. The radiation of Australian marsupials only reaches a level of taxonomic diversity comparable to that in South America after the continent drifted northward into lower latitudes and habitat diversity increased. This is deduced from changes in floral diversity and the projected timing of the diprotodontian divergences based on DNA hybridization data.

A leatherback sea turtle from the Eocene of Antarctica: implications for antiquity of gigantothermy in Dermochelyidae

(2003). A leatherback sea turtle from the Eocene of Antarctica: implications for antiquity of gigantothermy in Dermochelyidae. Journal of Vertebrate Paleontology: Vol. 23, No. 4, pp. 945-949.

The Oldest Genus of Scincid Lizard (Squamata) from the Tertiary Etadunna Formation of South Australia

Abstract Recent expeditions to the Lake Palankarinna area of South Australia resulted in the oldest and only known extinct skink genus in Australia. The holotype of a new genus and species, Proegernia palankarinnensis, was collected from the basal portion of the Late Oligocene Etadunna Formation from the Minkina Local Fauna. Additional scincid fossils previously recovered from higher levels in the formation include material that may be referable to Proegernia. Proegernia is placed in the Egernia group within the Australian Lygosominae, based mainly on its closed Meckelian groove with the apex of the splenial notch low on the lingual surface. However, the apex is markedly more anterior in Proegernia than in any living Egernia group taxon. Proegernia possesses characters suggestive of its position as a stem taxon for later occurring skinks of the Egernia species groups. Even so, overall stage of evolution suggests that the Scincidae of Australia had a long evolutionary history prior to the Late Oligocene, a contention supported by previous molecular studies.