Laura Chornogubsky

Persistence of a Mesozoic, non-therian mammalian lineage (Gondwanatheria) in the mid-Paleogene of Patagonia

We describe two isolated molariforms recovered from early–middle Eocene (early Lutetian) levels of northwestern Patagonia, Argentina. Comparisons with major lineages of therian and non-therian mammals lead us to refer them to a new genus and species of Gondwanatheria (Allotheria). There is a single root supporting each tooth that is very short, wide, rounded, and covered by cementum; the steep sidewalls, lack of a neck between the crown and root, and the heavily worn stage in both molariforms suggest that they were of a protohypsodont type. Both teeth are strongly worn at their centers, all along their length, with the labial edge less worn than the lingual; they show strong transverse crests that alternate with lingual grooves. The protohypsodont aspect of the teeth, as well as the strong, transverse crests, are suggestive of sudamericid affinities; on the other hand, the thin enamel layer and the occlusal pattern formed by the crests and grooves shows more similarities to molariform teeth of the Ferugliotheriidae. The new taxon adds evidence regarding the (1) extensive radiation of the Gondwanatheria throughout the Southern Hemisphere, (2) persistence of several lineages well after the Cretaceous/Paleogene boundary, and (3) early evolution of hypsodont types among South American herbivorous mammals.

A Brief History of South American Metatherians

With more than 100 species, living South American marsupials (Mammalia, Metatheria) give only a glimpse of the much higher taxonomic and ecological diversity acquired by metatherians throughout the Cenozoic Era. The term Metatheria designs a taxon within Mammalia that includes not only Marsupialia but also all therian mammals more related to Marsupialia than to Eutheria. Several features (e.g., epipubic ones) formerly considered as diagnostic of Metatheria are now regarded either a primitive condition or not present in all members of this group. Other derived features, such as the presence of a shelf-like, inflected angular process in the lower jaw, are consistently present in all metatherians. A brief characterization of all major South American, Cenozoic metatherian lineages is given: “basal ameridelphians,” Sparassodonta, Didelphimorphia, Paucituberculata, Microbiotheria, and Polydolopimorphia (the latter including Polydolopiformes and Bonapartheriiformes). Three periods can be distinguished in the history of our knowledge of Cenezoic South American Metatheria: the first one (1878–1930) is intimately linked to Florentino Ameghino, Argentina’s most notable paleontologist; much of our knowledge on extinct metatherians from South America was elaborated by him. The second period (1930–1977) occurred under the influence of George Gaylord Simpson’s ideas. Bryan Patterson and Rosendo Pascual also had an important imprint in South America’s Mammalian Paleontology. The third period (1977-present) is currently evolving under new phylogenetic, taxonomic, and paleobiogeographic paradigms; influences are multiple and major reviews of specific lineages are currently in the making. A final note on the incompleteness of Cenozoic South America’s fossil record is made: only the mid to high latitudes, basically in the Southern Cone, are moderately well-sampled in their terrestrial fossil record.

A reassessment of Antarctic polydolopid marsupials (Middle Eocene, La Meseta Formation)

Abstract New polydolopid marsupial specimens have been recovered from the La Meseta Formation, a late early Eocene to probably early Oligocene unit cropping out in the northern third of Seymour (Marambio) Island, at some 100 km off the northern Antarctic Peninsula. Our review of the original materials, as well as the new specimens from the same levels, led us to: 1) revalidate the genus Antarctodolops Woodburne & Zinsmeister 1984, 2) regard Eurydolops seymouriensis Case, Woodburne & Chaney 1988 as a junior synonym of Antarctodolops dailyi Woodburne & Zinsmeister, and 3) recognize a new species of this same genus: A. mesetaense. As previously stated, the polydolopid radiation might be related to the expansion of the Nothofagus flora, as both have the same spatial distribution in southern South America and West Antarctica.

A review of the molar morphology and phylogenetic affinities of Sillustania quechuense (Metatheria, Polydolopimorphia, Sillustaniidae), from the early Paleogene of Laguna Umayo, southeastern Peru

SUPPLEMENTAL DATA Supplemental materials are available for this article for free at www.tandfonline.com/UJVP Citation for this article: Chornogubsky, L., and F. J. Goin. 2015. A review of the molar morphology and phylogenetic affinities of Sillustania quechuense (Metatheria, Polydolopimorphia, Sillustaniidae), from the early Paleogene of Laguna Umayo, southeastern Peru. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2015.983238.

Dispersal of Vertebrates from Between the Americas, Antarctica, and Australia in the Late Cretaceous and Early Cenozoic

The early Paleocene diversity of metatherians in Tiupampan faunas of South America and the pre-Tiupampan Paleocene polydolopimorphian Cocatherium speak in favor of an earliest Paleocene or Late Cretaceous dispersal of metatherians from North America. No Late Cretaceous metatherian or eutherian mammals have been recovered to date in South America, but the late Campanian to Maastrichtian hadrosaurine dinosaurs in Argentina, as well as the late Maastrichtian of the Antarctic Peninsula, is evidence of a biotic connection to North America. Placental ‘condylarths’ in the Tiupampan may have been related to, and dispersed southward relative to, Puercan taxa in North America and perhaps reflect a somewhat later event in comparison to metatherians. Other than hadrosaurine dinosaurs, Late Cretaceous vertebrates of South America are basically Gondwanan in affinities and reflect (and survived) the pre-106 Ma connection between South America, Africa, and Antarctica. The potential for a Late Cretaceous dispersal of metatherians would be compatible with a continued dispersal to Australia at that time, also supported by plate tectonic relationships, notwithstanding the basically endemic coeval Australian dinosaur fauna, and recognizing the essential absence of a Late Maastrichtian land vertebrate record there. An early Paleocene connection between at least Antarctica and South America is documented by the presence of a monotreme in the Peligran fauna of Patagonia. This, coupled with the fact that post-Peligran mammal faunas in South America and the Antarctic Peninsula (from at least 52 Ma in that location) are composed of derived metatherian as well as placental mammals, suggests that dispersal of metatherians to Australia had been achieved prior to the Eocene. Such timing is compatible with the still plesiomorphic level of Australian metatherians from the early Eocene Tingamarra fauna of Australia, the marine sundering of the Tasman Gate at about 50 Ma and the development of a continuously marine southern coastline of Australia from about 45 Ma effectively foreclosed overland mammal and other vertebrate dispersal to Australia thereafter.

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.

New Palaeogene metatherians from the Quebrada de Los Colorados Formation at Los Cardones National Park (Salta Province, Argentina)

The Quebrada de Los Colorados Formation (Los Cardones National Park, Salta Province, north-western Argentina), is an Eocene–Oligocene unit well represented in the Calchaquí Valley area. Here we describe a new metatherian association recorded from the base of this formation, inferred as middle Eocene. Represented taxa are: Sparassodonta, family indet.: Patene simpsoni; Polydolopimorphia, Bonapartheriiformes, Argyrolagoidea: family, genus, and species indet.; and Bonapartherioidea, Prepidolopidae: Punadolops alonsoi and Coloradolops cardonensis gen. et sp. nov. Patene simpsoni is also found in the Tonco Valley, near Los Cardones, and at São José de Itaboraí, Rio de Janeiro, Brazil. Punadolops alonsoi was originally described from the Geste Formation in Salta and Catamarca provinces. Hitherto, no other Argyrolagoidea have been found in north-western Argentina in the Palaeogene. Coloradolops cardonensis is unique to the locality and levels here described. The preservation of part of the skull constitutes the best prepidolopid remains known. North-western Argentina is a key region for the study of the evolution of South American mammals, since it has a singular geographical location between the Neotropics and the southernmost part of South America. Due to biogeographical constraints, it may lead to a better comprehension of the continental distribution of metatherian lineages during Cenozoic times. http://zoobank.org/urn:lsid:zoobank.org:pub:7B4333A4-CB78-4F52-B479-571748A46488

Summary: Milestones in the Evolution of South American Metatherians

We summarize the configuration of plates, geographical barriers, and possible dispersal events during the Late Cretaceous–Cenozoic between North and South America, the Caribbean, Antarctica, and Australia. The arrival of metatherians in South America was a Late Cretaceous event, and probably a Maastrichtian one. There are few doubts that the first metatherians in this continent arrived from North America. We suggest that not only eutherian mammals but also metatherians may have reached South America from the north in a series of successive dispersal waifs. This FABI (First American Biotic Interchange) may have replicated the successive waif dispersal mood of the late Cenozoic GABI (Great American Biotic Interchange). The initial radiation of basal South American metatherian lineages (“Ameridelphia”) may have already occurred by Late Campanian-Maastrichtian times. We also suggest that a cooling pulse happening by the Latest Cretaceous (Late Maastrichtian, ca. 68–67 Ma) may have been involved in the origin of the Australidelphia, as part of the southern (Austral Kingdom) Nothofagus biota. Four out of six faunal phases were involved in the evolution of South American metatherians: (1) Early South American (Late Cretaceous to the Late Eocene), Late South American (Early Oligocene to late Miocene), Interamerican (Plio–Pleistocene), and Hypoamerican (Holocene). The first of these phases involved the arrival and expansion of many lineages and adaptive types. The global cooling by the Eocene–Oligocene Boundary implied the extinction of many (mostly tropical) lineages, as well as the diversification of several specialized ones. The third of these faunal phases transpired during a time lapse of ecological imbalance and global cooling, while the last phase saw already much impoverished metatherian associations throughout the continent.

Phylogeny and Diversity of South American Metatherians

The Metatheria include not only marsupials but all therians more related to Marsupialia than to the Eutheria. Marsupialia is considered as a metatherian crown group including all extant marsupials, their common ancestor and all of their descendants. “Ameridelphia” is not a natural group. Australidelphia includes the Microbiotheria and all Australasian marsupials. Several authors also argue that the Polydolopimorphia are Australidelphians as well. Relationships of Sparassodonta with other Metatheria are a matter of discussion. To several authors, they are more closely related to South American and Australian groups than to basal North American and/or Asian metatherians. Our concept of Didelphimorphia includes the Peradectoidea (Peradectidae and Caroloameghiniidae) and the Didelphoidea (Didelphidae and Sparassocynidae). In several analyses, the Paucituberculata appear as more closely related to the Australidelphia than to the Didelphimorphia. The relationships of the Microbiotheria within the Australidelphia have been subject of much discussion. They have been considered either as sister-taxa of all other Australidelphia, at the base of Diprotodontia, as a sister-taxon of Dasyuromorpha, as a sister-taxon of Phalangeriformes + Diprotodontia, or even related with part of the former.

South American Living Metatherians: Physiological Ecology and Constraints

South American living metatherians are relatively inconspicuous and comprise ca. 10 % of the region’s mammal species richness. Most of them are small-sized (<150 g), with long tails and grasping hands and feet, and resemble one another in their general appearance. Individuals are solitary, nocturnal or crepuscular, and most of them are arboreal or scansorial. Two orders are exclusive of South America (Microbiotheria and Paucituberculata), while a third (Didelphimorphia) ranges from Patagonia (Argentina) to the border between USA and Canada. The ecology, physiology, and reproductive traits of living South American marsupials have been poorly studied. This chapter describes several aspects of their natural history and how they influence their recent distribution, as they probably did so throughout the Cenozoic. Physiological constraints include variable energetic costs for regulating metabolic processes, due to low rates of metabolism, the possibility to enter torpor/hibernation, and the storage of different types of fat tissues available for those varied processes. Reproduction, which can be characterized by a short gestation period and long, energy-demanding, breeding period, shows specific differences in strategies (e.g., semelparity, partial semelparity, iteroparity) and their main traits (e.g., litters per year and litter size, teat number). South American marsupials make a complete use of the habitat available to them and have a broad, generally opportunistic and omnivorous diet. These adaptations, combined with a general small size, small energy expenditure on foraging and other daily activities, limit their distribution. Despite these constraints, South American marsupials seem to thrive in environments where competition with other animals might be strong (e.g., tropical and subtropical climates), or where a few small mammals can survive (e.g., temperate and temperate-cold climates) due to several environmental limitations.