Neotropical metatherian diversity around the Eocene-Oligocene Transition: the Shapaja section, Peruvian Amazonia
Résumé
Metatheria (Mammalia) were remarkable faunal components of South American Cenozoic history. Now represented solely
by the marsupials (e.g., opossums, kangaroos), metatherians evolved multiple adaptive types with greater past diversity and
abundance than today. One important chapter of this history is the Eocene–Oligocene transition, a period of considerable
changes worldwide, with marked extinctions, diversity changes, and drastic climatic processes (i.e., transition from the early
Cenozoic “Greenhouse World” to the post-Eocene “Icehouse World”). For Metatheria, the Eocene-Oligocene transition is
considered the major turnover point in their evolutionary history in South America, an assumption mainly based on the
fossil record from Argentinian Patagonia. However, the Eocene–Oligocene transition is scarcely known at tropical latitudes
of South America, like other time intervals, since this region is still poorly understood from a paleontological and geological
standpoint. This study aims at partly filling this knowledge gap, by reporting preliminary identifications of metatherians from
the late Eocene–early Oligocene Shapaja section, near Tarapoto, Peruvian Amazonia. Nine fossiliferous localities of the
Pozo Formation were dated by chemostratigraphy and explored through wet screening (1–2 mm meshes), which allowed
recovering small-sized fossils (plants, mollusks, decapods, fishes, amphibians, reptiles, and many mammals). Metatherians
were found in most localities and, according to their taxonomic composition, three assemblages could be recognized
throughout the section, a pattern also observed in rodents and fishes. The oldest assemblage (TAR-74, early late Eocene)
is composed of only one new prepidolopid polydolopimorphian. The second assemblage (TAR-20, 72, 21), latest Eocene,
encompasses scarce and fragmentary remains of another probable prepidolopid, numerous remains of a small gerbil-like
argyrolagid, and teeth of palaeothentoids, an extinct clade of paucituberculatans, with two basal forms plus two
palaeothentines. These records represent the oldest occurrences of Palaeothentinae and Argyrolagidae, and the
northernmost record of the latter clade. The earliest Oligocene localities yielded none (TAR-22) or few fragmented
metatherian remains (TAR-13), possibly due to taphonomic biases, being considered transitional. Finally, the third
assemblage (TAR-01, early Oligocene) includes one larger, rarer argyrolagid and several palaeothentoids (three basal taxa,
three palaeothentids, and one abderitid). Thus, the Shapaja section does not attest to a smaller diversity around the
Eocene–Oligocene transition. However, the changes in the taxonomic composition of the assemblages, along with other
geological and paleontological data, point to two biotic turnovers. The first one, during the late Eocene, has probably been
driven by regional processes related to the Andean orogeny, which led to an episodic marine incursion in this area. The
later biotic turnover, by contrast, seems to be related to the Eocene–Oligocene transition global processes, namely the
great drop in sea level and the onset of drier and cooler climates worldwide, with decreased precipitation and increased
seasonality. Indeed, fossil plants from Shapaja indicate the occurrence of multi-stratified rainforests during the latest
Eocene and more open, deciduous forests in the earliest Oligocene. Finally, the Shapaja section highlights the importance
of fieldwork and research efforts in northern South America, to get better correlations with middle and high latitudes
localities, and thus a refined paleobiodiversity picture in the whole continent