Neotropical metatherian diversity around the Eocene-Oligocene Transition: the Shapaja section, Peruvian Amazonia

Abstract

International audienceMetatheria (Mammalia) were remarkable faunal components of South American Cenozoic history. Now represented solelyby the marsupials (e.g., opossums, kangaroos), metatherians evolved multiple adaptive types with greater past diversity andabundance than today. One important chapter of this history is the Eocene–Oligocene transition, a period of considerablechanges worldwide, with marked extinctions, diversity changes, and drastic climatic processes (i.e., transition from the earlyCenozoic “Greenhouse World” to the post-Eocene “Icehouse World”). For Metatheria, the Eocene-Oligocene transition isconsidered the major turnover point in their evolutionary history in South America, an assumption mainly based on thefossil record from Argentinian Patagonia. However, the Eocene–Oligocene transition is scarcely known at tropical latitudesof South America, like other time intervals, since this region is still poorly understood from a paleontological and geologicalstandpoint. This study aims at partly filling this knowledge gap, by reporting preliminary identifications of metatherians fromthe late Eocene–early Oligocene Shapaja section, near Tarapoto, Peruvian Amazonia. Nine fossiliferous localities of thePozo Formation were dated by chemostratigraphy and explored through wet screening (1–2 mm meshes), which allowedrecovering small-sized fossils (plants, mollusks, decapods, fishes, amphibians, reptiles, and many mammals). Metatherianswere found in most localities and, according to their taxonomic composition, three assemblages could be recognizedthroughout 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-likeargyrolagid, and teeth of palaeothentoids, an extinct clade of paucituberculatans, with two basal forms plus twopalaeothentines. These records represent the oldest occurrences of Palaeothentinae and Argyrolagidae, and thenorthernmost record of the latter clade. The earliest Oligocene localities yielded none (TAR-22) or few fragmentedmetatherian remains (TAR-13), possibly due to taphonomic biases, being considered transitional. Finally, the thirdassemblage (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 theEocene–Oligocene transition. However, the changes in the taxonomic composition of the assemblages, along with othergeological and paleontological data, point to two biotic turnovers. The first one, during the late Eocene, has probably beendriven by regional processes related to the Andean orogeny, which led to an episodic marine incursion in this area. Thelater biotic turnover, by contrast, seems to be related to the Eocene–Oligocene transition global processes, namely thegreat drop in sea level and the onset of drier and cooler climates worldwide, with decreased precipitation and increasedseasonality. Indeed, fossil plants from Shapaja indicate the occurrence of multi-stratified rainforests during the latestEocene and more open, deciduous forests in the earliest Oligocene. Finally, the Shapaja section highlights the importanceof fieldwork and research efforts in northern South America, to get better correlations with middle and high latitudeslocalities, and thus a refined paleobiodiversity picture in the whole continen