South American Land Mammal Ages

The South American mammalian record is relatively incomplete, with discontinuities between all the Paleogene South American Land-Mammal Ages (SALMAs). Nevertheless, a seemingly stable sequence of Cenozoic SALMAs of presumed age has been in use for decades. In the Paleogene, the following sequence has long been recognized: Riochican (late Paleocene), Casamayoran (early Eocene), Mustersan (middle Eocene), Divisaderan (middle or late Eocene), and Deseadan (early Oligocene; Simpson, 1948; Patterson and Pascual, 1968). Relative ages were assigned mainly by strati-graphic position and stage of evolution, as the faunas are entirely endemic. Over the last 30 years or so, however, magnetostratigraphic studies coupled with radioisotopic dates, together with new fossil discoveries, have forced significant revisions in the SALMAs, with particular impact on those of the Paleogene.

Three additional Paleocene land-mammal ages or sub-ages are now recognized that precede the classic late Paleo-cene Riochican: Itaboraian, Peligran, and Tiupampan (see Fig. 1.7). The Tiupampan fauna was initially thought to come from the El Molina Formation of Late Cretaceous age, but it is now known to come from the overlying Santa LucĂ­a Formation of Paleocene age (Marshall et al., 1995). The Itabo-raian is presumed to be earlier late Paleocene (but it derives from fissures, which are difficult to date accurately), whereas the Tiupampan and Peligran are considered successive early Paleocene land-mammal ages (e.g., Flynn and Swisher, 1995). The Riochican appears to correlate with late Paleo-cene marine strata, but radiometric dates indicate only that it is younger than 63 million years. It may correlate approximately with magnetochron C25n. Low-precision radio-metric dates confirm Paleocene age for the three underlying ages as well, but their durations and precise placement within the Paleocene are speculative. Recently, for example, Marshall et al. (1997), using magnetostratigraphy, recalibrated the Paleocene SALMAs and considered all four to be of late Paleocene age, about 55.5-60 million years ago. Furthermore, they concluded that the actual sequence is Peligran-Tiupampan-Itaboraian-Riochican. Because the original Riochican section spanned the entire late Paleocene, they considered all four to be subages of a single late Paleo-cene Riochican Land-Mammal Age. Most researchers, however, have accepted an early Paleocene age for the Tiupampan and consider it to be the oldest Cenozoic SALMA. This consensus is followed in this volume.

The Peligran Land-Mammal Age is especially problematic. Thought to correlate approximately with the Torrejonian NALMA, it is founded on a new Argentine "fauna" consisting of a few very fragmentary specimens of five mammalian species, together with frogs, turtles, and crocodilians

(Bonaparte et al., 1993). The mammal species include the gondwanathere Sudamerica (an enigmatic group whose affinities are very uncertain), the only non-Australian monotreme, and three supposed condylarths, one of which could instead be a dryolestoid. In some respects this assemblage has more of a Mesozoic than Paleocene aspect. Whether this enigmatic fauna proves to be older or younger than Tiupampan, the available fossils are an inadequate basis for establishing a land-mammal age.

Recently there has been even greater change in the concepts of the Eocene SALMAs. New 40Ar/39Ar dates on rocks from the later part of the Casamayoran SALMA (Bar-rancan subage), conventionally considered early Eocene, yielded the surprising result that they could be as young as late Eocene (35.3-37.6 Ma), almost 20 million years younger than previously thought (Kay et al., 1999). This finding would indicate that the Casamayoran extended much later in time than previously thought and that the Mustersan SALMA is latest Eocene. It also raises the possibility that Riochican could be Eocene, and that there might be an even longer gap in the South American Eocene record than has been acknowledged. But the early Casamayoran fauna (Vacan subage; Cifelli, 1985) is more similar to the Riochican fauna, suggesting that the hiatus is more likely between the Vacan and the Barrancan subages. Flynn et al. (2003) reinterpreted the Casamayoran radioisotopic evidence to indicate a minimum age of 38 million years, and indicated that the lower boundary could be anywhere down to 54 million years ago (see Fig. 1.7), which would equate Casamayoran with most of the early and middle Eocene. This calculation of its duration may be too long, but age constraints are so poor that a more precise estimate is not yet possible.

The revised age estimates for the Casamayoran compress the Mustersan and Divisaderan into a short interval at the end of the Eocene. The relative age and even the validity of the Divisaderan are especially tenuous. Finally, high-precision 40Ar/39Ar dates for the recently proposed Tin-guirirican SALMA indicate that it either bridges the Eocene/ Oligocene boundary (Flynn and Swisher, 1995) or is of early Oligocene age (Kay et al., 1999). The younger age was upheld by Flynn et al. (2003), who dated the Tinguirirican at 31-32 million years ago but indicated that it might extend back as far as 37.5 million years ago (latest Eocene). Radio-metric dates also show that the Deseadan is much younger than long believed, shifting it to late Oligocene (Flynn and Swisher, 1995). Figure 1.7 follows Flynn et al. (2003) for the Eocene SALMAs.

Note, however, that most of these revisions are so recent that they were not known at the time of McKenna and Bell's (1997) compilation, and obviously were unknown to Simpson and other earlier workers. Therefore occurrences and ranges of South American taxa in this text reflect the traditional terminology, namely, that Casamayoran was equivalent to early Eocene, Mustersan and Divisaderan to middle Eocene, and Tinguirirican to the Eocene/Oligocene boundary. Wherever possible, the age of fossils is clarified with the SALMA of origin to avoid confusion.

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