North American Land Mammal Ages

The sequence of NALMAs initially proposed by Wood et al. (1941) has been widely applied and provides a useful and well-documented biochronology for mammals of North America. Excellent summaries of the NALMAs and their mammal assemblages are found in the two volumes edited by Woodburne (1987, 2004). The NALMAs of interest in

Fig. 1.7. (opposite) Early Cenozoic mammalian geochronology and biochronology Chart shows the time period emphasized in this book (Paleocene-Eocene), approximate age in millions of years (Ma), and correlation with the geomagnetic polarity time scale (GPTS), standard stage/age (commonly used in Europe), and land-mammal ages in North America (NALMA), Asia (ALMA), and South America (SALMA). White bands in GPTS column are intervals of reversed polarity (r), which precede the normal (n, black) interval of the same number. Hatching and dashed lines in ALMA and SALMA denote uncertain boundaries. The position of the boundary between Arshantan and Irdinmanhan ALMAs is unknown. Upper and (especially) lower limits of the Casamayoran SALMA are uncertain. The long span shown reflects this uncertainty, and may overestimate the actual duration of this land-mammal age. (Drafted by W v. Koenigswald and T. Smith, based on Berggren et al., 1995b; Flynn and Swisher, 1995; McKenna and Bell, 1997; Aubry et al., 2003; Dawson, 2003; Flynn et al., 2003.)

this volume are those of the Paleocene (Puercan, Torre-jonian, Tiffanian, and Clarkforkian) and Eocene (Wasatchian, Bridgerian, Uintan, Duchesnean, and Chadronian). These land-mammal ages have been subdivided into sequential biochrons that are variously based on first or last appearances, lineage segments, abundance zones, or assemblage zones. The North American Paleocene-Eocene record is the most nearly continuous in the world, although it is largely concentrated in the region of the Rocky Mountains.

In addition to the details discussed in the preceding sections, the following observations and changes concerning the original concepts may be noted. The Paleocene Puercan and Clarkforkian Land Mammal Ages are the shortest ages, about 1 million years each (Lofgren et al., 2004). Of the Paleocene NALMAs, however, only the Puercan is constrained by radiometric dates, whereas the duration of the others, including the Clarkforkian, is estimated (Clarkforkian was considered to be only half a million years long by Wood-burne and Swisher, 1995). The current convention of dividing the Paleocene into only early and late portions (e.g., Berggren et al., 1995a; McKenna and Bell, 1997) results in shifting the Torrejonian NALMA, long considered middle Paleocene, into the early Paleocene. This practice is largely responsible for the apparent temporal range extensions of many mammals discussed later in the volume, although in some cases new evidence has actually extended the range stratigraphically lower into sediments of Puercan age. Land-mammal age occurrences are specified in the text where there might be confusion. The Tiffanian and Clarkforkian together make up the late Paleocene and are believed to account for a little more than half of Paleocene time.

The beginning of the Wasatchian Land-Mammal Age now coincides with the onset of the global CIE, which is also designated as the beginning of the Eocene. Although the exact date of that event is uncertain (but most likely between 55.0 and 55.8 Ma), several 40Ar/39Ar dates are now known from tuffs and volcanic ashes of latest Wasatchian age in the Bighorn and Greater Green River basins of Wyoming, ranging from about 50.7 to 52.6 million years ago (Wing et al., 1991; M. E. Smith et al., 2003, 2004). The Wasatchian/ Bridgerian boundary appears to be at about 50.6-51.0 million years ago (Smith et al., 2003; Machlus et al., 2004). The Bridgerian, long considered equivalent to the middle Eocene, now straddles the early/middle Eocene boundary; nonetheless, all Bridgerian occurrences were listed as middle Eocene by McKenna and Bell (1997), which could affect some ranges discussed in later chapters. Numerous dates for the Bridger-ian range up to slightly younger than 47 million years ago, and the Bridgerian/Uintan boundary is situated in chron C21n at about 46.7 million years ago (Smith et al., 2003).

With the shift of the Eocene/Oligocene boundary to the beginning of the Orellan, the Chadronian (formerly early Oligocene) is now late Eocene; and it is 3 million years long, not 5 million, as previously believed. The Uintan and Duchesnean NALMAs (long considered late Eocene in age) are now correlated with middle Eocene. Several 40Ar/39Ar dates on ashes and ignimbrites from Texas and New Mexico indi-

cate that the Duchesnean spanned from 37 to almost 40 million years ago (Prothero, 1996a; Prothero and Lucas, 1996). The Duchesnean/Chadronian boundary is situated near the top of chron Cl7n.

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