404 P. S. MARTIN 



Extinction Marks the Boundary between the Pleistocene and 

 Recent 



In the Rocky Mountain region Hunt (1953) reported that the 

 disappearance of such large mammals as elephants, camels, and 

 horses coincides with a widespread unconformity in the late Quater- 

 nary deposits. He correlated this break with the drought of the 

 Altithermal and finds that extinction immediately predates it. 

 While this relationship may be of geological utility in western North 

 America, chronological detail does not bear out such a distinction 

 between "Pleistocene" and "Recent" elsewhere on the continent. 

 In Alaska thin gravels and clays containing remains of extinct 

 mammals are at least of Late-glacial age or older. Organic material 

 overlying the remains of elephant, horse, and extinct bison has been 

 dated at 10,200±800 (L-137G) and 9,400±750 (L-137N) years 

 B.P., (see discussion by Sigafoos and Hopkins in Broecker et al., 

 1956, pp. 156-157). Horn sheaths of Bison crassicornis were dated 

 at 16,400 ±2000 (M-38). 



In Mexico the Upper Becerra Peat, containing remains of the 

 mammoth M. imperator is also considered as older than 10,000 

 B.P. (Wormington, 1957, pp. 91-99). Hibbard (1955) considered it 

 early Wisconsin. MacNeish's important and, in large part, unpub- 

 lished studies in southern Tamaulipas (1950, 1955) have revealed 

 leaflike points associated elsewhere with the Becerra mammoths, but 

 there is no evidence of extinct animals in his radiocarbon -dated 

 middens, which cover the entire Post-glacial period (personal com- 

 munication). Apparently extinction in both Mexico and Alaska 

 preceded that in the Rocky Mountains. 



In Florida and South America extinction postdated the Alti- 

 thermal. Unquestionably this is the most controversial aspect of the 

 extinction chronology, partly because it all but eliminates climatic 

 change as an extinction cause. The vastly rich fauna of the Mel- 

 bourne and Seminole beds of Florida was dated on archaeological 

 and geochronological grounds by Rouse (1952) at 4,000 to 2,000 

 years ago. A radiocarbon date, L-2N, 2,040 ±90, of charcoal from a 

 newly exposed canal is "associated with extinct Seminole Field 

 mammals, an 'archaic' spear point, flint chips, and burned bone . . . 

 the date seems anomalously low in view of the extinct fauna" (field 

 description from unidentified collector in Broecker et al, 1956, p. 

 161). The fauna of the Seminole field includes the porcupine, capy- 



