PLEISTOCENE-AGE GIANT BEAVER (CASTOROIDES OHIOENSIS) AND EXTANT BEAVER (CASTOR CANADENSIS) ENVIRONMENTS OF SOUTHERN WISCONSIN

The Erb site in Jefferson County, Wisconsin uniquely provides fossil evidence of both extinct (Castoroides ohioensis) and extant (Castor canadensis) beavers and their environments during the terminal Pleistocene. Radiocarbon dating of dentine obtained from a giant beaver tooth provides an age of 12,430 +/- 35 (~14,500 cal yr BP) and a delta 13C value of -17.2 ‰ PDB, the latter suggesting that this extinct rodent primarily consumed aquatic plants. Pollen and plant macrofossil data from this basal level similarly reconstruct a cold, open swampy environment dominated by species of Carex (sedges) and the submerged aquatic Najas flexilis (naiad). Interpretations from these paleontological and paleobotanical data agree with the sedimentological shift from mud to marl deposition, indicating glacial Lake Scuppernong was draining, creating marshlands and hard water springs. Subsequently, Castor canadensis, an extant species of beaver, occupied the swampy locale sometime between ca. 13,800 and 12,200 cal yr BP, as evidenced by Populus (poplar/aspen) logs bearing chew marks in overlying pond sediments. Other tree species present at this time included Fraxinus nigra-type (black ash), Picea glauca (white spruce), P. mariana (black spruce), and Abies balsamea (fir). These trees along with several species of temperate-adapted shrubs and herbs suggest warming temperatures during the Bolling-Allerod. During the subsequent Younger Dryas, Picea mariana (black spruce) and Larix laricina (tamarack) dominated the tree flora from ca. 13,300 to 11,500 cal yr BP as the marsh shallowed under cool and dry conditions. Betula (birch) invaded the drying marsh, depositing logs and a reddish woody peat that date from ca. 11,100 to 10,800 cal yr BP. This peat contains pollen and macrofossils of several additional tree, shrub and herb species, which suggests the development of a hardwood-dominant forest. The fossil record is truncated at ca. 10,600 cal yr BP. However, fossils from the older deposits offer a detailed record of late-glacial vegetation and environmental changes in southern Wisconsin during the demise of the extinct giant beaver and the local arrival of “modern” beavers.