INTRODUCTION

  FOSSIL mammals are known only from the Late Pleistocene of Michigan. If earlier Cenozoic deposits ever were present in Michigan they were destroyed by glacial ice that four times overrode the state. The American mastodon and Jefferson mammoth were the largest Pleistocene animals to walk on Michigan soil. Most of the information concerning these large mammals in the state must be sought in newspaper reports of individual finds. However, 0. P. Hay (1923) reported upon all Michigan Pleistocene mastodon and mammoth remains known to him. Case and others (1935) reported the occurrences of the Jefferson mammoth from Glacial Lake Mogodore in Cass County. Later in the same year, Case and Stanley (1935) reported on the Bloomfield Hills mastodon. Archie MacAlpin (1940) summarized all known finds of the American mastodon in Michigan but did not publish on the mammoths. The purpose of this paper is to place on record all known occurrences of both the American mastodon and Jefferson mammoth in the state, including those discovered since 1940, and to make available to the citizens of Michigan an account of these interesting mammals.

           Most of the fossil remains of these animals found in the state have occurred in swamps and bogs. Such areas owe their origin to the manner in which the last glacier melted; the majority of the lake depressions were formed at the time of the retreat of the last ice sheet. As the ice front retreated large blocks of ice were left behind, buried under the glacial outwash sediments. For a time such ice blocks were protected from the warmth of the sun by the gravel, sand, and silt that covered them. Eventually, however, these ice remnants melted, leaving depressions in their place. Although most of these were of small size, others were quite large. Geologists refer to the smaller ones as kettles or kettle holes. If silt or clay washed into these depressions it sealed them, and water would then accumulate to form a pond or lake. In most cases such depressions contained water only in the spring, but if they went below the water table a permanent pond or lake resulted. Over the years such ponds lakes continued to receive sand, silt, and clay from flood waters, additional material from vegetation growing around their edges. In time at least some of these bodies of water, particularly the shallower ones, became filled to levels where aquatic and semiaquatic vegetation could encroach upon the open water. Thus a bog was formed where an open pond or lake had once existed. Wherever water was shallow, plant material extended down through the water to the bottom, but where the water deepened the vegetation often formed a mat that floated over part of the water surface. Partially or completely filled depressions of this sort exist in Michigan today and are called "quaking bogs." The process of filling, however, is one that still continues and depressions in all stages can be found in the state. Floating mats of the nature described are unsafe; a heavy animal venturing too far out on one often breaks through. Many times such an animal is unable to climb back onto the mat, becomes mired, and eventually drowns. These bogs apparently were places of retreat for mastodons and some mammoths during the winter months. They would have been protected from strong winds, and the surrounding forest would have offered abundant food. But the deep muck and peat and the floating mats of vegetation, though partly frozen, were potential traps.

        Most mastodon remains in Southern Michigan are associated with muck, peat, and marl. Chances for preservation are good in bogs because there is immediate burial. The body of an animal that dies on high ground is seldom preserved because predators and scavengers scatter the bones. Such exposed bone usually becomes decayed or badly weathered before it can be carried downslope to a lake or stream to be preserved in the sediments deposited there. Bone that comes from a bog is usually very dark colored, dark brown or black, due to the acids produced by the decaying organic matter.

        Because of their larger size, mastodons are the mammals most commonly found fossil in Pleistocene deposits of Michigan. Smaller animals such as elk, deer, wolves, and beavers, are probably much more abundant in bog deposits, but because of their smaller size their remains may be overlooked or considered those of recent forms. Moreover, these smaller bones often look very much like the pieces of branches or roots that, are sometimes present in the muck or peat. Recovery of the smaller Pleistocene mammals of Michigan would do much to increase our understanding of these Ice Age forms.