The Amphibians and Reptiles of Michigan: A Quaternary and Recent Faunal Adventure

Part 3. Quaternary Remains of Michigan Amphibians and Reptiles

211 Introduction This part of the book deals with Michigan amphibian and reptile remains that have been excavated from Quaternary paleontological and archaeological sites in Michigan.The Quaternary period represents the last 1.8 million years before the present (Ma BP).The Quaternary period itself is divided into two epochs: the Pleistocene epoch or “Ice Age” (1.8 million to 10,000 years ago) and the Holocene epoch (10,000 years ago to the present). Michigan amphibian and reptile remains have been found only in the last few thousand years of the Pleistocene and the Holocene.The reason is that much of the Michigan vertebrate fossil record from the end of the Pennsylvanian period to the late Pleistocene (about 290 million years to about 50,000 years ago) was eliminated by erosion of an uplifted Michigan as well as by the scouring effect of the Pleistocene glaciers (Holman 1995a). The evolution of the modern species making up the herpetofauna of Michigan took countless millions of years. Plants and animals did not become truly diverse until the Cambrian period about 540 Ma BP. Four-legged vertebrates (tetrapods) did not evolve from fishes until the Late Devonian period about 360 Ma BP. Salamanders, frogs, and turtles appeared by the end of theTriassic period about 206 Ma BP.True lizards appeared during the Jurassic, which lasted from the end of theTriassic until about 144 Ma BP. Snakes were the latecomers, appearing for the first time in the Cretaceous, about 100 Ma BP.The ancestors of our own amphibians and reptiles, as you can see, go back a long, long way. Many of the genera and even species of Michigan amphibians and reptiles have been around a long time, as represented by fossils found in other parts of North America (Holman 1995b). Necturus (the Mudpuppy genus) and Apalone (the softshell turtle genus) are both known from the Paleocene epoch, which lasted from 65 to 54.8 Ma BP. Later, in the Eocene epoch, about 35 Ma BP, Ambystoma (the Mole Salamander genus) and Hyla (the Treefrog genus) are known from Saskatchewan, Canada. The Skink genus (Plestiodon) is known from the Oligocene epoch, about 25 Ma BP.The Miocene epoch that ranged from 23.8 to 5.3 Ma BP produced fossils of the Lungless Salamanders (genus Plethodon), the toad genus (Bufo), and the modern turtle genera Chelydra (Snapping Turtles), Chrysemys (PaintedTurtles), Emydoidea (Box Turtles), and Sternotherus (MuskTurtles). Snake genera present in the Miocene included Heterodon (Hog-nosed Snakes), Diadophis (Ring-necked Snakes), Coluber (Racers), Pantherophis (Ratsnakes), Lampropeltis (Kingsnakes), and Nerodia (Watersnakes). During the Pliocene (5.3 to 1.8 Ma BP), the North American herpetofauna became essentially modern, with many existing species being known in the fossil record (Holman 1995b). Pleistocene Fossil Sites The Late Pleistocene (Wisconsinan) glaciation in Michigan left thousands of kettles and other glacially derived basins in its wake.These features first filled with essentially sterile glacial meltwater, but with the passage of time they developed into ponds that supported biological assemblages of bacteria, single-celled organisms (protists), fungi, and true plants and animals. Eventually, these ponds filled in by natural ecological processes. Such infilled Pleistocene kettles and basins depict the succession stages from the birth to the death Quaternary Remains of Michigan Amphibians and Reptiles 3 The Amphibians and Reptiles of Michigan 212 of these ponds (see Holman 2001a).The organically rich layers of these infilled structures normally yield the fossil remains of seeds, nuts, leaves, sticks, barks, and bits of broken tree limbs (sometimes even the remains of wood gnawed by fossil beavers) as well as snails and small clams by the thousands. Fossil beetle wings and other bits of insects are occasionally found in such sites. Oddly, although vertebrate fossils are relatively rare in these deposits, huge elephant-like mammals, the mastodonts and the mammoths, are the most numerous vertebrates found (Holman 1995a). Other large, extinct mammals, such as giant beavers and extinct musk oxen, are found in these sites in Michigan but more rarely than mastodonts and mammoths. Small vertebrate animals, including amphibians and reptiles, are exceedingly rare in Michigan Pleistocene kettle and other shallow basin sites. In fact, only frogs and turtles have been found in such situations. Several ancient “great lakes” existed in Michigan in the prehistoric part of the Quaternary period.These older lakes differ in their conformation from those of the present Great Lakes and have been given separate names (see...