1 The Fossil Record and Horse Evolution Learning Objectives  To understand the reasons why the fossil record is incomplete  To synthesize the relationship between form and function in morphology  To apply the principles of natural selection to describe how changing environments can exert selective pressures on individuals that result in adaptive changes Introduction All living things are related to each other through an enormously complex lineage of descent that stretches back more than 3.5 billion years. Disentangling this convoluted history and describing the evolutionary changes that have occurred over this long time period is within the realm of the science of Paleontology. Reconstructing evolutionary history is no easy task. While it is difficult enough for historians to reconstruct events that occurred only hundreds to a few thousands of years ago, paleontologists are faced with the task of reconstructing events that happened millions or even billions of years ago. The most direct source of information about the evolutionary history of organisms comes from the fossil record. By studying this evidence, paleontologists have been able to reconstruct how living organisms are related to each other, how groups of organisms have changed over time, and how changes in organisms are related to changes in past environments. In this lab, we will use fossils to study evolutionary history in animals. Because the fossil record of horses is very rich, it provides a good case study that can help elucidate the pattern of past evolutionary change. As you go through the exercises, remember that natural selection results in adaptive changes over time that reflect the organisms’ environment. See if you can determine how horse evolution may have paralleled changes in its environment over time, and think about how characteristics you observe in other animals may be adaptations for their environments at this particular time in their evolutionary history. Fossil Formation Broadly speaking, fossils are any preserved remnant trace of a once-living organism. This includes the cured remains of early hunter-gatherer humans found in Scottish peat bogs, the jeweled remains of insects trapped in amber, the intricate impressions of early bird feathers made in German limestone, and the mineralized bones of dinosaurs that now fill the halls of natural history museums. Los Angeles contains a great repository of fossils at the La Brea Tar Pits. Despite the variety of fossil types, fossils form only rarely, and usually represent a biased sample of living communities. Once an organism dies, all traces of its existence generally are obliterated by scavengers, decomposers, and physical erosion. Preservation only occurs in environments where the normal forces of decomposition are greatly reduced, such as the bottom of an anoxic lake or the margin of a tar pit. Typically, only hard structures such as bone fossilize, although soft tissue is sometimes also recorded (Fig. 1). Even after preservation, geologic forces such as2 erosion and faulting (and, to a lesser extent, volcanic activity) can destroy the preserved remains and, moreover, surviving fossils must get exposed on the surface so that paleontologists can find them (Fig. 2). Because of these difficulties, the overall fossil record can be viewed as annoyingly incomplete. Of all the species existing today, less than 10% are expected to be preserved as fossils. Still, given all these difficulties the fossil record is remarkably useful, and for some groups (such as horses) the fossils show a clear history of evolutionary change and relationships. Geologic Time Perhaps the most remarkable thing about the fossil record is its length. Fossil evidence of life has been found in some of the oldest rocks ever discovered on earth, around 3.5 billion years old. The magnitude of “geologic time,” measured in millions or even billions of years, can be hard to comprehend as we measure our lives in days, weeks, and months. Nonetheless, all of the life we see around us today is the product of 3.5 billion years of continuous change: extant species are the surviving tips of the enormous, ancient tree of life. To organize this overwhelming expanse of time, paleontologists divide geologic time into smaller units based on the fossils characteristically found during each time period. Geologic time is divided into major units called eons. The Precambrian eons (the Archean and the Proterozoic) are the longer and least understood. Combined, they lasted some 4 billion years, from the formation of the planet to the first appearance in the fossil record of hard-bodied, multicellular organisms around 700-570 million years ago (Fig. 3). Figure 1. Fossil formation. http://www.cssforum.com.pk/css-optional-subjects/group-d/zoology/14536-notes-zoology-16.html Figure 2. Sedimentation, erosion and the fossil record http://kvhs.nbed.nb.ca/gallant/biology/biology.html3 The Phanerozoic eon extends to the present, during which most of the major forms of life evolved. The Phanerozoic is further divided into three eras. The Paleozoic lasted from 590 million to 248 million years ago and is the time during which the first vertebrates, such as fish, first appeared. The Mesozoic lasted from 248 to 65 million years ago and is the time period during which dinosaurs first appeared and came to dominate the terrestrial fauna. The Cenozoic Era is the present era and is the time period during which the diversity of mammals (including horses and humans) was generated. Eras are further divided into smaller units called periods and epochs. Figure 3. Geologic time. http://www.manitoba.ca/iem/mrd/min-ed/kidsrock/origins/images/time_chart.gif4 General Pattern of Horse Evolution The story of horse evolution is part of a broader story of the evolution of mammals. The first mammals were shrew-like insectivores that first appeared during the Mesozoic, 200 million years ago (MYA). Their diversity paled in comparison with the profusion of dinosaur forms. This changed abruptly 65 MYA when a massive asteroid slammed into the planet triggering a mass extinction that included most of the dinosaurs. Smaller animals such as the early mammals had a better chance of surviving in the aftermath of the impact. Mammals subsequently moved into the newly vacant habitats and niches, diversifying from 40 genera in the Late Cretaceous to over 200 in the Early Eocene. This rapid evolution of