Mammals: Synapsid Amniotes PDF

## 22 Mammals: Synapsid Amniotes ### 22.1 Evolutionary Perspective **Learning Outcomes:** 1. Describe the characteristics of the members of the class Mammalia. 2. Assess the importance of two mass-extinction events in the evolution of modern mammals. The fossil record that documents the origin...

## 22 Mammals: Synapsid Amniotes ### 22.1 Evolutionary Perspective **Learning Outcomes:** 1. Describe the characteristics of the members of the class Mammalia. 2. Assess the importance of two mass-extinction events in the evolution of modern mammals. The fossil record that documents the origin of the mammals from ancient reptilian ancestors is very complete and relatively uncontroversial. It is being used to test, and has confirmed, many macroevolutionary theories. The beginning of the Tertiary period, about 70 mya, was the start of the “age of mammals.” It coincided with the extinction of many reptilian lineages, which led to the adaptive radiation of the mammals. Tracing the roots of the mammals, however, requires returning to the Carboniferous period 320 mya, when the synapsid branch of the amniote lineage diverged from the reptilian branch of this lineage. The fossil record is conclusive-the synapsid lineage was the first amniote lineage to diversify, beginning about 320 mya. Synapsids quickly became very diverse and widespread. They were the dominant, largebodied animals on the earth for more than 100 million years, through the remaining Carboniferous and Permian periods. Mammalian characteristics evolved gradually over a period of 200 million years. Most of what we know about early synapsids is based on skeletal characteristics. Other mammalian features like hair, mammary glands, and endothermy do not preserve well in the fossil record. Early synapsids had a sprawling gait and were probably ectothermic. The large sails on some, like *Dimetrodon*, probably helped these synapsids raise body temperature after a cool night. These sails are also an evidence that early synapsids lacked hair. Early synapsids were probably also egg-layers. Some were herbivores; others showed skeletal adaptations reflecting increased effectiveness as predators. The anterior teeth of the upper jaw were large and were separated from the posterior teeth by a gap that accommodated the enlarged anterior teeth of the lower jaw when the jaw closed. The palate was arched, which strengthened the upper jaw and allowed air to pass over prey held in the mouth. By the middle of the Permian period, other successful synapsids had arisen. They were a diverse group known as the therapsids. Some were predators, and others were herbivores. In the predatory therapsids, teeth were concentrated at the front of the mouth and enlarged for holding and tearing prey. The posterior teeth were reduced in size and number. The jaws of some therapsids were elongate and generated a large biting force when snapped closed. The teeth of the herbivorous therapsids were also mammal-like. Some had a large space, called the diastema, separating the anterior and the posterior teeth. The posterior teeth had ridges and cutting edges that were probably used to shred plant material. Unlike other synapsids, therapsids held hindlimbs directly beneath the body and moved them parallel to the long axis of the body. Changes in the size and shape of the ribs suggest the separation of the trunk into thoracic and abdominal regions and a breathing mechanism similar to that of mammals. About 240 mya, most of the very successful therapsids were wiped out during a major extinction event at the Permian-Triassic boundary-possibly as a result of huge Siberian volcanic events. Only a few cynodont therapsids survived this extinction event. By this time, however, the reptilian (diapsid) amniote lineage had also emerged. The archosaurs (dinosaurs, crocodiles, and eventually the birds) also survived this extinction event, and these reptiles became the dominant large animals on terrestrial landscapes through the Mesozoic era, which ended about 65 mya. Cynodonts became increasingly smaller, probably nocturnal, and more mammal-like. The smaller size and development of hair and endothermy were probably selected for as these mammal precursors exploited niches not occupied by much larger dinosaurs and smaller diurnal reptiles living at the same time. Other mammalian characteristics evolved during the Jurassic period. Teeth became highly specialized to facilitate rapid food processing. There were changes in the structure of the middle ear and regions of the brain devoted to hearing and olfaction. The fact that most mammals lack color vision also reinforces the idea that these early mammals were nocturnal. The first "true mammals" were present in the Jurassic period. During the Mesozoic era, mammal populations were relatively diverse, but they were not particularly abundant. New molecular and fossil data suggest that ecologically diverse representatives of all 18 extant mammalian orders were present in the late Mesozoic era. About 65 mya another mass extinction occurred-probably associated with asteroid impact in what is now Central America. Dinosaurs, many ancient birds, and many other taxa became extinct, but at least some early mammals survived this second mass extinction event of synapsid history. This Cretaceous-Tertiary extinction allowed surviving mammals to continue the diversification that began in the Mesozoic era and expand into niches formerly occupied by the dinosaurs. The Tertiary period became the “age of mammals.” ### 22.2 Diversity of Mammals **Learning Outcomes:** 1. Explain the role of continental movements in influencing mammalian evolution. Hair, mammary glands, specialized teeth, three middle-ear ossicles, endothermy, and other characteristics listed in table 22.1 characterize the members of the class *Mammalia*. There are about 5,400 species of mammals that range in size from the bumblebee bat to the blue whale. They are the dominant large terrestrial animals on all continents of the earth, and some have extended their habitats into the oceans and the air. There are two lineages of living mammals (figure 22.3). The subclass Prototheria contains the surviving infraclass Ornithodelphia, commonly called the monotremes. These names refer to the fact that monotremes, unlike other mammals, possess a cloaca and generate a large biting force when snapped closed. The teeth of the herbivorous therapsids were also mammal-like. Some had a large space, called the diastema, separating the anterior and the posterior teeth. The posterior teeth had ridges (cusps) and cutting edges that were probably used to shred plant material. Unlike other synapsids, therapsids held hindlimbs directly beneath the body and moved them parallel to the long axis of the body. Changes in the size and shape of the ribs suggest the separation of the trunk into thoracic and abdominal regions and a breathing mechanism similar to that of mammals ### 22.3 Evolutionary Pressures **Learning Outcomes:** 1. Justify the statement that “easily recognizable mammalian characters are epidermal in origin.” 2. Compare the usefulness of the study of tooth structure by a mammalogist, an ornithologist, and a herpetologist. 3. Compare the function of menstrual cycles in female primates to estrus cycles in other female mammals. Mammals are naturally distributed on all continents except Antarctica, and they live in all oceans. This section discusses the many adaptations that have accompanied their adaptive radiation. ### 22.4 Human Evolution **Learning Outcomes:** 1. Explain the global conditions that influenced the evolution of bipedal locomotion in early apes in Africa. 2. Describe a sequence of hominins and time frames that are important in understanding events of human evolution. 3. Describe the role of cultural evolution in the development of human societies. * * *