Chapter 28, Problem 1RQ

Describe the evolution of mammals, tracing their synapsid lineage from early amniote ancestors to true mammals. How would you distinguish pelycosaurs, early therapsids, cynodonts, and mammals?

Summary Introduction

To describe: Mammals’ evolution by outlining synapsid lineage starting from early amniote ancestors to that of true mammals.

Introduction: The living amniotes are classified under two clades, namely, Mammalia, and Reptilia (which include birds, crocodilians, lepidosaurs, and turtles). The mammals, as well as the extinct closest relatives of mammals, contain a single pair of openings in the skull’s temporal region; therefore, they are called synapsids.

Explanation of Solution

The first group of amniotes that diversified extensively into terrestrial habitats is the synapsids. Pelycosaurs were the earliest synapsids found in the early Permian period, and they resembled lizards, but they do not share the features of lizards. From one of the groups of pelycosaurs, therapsids have emerged and are the only group of synapsids that entered the Mesozoic era. Therapsids evolved and diversified into many carnivorous and herbivorous forms. They became extinct at the end of the Permian period. However, one group of therapsids survived and entered into the Mesozoic era, and this group was termed as cynodonts. They diversified and gave rise to mammals during the Triassic period. The evolution of mammals was accompanied by the derivation of many important features. The diversification of mammals started in the Cretaceous period. The modern orders emerged in the Cenozoic era. Large-bodied mammals were greatly diversified during the ice ages of Pleistocene. At the Pleistocene epoch’s end, most of the diversified mammals became extinct due to habitat changes and hunting by modern humans. 

Summary Introduction

To describe: The way in which mammals, cynodonts, early therapsids, and pelycosaurs are distinguished.

Explanation of Solution

Pelycosaurs: They are characterized based on their resemblance to lizards.

Early therapsids: They had an erect gait having upright limbs that is positioned beneath their body. The cerebellum, which is the brain’s muscular coordination center, assumed an extended role.

Cynodonts: They evolved certain characteristics such as powerful jaw musculature, nasal cavity’s turbinate bones, heterodont teeth, and secondary palate.

Mammals: Mammals are differentiated by their mammary glands, hair, ear ossicles, and diphyodont teeth.