2. The Origin and Early Evolution of Mammals: Synapsids (Protomammals or Stem Mammals)

Donald R. Prothero

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2. The Origin and Early Evolution of Mammals

Synapsids (Protomammals or Stem Mammals)

Donald R. Prothero
Donald R. Prothero

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This chapter is in the book The Princeton Field Guide to Prehistoric Mammals

20chapter 2THE ORIGIN AND EARLY EVOLUTION OF MAMMALSSYNAPSIDS (PROTOMAMMALS OR STEM MAMMALS)Mammals have an extensive fossil record going back to the be-ginning of the Age of Dinosaurs. But we can trace their ancestry even further back to fossils from the Carboniferous (Fig. 1.3), the “age of coal swamps.” Although these early mammal rela-tives are often referred to as “mammal-like reptiles,” the proper name for this group, which includes the ancestors of mammals and all their mammalian descendants, is Synapsida. If you prefer a casual term, some people call them “protomammals” or “stem mammals.” The Synapsida split off from the reptiles in the Early Carboniferous, at 315 Ma. Both the earliest members of the synapsid lineage (Protoclepsydrops and Archaeothyris) and the earliest members of the reptile lineage (Hylonomus and Westlo-thiana) evolved side by side at this time, but at no time were the ancestors of mammals ever reptiles. People still use the obsolete term “mammal-like reptiles” out of habit, or from copying outdated sources, but no up-to-date paleontologist uses the term any more.Most people are familiar with the “finback” fossils such as Dimetrodon and Edaphosaurus, which appear in many dinosaur books for kids and often in toy kits of dinosaurs as well (Fig. 2.1). Except these animals are not dinosaurs—they are among the ear-liest synapsids. These creatures are part of the first great evo-lutionary radiation of synapsids (Fig. 2.2) and are often called Figure 2.1. Reconstructions of some protomammals or synapsids (formerly but incorrectly called “mammal-like reptiles”). On the right in the background is the fin-backed predatory “pelycosaur” Dimetrodon, and on the left is the fin-backed herbivorous “pelycosaur” Edaphosaurus. In front on the left is the huge predatory “therapsid” gorgonopsian Gorgonops, and behind it is the herbivorous “therapsid” dinocephalian Moschops. Behind the human is the dinocephalian Estemnosuchus, with the bizarre crests and tusks on its face. In the right front are the wolf-size predatory “cynodont” Cynognathus, and the cow-size herbivorous dicynodont “therapsid” Kannemeyeria, which has a toothless beak except for canine tusks.

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20chapter 2THE ORIGIN AND EARLY EVOLUTION OF MAMMALSSYNAPSIDS (PROTOMAMMALS OR STEM MAMMALS)Mammals have an extensive fossil record going back to the be-ginning of the Age of Dinosaurs. But we can trace their ancestry even further back to fossils from the Carboniferous (Fig. 1.3), the “age of coal swamps.” Although these early mammal rela-tives are often referred to as “mammal-like reptiles,” the proper name for this group, which includes the ancestors of mammals and all their mammalian descendants, is Synapsida. If you prefer a casual term, some people call them “protomammals” or “stem mammals.” The Synapsida split off from the reptiles in the Early Carboniferous, at 315 Ma. Both the earliest members of the synapsid lineage (Protoclepsydrops and Archaeothyris) and the earliest members of the reptile lineage (Hylonomus and Westlo-thiana) evolved side by side at this time, but at no time were the ancestors of mammals ever reptiles. People still use the obsolete term “mammal-like reptiles” out of habit, or from copying outdated sources, but no up-to-date paleontologist uses the term any more.Most people are familiar with the “finback” fossils such as Dimetrodon and Edaphosaurus, which appear in many dinosaur books for kids and often in toy kits of dinosaurs as well (Fig. 2.1). Except these animals are not dinosaurs—they are among the ear-liest synapsids. These creatures are part of the first great evo-lutionary radiation of synapsids (Fig. 2.2) and are often called Figure 2.1. Reconstructions of some protomammals or synapsids (formerly but incorrectly called “mammal-like reptiles”). On the right in the background is the fin-backed predatory “pelycosaur” Dimetrodon, and on the left is the fin-backed herbivorous “pelycosaur” Edaphosaurus. In front on the left is the huge predatory “therapsid” gorgonopsian Gorgonops, and behind it is the herbivorous “therapsid” dinocephalian Moschops. Behind the human is the dinocephalian Estemnosuchus, with the bizarre crests and tusks on its face. In the right front are the wolf-size predatory “cynodont” Cynognathus, and the cow-size herbivorous dicynodont “therapsid” Kannemeyeria, which has a toothless beak except for canine tusks.

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