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The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian sub-period of the late Carboniferous period. By the mid- Triassic, there were many synapsid species that looked like mammals. The lineage leading to today's mammals split up in the Jurassic; synapsids from this ...
- Overview
- The evolution of the mammalian condition
Mammals were derived in the Triassic Period (about 252 million to 201 million years ago) from members of the reptilian order Therapsida. The therapsids, members of the subclass Synapsida (sometimes called the mammal-like reptiles), generally were unimpressive in relation to other reptiles of their time. Synapsids were present in the Carboniferous Period (about 359 million to 299 million years ago) and are one of the earliest known reptilian groups. They were the dominant reptiles of the Permian Period (299 million to 252 million years ago), and, although they were primarily predaceous in habit, the adaptive radiation included herbivorous species as well. In the Mesozoic Era (about 252 million to 66 million years ago), the most important of the synapsids were the archosaurs, or “ruling reptiles,” and the therapsids were, in general, small active carnivores. Therapsids tended to evolve a specialized heterodont dentition (that is, a set of teeth separated into molars, incisors, and canines) and to improve the mechanics of locomotion by bringing the plane of action of the limbs close to the trunk. A secondary palate was developed, and the temporal musculature, the muscle involved in closing the jaw, was expanded.
The several features that separate modern reptiles from modern mammals doubtless evolved at different rates. Many attributes of mammals are correlated with their highly active habit—for example, efficient double circulation with a completely four-chambered heart, anucleate and biconcave erythrocytes, the diaphragm, and the secondary palate (which separates passages for food and air and allows breathing during mastication or suckling). Hair for insulation is a correlate of endothermy, or warm-bloodedness, the physiological maintenance of individual temperature independent of environmental temperature. Endothermy allows high levels of sustained activity. The unique characteristics of mammals thus would seem to have evolved as a complex interrelated system.
Mammals were derived in the Triassic Period (about 252 million to 201 million years ago) from members of the reptilian order Therapsida. The therapsids, members of the subclass Synapsida (sometimes called the mammal-like reptiles), generally were unimpressive in relation to other reptiles of their time. Synapsids were present in the Carboniferous Period (about 359 million to 299 million years ago) and are one of the earliest known reptilian groups. They were the dominant reptiles of the Permian Period (299 million to 252 million years ago), and, although they were primarily predaceous in habit, the adaptive radiation included herbivorous species as well. In the Mesozoic Era (about 252 million to 66 million years ago), the most important of the synapsids were the archosaurs, or “ruling reptiles,” and the therapsids were, in general, small active carnivores. Therapsids tended to evolve a specialized heterodont dentition (that is, a set of teeth separated into molars, incisors, and canines) and to improve the mechanics of locomotion by bringing the plane of action of the limbs close to the trunk. A secondary palate was developed, and the temporal musculature, the muscle involved in closing the jaw, was expanded.
The several features that separate modern reptiles from modern mammals doubtless evolved at different rates. Many attributes of mammals are correlated with their highly active habit—for example, efficient double circulation with a completely four-chambered heart, anucleate and biconcave erythrocytes, the diaphragm, and the secondary palate (which separates passages for food and air and allows breathing during mastication or suckling). Hair for insulation is a correlate of endothermy, or warm-bloodedness, the physiological maintenance of individual temperature independent of environmental temperature. Endothermy allows high levels of sustained activity. The unique characteristics of mammals thus would seem to have evolved as a complex interrelated system.
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The earliest known mammals were the morganucodontids, tiny shrew-size creatures that lived in the shadows of the dinosaurs 210 million years ago. They were one of several different mammal lineages ...
- Rick Gore
Oct 31, 2023 · Figure 29.6B. 1 29.6 B. 1: Cynodonts: Cynodonts, which first appeared in the Late Permian period 260 million years ago, are thought to be the ancestors of modern mammals. Since Juramaia, the earliest-known eutherian, lived 160 million years ago in the Jurassic, this divergence must have occurred in the same period.
Apr 2, 2020 · The Mammal-like Reptiles, or Therapsids, first appeared about 285 million years ago – near the beginning of the Permian (which is well before the dinosaurs). They evolved quickly and many different groups arose. They were very successful until about the end of the Permian, about 245 million years ago. Then something catastrophic affected the ...
Oct 16, 2021 · In fact, the first mammals evolved from a population of vertebrates called therapsids (mammal-like reptiles) at the end of the Triassic period and coexisted with dinosaurs throughout the Mesozoic Era. But part of this folktale has a grain of truth. It was only after the dinosaurs went kaput that mammals were able to evolve beyond their tiny ...
Feb 13, 2017 · The first mammals survived the Triassic-Jurassic extinction event that occurred 201.3 million years ago. This mass extinction wiped out around half of the Earth’s species. Most of the archosaurs that weren’t dinosaurs became extinct, as did many therapsids. The dinosaurs, early crocodiles and pterosaurs survived.
