Origin and Evolution of Mammals

Origin and Evolution of Mammals
The evolutionary descent of mammals from their earliest amniote ancestors is perhaps the most fully documented transition in vertebrate history. From the fossil record, we can trace the derivation over 150 million years of endothermic, furry mammals from their small, ectothermic, hairless ancestors. Skull structures and especially teeth are the most abundant fossils, and it is largely from these structures that we can identify the evolutionary descent of mammals.

The structure of the skull roof permits us to identify three major groups of amniotes that diverged in the Carboniferous period of the Paleozoic era, the synapsids, anapsids, and diapsids. The synapsid group which includes the mammals and their ancestors, has a pair of openings in the skull roof for the attachment of jaw muscles (Figures 30-1, 30-2, and 30-3). Synapsids (Figure 30-1B) were the first amniote group to radiate widely into terrestrial habitats. The anapsid group is characterized by solid skulls and includes the turtles and their ancestors (Figure 30-1A). The diapsids have two pairs of openings in the skull roof (Figure 30-1C; see also Figure 28-2) and this group contains the dinosaurs, lizards, snakes, crocodilians, birds, and their ancestors.

The earliest synapsids radiated extensively into diverse herbivorous and carnivorous forms that are often collectively called pelycosaurs (Figures 30-2 and 30-3). These early synapsids were the most common amniotes in the early Permian. Pelycosaurs share a general outward resemblance to lizards, but this resemblance is misleading. Pelycosaurs are not closely related to lizards, which are diapsids, nor are they a monophyletic group. From one group of early carnivorous synapsids arose the therapsids (Figure 30-3), the only synapsid group to survive beyond the Paleozoic. With therapsids we see for the first time an efficient erect gait with upright limbs positioned beneath the body. Since stability was reduced by raising the animal from the ground, the cerebellum, muscular coordination center of the brain, assumed an expanded role. Therapsids radiated into numerous herbivorous and carnivorous forms; however most disappeared during the great extinction event at the end of the Permian.


Only the last therapsid subgroup to evolve, the cynodonts, survived to enter the Mesozoic era. Cynodonts evolved several novel features including a high metabolic rate, which supported a more active life; increased jaw musculature, permitting a stronger bite; several skeletal changes, supporting greater agility; and a secondary bony palate (Figure 30-4), enabling the animal to breathe while holding prey or chewing food. The secondary palate would be important to subsequent mammalian evolution by permitting the young to breathe while suckling. Along with the improved biomechanical shift to upright posture in cynodonts, the long bones became more slender and developed bony processes at the joints for firmer muscle attachment. Flexibility of the spinal column was probably improved by reduction in number of ribs. Within the diverse cynodont clade (Figure 30-3), a small carnivorous group called trithelodontids most closely resembles the mammals, sharing with them several derived features of the skull and teeth.

The earliest mammals of the late Triassic period were small mouse- or shrew-sized animals with enlarged crania, jaws redesigned for a shearing action, and a new type of dentition, called diphyodont, in which teeth were replaced only once (deciduous and permanent teeth). This event contrasts with the primitive amniote pattern of continual tooth replacement throughout life (polyphyodont teeth). The earliest mammals were almost certainly endothermic, although their body temperature would have been rather lower than modern placental mammals. Hair was essential for insulation, and the presence of hair implies that sebaceous and sweat glands must also have evolved at this time to lubricate the hair and promote heat loss. The fossil record is silent on the appearance of mammary glands, but they must have evolved before the end of the Triassic. The young of early mammals probably hatched from eggs in a very immature condition, totally dependent on maternal milk, warmth, and protection. This mode of reproduction occurs today only in the monotremes (echidnas and platypus).

Oddly, early mammals of the mid- Triassic, having developed nearly all of the novel attributes of modern mammals, had to wait for another 150 million years before they could achieve their great diversity. While the dinosaurs became diverse and abundant, all nonmammalian synapsid groups became extinct. But mammals survived, first as shrewlike, probably nocturnal, creatures. Then, in the Cretaceous period, especially during the Eocene epoch that began about 54 million years ago, modern mammals began to expand rapidly. The great Cenozoic radiation of mammals is partly attributed to numerous habitats vacated by the extinction of many amniote groups at the end of the Cretaceous. Mammalian radiation was almost certainly promoted by the facts that mammals were agile, endothermic, intelligent, adaptable, and gave birth to living young, which they protected and nourished from their own milk supply, thus dispensing with vulnerable eggs laid in nests.

Characteristics of Class Mammalia

1. Body covered with hair, but reduced in some
2. Integument with sweat, scent, sebaceous, and mammary glands
3. Skull with two occipital condyles and secondary bony palate; middle ear with three ossicles (malleus, incus, stapes); seven cervical vertebrae (except some xenarthrans [edentates] and the manatee); pelvic bones fused
4. Mouth with diphyodont teeth (milk, or deciduous, teeth replaced by a permanent set); teeth heterodont in most (varying in structure and function); lower jaw a single enlarged bone (dentary)
5. Movable eyelids and fleshy external ears (pinnae)
6. Four limbs (reduced or absent in some) adapted for many forms of locomotion: terrestrial, aquatic, aerial
7. Circulatory system of a four chambered heart, persistent left aorta, and nonnucleated, biconcave red blood corpuscles
8. Respiratory system of lungs with alveoli, and voice box (larynx); secondary palate (anterior bony palate and posterior continuation of soft tissue, the soft palate) separates air and food passages (Figure 30-4); muscular diaphragm for air exchange separates thoracic and abdominal cavities
9. Excretory system of metanephros kidneys with ureters that usually open into a bladder
10. Brain highly developed, especially neocerebrum; 12 pairs of cranial nerves
11. Endothermic and homeothermic
12. Cloaca present only in monotremes (present as shallow cloaca in marsupials)
13. Separate sexes; reproductive organs of a penis, testes (usually in a scrotum), ovaries, oviducts, and vagina; sex determination by males (heterogametic)
14. Internal fertilization; eggs develop in a uterus with placental attachment (placenta rudimentary in marsupials and absent in monotremes); fetal membranes (amnion, chorion, allantois)
15. Young nourished by milk from mammary glands