Evolution__3rd_Edition

542 PART 5 / Macroevolution
The amniotic egg evolved in reptiles
Mammals have several differences
from their reptilian ancestors
Modern tetrapods presumably just happen to be derived from five-digit ancestors, and
have retained that condition.
The next big step in terrestrial vertebrate evolution was the origin of the amniotic
egg: reptiles, birds, and mammals are amniotes, and members of these groups, unlike
most amphibians, do not return to water for the early stages of the life cycle. The origin
of egg types cannot be traced directly in the fossil record; however, at the origin of the
reptiles there were changes in skeletal morphology as well as egg type and there is good
evidence for the former. The reptiles probably evolved in the Carboniferous. The small
lizard-like creature called Hylonomus, from fossil deposits in Nova Scotia, is an early
reptile.
After the origin of the reptiles the two main events in vertebrate evolution were the
origin of bird flight and the origin of mammals. We shall not look into bird evolution
here (Section 10.4.2, p. 264, discusses how feathers are another example of preadaptation),
but we shall look at the origin of mammals. It is the best documented of any of
the major transitions in evolution, being even better documented in the fossil record
than the origin of tetrapods.
18.6.2 Mammals evolved from the reptiles in a long series of
small changes
Figure 18.10 (opposite)
The evolutionary radiation of the mammal-like reptiles. There
were three main phases: (a) pelycosaurs (sphenacodontids
and ophiacodontids in this picture), (b) therapsids, and (c)
cynodonts. Within each phase there were many smaller
The mammals are a distinct group of vertebrates in many respects: (i) they have warm
blood and a constant body temperature, and the high metabolic rate and homeostatic
mechanisms that go with it; (ii) they have a characteristic mode of locomotion, or gait,
in which the body is held upright with the legs underneath (in contrast to the “sprawling”
gait of reptiles, such as lizards, in which the legs stick out sideways); (iii) they have
large brains; (iv) their method of reproduction, including lactation, is also distinctive;
and (v) the active metabolism of mammals demands efficient feeding, so mammals
have powerful jaws and a set of relatively durable teeth, differentiated into a number of
tooth types. Therefore, when the mammals evolved from the reptiles, there had to be
changes on a large scale in many characters. How did this transition take place?
Not all of the distinctively mammalian characters are preserved in the fossil record.
The earliest mammalian fossils, such as Megazostrodon (Figure 18.10, at the top), date
back to the late Triassic, about 200 million years ago. Whether Megazostrodon was
viviparous and lactated is not known directly. But we can see that that it had a mammalian
jaw, gait, and tooth structure, and can therefore infer that it probably also
had warm-blooded physiology. The origin of the mammals can be traced back before
200 million years ago, through a series of reptilian groups informally called the
mammal-like reptiles and formally called the Synapsida. They evolved over an approximately
100 million-year period from the Pennsylvanian to the end of the Triassic,
when the first true mammals appeared. Some synapsids persisted into the Jurassic, but
evolutionary lineages. Some fossil forms are illustrated. Note
again the evolution of the more powerful and precision-action
mammalian jaw, and the change from a sprawling gait in
Dimetrodon to an upright gait in Probelesodon. Redrawn, by
permission of the publisher, from Kemp (1999).
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