Encyclopedia of Evolution.pdf - Online Reading Center

mammals, evolution of
the reptilian articular bone; and the stapes. In cynodonts,
these bones are intermediate between the upper jaw bones
of reptiles and the inner ear bones of mammals. In fact,
juvenile opossums (genus Didelphis) have inner ear bones
that closely resemble those of Morganucodon.
• Jaw joint. In early synapsid reptiles, as in modern reptiles,
the quadrate bone articulates with the articular bone of the
skull, but in mammals it is the dentary bone that articulates
with the skull. This means that during the evolution of
mammals, the location of the jaw joint had to change from
quadrate-articular to dentary-skull. In cynodonts, both jaw
joints were functional.
Because the term reptile usually excludes mammals and
birds, “reptile” is not a coherent taxonomic group (see cladistics;
reptiles, evolution of). However, mammals are a
coherent taxonomic group, representing all of, and only, the
modern descendants of the cynodonts.
By the Jurassic period, there were three major lineages
of mammals: the monotremes, the multituberculates, and
the therians. The modern descendants of the Jurassic monotremes
are the duck-billed platypus and the spiny anteater
(echidna) of Australia and nearby islands. They retain the
primitive feature shared by most (but not all) reptiles of laying
eggs. The mothers produce milk but do so from glands
rather than from breasts. The multituberculate lineage
became extinct during the Oligocene epoch of the Tertiary
period. Other modern mammals evolved from the Jurassic
therian lineage.
By the Cretaceous period, the therian lineage had
already split into its two modern branches: the marsupials,
which bear their young at a very early developmental
stage, and the young nurse inside of a pouch; and the placentals,
in which the fetus develops inside the uterus of the
mother and is nourished through a placenta. The placenta,
a structure vascularized by both the mother’s and the
fetus’s blood vessels, allows the mother’s blood to come
close to but not in contact with the fetus’s blood. This prevents
the mother’s body from launching an immune attack
against the fetus. Therefore the fetus can stay inside the
mother’s body until highly developed. When born, the offspring
has a relatively high chance of survival. In marsupials,
on the other hand, the fetus must be born before the
mother’s body launches the immune attack. The fetus must
crawl from the birth canal to a nipple, where it attaches
and nurses until it is able to survive on its own. It is protected
inside a pouch of skin.
Most of the evolutionary history of mammals occurred
during the Mesozoic era, alongside the dinosaurs. The
dinosaurs were, on the average, larger and ruled the day,
while most mammals were smaller and were probably most
active at night. While dinosaurs probably often ate mammals,
occasionally a large mammal would eat a dinosaur. In 2005
scientists published an account of a three-foot- (1-m-) long
Kepenomamus robustus mammal that had eaten a baby dinosaur
130 million years ago.
The Cretaceous extinction caused the extinction of
the dinosaurs. When the dust settled, the surviving mammals
encountered a world in which dinosaur competitors had been
cleared away. During the Paleocene epoch of the Tertiary
period, many kinds of mammals evolved, although it was not
until the beginning of the Eocene epoch that the modern lineages
of mammals began to diversify. Because this was a time
of rapid evolution (which can be considered a punctuation; see
punctuated equilibria), it is difficult to reconstruct those
early steps, whether from fossils or from DNA (see DNA [evidence
for evolution]). Although all modern orders of mammals
existed in the early Tertiary, the dominant animals were
in families that no longer exist, and the representatives of modern
orders did not look like their modern counterparts. That
is, modern orders existed, but modern families and genera
did not. In contrast, most of the genera of early Tertiary trees
would have been recognizable to an experienced naturalist.
Surviving and recently extinct marsupials included or
include many forms that closely resemble placental mammals.
The marsupial counterparts of bats, mice, dogs, and
cats represent a striking case of convergence. Kangaroos
can be considered the marsupial equivalent of deer, a convergence
of lifestyle but not of appearance as much as in the
other examples.
Modern placental mammals include:
• Edentates such as sloths, anteaters, and armadillos
• Insectivores such as shrews and hedgehogs
• Bats, including fruit bats and pollinator bats with excellent
night vision, and other bats that rely primarily on echolocation
to capture flying insects
• Primates, with excellent vision (see primates)
• Carnivores, including dogs, cats, weasels, raccoons, bears,
seals, and walruses
• The elephants and sea cows
• Odd-toed ungulates such as rhinoceroses and horses (see
horses, evolution of)
• Even-toed ungulates such as pigs, hippopotamuses, camels,
deer, antelopes, and cows; the cetaceans such as whales and
dolphins are close relatives of the hippopotamus (see cladistics;
whales, evolution of)
• Rabbits
• Rodents, which comprise half of modern mammal species,
not only mice and rats but also porcupines, beavers, and
squirrels. The continuously growing incisors allow gnawing,
which, along with their high reproductive rate, has
made rodents very successful.
Competition between marsupials and placentals. Wherever
marsupials and placentals have been in contact, the placentals
seem to eventually prevail:
• In the Mesozoic era, marsupials were widespread in the
world. After the Oligocene, however, most marsupials were
found in Australia and South America. Placental mammals
may have outcompeted them in other places.
• After the Panama land bridge formed, connecting North
and South America for the first time since the breakup of
Pangaea during the Mesozoic era, many marsupial mammals
migrated south from North America and outcompeted
marsupials in South America. Few South American marsu