Mammals & Dinosaurs
Mammals first appear
at about the same time as Dinosaurs in the Triassic period.
Dinosaurs evolved into many forms and came to dominate many
Mesozoic ecosystems. During the entire reign of the dinosaurs,
mammals remained small never getting any bigger than a cat.
The evolutionary biologist
Stephen J. Gould (1988) commented, “Mammals were
relatively rare…They lived under the foot of dinosaurs,
in the nooks and crannies of a dinosaur world.” Most
dinosaurs did not survive the extinction event at the end of
the Cretaceous although; a small group would give rise to the
birds. Many niches were opened up to the survivors of this
extinction event. Mammals, in particular, possessed important
characteristics that would
allow them to evolve into many forms that could occupy these
empty niches. Mammals have been so successful during the Cenozoic
era that it is often referred to as the “age of mammals”.
What are some of the characteristics that have made mammals
Characteristics of Mammals
Mammals nourish their
young with milk secreted from mammary glands on the mother’s
chest. The milk is a source of water, proteins, carbohydrates,
fat, and minerals.
glands allow the mother to feed its young without leaving
nest or burrow. The young is given an easily digestible source
of food. While feeding on the milk the young also benefits
from the parents presence, which provides warmth and protection.
Most other vertebrates lay eggs and the hatchlings are often
independent at birth and must survive on their own.
Mammals, like birds,
are “warm-blooded”, that
is they maintain a constant body temperature through the rapid
metabolism of food. A constant body temperature allows mammals
to be active at night or day, live in cold environments, and
sustain activities that require high levels of energy like
running long distances and flying. Maintaining a constant body
temperature by fueling a high metabolism comes at a cost, it
requires on average 10 times as much food as a “cold-blooded” organism
of comparable size. Mammals have many adaptations for maintaining
a constant body temperature, one of which is insulating hair.
Most mammals have live
birth, that is, the young are born alive after developing
inside the mother. Most other vertebrates
lay eggs although; live birth has evolved independently many
times in fish, amphibians, reptiles, and mammals, but not in
birds (Cowen, 2005, p220). Live birth does have some advantages
over laying eggs. A fetus that grows inside the mother has
an unlimited supply of water, oxygen, and nutrients. Waste
can be easily removed. The fetus is protected from predation
and disease. The fetus is easily kept at a constant temperature.
A fetus is more flexible and can squeeze through the birth
canal easier than a shelled egg. Although the newborn is not
independent it can be nourished
by the mammary glands of the mother.
Mammals have fewer offspring, but invest heavily in their
young through extended parental care. Mammals have a prolonged
period of development during which time the parents protect
and care for their young. The young learn to care for themselves
under the tutelage of their parents. Using parental care for
a small number of offspring increases the chances that they
will survive. Many other vertebrates do not exhibit parental
care and increase the chances that some of their young will
survive by producing large numbers of offspring.
The above characteristics are not always easy to determine
using fossil evidence. Fortunately, mammals also share many
features that are more easily determined from fossil skeletal
Mammals have an enlarged outer folding of the brain called
the neocortex. Enhanced senses of vision, hearing, and smell
along with a more active lifestyle require a more sophisticated
brain. Fossil skulls can be used to make inferences about the
Mammals have three inner ear bones, the malleus, incus, and
stapes, which transmit sound from the eardrum to the oval window.
The malleus and incus are remnants of the bones in the lower
jaw of primitive amniotes. The three inner ear bones increase
the ability of mammals to hear high frequency sounds like the
buzz of insects. This would have been important for early mammals
that ate mostly insects.
Mammals have a single bone making up the lower jaw with a
powerful masseter muscle, which improves chewing efficiency.
The primitive amniotes and reptiles have a lower jaw made of
multiple bones. Mammals have two sets of teeth, milk and permanent.
Mammal teeth are specialized into incisors, canines, premolars,
and molars. The upper and lower molars of mammal teeth fit
together well making them efficient at processing food. The
molars of mammals have branched roots that help to secure them
in the jaw; losing a permanent tooth is a serious hazard. Many
other vertebrates, like the reptiles, have teeth that all look
pretty much the same and are constantly replaced during their
lifetime. A reptile may have a mixture of older and younger
teeth in their jaw that do not meet precisely. Mammal teeth
are so specialized that you can use them to determine their
diet and even use them for classification. Teeth are important
for studying mammal evolution because they are the most common
Living mammals are divided
into egg-laying mammals, pouched mammals, and placental mammals.
Monotremes are egg-laying
and are represented by the duck-billed platypus from Australia
and the echidna of Australia and New Guinea. Monotreme
young develop within a protective shelled egg. The eggs are
kept inside the mother’s body and nourished for a short
period of time before they are laid. After the eggs are laid
the female incubates them with the warmth of her body. Monotremes
have an egg tooth that helps them break through their shell.
Monotremes do not possess nipples, so they do not suckle their
young. Instead, the young must lap up the milk as it is secreted
from the mother’s mammary glands. Monotremes are the
most primitive living mammal. Like the monotremes primitive
mammals probably laid eggs and had poor thermoregulation. The
Monotreme fossil record is scant and dates back to the Cretaceous.
Pouched mammals are
known as marsupials. Today, most marsupial species live in
Australia, which include the
and kangaroos. North America has one marsupial, the opossum.
South America is home to many species of opossums. Marsupial
embryos are nourished inside the females’ body for a
short time, but do not attach to the uterine wall. Marsupials
are born underdeveloped, blind and very small. Newborn marsupials
use their front legs to crawl into the mother’s pouch
or marsupium where they attach to a nipple and complete their
The marsupial fossil record dates back to the Mid-Cretaceous
of North America. Cretaceous marsupials of North America were
numerous and diverse, but remained small. During the Paleocene
South American marsupials evolved into small insectivores,
omnivores, and medium to large carnivorous forms with lifestyles
like dogs, cats, and bears. Australian marsupials first appear
in the Eocene. Australian marsupials of the Early Cenozoic
evolved without competition from placental mammals and, unlike
South America, their adaptive radiation included the evolution
of middle to large herbivores as well as carnivores (Kemp,
2005, p. 208). However, as in South America convergent evolution
resulted in Australian marsupials with body forms and lifestyles
similar to placental mammals on other continents. Marsupial
equivalents of rodents, wolves, tapirs, anteaters, and cats
are easily recognized.
flourished and did not experience significant competition
from placental mammals until
during the Pleistocene 40,000 years ago. Europeans introduced
more invasive placental mammals during the 1780’s. Today,
habitat destruction and competition from invasive species endanger
Australia’s marsupial populations.
More than 95% of all
living mammals are placentals. The embryos of placental mammals
become embedded in the wall
of the uterus.
An organ called the placenta grows and connects the embryo
to the uterine wall. The circulatory system of the developing
embryo and mother are not directly connected; however, the
exchange of nutrients and waste does occur through the placental
tissues. The embryo is not limited by a fixed amount of food
from a yolk and it is protected from the mother’s immune
system, so it can develop within the womb for a longer period
of time. The increased gestation permits the formation of
a complex brain and nervous system.
The fossil record of
mammals dates back to the Early Cretaceous. Placental mammals
in North America, Europe and Asia underwent a great adaptive
radiation during the Paleocene. Most of mammalian orders
that flourished during this time would become extinct.
However, a few mammals that evolved during the Paleocene
orders, like flying lemurs, bats, primates, odd-toed ungulates,
and rodents. Representatives of all other modern orders
of placental mammals appear by the early Eocene (Benton,
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