29.6 Mammals

• The Enantiornithes did not survive past the Cretaceous period.

• Diseases, disorders, and injuries in animals are what veterinarians are concerned with.
• Animals are treated in zoos and laboratories.
• Animals that are kept as pets are often taken care of by veterinarians.
• Animals that veterinarians care for are pigs, goats, cows, sheep, and horses.

• In order to complete a degree in veterinary medicine, veterinarians must take courses in comparative zoology, animal anatomy and physiology, microbiology, and pathology, as well as many other courses in chemistry, physics, and mathematics.

• It takes an understanding of the vastly different anatomies of various species for veterinarians to perform surgery on many different species.
• The stomach of a cow has four "compartments" compared to one for non-ruminants.
• Birds have unique adaptations that allow for flight, which requires additional training and care.

• Enhancing our knowledge of animals and medical science is conducted by some veterinarians.
• The transmission of the avian flu virus to humans is a concern.
• The H5N1 bird flu strain has been spreading in birds in Asia, Europe, Africa, and the Middle East.
• There have been cases of bird-to-human transmission.
• More research is needed to understand how the virus can spread and how it can be stopped.

• The jaw, skeleton, integument, and internal anatomy are all distinctive to mammals.

• Monotremes, marsupials, and eutherians are the three clades of modern mammals.

• Hair has many important functions for most mammals, even though it is not obvious on some species.
• Hair traps a boundary layer of air close to the body, keeping heat from being generated.
• Along with insulation, hair can serve as a sensory mechanism.

• It is useful to nocturnal or burrowing mammals to attach nerves that transmit information about sound sensation.
• When an animal's hair stands "on end" to warn enemies, or to make the mammal look bigger to predators, hair can be part of social signaling.

• The skin of mammals has a number of different types of secretory glands.
• Most of the body has sbexras.
• Some mammals don't have eccrine glands at all, and most mammals do.
• Sweat glands are located over most of the body surface and figure prominently in regulating the body temperature through cooling.
• In both eutherians and monotremes, both males and females have mammary glands.
• The evolutionary origin of the sebaceous or eccrine glands is not clear.

• The jaws of other animals are made up of several bones, including the quadrate bone at the back of the skull and the articular bone at the back of the jaw.
• The stapes are located in the ear of other animals.
• The quadrate and articular bones have moved into the middle ear in mammals.
• The malleus is derived from the articular bone, while the incus is from the quadrate bone.

• The arrangement of jaw and ear bones helps to distinguish fossils of other synapsids.

• The middle ear bones of mammals and reptiles are different.
• The malleus and incus are found in mammals, whereas the stapes are found in birds.

• The temporalis and masseter are the major muscles in mammals.
• The muscles that allow up-and-down and side-to-side movements of the jaw are unique to mammals.
• Heterodont teeth mean that mammals have different types and shapes of teeth, rather than just one type and shape of tooth.
• Most other animals with teeth are polyphyodonts, which means they have their teeth replaced throughout their life.

• A group of cardiac cells located in the walls of the right atrium are called the sinoatrial node and determine the rate at which the heart beats.
• The erythrocytes of mammals are nucleated, whereas the erythrocytes of other mammals are not.

• mammals have a portion of the nephron called the loop of Henle, which allows them to produce urine with a higher concentration of solutes than blood.
• There is a system of veins that move blood from the hind or lower limbs to the kidneys in mammals.
• Only jawless fishes have portal systems.
• All mammals have a urinary bladder.

• The skulls of mammals have two occipital condyles, bones at the base of the skull that articulate with the first vertebra, as well as a secondary palate at the rear of the pharynx that helps to separate the pathway of swallowing from that of breathing.
• Turbinate bones are located along the sides of the nose and help warm and moist the air.
• Some mammals have fused bones in their Pelvis, while others do not.
• The eyelids and ears of mammals are similar to those of birds.
• There is a lack of a muscular diaphragm in birds.

• The brains of mammals are different from the brains of other animals.
• The cerebral cortex, the outermost part of the cerebrum, is highly convoluted and folded, allowing for a greater surface area than is possible with a smooth cort.
• The two cerebral hemispheres are linked by the corpus callosum, a specialized structure found inutherian mammals.
• The left and right cerebral cortexes have different functions.

• A synapsid is a mammal that has a single, ancestrally fused, postorbital opening in the skull.
• They are the only living synapsids, as earlier forms became extinct.
• The pelycosaurs and the therapsids are two groups of synapsids.
• The cynodonts are thought to be the ancestors of mammals.

• The ancestors of modern mammals are thought to be Cynodonts, which first appeared 260 million years ago.
• There are holes in the upper jaws of cynodonts that suggest they had whiskers.
• Changes to certain bones that improved food processing and ambulation are likely to be related to the increased metabolism required to modify body temperature.
• The later synapsids, which had more evolved characteristics unique to mammals, possess cheeks for holding food and teeth that are specialized for chewing and releasing heat.

• The ability to breathe at the same time is aided by the presence of a secondary palate.
• The secondary palate separates the area of the mouth where chewing takes place from the area above where breathing takes place.
• There is a secondary palate in mammals and cynodonts.
• Changes from early synapsids to later ones can be seen in the jawbone.
• The cheekbone is present in mammals but not in pelycosaurs.

• The shoulder girdle of therian mammals is different from that of other mammals in that it does not have a procoracoid bone or an interclavicle.

• The earliest known mammal fossils are from the early Jurassic period, 205 million years ago.

• Both reptilian and mammal jaws were present in the jaws of morganucodonts.
• Like modern mammals, the morganucodonts had differentiated teeth.
• From the middle of the Dinosaur era to the end of the Cretaceous period, mammals diversified.
• There are some small mammals in the fossil record.
• By the end of the Mesozoic, most of the dinosaurs were gone.
• About 65 million years ago, another radiation of mammals began and continued through the Cenozoic era.

• The Megazotrodon, an extinct mammal, may have been nocturnal.

• The Jaw of a morganucodont has a double hinge between the dentary and squamosal and between the articular and quadrate bones.
• The middle ear has bones in it.

• There are three major groups of mammals.
• The eutherians and the marsupials form a sister clade to the metatherians and eutherians.

• The platypus and one species of echidna are both found in Australia.
• Monotremes lay eggs rather than giving birth to young children.
• The leathery shell of their eggs is similar to the shell of reptile eggs.

• Monotremes keep their eggs for about two-thirds of the developmental period.
• There is a yolk-sac in the uterus.
• The babies hatch in a fetal state and are nourished by milk from the nest.
• Young platypuses do not have teeth.

• The average body temperature of the three monotreme species is less than that of the other mammals.