Mammals

Evolutionary history Mammals came from synapsid amniotes. A therapsid reptile stock thrived in the Triassic, radiated widely in the Cenozoic, and today are the mammals, the dominant terrestrial vertebrates. Difficult to trace evolution through fossil record because many of the features that identify an animal as a mammal are in the soft tissues which aren't preserved in fossils.

Characteristics Hair. Birds have feathers, but all mammals have hair (even if it is reduced to a few whiskers as in the whales). Primarily for insulation. = a diagnostic feature.

Characteristics Mammary glands. Living female mammals use to feed their young - rudimentary in males. Milk is ejected from openings on various parts of ventral surface. Habit connected with viviparous condition in most.

Viviparous Placenta connects fetus to mother. Formed partly from the allantois membrane and partly from the uterus. Placental mammals form the Eutheria, the largest subclass. In the Metatheria (marsupials) there is no placenta (or only a temporary one. Newborn is still a fetus and very small (6 foot adult kangaroo gives birth to a 1/2 inch long baby). Crawls up mother's belly along track that she licks and attaches to teat where remains for months.

Prototheria (Platypus and Echidnas) Prototheria are the egg laying mammals. Eggs similar to reptiles. Mammary glands different from other mammals, open all over ventral surface and young lap up the milk.

Diagnostic features in the skeleton Lower jaw is of only one bone on each side (dentary). In reptiles and birds, always several. Jaw articulates with squamosal. In other terrestrial vertebrates it articulates with the quadrate and the articular bones. These bones are not present in mammals. Three small bones carry sound across the middle ear (there is only one in other terrestrial vertebrates). There is much evidence that these two extra ear bones are from the missing quadrate and articular bones.

The skull Basically like synapsid reptiles, but with a greatly expanded brain case to accommodate the large brain. The teeth are heterodont (of different types) and there are two sets, deciduous and permanent. The young are typically born toothless, which correlates with the habit of suckling.

Characteristics Heart has two completely separated ventricles so is four-chambered, but evolved differently from heart of birds. Can tell by aortic arches. The left is the only arch in mammals, but it is the right which is retained in birds.

Characteristics Respiration is efficient, but not as efficient as in the birds. There is a diaphragm which helps the negative pressure system of air intake and alveoli in the lungs increase the surface area for gas exchange. Much dead air.

 Endothermy Probably evolved to keep nocturnal ancestors warm, but didn't get much warmer than ambient temperature. When became diurnal raised temperature rather than invest in means of cooling. Now had to maintain this temperature when it was cold and increased metabolic drain was the result.  The hair, mammary glands, heart and respiratory system are all correlated with endothermy

Endothermy Hair and fat insulate to prevent heat loss. Temperature also maintained by shivering and increasing rate of metabolism (expensive, so used only when ambient air so cool that insulators are no longer effective). Sweat and panting, increases blood flow to the surface and piloerection can increase heat loss and cool the body. 

Endothermy After this need metabolic increase to cool.  An increased blood supply to the surface and panting and sweating get expensive if have to be carried to extremes.  Mammary glands and long period of caring for young are also correlated with endothermy. Young must be protected while they develop to be able to maintain the body temp. on their own. Result is fewer young and increased parental care.

Subclasses There are two subclasses of mammals = Prototheria (living monotremes, the egg-layers) and Theria (viviparious) The living therians are divided into the Metatheria (the marsupials) and the Eutheria (placental mammals). The most significant of the evolutionary trends in the Eutheria are:

Evolutionary trends in the Eutheria Increase in tooth size Reduction on tooth number Reduction of toes Increase in size of the forebrain and complexity of the cerebral cortex.

Evolutionary trends in the Eutheria Much of the eutherian diversity is believed to be associated with the adaptability of their dentition and limbs and feet to a variety of life styles and food sources. It is also assumed that the increase in number and complexity of the integrative pathways and centers in the central nervous system, particularly the cerebral cortex, has been responsible for their success.

The end! Notice that I have kept this as short as possible.  It is up to you to learn more about mammals on your own.  In this course you have developed the tools to do so.