The Compass Rose, Vol. 1, No. 1, Summer 2000

Mammals are members of the Phylum Chordata. Chordates are animals that at some point in their lives have a notochord, a dorsal hollow nerve cord, pharyngeal gill slits, and a muscular postanal tail. Mammals are also part of the subphylum Vertebrata, which has the added characteristics of increased cephalization, the cranium and vertebral column has replaced the notochord as the main axis of the body, and the adaptations of vertebrate support greater metabolic rate and an increase in size and activity.

All mammals have hair, a body covering made of keratin. The covering is used for insulation and aids in helping the endothermic mammals in keeping a constant body temperature. Mammals produce milk to feed their young (Campbell 1996). They have a four-chambered heart that completely separates arterial blood from venous blood. Mammals also have a diaphragm in order to increase the intake of air. Two occipital condyles are located at the rear of the skull, and a bony secondary palate separates the nasal passages from the mouth (Strickberger 1996).

Mammals are descended from the extinct reptile subclass Synapsida ("Those with Fused Arches"; in reference to the incorrect assumption that the euryapsid condition resulted from the fusing of the two arches in a diapsid ancestor). This subclass of reptiles first made its appearance in the Permian. Synapsida is broken down into two orders, Pelycosauria and Therapsida.

The earlier order, Pelycosauria ("Bowl-lizards") was one of the earliest reptiles capable of some speed and higher temperatures needed for activity. This tendency towards speed, greater need of prey, and efficiency in catching the prey resulted in the lengthening of the jaw in early members of the group.

Pelycosaurs evolved longer, sharper teeth of different sizes to provide a better hold on the active insects on which they were preying. Early in Pelycosaur history, a suborder, Edaphosauria appeared. Edaphosauria showed a tendency toward eating plants. The ability to eat plants gave these reptiles the ability to grow larger, and consequently slower--at least until equally large predators began to appear on the scene.

A problem brought about by the growing size of the reptiles was that as their total volume increased, the volume to body surface area decreased (Volume increases by a power of three, while surface area increase by only a power of two.). Some of the edaphosaurs chose to remain in and near water, where temperature fluctuations were much smaller due to a property of water. But most of the large reptiles began to evolve immense sails, a trait for which they are now famous. These sail-wearing reptiles are often confused as dinosaurs, but they belong to a very different subclass of reptiles.

With a highly diverse and prosperous group of herbivores, a group of carnivores soon follows. The smaller, non-sailed line of synapsida soon began to produce effective predators. The new order of reptiles began to develop a more upright stance, carrying the body more above the legs rather than between the legs. These animals also began to show large canine teeth, a characteristic present in many of their descendents-mammals. The new order, Therapsida (named for the mammal-like arch of the cheekbones), were about the size of a dog. They had advanced cheekbone arches, which meant they had stronger jaw muscles and consequentially could bite harder.

Early therapsids began to expand the habitats of reptiles farther and farther from their traditional homes in the swamps. The more erect posture indirectly raised their internal body temperature, allowing them to move into cooler climates than reptiles had ever moved before.

As the therapsid carnivores became more and more effective, the early prey of their more primitive relatives, the pelycosaurs and other early reptiles, such as the cotylosaurs, began to decrease. In response many of the therapsids turned toward being omnivorous and finally to a herbivorous lifestyle of their own.

This herbivorous suborder became known as Anomodontia. Many of these reptiles evolved bony projections on the skull, probably used as defense against their carnivorous relatives. They often had immense skulls and bones, indicating that they were built very solidly. The reptiles were probably much like today's rhinoceroses. They most likely moved in herds and gave rise to another radiation of reptiles. Anomodonts ranged in size from about the size of a rhinoceros to only a few pounds.

The predatory lines of therapsids evolved into two separate lineages, the Gorgonopsians and Therocephalia.

The Gorgonopsians were significant because they were the first creatures able to trot. They had comparatively longer legs than their ancestors and were the fastest land vertebrate of their time. The Therocephalians also had a similar gait to the Gorgonopsians but were generally larger with long teeth. There is even evidence that the animals were the first to experiment with venom (McLoughlin 1980).

From the Therocephalians and Gorgonopsians arose the baurimorphs and the cynodonts, two groups with convergent evolution of a very important higher vertebrate trait. The increased need for greater metabolism caused a higher intake of both food and oxygen. In primitive reptiles, air flows into the nostrils, which lead to the mouth and then flows down the esophagus and into the lungs. When one of these reptiles was eating, they could not breathe. In these two groups the evolution of the secondary palate allowed for digestion and breathing to take place. This plate of bone in the roof of the mouth allowed for air to pass from the nostrils into the back of the mouth behind the area where food was being chewed (Strickberger 1996).

Cynodonts experienced a large adaptive radiation. Many of the species came to resemble mammals so closely that taxonomists had to erect a formal "barrier" between jaw-joints to designate the two. The barrier is defined as being at "the point where the 'reptilian' quadrate-articular jaw joint is replaced by one located at the juncture of the squamosal (rear cheek bone) and the dentary." Mammals and cynodonts seem so similar that many taxonomists have argued to redefine mammals as part of the class Theropsida ("Those Who Look Like Beasts"). The changes toward mammalian adaptations reflected the need for a higher metabolism and endothermy in the carnivorous groups. The cynodonts have left us such a complete fossil record of their change from reptile to mammal that they are often called an "infraorder of missing links" (McLoughlin 1980).

Cynodonts also developed more specialized teeth. They were the first to have specialized molars for chewing of food. They were triple blades that sheared the food as they moved past one another. These early molar bearing reptiles were also the first to display diphyodonty (a single tooth-replacement as the animal aged; Strickberger 1996).

Small holes (foramina) were beginning to be found on the skulls of some individuals. Nerves and blood vessels were believed to have passed through the tiny foraminas that led to active cheek and lip muscles, and even to whiskers. Although the fossil record does not display the origin of hair, many paleontologist believe that the gradual spread of the sensitive whiskers may have been the start of hair.

At the end of the Permian, a vast change in the plant life began to take over. Cone bearing and flowering plants (McLoughlin 1980) were quickly replacing ancient plants, such as seed ferns, which were pollinated by rainstorms. A mass extinction event occurred as the continents closed together and formed Pangaea, which caused global weather fluctuations on a mass scale (Benton 1991). Therapsids were not quick to adapt to these changes and consequently receded to a very minor role, while other reptiles, most noticeably the dinosaurs, took to the forefront.

The dinosaurs dominated the planet for the next 120 million years. The therapsids that did survive were no larger than the house cats of today. Fossil evidence of the tiny animals is scarce, usually restricted to a few teeth. The tiny therapsids were nocturnal, active insect-eaters or herbivores.

The therapsids were likely forest-dwellers and needed to develop senses other than sight to help them navigate through their darkened forest home. The brain began to expand the cerebellum allowing for finer senses. The ear began to change, as the tiny, complex bones in the jaws of reptiles began to shift and move. These tiny bones eventually became the malleus (hammer) and incus (anvil) of the ear.

Endothermic animals are born much more under developed than ectothermic animals. Without the hair needed for warmth at birth, many of the mammalian ancestors would have died of hypothermia. Consequently, parental care became a need for these newborns (McLoughlin 1980).

The shift from polyphyodonty to diphyodonty allowed only one replacement of teeth in mammals per lifetime. These deciduous or milk teeth include incisors, canines, and deciduous molars. When the animals are young the deciduous molars perform the active chewing function. When the mammal has matured, the new set of teeth includes permanent premolars and molars to perform chewing (Strickberger 1995).

Young skulls grow so rapidly that a teeth replacement would be needed almost immediately should the young be born with fully formed and functional teeth. The need for a non-chewing food for the young arose.

Later therapsids were already likely covered in hair and had the ability to aid in temperature regulations through skin secretions (i.e., sweat). These secretions, if close to the modern forms, contained fats, salts, and other nutrients. In modern day Monotremes this condition is still retained. The young do not cling to nipples, but rather lick stiff hairs protruding from skin pores where the nourishment drips. The "milk" is thick enough to cling to the hairs until the young take it. Monotreme milk is a fatty compound containing no true milk sugars and is likely similar to the original therapsid method of feeding young (McLoughlin 1980).

Monotremes arose from the ancient subclass Prototheria. They retain much of their reptilian ancestor's characteristics to the point that it is often debated as to whether or not they are truly mammals. Most extreme is that monotremes retain a cloaca, a single opening for excretion, urination, and reproduction (Strickberger 1996). (There is some evidence that monotremes arose later from the Haramiyids, a group related, but much more unsuccessful to multituberculates. These tiny mammals blurred the line of mammalian-reptile as much as today's modern monotremes and may have stemmed the egg-laying mammals of today. Much of the support for this lineage comes from lack of evidence of monotreme fossils at the time they were supposed to have arose from the Prototheria; Jenkins et al. 1997.)

Toward the end of the Triassic, three orders of nominal mammals (mammals whose jaws articulated through a dentary-squamosal joint): Multiberculata, Triconodonta, and Symmetrodonta. These little mammals were virtually undistinguishable from modern shrews. Their top size was ten centimeters in length. Milk teeth were found, establishing that a near defenseless infancy stage was already fairly common. These animals had narrow heads with a very strong olfactory sense. They developed a quick, bounding gait to replace the ungainly trot of their ancestors. The bounding gait required an alteration to the spinal column to allow more movement. In reptiles, ribs are attached to each vertebrate between the neck and the pelvis. In mammals lumbar vertebrates (the vertebrate of the lower back) have lost their ribs and are much freer to flex.

Adaptations toward the tiny early mammalian way of life also caused the ending of the reptile way of growing till death. The pressures of the dinosaurs on the early mammals forced them to remain tiny. Skeletal growth ended at adulthood allowing small size to remain small throughout their life.

The multituberculates became the dominant group of the time. They gradually replaced all of their thecodont relatives, driving them finally to extinction (McLoughlin 1980). The name multituberculates refer to the uncommon amount of cusps or tubercules on their molars. They roughly resembled today's rodents and sported the same large permanently growing incisors. They likely occupied the same niches as modern rodents. Multituberculates were found on every continent and comprised more than 75% of the mammal fossils found throughout the Mesozoic. Multituberculates were the longest living order of mammals, surviving 130 million years before being replaced by members of order rodentia (Monastersky 1996).

During the Jurassic, two new orders of mammals arose, the Docodonta and the Pantotheria. Docodonts became extinct as quickly as Triconodonta, and Symmetrodonta, but from Pantotheria arose the two largest modern order of mammals, the metatheres ("between-beasts", marsupials) and eutheres ("true-beasts", placentals). The subclass Theria was created to encompass Pantotheria, Metatheria, and Eutheria.

Metatheria and Eutheria both bear live young. This suggests that the bearing of young arose somewhat earlier than the two orders, probably amongst their pantothere ancestors. Originally eggs were retained internally until hatching, but gradually, the shell and yolk sac disappeared. A method of direct exchange of nutrients and waste between the mother and the embryo was formed.

The first metatherians appeared around 80 million years ago. They probably resembled modern opossums, and, in fact, the subclass Didelphimorpha arose at this time. Opossums, like their other relatives, were restricted in their success only by the method that they bear their young. Marsupials give birth to embryonic young who attach themselves to their mother's teats inside of a protective skin pouch for several months. At the end of this time, the metatherian reaches a stage similar to the stage of development eutherians are at when born. During the Cretaceous Australia and South America broke away from the rest of the world, possibly saving the metatherians from going extinct from competition with their placental relatives.

South America eventually rejoined with North America, causing the extinction of many of its marsupial occupants as placentals migrated on the continent. Australia, however remained separate from the other continents until human beings began to populate it. On Australia arose a metatherian fauna as diverse as the eutherians (McLoughlin 1980). Evidence of a possible placental mammal has been found on Australia in recent years. The find has been hotly debated, because the placement of a placental mammal in Australia would upset all preexisting ideas of mammalian dispersion (Wuetrich 1997).

Eutherians were so successful that they quickly diversified into several groups, the most prominent being the insectivores, which still exist today in the forms of shrews and moles. Insectivores developed highly sensitive hearing and smell. Many of them had poison glands in which to stun their prey (McLoughlin 1980). These glands still occur in some of their descendents such as Blarina brevicauda, the short-tailed shrew common of our area of North America (Kurta 1995).

Since no dinosaurs had ever populated the trees, many of the first arboreal mammals took to the trees. These new mammals, active again in the day, began to develop highly advanced sight. To find their food of fruit and insects, they likely developed colored vision. The bounds and leaps they performed between branches required depth perception. From these tree dwellers evolved the earliest primates, our own order.

Until the end of the Cretaceous, mammals remained tiny animals, hidden from the dominant dinosaurs. Most (with the exception of the primates) remained as nocturnal animals. They stayed this way until the catastrophe that ended the reign of the dinosaurs and the end of the Mesozoic era (McLoughlin 1980).

After the extinction of the dinosaurs the mammals were the quickest group to pick up the evolutionary torch. They, along with the other endothermic vertebrate class, Aves (birds), expanded to all of the niches vacated by dinosaurs. Today there are three living infraclasses and over forty-five different orders (Eisenberg 1981). Mammals are the dominant class of animal on earth. From lumbering reptile to nocturnal shrews to ourselves, mammals have undergone a long and intense evolutionary origin. Today we are undergoing an intense extinction event as disastrous as the one that killed the dinosaurs. Mammals survived the last one and multiplied, but will we be able to do it again?