Today there are over 5,000 identified living species of mammals, which represents roughly less then 10% of all vertebrates and 0.4% of all animals. So, measured by the number of species mammalian diversity is not great; however, mammals exhibit great diversity in form, which has allowed adaptation to a wide variety of niches exemplified by organisms as diverse as humans, whales and bats. What are some of the characteristics that unite this diverse and successful group?

• Most mammals, like birds, are homeothermic endotherms. Homeothermic organisms use behavioral and physiological strategies to maintain a constant body temperature regardless of the environmental temperature. Endothermic organisms use high metabolic rates to generate internal body heat. In contrast, many reptiles are poikilothermic ectotherms. Poikilothermic organisms have internal body temperatures that vary with the ambient environmental temperature. Ectothermic organisms rely on environmental sources of heat to control body temperature. Among other advantages a constant body temperature allows mammals to be active in a wide variety of environmental temperatures. This advantage comes at a cost; mammals must eat up to 10 times as much as an ectothermic organism to fuel this high metabolism.
  
  
• Mammal skin is watertight and possesses follicles that produce keratinous hair or fur. Mammal skin may also possess a variety of glands for sweating, scent production and oil to maintain fur. A subcutaneous layer of fat is found beneath the skin. Hair, fat, and sweat glands are important adaptations for homeothermy.
  
  
• Mammals have a four-chambered heart with a single artery leaving the heart (aortic arch) that curves to the left. The four-chambered heart is part of a two-circuit circulatory system that keeps oxygenated blood separate from deoxygenated blood. The erythrocytes (red blood cells) of mammals are enucleated (do not contain nuclei). This system improves the efficiency of delivering oxygen to cells, an important adaptation for endothermy.
  
  
• Mammal kidneys possess the loop of Henle, which creates concentrated urine, enabling mammals to get rid of nitrogenous waste with little loss of water. A high metabolic rate generates a greater amount of metabolic waste; the loope of Henle allows mammals to rid themselves of this waste.
  
  
• Mammals possess a muscular diaphragm separating the thoracic and abdominal cavity that helps to ventilate the lungs. The lung tissue has a high surface area to increase oxygen absorption.
  
  
• 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.

The characteristics above are not usually preserved in the fossil record, although some, such as brain structure and presence of a diaphragm can be inferred using skeletal clues. Paleontologists must look to skeletal structures shared by mammals when studying mammalian evolution, we will mention just a few:

• The lower jaw of mammals is composed of a single bone the dentary, which articulates with the squamosal bone of the skull. The lower jaw of primitive amniotes and reptiles is composed of multiple bones.
  
  
• Mammal teeth are highly differentiated (heterodont dentition) into incisors, canines, premolars, and molars. Mammal molars exhibit a tricuspid pattern and posses multiple roots. Primitive amniotes and reptiles possess homodont dentition, having single tooth morphology (all the teeth have pretty much the same form).
  
  
• Mammals exhibit diphyodont dentition. This means they have two successive sets of teeth, deciduous (milk teeth) and permanent (adult teeth). The first generation teeth (milk teeth) include incisors, canines, and premolars. Incisors, canines, and premolars are replaced to form the second generation (adult teeth). A single set of permanent molar teeth is also added to the adult teeth. Primitive amniotes and reptiles continually replace their teeth during their lifetime (polyphydont).
  
  
• Mammals have a secondary palate over the roof of the mouth that separates the nasal and food passages. The secondary palate allows mammals to breathe while they swallow. Primitive amniotes and reptiles must hold their breath when swallowing.
  
  
• Mammals have a single external nasal opening.
  
  
• The mammal cranium is enlarged and connected to the vertebrae column by a double occipital condyle. The cranium and the mandible of mammals are fused.
  
  
• 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 articular and quadrate bones in the lower jaw of primitive amniotes. Mammals also tend to have fleshy ears that funnel sound into the head.
  
  
• Mammal vertebrae are differentiated into cervical, thoracic, and lumbar regions. Most mammals have 7 cervical vertebrae. Mammals have no ribs on their lumbar or cervical vertebrae.
  
  
• Mammals share important characteristics exhibited by the shoulder and pelvic girdles that reflect a more erect gait with limbs tucked underneath the body. The head of the thighbone (femur) is positioned to one side to accommodate this gait. The vertebral column is articulated for an up and down flexion instead of side to side.
  
  
• Mammals grow rapidly after birth and slow to a terminal, adult growth stage (most other animals grow continuously throughout their lives). Growth patterns are recorded within the bones.

Mammal Origins

In the Early Carboniferous amniotes split into two lineages the synapsids (class Synapsida) and the reptiles (class Sauropsida). Traditionally, synapsids have been referred to as mammal-like reptiles. Synapsids did not evolve from reptiles, but both groups share a common ancestry with basal amniotes. Since synapsids did not evolve from reptiles, the non-taxonomic term protomammals is preferred over mammal-like reptiles (Prothero, 2004, p. 394).

Synapsids are traditionally divided into two successive groups, the pelycosaurs (order Pelycosauria Greek, “bowl lizard”) that span from the Mid-Carboniferous to the Late Permian and the therapsids (order Therapsida) that span from the Mid Permian to the Mid Jurassic. Many skeletal features common to mammals evolved among the therapsids. One therapsid evolutionary line, known as the cynodonts (suborder Cynodontia), would give rise to mammals sometime during the Triassic.

The synapsid fossil record is rich with transitional forms illustrating evolutionary trends in skeletal structure, which lead to and define mammals. The synapsid article on our website explores some of these skeletal transitions (jaw and ear) revealed by the fossil record. Triassic carnivorous cynodonts over time acquired an increasing number of mammalian skeletal features. Determining when these organisms became mammals is difficult because they exhibit a mosaic of mammalian and primitive amniote characteristics. So, paleontologists must establish a decision based upon evidence and logic.

There is a debate regarding how to define the mammalian clade. Some paleontologists say that logically, we should define mammals as all of the descendants of the last common ancestor of monotremes (egg laying mammals) and therians (marsupials and placentals) (Benton, 2005, p. 289). Under this definition what traditionalists would call basal or ancient mammals are referred to as mammaliformes or mammaliaformes (closest relatives of mammals). Although the definition is logical pinpointing which, if any fossil fits this criterion is problematic.

The traditional paleontological view suggests the dentary-squamosal jaw joint is the key mammalian character that defines the node for the clade mammalia, although others use the presence of an incus and malleus in the middle ear to define a mammal (Prothero, 2004, p. 399). The node at which morganucontids branched off the mammalian line marks the base of the mammal clade if one uses the dentary-squamosal jaw criterion. This practical definition is supported by evidence that is easily fossilized; teeth and jaws are the most common mammalian fossil. However, basal mammals that meet this criterion possess skeletal structures that consist of a mosaic of mammalian and primitive amniote characteristics.

We adopt the traditional view in our museum, using the presence of the dentary/squamosal jaw joint to recognize the first fossil mammals. The dentary/squamosal jaw joint first appears in shrew-like organisms such as Sinoconodon and Morganucodon.

Benton, M.J. (2005). Vertebrate Palaeontology [3rd edition]. Main: Blackwell Publishing.

Prothero, D.R. (2004). Bringing Fossils to Life: An Introduction to Paleobiology [2nd edition]. New York: McGraw-Hill.

Prothero, D.R. (2007). Evolution: What Fossils Say and Why It Matters. New York: Columbia University Press.