The characteristics that define a mammal have been refined throughout the course of evolution in order to better suit the environmental, locomotive, an general survival needs of the organisms. Mammals are chordates, vertebrates, aminotes, tetrapods, synapsids, cynodonts, and can be proto-, meta-, or eutherians (which are the three clades of extant mammals). The chordates arose during the Precambrian time period approximately 550 million years ago. During the Precambrian period, many symplesiomorphies (ancestral characteristics) such as a dorsal tubular nerve cord and notochord allowed for the evolution of vertebrates. The subphylum Vertebrata, which evolved approximately 500 million years ago, involved further complicating these symplesiomorphies into synapomorphic features that allowed for the marine organisms to move on to land. The diversification of vertebrates occurred during the Cambrian Explosion, and during this time the synapomorphies that evolved part of the phylum vertebrata even further specialized to accommodate for the increasing needs of the organisms. Organisms continued to evolve to have amniotic eggs in order to lay eggs outside of the water, sprawling out limbs to walk on land, and increasingly specialized features to accommodate for the survival needs of the organisms. The first terrestrial organism to emerge on land, called Tiktaalik, was the first in the lineage of Retiliomorphes, which eventually caused for the arising of mammals. The primitive forms of mammalian synapomorphies can be seen in reptiliomorpha such as the derivation of upright/erect posture, three middle ear ossicles, and heterodont teeth patterning.
The tetrapod emergence on top land began with a sprawling posture with sprawled limbs that extend laterally in place of fins. The sprawled out legs were a primitive characteristic of mammalian reptiliomorphes, the pelycosaurs. There are many extant vertebrates that still maintain these structures such as lizards, however, mammals have upright and erect limb orientation. A sprawling posture limits the range of movements and speed the organism is able to go, through the development of the erect posture, mammalian movements have become more various and dynamic. Mammals developed limbs through the expression of Hox genes and its different flavors during the pharyngula stage of development, which allowed for limbs to develop below the body, and support the majority of the body weight. Mammals have a variety of gaits and a large range of movements depending on their lifestyle through the evolution of upright posture.
The skull is a collection of components that support several systems throughout the body. The middle ear ossicles of mammals are part of the auditory apparatus, which allows mammals to hear different sounds with high sensitivity. In early pelycosaurs, the skeletal elements that give rise to the ossicles are part of the lower jaw and support mastication. The three derived mammalian middle ear ossicles consist of malleus, incus, and stirrup shaped stapes, which further advanced the modern mammal sensory system. In its primitive state, the ossicles were part of the lower dentary jaw bone in the form of the malleus and the articular bone, along with the columella. The columella was the exclusive component used for hearing in the primitive state of mammals, and is even still retained in modern day reptiles and birds. Due to the expansion of the jaw, the primitive structures migrated posteriorly, allowing for the joint to form into the three ossicles. The synapsids further evolved to develop three middle-ear ossicles, and continued to evolve developing a stirrup shaped stapes. The evolution of the three middle ear ossicles are largely related to changes that occured in the jaw-joint and teeth shape/patterns of the mammals and also for better hearing.
Another trait that is used to distinguish mammals is the heterodont teeth patterning. The shape and patterning of the teeth is vital to the organism as it is involved in the first part of the digestion, chewing. The variations in teeth were a result of specific diets and other ecological pressures different organisms experienced throughout the course of evolution. The size and positioning of the coronoid process and dentary-squamosal jaw joint also varies largely based on diet. Reptiliomorphs had homodont teeth patterning, in which the teeth form is the same form throughout the entire mouth. Cynodontia, which are descendants of therapsids and gave rise to mammals and mammaliaformes, contained double-rooted cheek teeth, an increasing dentary size, and tricuspid teeth. Teeth further evolved to be more complex in form allowing for chewing and are largely based on the organism’s diet. Herbivores animals have large tricuspid molars that allow for grazing, while organisms that are carnivorous have enlarged canines that can be used for tearing meat and capturing prey.
There are many more features that distinguish mammals from other organisms such as hair, upright posture, thoracic ribs, mammary and sweat glands, and a distinct phalangeal formula. Mammals are highly derived organisms that exist in every continent and even in the water. Mammals are very unique endothermic organisms that are considered the highest ranking throughout the kingdom Animalia.