Evolution and Classification of Mammals

Animals that belong to the class Mammalia are termed mammals. It is one of the evolved species of the animal kingdom and is categorized under Vertebrata. Mammals are present all over the world since they have adapted well to all kinds of environments and surroundings, right from oceans, rivers, and polar regions to rainforests and deserts. Let’s understand in detail mammal evolution and classification.

Origin of Mammals

The origin of mammals dates back to the Triassic Period which was about 252 million to 201 million years ago. Mammals have evolved from members of the reptilian species Therapsida, who were members of the subspecies Synapsida or the mammal-like reptiles. At the time of their origin, mammals were not as impressive as the other reptiles of that era. The first mammals on earth were the morganucodontids. They were tiny creatures living 210 million years ago in the shadows of the dinosaurs.

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Synapsids

Synapsids lived about 359 million to 299 million years ago in the Carboniferous Period. They were one of the earliest and also a dominant reptilian group of the Permian Period ( about 299 million to 252 million years ago). Synapsids were primarily predaceous but adaptive radiation led to some members of this group becoming herbivores. The predominant synapsids in the Mesozoic Era ( 252 million to 66 million years ago), were the archosaurs, also known as the ruling reptiles. 

Pelycosaurs

During the first half of the Permian Period, Pelycosaurs were synapsids that were the most common land vertebrates. A common pelycosaur genus called Dimetrodon walked like a lizard with its sprawling legs. It had a small brain but it gradually started to develop some of the mammalian traits like developing teeth of different types.

Therapsids

Some pelycosaurs evolved into the first mammals called therapsids, some of whom lived about 260 million years ago. Therapsids were the most diverse and common land vertebrates of the second half of the Permian Period. Initially, the therapsids used to resemble Dimetrodon but after a while, they started looking like mammals, developing several mammalian traits like legs that were positioned under the body and not along the sides. They were small active carnivores that underwent an evolution process of specialized heterodont dentition. A set of their teeth separated into molars, incisors, and canines. Another change was an improvement in the mechanics of locomotion as the plane of action of the limbs was brought close to the trunk. There was the development of a secondary palate and expansion of the temporal musculature, the muscle responsible for closing the jaw.

Mass extinction ended the Permian Period about 250 million years ago and along with it most but not all therapsids also became extinct. Their place was taken over by sauropsids who were amniotes that evolved into reptiles, dinosaurs, and birds. The remaining therapsids did not face much competition from others of their kind and some of them gradually evolved into mammals.

Cynodonts

The surviving therapsids were small animals, some of which evolved into cynodonts. During the first half of the Triassic Period, cynodonts thrived worldwide. They were nocturnal and ate insects. They were saved from extinction mainly because of being nocturnal. Animals in that niche were not taken over by the dinosaurs in the Triassic Period.

As Cynodonts continued to develop mammal-like traits, their evolution adapted them to their nocturnal niche in several ways:

• They developed the ability to regulate body temperature which allows nocturnal animals to remain active in the night when the temperature falls.

• They developed a good sense of hearing because it is more useful than a good vision if they are hunting in the dark.

By the end of the Triassic Period, cynodonts had shrunk in size and had many mammalian traits like:

• A larger brain than before

• Four different types of teeth

• Endothermy

• Three tiny bones in the middle ear

• A diaphragm for breathing

• Lactation

• Hair on the body

Although Cynodonts gave rise to mammals about 200 million years ago, they are not considered mammals themselves. They became extinct during the Jurassic or Cretaceous Period because of competition with early mammals.

Evolution of Early Mammals

Early mammals evolved from cynodonts and they continued to evolve. The first to split off were Monotreme mammals, followed by marsupials. The last evolution of mammals was that of Placental mammals.

Evolution of Monotremes

Early monotremes evolved about 150 million years ago. They retained some of the traits of therapsids like laying eggs and having a cloaca. These traits are also found in modern monotremes.

Evolution of Marsupials

Early marsupials evolved about 130 million years ago and one of the earliest of this kind was the extinct genus Sinodelphys. It was about 15 centimeters (6 inches) long and its limb structure in the fossil found hinted at it being a climbing animal. Climbing trees was a means for them to escape from predators by climbing into trees. It survived on a diet of insects and worms.

Evolution of Placental Mammals

The ancestor of placental mammals may have been the extinct genus Eomaia. Early placental mammals evolved about 110 million years ago and fossils of Eomaia suggest that it may have had several traits of placental mammals. It may have been only about 10 centimeters (4 inches) long and was a tree climber that ate insects and worms.  

Evolution of Modern Mammals

The gradual and phased-out evolution of mammals led to the development of several features that differentiated between modern mammals and modern reptiles. Several mammalian traits can be attributed to their high levels of activity. A good example of this would be the system of double circulation with a four-chambered heart, the diaphragm, anucleate and biconcave erythrocytes, and the secondary palate that separates the food and air passages and facilitates breathing during suckling or mastication. Hair on their body provides insulation which is known as endothermy or warm-bloodedness. It helps in maintaining the body temperature of an individual independent of the temperature of the external environment. Endothermy also allows mammals to have high levels of sustained activity. The unique characteristics of mammals are a result of the evolution of their complex and interrelated system.

The mammals and reptiles evolved at a different pace and this led to distinct characteristics that separate them. This evolution of mammals occurred as a response to a variety of interrelated conditions. At certain junctures in the period of transition from reptiles to mammals, some forms combined the characteristics of both groups. This pattern of evolution is termed mosaic and is common in transitions that mark the origin of a new and major adaptive species. 

Some authors suggest that to have simplified definitions and strict delimitation of the Mammalia, only one trait like the teeth must be the base for creating a boundary. By using a single osteological character, the placement of numerous fossil species can be done in a logical classification. Other mammalian characteristics such as the nursing of the young, degree of endothermy, and the condition of the internal organs may not be evaluated. If the advanced therapsids were alive today, it would have been difficult for taxonomists to classify which ones were to be placed in the Mammalia and which in the Reptilia category.

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Basis of Classification of Mammals

Scientists have done the higher classification of Mammalia based on a broad array of characteristics. Evidence from comparative anatomy was considered extremely important earlier, but recent information from other disciplines like serology (study of immune reactions in body fluids), physiology, and genetics has provided insights that can be used while considering relationships. Paleontological findings have also supplemented the comparative study of living organisms. The fossil records provide a historical dimension to the existing information on mammalian relationships. In some mammals like horses, the fossil record has been used in detail to trace the lineages.

The fossil record of mammals is better than that of other major vertebrate groups. Several factors affect fossilization, the most important ones being the habitat of the organism, its structure, and conditions at the time of death. The teeth and the related jaw bones and the skull are the most common remains of mammals. The enamel that covers the mammalian tooth consists of prismatic rods of crystalline apatite. It is the hardest tissue in the body of mammals and is highly resistant to physical and chemical weathering. A large number of teeth were found in deposits of fossil mammals. They were the reason for using dental characteristics in the interpretation of mammalian relationships and phylogeny. Dental features exhibit the broad radiation of mammalian feeding specializations from when they were predaceous. It is the reason why they are ideal for this key role in classification.

Classification Based on Reproduction

Based on their reproduction process, mammals can be classified into three classes: Eutheria, Prototheria, Metatheria. 

• Eutheria

Mammals under this category produce young ones that develop inside the body of the mother in the form of babies. They receive nutrition through the placenta of the mother. This subclass can be classified further into 19 orders. The primary ones include:

• Metatheria

Mammals falling under this classification give birth to immature babies who then stay inside a pouch present externally on the mother’s body. Kangaroos are a good example of Metatheria. This classification is divided into seven different orders:

• Prototheria

Mammals under this subclass reproduce by laying eggs. This classification has only one order with 6 species. Examples: Duck-Billed platypus, Echidna. 

Order: Monotremata

Characteristics of Mammals

Mammals possess distinct characteristics that set them apart from other species. These include:

• They are the most dominant species and because of their adaptation skills, are found in all kinds of habitats.

• Mammals are warm-blooded and reproduce by giving birth to their younger ones.

• They can also lay eggs and such mammals are known as viviparous.

• Their mammary glands help them produce milk for feeding their babies.

• There are oil glands, sweat glands (sudoriferous glands) and (sebaceous glands) on their skin.

• The presence of a distinct part of the brain called the Neocortex.

• Mammals are heterodont which means that they have different types of teeth.

• They have single-boned lower jaws.

• The skull of mammals is dicondylic which is when the skull is attached to the body by two articulatory condyles.

• The body of Mammalia is covered with fur of hair which helps them adapt to their environment.

• They have cervical vertebrae.

• Mammals breathe through their lungs.

• The trunk is divided into thorax and abdomen.

• The diaphragm in mammals is more advanced than other species.

• The 3 middle ear bones in mammals aid good hearing.

• The heart of mammals has four chambers. Sinus venous and renal portal system is not present.

• A mammal’s brain is well developed and is divided into cerebrum, cerebellum, and medulla.

• Mammals have 12 pairs of cranial nerves.