There are other striking similarities between reptiles and birds in their anatomies and reproductive systems.
This is not surprising because birds are descendents of theropod dinosaurs two-legged mostly carnivorous dinosaurs. Birds class Aves Dogs, cats, bears, humans and most other large animals today are members of the vertebrate class Mammalia.
All mammals conceive their young within the reproductive tract of the mother and, after birth, nourish them with milk produced by their mammary glands. Mammals are heterodonts with strong jaws. That is to say, they have a variety of specialized teeth incisors, canines, premolars, and molars. This allows them to chew their food into small pieces before swallowing it.
Subsequently, they can eat any size plant or animal. Many reptiles must swallow their prey whole, which limits them to hunting smaller game. Like birds, mammals are endothermic , or warm blooded. They are able to maintain a relatively constant body temperature regardless of external environmental conditions mainly by using internal physiological mechanisms. In other words , they are homeothermic , or stable in core body temperature, as a result of endothermy.
All of the living species of insects, fish, reptiles, and amphibians are ectothermic , or cold blooded. They keep their body temperature in a normal range mainly by avoiding exposure to environmental temperature extremes.
For instance, reptiles usually remain in shaded areas on hot days to prevent fatal overheating. On cold nights, their lowered body temperature can cause them to become sluggish and inactive. In contrast, endothermic animals are able to remain active at night and often in the winter when the air temperatures are especially cold. They can also move about in the heat of very warm days. This ability most likely provided an advantage for the early small mammals in surviving alongside dinosaurs and other large reptiles , which apparently were mostly ectothermic.
The downside of endothermy is the need to consume far more calories relative to body size in order to maintain a constant core body temperature. Small mammals, such as moles with their rapid metabolism rates, must eat insects or other high calorie foods every half hour or so in order to stay alive. By comparison, cold blooded rattlesnakes usually eat only once every weeks and have been known to go without food for as long as two years.
Aiding in mammal body temperature control is their insulating hair and sweat glands. Sweating helps to dissipate heat by evaporative cooling. Compared to most other land mammals, humans are relatively hairless, but they have far more sweat glands.
Mammals have four chambered hearts like birds , complex nervous systems, and large brains relative to the size of their bodies. This broad range of useful features has made mammals highly adaptive and successful. They first appeared about ,, years ago , early in the age of dinosaurs , and replaced reptiles as the dominant class of land animals after 65,, years ago.
As the rapidly changing environment at that time led to the mass extinction of most large reptiles, it left vast evolutionary possibilities which mammals took advantage of by rapidly diversifying through adaptive radiation.
Important to mammalian success is their reproductive system. Their bodies took the amniote egg revolution of reptiles and birds one step further. In effect, the uterus functions as the protective eggshell.
Young mammals spend a long period of their early development within their mother's uterus. After birth, they are provided with protein and fat rich milk to eat and are usually protected until maturity. Pregnancy and milk production require mothers to significantly increase their own calorie consumption in order to provide nutrients for their infant s. Despite their success, mammals still only make up about.
However, the subphylum Vertebrata is distinguished from the phylum Chordata by the development of the notochord into a bony backbone. Vertebrates include the amphibians, reptiles, mammals, and birds, as well as the jawless fishes, bony fishes, sharks, and rays.
More than 64, species of vertebrates have been described, but the extant vertebrate species represent only a small portion of all the vertebrates that have existed. Vertebrates range in size from the frog species Paedophryne amauensis as small as 7.
Vertebrates comprise about 4 percent of all described animal species; the remainder are invertebrates, which lack backbones. All vertebrates are built along the basic chordate body plan: a stiff rod running through the length of the animal vertebral column , with a hollow tube of nervous tissue the spinal cord above it and the gastrointestinal tract below.
In all vertebrates, there is a mouth at anterior end of the animal and an anus before the posterior end of the body. Vertebrates are defined by the presence of the vertebral column. In vertebrates, the notochord develops into the vertebral column or spine: a series of bony vertebrae each separated by mobile discs. These vertebrae are always found on the dorsal side of the animal. However, a few vertebrates have secondarily lost their vertebrae and, instead, retain the notochord into adulthood e.
Vertebrates are also the only members of Chordata to possess a brain. In chordates, the central nervous system is based on a hollow nerve tube that runs dorsal to the notochord along the length of the animal. In vertebrates, the anterior end of the nerve tube expands and differentiates into three brain vesicles. Vertebrates are the largest group of chordates, with more than 62, living species. Vertebrates are grouped based on anatomical and physiological traits.
Tetrapods can be further divided into two groups: amphibians and amniotes. Amniotes are animals whose eggs are adapted for terrestrial living; this group includes mammals, reptiles, and birds.
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