Class Mammalia

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In biological classification,Mammal (Mammalia /məˈmeɪli.ə/  Latin: mamma) is a taxonomic rank, as well as a taxonomic unit, a taxon, in that rank. Mammals are characterized by the presence of milk-producing mammary glands for feeding their young, a neocortex region of the brain, fur or hair, and three middle ear bones. These characteristics distinguish them from reptiles and birds, from which their ancestors diverged in the Carboniferous Period over 300 million years ago. Around 6,400 extant species of mammals have been described and divided into 29 orders.

A class of the subphylum Vertebrate, commonly known as mammals. Most mammals are vertebrates covered with fur, fast-moving, viviparous at constant temperature, and with a diaphragm in their bodies. They are the animal group with the body structure in vertebrates. Mammals can be divided into protozoa, true animal subclass and post animal subclass. Mammals are distributed all over the world, and live in various lifestyles such as land, underground, water and air; there are two types of nutrition: herbivorous and carnivorous.

The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes (echidnas and platypuses) and Therian mammals (marsupials and placentals) and all descendants of that ancestor. Since this ancestor lived in the Jurassic period, Rowe's definition excludes all animals from the earlier Triassic, despite the fact that Triassic fossils in the Haramiyida have been referred to the Mammalia since the mid-19th century. If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others. Ambondro is more closely related to monotremes than to therian mammals while Amphilestes and Amphitherium are more closely related to the therians; as fossils of all three genera are dated about 167 million years ago in the Middle Jurassic, this is a reasonable estimate for the appearance of the crown group.

T. S. Kemp has provided a more traditional definition: "Synapsids that possess a dentary–squamosal jaw articulation and occlusion between upper and lower molars with a transverse component to the movement" or, equivalently in Kemp's view, the clade originating with the last common ancestor of Sinoconodon and living mammals. The earliest known synapsid satisfying Kemp's definitions is Tikitherium, dated 225 Ma, so the appearance of mammals in this broader sense can be given this Late Triassic date.

The earliest mammal fossil is Hadrocodium wui found in China, which lived in the Jurassic period 200 million years ago. From the perspective of fossils, a very important difference between mammals (especially early mammals) and reptiles is their teeth. In reptiles each tooth is identical and indistinguishable from one another, whereas in mammals the teeth are differentiated into different forms according to their position on the jaw. In addition, the teeth of reptiles are constantly renewed, and the teeth of mammals are no longer renewed except for deciduous teeth. In the animal kingdom, only mammals have three bones in their ears. They evolved from the two jaws of reptiles.

All mammals of the Mesozoic Era were small. Mammals occupied many ecological niches after the extinction of the dinosaurs. By the time of the Quaternary, mammals had become the dominant animals on land.

Like birds, mammals evolved from reptiles.

The earliest mammals evolved from the theropods among reptiles. True mammals appeared at the end of the Triassic in the Mesozoic Era. After entering the Cenozoic Era, mammals replaced dinosaurs to occupy the ecological niche and evolved into today's diverse mammal populations.

history

According to the fossil evidence currently available to humans, mammals are believed to first appear in the Triassic period of the Mesozoic Era more than 200 million years ago, and began to flourish in the Cenozoic Era, becoming the dominant animal on land until now.

Mesozoic

There is fossil evidence that in the mid-Triassic, the skulls of some therapsids began to develop many new features. In reptiles, the multi-bone jaw evolved into a single tooth, and the teeth evolved from a single shape to multiple shapes depending on their position in the mouth. The cheekbone arch becomes more developed to support the need for masticatory muscle strengthening. The bones of their bodies became light and flexible, and their limbs stood upright instead of stretching out to the side like reptiles. They have well-developed fur, so they may have been warm-blooded. They can be considered a stage in the process of transitioning to true mammals.

In the late Triassic period about 200 million years ago, early mammals and dinosaurs officially appeared almost at the same time. At that time, the large and small dinosaurs that ruled the land occupied most of the ecological niches, while the early mammals were insignificant in size and mainly survived on small prey such as insects that lived in the jungle. By the end of the Cretaceous period 65 million years ago, dinosaurs may have become extinct rapidly due to climate change, while mammals had constant temperature characteristics, and their small bodies reduced heat loss, helping them survive in low temperatures and dark nights.

new generation

All mammals before the Paleogene were small in size, and they were all small hunters with a body length of about 12 centimeters and a size similar to that of mice. After entering the Cenozoic era, dinosaurs declined, so mammals were able to occupy more ecological niches, and rapidly evolved thousands of different species with rich diversity. Early bats and whales appeared 50 million years ago; by 40 million years ago, all orders under Mammalia basically appeared.

By the Quaternary, mammals had become the dominant animals on land. Although the ice age wiped out a group of large mammals, including mammoths, most of the species that survive today have not changed much.

Mammal classification has been through several revisions since Carl Linnaeus initially defined the class, and at present, no classification system is universally accepted. McKenna & Bell (1997) and Wilson & Reeder (2005) provide useful recent compendiums. Simpson (1945) provides systematics of mammal origins and relationships that had been taught universally until the end of the 20th century. However, since 1945, a large amount of new and more detailed information has gradually been found: The paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics. Though fieldwork and lab work progressively outdated Simpson's classification, it remains the closest thing to an official classification of mammals, despite its known issues.

Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders in numbers of species are Rodentia: mice, rats, porcupines, beavers, capybaras, and other gnawing mammals; Chiroptera: bats; and Soricomorpha: shrews, moles, and solenodons. The next three biggest orders, depending on the biological classification scheme used, are the Primates: apes, monkeys, and lemurs; the Cetartiodactyla: whales and even-toed ungulates; and the Carnivora which includes cats, dogs, weasels, bears, seals, and allies. According to Mammal Species of the World, 5,416 species were identified in 2006. These were grouped into 1,229 genera, 153 families and 29 orders. In 2008, the International Union for Conservation of Nature (IUCN) completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495, including 96 recently extinct.

Existing mammals are divided into three subclasses:

1. Protozoa

·Single hole

2. Post-animal subclass

· Possumes

· Shrew Possums

·Quellales

· Bandicoot

· Marsupiales

·Kangaroo

· Chilean opossums

3. True beasts

· Insectivora

·Tree shrews

·Dermoptera

· Chiroptera

Carnivora (now the pinnipeds are merged into the carnivorous order and become its subordinate pinnipeds)

·Manatee

· Proboscis

· Perissodactyla

· Hyraxes

· Angiodentidae

·Cetacea-Artiodactyla (Cetacea and Artiodactyla merged)

· Lepidoptera

·Cathera (formerly belonged to the order Apodent)

·Pelopodidae

· Rodents

· Lagomorpha

·Elephant shrews

· Primates

Research data show that at least 351 species of mammals have become extinct in the past 126,000 years, 80 of which are known from historical data after 1500 AD, and others are only determined by fossils. This suggests that the rate of animal extinction has been increasing over the years, as evidenced by observations of birds, reptiles, amphibians and ray-finned fish.

Experts at the University of Gothenburg in Sweden collected data on mammal extinctions over nearly 126,000 years to assess the magnitude of the threat posed to biodiversity by current trends. They analyzed changes in extinction rates, identifying specific periods of accelerated extinction rates and comparing them with changes in human populations over the same period. The results were published in the journal Science Advances.

Experts found that the current extinction rate is about 1,700 times that of the beginning of the Late Pleistocene. If it is based on the current level, it will only take 8.10 years for 351 species of animals to become extinct, but if the speed of 126,000 years ago is maintained, it will take about 1.75 million years for 351 species of animals to become extinct. In addition, research shows that there are four time periods where the extinction rate has accelerated significantly, namely 63,800-32,200 years ago, 16,000-9,500 years ago, 2300-600 years ago, and 180-120 years ago.

Computer model data shows that the first two time periods partly coincide with the colonization of Australia (65,000-44,000 years ago) and the Americas (24,000-12,000 years ago). Experts therefore judge that human activities are the main cause of animal extinction. They studied the impact of human population size and area of human activity on the rate of extinction of animals. The results showed that population density reflected extinction rates with around 97 percent accuracy. Experts believe that this figure is also related to other human factors, including the development of hunting and agriculture, the use of fire, deforestation and urbanization.

Experts say the findings so far pale in comparison to projections of animal extinctions over the next 80 years. They believe that by 2100, threats to biodiversity will increase significantly, with 502 to 610 species of mammals becoming extinct. Africa, the Americas, and Eurasia are likely to see large extinctions: The continents' extinction rates remain low, but many species are threatened with extinction. Strong environmental policies can help reduce extinction threats.