Information:  - Birds (Aves), also known as avian dinosaurs, are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. Birds live worldwide and range in size from the bee hummingbird to the ostrich. They rank as the class of tetrapods with the most living species, at approximately ten thousand, with more than half of these being passerines, sometimes known as perching birds or, less accurately, as songbirds.  - Mammals are any vertebrates within the class Mammalia (from Latin "mamma" "breast"), a clade of endothermic amniotes distinguished from reptiles and birds by the possession of a neocortex (a region of the brain), hair, three middle ear bones and mammary glands. The sister group of mammals may be the extinct "Haldanodon." The mammals represent the only living Synapsida, which together with the Sauropsida form the Amniota clade. The mammals consist of the Yinotheria including monotrema and the Theriiformes including the theria.  - Theriiformes is a subclass of mammals. The term was coined in 1997 by McKenna & Bell in their classification of mammals.  - Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus "Rattus", the most important of which to humans are the black rat, "Rattus rattus", and the brown rat, "Rattus norvegicus". Many members of other rodent genera and families are also referred to as rats, and share many characteristics with true rats.  - Thomas Henry Huxley (4 May 1825  29 June 1895) was an English biologist (comparative anatomist), known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution.  - A mammary gland is an exocrine gland in mammals that produces milk to feed young offspring. Mammals get their name from the word "mammary". The mammary glands are arranged in organs such as the breasts in primates (for example, humans and chimpanzees), the udder in ruminants (for example, cows, goats, and deer), and the dugs of other animals (for example, dogs and cats). Lactorrhea, the occasional production of milk by the glands, can occur in any mammal, but in most mammals lactation, the production of enough milk for nursing, occurs only in phenotypic females who have gestated in recent months or years. It is directed by hormonal guidance from sex steroids. In a few mammalian species, male lactation can occur.  - Ants are eusocial insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the Cretaceous period, about 99 million years ago and diversified after the rise of flowering plants. More than 12,500 of an estimated total of 22,000 species have been classified. They are easily identified by their elbowed antennae and the distinctive node-like structure that forms their slender waists.  - Charles Robert Darwin, (12 February 1809  19 April 1882) was an English naturalist and geologist, best known for his contributions to the science of evolution. He established that all species of life have descended over time from common ancestors, and in a joint publication with Alfred Russel Wallace introduced his scientific theory that this branching pattern of evolution resulted from a process that he called natural selection, in which the struggle for existence has a similar effect to the artificial selection involved in selective breeding.  - Metatheria is one of two mammalian clades , the other being Eutheria , with extant members that diverged in the Early Cretaceous or perhaps the Late Jurassic , and which includes all mammals more closely related to marsupials than to placentals . First proposed by Thomas Henry Huxley in 1880 , it is a slightly more inclusive group than the marsupials ; it contains all of the living mammals with abdominal pouches ( most female marsupials ) as well as their more primitive ancestors and relatives . Metatherians are one of three main classes of extant mammals : monotremes ( egg laying mammals like the platypus and the echidna ) , metatheria ( or marsupials , which includes the three American orders ( Didelphimorphia , Paucituberculata and Microbiotheria ) and the four Australasian orders ( Notoryctemorphia , Dasyuromorphia , Peramelemorphia and Diprotodontia ) ) , and the eutherians ( or placental mammals ) consisting of twenty one orders , divided into four superorders . Metatherians belong to a subgroup of the northern tribosphenic mammal clade or Boreosphenida . They differ from all other mammals in certain morphologies like their dental formula , which includes about five upper and four lower incisors , a canine , three premolars , and four molars . Other morphologies include skeletal and anterior dentition , such as wrist and ankle apomorphies ; all metatherians share derived pedal characters and calcaneal features .  - Evolution is change in the heritable characteristics of biological populations over successive generations. Evolutionary processes give rise to biodiversity at every level of biological organisation, including the levels of species, individual organisms, and molecules.  - The neocortex, also called the neopallium and isocortex, is the part of the mammalian brain involved in higher-order brain functions such as sensory perception, cognition, generation of motor commands, spatial reasoning and language.   - Synapsids (Greek, 'fused arch'), synonymous with theropsids (Greek, 'beast-face'), are a group of animals that includes mammals and every animal more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having a temporal fenestra, an opening low in the skull roof behind each eye, leaving a bony arch beneath each; this accounts for their name. Primitive synapsids are usually called pelycosaurs or pelycosaur-grade synapsids; more advanced mammal-like ones, therapsids. The non-mammalian members are described as mammal-like reptiles in classical systematics; they can also be called stem mammals or proto-mammals. Synapsids evolved from basal amniotes and are one of the two major groups of the later amniotes; the other is the sauropsids, a group that includes modern reptiles and birds. The distinctive temporal fenestra developed in the ancestral synapsid about 312 million years ago, during the Late Carboniferous period.  - Sauropsida ("lizard faces") is a group of amniotes that includes all existing birds and reptiles as well as their fossil ancestors and other extinct relatives. Large land animals are either in this group or in its sister group, Synapsida, which includes mammals and their fossil relatives. This clade includes Parareptilia and other extinct clades. All living sauropsids are members of the sub-group Diapsida, the Parareptilia clade having died out 200 million years ago. The term originated in 1864 with Thomas Henry Huxley, who grouped birds with reptiles based on fossil evidence.  - A clade (from , "klados", "branch") is a group of organisms that consists of a common ancestor and all its lineal descendants, and represents a single "branch" on the "tree of life".  - Vertebrates comprise all species of animals within the subphylum Vertebrata (chordates with backbones). Vertebrates represent the overwhelming majority of the phylum Chordata, with currently about 64,000 species described. Vertebrates include the jawless fish and the jawed vertebrates, which include the cartilaginous fish (sharks and rays) and the bony fish.   - Reptiles are tetrapod (four-limbed vertebrate) animals in the class Reptilia, comprising today's turtles, crocodilians, snakes, amphisbaenians, lizards, tuatara, and their extinct relatives. The study of these traditional reptile orders, historically combined with that of modern amphibians, is called herpetology.  - Theria (Greek: "", wild beast) is a subclass of mammals amongst the Theriiformes (the sister taxa to Yinotheria). Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials).    After reading the paragraphs above, we are interested in knowing the entity with which 'metatheria' exhibits the relationship of 'taxon rank'. Find the answer from the choices below.  Choices: - branch  - class  - family  - form  - group  - order  - phylum  - species  - subclass  - superfamily
Answer:
subclass