Which of following belongs to non-extinct infraclass of mammalia?

Question

Accepted Answer

The correct option is 3: Ornithodelphia

The class Mammalia is broadly classified into subclasses, and further refined into infraclasses, to delineate the major evolutionary lineages. The three non-extinct (extant) groups of mammals are the Monotremes, the Marsupials, and the Placentals. The infraclass **Ornithodelphia** is the formal taxonomic term that contains the modern Monotremes, which include the duck-billed platypus and four species of echidna (spiny anteaters). These are the only mammals that reproduce by laying eggs, a primitive trait retained from their reptilian ancestors, and they represent the most evolutionarily distinct and earliest diverging lineage of living mammals. Because this infraclass includes currently living species, it is the only group among the choices that belongs to a *non-extinct* lineage, thus satisfying the condition of the question. For completeness, the other two non-extinct infraclasses are Metatheria (Marsupials) and Eutheria (Placental mammals).

The incorrect option is 1: Eotheria

The term Eotheria is an outdated and largely obsolete classification in mammalian paleontology. Historically, it was used to refer to a very early and primitive grouping of Mesozoic mammals, often encompassing species that are basal to the main mammalian family tree or are classified in other extinct orders today. Regardless of the specific historical usage, Eotheria does not represent a natural or monophyletic group that has any living members. Since all animals ever classified under Eotheria are known only from the fossil record and are entirely extinct, this choice is incorrect.

The incorrect option is 2: Allotheria

Allotheria is a formally recognized infraclass of Mammalia, but it is an entirely **extinct** group. This infraclass is famously characterized by the order Multituberculata, an ancient and remarkably successful group of rodent-like mammals. Multituberculates were a dominant component of the mammalian fauna for approximately 120 million years, from the Middle Jurassic all the way through the Paleocene. Despite their widespread presence and long existence, the last of the Allotherians died out during the Eocene epoch, likely due to competition from true rodents. Therefore, Allotheria is strictly a collection of extinct species, making it an incorrect answer.

The incorrect option is 4: Pantotheria

Pantotheria is another term used to describe a broad, often paraphyletic, assemblage of primitive mammals from the Mesozoic Era. This group is considered significant because its members are thought to be closely related to the ancestral stock that gave rise to the modern subclass Theria (Marsupials and Placentals). As with the other incorrect options, all species considered part of Pantotheria are known solely from the fossil record and became extinct by the end of the Cretaceous period. The term denotes a transitional, but entirely vanished, evolutionary grade, and thus is an **extinct** infraclass, or possibly a collection of extinct orders, not a non-extinct one.