Prototheria

mammal subclass

Written by

The Information Architects of Encyclopaedia Britannica

Last updated

July 13, 2026

Quick Summary

Prototheria is a subclass of mammals that includes the monotremes, which are the platypus and echidnas. These animals are unique among mammals because they lay eggs. Monotremes are considered the most ancient order of mammals, possessing a combination of primitive and specialized features. They are found in Australia, Tasmania, and New Guinea. Both sexes have a single opening for reproduction and excretion. While they share these primitive traits, they have evolved distinct adaptations for their specific ecological niches.1234

Summary created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

6 Britannica Sources

Ask Britannica AI about this topic

Britannica doesn't have a full article on this topic, so we've curated a list of our relevant coverage.

vertebrate
Annotated classification
Subclass Prototheria Primitive; egg-laying; hair; mammary glands without nipples; pectoral girdle; separate oviducts that open into cloacal chamber that is shared with excretory ducts; oviparous. Subclass Theria Mammary glands with nipples; functional teeth; oviducts partly fused; with or without a cloaca; uterus and vagina; viviparous.(Ed.)
Read More
Subclass Prototheria Primitive; egg-laying; hair; mammary glands without nipples; pectoral girdle; separate oviducts that open into cloacal chamber that is shared with excretory ducts; oviparous. Subclass Theria Mammary glands with nipples; functional teeth; oviducts partly fused; with or without a cloaca; uterus and vagina; viviparous.(Ed.)
Read More