Prehistoric Mammals

From rat-sized Mesozoic survivors to giant Ice Age megafauna — the Age of Mammals spanned 66 million years after the dinosaurs.

Time Range

225 Ma–Present

Cenozoic peak

66–2 Ma

Largest

Paraceratherium ~20t

Status

Extant class

Overview

Mammals began as small, nocturnal insectivores living alongside the dinosaurs, then radiated after the K-Pg extinction into whales, bats, horses, elephants, primates and the giant Ice Age megafauna. Their fossil record documents the rise of the lineage that now includes us.

Quick Facts

Geological Range

Triassic stem-mammals (~225 Ma); crown-group Mammalia by the Jurassic (~200 Ma) – Present

Major Habitat

Terrestrial, aerial, freshwater and marine — virtually every environment

Known Genera

Species Documented

Registry Specimens

0 — grows with the Global Registry

Countries Represented

Data currently being expanded.

Key Characteristics

•Differentiated teeth (incisors, canines, premolars, molars) — the most common fossils
•Single lower jaw bone and dentary–squamosal jaw joint
•Milk secretion and (in most) live birth
•Rapid evolution after the K-Pg extinction produced giants like Paraceratherium and mammoths

Fossil Record

Teeth are abundant and highly diagnostic; complete skeletons come from tar pits (Rancho La Brea), river gravels, cave deposits and Lagerstätten like Messel (Germany). Whale evolution is spectacularly documented in Pakistan, Egypt and North America.

Global Distribution

Global — Cenozoic continental deposits on all continents, with distinctive isolated radiations in Australia and South America.

Evolutionary & Scientific Importance

Mammal fossils record recovery after mass extinction, the origin of whales from land mammals, primate evolution, and the human-induced late Quaternary megafauna extinctions.

Classification

Kingdom

Animalia

Phylum

Chordata

Clade

Synapsida → Mammalia

Major groups

Monotremes · Marsupials · Placentals (whales, elephants, primates, carnivores and more)