Fossil Mammals and Synapsids

The synapsid lineage records the long transition from early amniotes to mammals and the Cenozoic radiation of mammals after the dinosaurs.

Definition

Synapsids are the amniote lineage characterized by a single temporal skull opening that includes mammals and their extinct relatives; fossil mammals are the members and stem relatives of the mammalian crown group.

Scope

This topic covers the synapsid clade from pelycosaurs and therapsids through cynodonts to true mammals, the origin of mammalian features such as the jaw joint and middle ear, the small mammals of the Mesozoic, and the explosive Cenozoic diversification of placentals and marsupials.

Core questions

• How did mammals evolve from earlier synapsids?
• What skeletal changes mark the origin of the mammalian jaw and ear?
• What were Mesozoic mammals like and how did they live?
• How did mammals radiate after the end-Cretaceous extinction?

Key concepts

• Synapsid temporal fenestra
• Cynodont-mammal transition
• Jaw joint to middle ear bones
• Cenozoic mammalian radiation

Key theories

Origin of the mammalian middle ear

Bones of the reptilian jaw joint were progressively incorporated into the mammalian middle ear, a classic transition documented across cynodont and early mammal fossils.

Post-Cretaceous mammalian radiation

Released from dinosaur dominance, mammals diversified rapidly in the early Cenozoic into the major living orders.

Clinical relevance

Fossil synapsids document one of the best-recorded major evolutionary transitions, and Cenozoic mammal faunas serve as sensitive indicators of past climate, biogeography, and ecosystem change.

History

The mammal-like reptiles of the Karoo Basin and elsewhere were described from the nineteenth century onward and became the textbook example of a gradual evolutionary transition. Discoveries of Mesozoic mammals from China and Mongolia have transformed understanding of early mammalian diversity.

Debates

Timing of crown-mammal diversification

Whether the major modern mammal groups diverged before or only after the end-Cretaceous extinction is debated between fossil and molecular evidence.

Key figures

• Tom S. Kemp
• Zofia Kielan-Jaworowska
• Zhe-Xi Luo

Seminal works

• kemp2005
• kielanjaworowska2004

Frequently asked questions

What are mammal-like reptiles?

They are extinct synapsids, the lineage that led to mammals; the term is now usually replaced by non-mammalian synapsids such as pelycosaurs and therapsids.

When did mammals become dominant?

Mammals existed alongside dinosaurs as mostly small animals, then radiated rapidly into large and diverse forms in the Cenozoic after the dinosaurs went extinct.