Curated timeline

Mammal Evolution

From Synapsids to the Ice Age: A chronological guide to the origin, diversification, and dominance of mammals across 320 million years of Earth history.

• Origin of Synapsida (about 320 Million Years Ago)

  Synapsids — the lineage that would eventually give rise to mammals — diverge from other amniotes during the Carboniferous period.

• Pelycosaurs Dominate (about 300 Million Years Ago)

  Pelycosaurs, early sail-backed synapsids such as Dimetrodon, become the dominant large land vertebrates of the early Permian.

• Rise of Therapsida (about 270 Million Years Ago)

  Therapsids replace pelycosaurs as the dominant land vertebrates. More mammal-like in posture and metabolism, they represent a key step toward true mammals.

• Appearance of Dicynodonts (about 265 Million Years Ago)

  Dicynodonts, herbivorous therapsids with turtle-like beaks and two tusks, emerge and soon become among the most successful land animals of their time.

• Cynodonts Appear (about 260 Million Years Ago)

  Cynodonts, a group of therapsids with differentiated teeth and a more erect posture, emerge. They are the direct ancestors of all modern mammals.

• Permian-Triassic Mass Extinction (about 252 Million Years Ago)

  The 'Great Dying' eliminates ~90% of species. Therapsids are devastated but some cynodonts and dicynodonts survive, setting the stage for the next phase of mammal evolution.

• Cynodont Diversification in the Triassic (about 245 Million Years Ago)

  Surviving cynodonts rapidly diversify in the early Triassic. Some develop traits including secondary palates, differentiated teeth, and possible endothermy.

• Appearance of Eucynodontia (about 240 Million Years Ago)

  Eucynodonts, a more derived cynodont group, arise. They include both the direct mammalian ancestors and the non-mammalian chiniquodontids.

• Origin of Mammaliaformes (about 225 Million Years Ago)

  Mammaliaformes arise — a clade that includes true mammals and their closest relatives. Key features include a dentary-squamosal jaw joint and tribosphenic-like teeth.

• Appearance of Adelobasileus (about 225 Million Years Ago)

  Adelobasileus cromptoni from Texas is one of the earliest known mammaliaforms, known from partial skull material dated to the Late Triassic.

• Triassic–Jurassic Mass Extinction (about 201 Million Years Ago)

  A major extinction event eliminates many large archosaurs and non-mammalian synapsids, opening niches that early mammals would gradually exploit.

• Appearance of Morganucodon (about 200 Million Years Ago)

  Morganucodon, one of the best-studied early mammals, lived alongside early dinosaurs. It was small, likely nocturnal, and had a fully mammalian jaw joint.

• Origin of Monotreme Lineage (about 190 Million Years Ago)

  The lineage leading to monotremes (egg-laying mammals like the platypus) diverges from the therian line, representing the earliest split in crown mammals.

• Haramiyids and Earliest Herbivory (about 190 Million Years Ago)

  Haramiyids represent some of the earliest mammaliaforms to evolve specialized herbivorous teeth, showing dietary diversification very early in mammal history.

• Evolution of Docodonts (about 175 Million Years Ago)

  Docodonts arise with complex molar cusp patterns, suggesting dietary specialization. Fossils like Castorocauda show early mammals were already exploiting aquatic environments.

• Rise of Multituberculates (about 170 Million Years Ago)

  Multituberculates, rodent-like mammals with highly specialized molars, appear. They become the longest-surviving mammal order, lasting over 100 million years.

• Therian Mammals Diverge (about 160 Million Years Ago)

  The two main therian lineages — marsupials (metatherians) and placentals (eutherians) — diverge from their common ancestor during the Middle to Late Jurassic.

• Jurassic Gliding Mammals: Volaticotherium (about 160 Million Years Ago)

  Volaticotherium antiquus, a Jurassic gliding mammal from China, reveals that powered or gliding flight evolved in mammals far earlier than previously thought.

• Earliest Eutherians (about 125 Million Years Ago)

  Eomaia scansoria from China is one of the earliest known eutherians — the lineage leading to all placental mammals — dating to the Early Cretaceous.

• Earliest Metatherians (about 125 Million Years Ago)

  Sinodelphys szalayi from China is among the earliest known metatherians, the lineage that leads to modern marsupials, from the Early Cretaceous.

• Mammals Spread Across Gondwana (about 120 Million Years Ago)

  As continental breakup continues, mammal populations diversify in geographic isolation across Laurasia and Gondwana, driving early biogeographic diversification.

• Repenomamus: A Large Predatory Mammal (about 130 Million Years Ago)

  Repenomamus robustus from China is one of the largest known Mesozoic mammals. A specimen was found with baby dinosaur remains in its gut, showing mammals did eat dinosaurs.

• Marsupials Enter North America (about 100 Million Years Ago)

  Metatherians diversify and spread into North America during the Cretaceous, with a variety of forms occupying different ecological niches.

• Placentals Diversify in the Cretaceous (about 90 Million Years Ago)

  Eutherian mammals undergo significant diversification during the Late Cretaceous, though still mostly small and ecologically subordinate to dinosaurs.

• Cretaceous–Paleogene Mass Extinction (about 66 Million Years Ago)

  An asteroid impact and associated volcanism wipes out non-avian dinosaurs and ~75% of species. Most mammals survive, setting the stage for their explosive diversification.

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