Life › Seas, forests & the move to land › Vertebrates move onto land (tetrapods)
Synapsids: the mammal line
from c. 310 million years ago· Organism or group· Animals
Well establishedStrong fossil, genetic or geological evidence.
The branch of amniotes that leads to mammals, marked by one skull opening behind each eye.
See it on the Life map →What happened
Soon after the first amniotes appeared, the lineage split in two. Synapsids ("fused arch") have a single opening behind each eye where jaw muscles attach; reptiles and birds come from the other branch, the sauropsids. Early synapsids such as Archaeothyris, about 306 million years ago, and later Dimetrodon were once called "mammal-like reptiles", but they were never reptiles. By the late Permian, therapsids, including tusked plant-eaters and saber-toothed predators, were the dominant land animals.
Why it matters
This is our own lineage. Mammal traits such as varied teeth, a stronger bite and eventually hair and warm blood built up step by step in synapsids over about 100 million years.
The evidence (fossils, genes, rocks)
A dense fossil sequence, especially from South Africa's Karoo Basin and Russia, records gradual changes in jaws, ears and teeth from early synapsids to mammals.
Still debated
When synapsids gained hair and warm-bloodedness is uncertain; estimates range from the late Permian to the late Triassic.
Inside this (1)
Connections (4)
▸The amniotic egg was inherited by Synapsids: the mammal lineWell established
The connection
Synapsids were among the first amniotes and inherited the egg's membranes from the common ancestor of all amniotes. Mammals still keep those membranes: platypuses and echidnas lay leathery eggs, while in placental mammals, including humans, the amnion surrounds the fetus inside the mother.
When and how
Late Carboniferous, about 310–306 million years ago, soon after amniotes appeared; early synapsids such as Archaeothyris come from Nova Scotia.
The evidence
All living mammals share the amnion and other egg membranes with reptiles and birds, and skeletons place synapsids inside the amniote family tree.
Debates and open questions
No early synapsid eggs have been found, so whether they laid hard or soft eggs, or held eggs in the body, is unknown.
▸Synapsids: the mammal line include DimetrodonWell established
The connection
Dimetrodon belongs to the early synapsids once lumped as "pelycosaurs". Its skull shows the single opening behind each eye that defines synapsids. Its family, the sphenacodontids, is the closest known relative of the therapsids, the later synapsids that led to mammals, so it sits near our own ancestral line.
When and how
Early Permian, about 295–272 million years ago, in the Red Beds of Texas and Oklahoma and in Germany; named by Edward Drinker Cope in 1878.
The evidence
Skull, jaw and tooth anatomy, studied since the 1800s and tested in modern family-tree analyses, place Dimetrodon firmly among synapsids.
Debates and open questions
Sails evolved separately in Dimetrodon and the plant-eating Edaphosaurus; why sails arose more than once is still studied.
▸The Great Dying (end-Permian extinction) devastated Synapsids: the mammal lineStrong evidence
The connection
Before the extinction, therapsid synapsids ruled land, from tusked dicynodont plant-eaters to saber-toothed gorgonopsian predators. Most of their lineages died out. A few survivors, notably Lystrosaurus and small cynodonts on the line to mammals, carried synapsids into the Triassic, where archosaurs, the reptile line leading to dinosaurs, soon took the lead.
When and how
Land ecosystems across Pangaea about 252 million years ago, best recorded in South Africa's Karoo Basin and in Russia.
How sure are we?
Fossil counts from the Karoo and elsewhere show most synapsid species disappearing at or near the boundary.
Debates and open questions
Whether land losses were simultaneous with sea losses, and whether the Karoo shows one sharp pulse or a drawn-out decline, are debated.
▸Synapsids: the mammal line led to First mammalsWell established
The connection
Over about 100 million years, synapsids gradually gained mammal traits: first varied teeth, then a bony palate for breathing while chewing, then a new jaw joint. In late Triassic cynodonts, two old jaw bones shrank and moved into the middle ear, and the first mammals emerged.
When and how
Late Triassic, about 225–200 million years ago; key fossils come from South Africa, Wales and China.
The evidence
A dense series of fossils shows each step, and embryos of living mammals still build the middle-ear bones from parts of the early jaw.
Debates and open questions
Exactly which cynodont group gave rise to mammals is debated, and where to draw the line called "mammal" varies between researchers.