What Were the First Mammals? Two Hundred Million Years in the Shadows
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Mammals did not arrive after the dinosaurs died. They arrived at the same time as the dinosaurs, around 210 million years ago, and then spent the next 145 million years living beneath them, mostly shrew-sized, mostly at night, eating insects and being eaten. The traits that define mammals today, fur, warm blood, milk, a big brain relative to body size and a keen sense of smell and hearing, were shaped by that long apprenticeship in the dark.
Descended from the losers of the Permian
The mammal line does not come from reptiles. It comes from the synapsids, a group that split from the reptile line over 300 million years ago and dominated the land in the Permian period, when Dimetrodon and the therapsids that followed it were the largest animals alive. The end-Permian extinction 252 million years ago wiped out most of them, and in the Triassic recovery the surviving synapsids were pushed aside by the archosaurs, the group that produced crocodiles and dinosaurs.
The cynodonts, the synapsids that survived, shrank. Fossils from the late Triassic show a sequence of small animals acquiring the mammal package piece by piece: teeth of different kinds for cutting and grinding, a secondary palate that allows breathing while chewing, a jaw hinged on a single bone, and the two bones that had formed the old jaw joint migrating into the ear to become the tiny hammer and anvil that give mammals their sharp hearing. The transition is one of the best documented in the fossil record.
The first true mammals
Where exactly to draw the line is a matter of definition, but by around 205 million years ago there were animals such as Morganucodon, found in Wales and China, that most palaeontologists call mammals: about ten centimetres long, weighing less than a mouse, with a mammalian jaw joint, fur suggested by the fine preservation of relatives, and teeth that were replaced only once, which points to a mother feeding milk to young that did not yet need to chew.
They were nocturnal, and that fact shaped everything. Working by night favoured hearing and smell over sight, and the parts of the brain handling those senses grew. Keeping warm through a cold night favoured fur and a fast metabolism. Being small and hunted favoured many offspring and fast growth. The mammal design was an answer to the question of how to make a living while the dinosaurs held the day.
Not as dull as the textbooks said
The picture of Mesozoic mammals as an unchanging line of shrews has collapsed in the last twenty-five years, largely because of fossils from northeastern China preserved with fur and stomach contents. Castorocauda, from 165 million years ago, was a beaver-like swimmer with a flat tail. Volaticotherium glided on a skin membrane like a flying squirrel. Fruitafossor dug for termites with mole-like forelimbs. Repenomamus, the size of a badger, had a young dinosaur in its stomach.
The three living groups had also separated by the middle of the age of dinosaurs. The egg-laying monotremes, today's platypus and echidnas, branched off first. The marsupials, which give birth to tiny young that finish developing in a pouch, and the placentals, which carry their young to a much later stage inside the body, had diverged from each other by around 160 million years ago. Juramaia, a 160-million-year-old fossil from China, is the earliest known member of the placental line, the one that leads to us.
The morning after the asteroid
The asteroid that ended the Cretaceous 66 million years ago killed every animal larger than a dog on land, and the small, burrowing, seed-and-insect-eating mammals were among the survivors. What followed was the fastest diversification in mammal history. Within a few hundred thousand years mammals were reaching the size of a sheep; within ten million years there were mammals the size of rhinos, along with the first primates, bats, whales and horses.
Body size, which had been capped at a few kilograms for 150 million years, became unlimited once the dinosaurs were gone, and the brain, already large for the body, scaled up with it. The mammal traits that had been adaptations to living small in the dark turned out to work at every size, in the sea and in the air. The main stages:
- •300 million years ago: synapsids split from the reptile line
- •252 million years ago: the Permian extinction leaves a few cynodont survivors
- •205 million years ago: the first true mammals, mouse-sized and nocturnal
- •160 million years ago: monotremes, marsupials and placentals have separated
- •66 million years ago: the dinosaurs go, and mammals fill the empty world
The takeaway
The first mammals appeared around 205 million years ago from small synapsid survivors of the Permian extinction, and lived for 140 million years as nocturnal animals in the shadow of the dinosaurs, evolving fur, milk, sharp hearing and large brains in the process. The living groups had already split before the asteroid struck, and when it did, the mammals were ready.