LIFE

The First Mammals

~220–205 million years ago

Phanerozoic · Mesozoic · Triassic

Animals the size of a shrew, probably out at night, with a jaw that had come to hinge directly on the skull, and molars that met precisely enough to chew. They are usually described as generalist insect-eaters, and two of the earliest turn out not to be: measured for bite force and for the wear on their teeth, Morganucodon was cracking hard prey, beetles among them, while Kuehneotherium, alongside it, took soft-bodied insects such as moths.

Why it matters

Two-thirds of mammalian history unfolded in the shadow of the dinosaurs, and it is tempting to read that as time spent waiting. It was not. The teeth were specialising from the very start, and it was across that long stretch that our lineage acquired its way of chewing, hearing and burning energy, well before it had room to grow large.

Dating & uncertainty

The mammaliaforms are a wider group than the mammals, and the boundary between them has shifted with each revision of the family tree, so the first mammal has no fixed date. The bones left over from the old jaw hinge became the small bones of the middle ear, a transition that can be followed fossil by fossil, which is unusual.

Sources

1. Gill et al. (2014), Nature · Dietary specializations and diversity in feeding ecology of the earliest stem mammals
2. Bi et al. (2014), Nature · Three new Jurassic euharamiyidan species reinforce early divergence of mammals
3. Luo et al. (2001), Science · A new mammaliaform from the Early Jurassic and evolution of mammalian characteristics

Earth history as one day

22:53:231 h 7 min before midnight

if Earth’s 4.54 billion years were compressed into a single day