LIFE

The First Mammals

~220–205 million years ago

Phanerozoic · Mesozoic · Triassic

The first mammaliaforms, a group wider than mammals, were shrew-sized and probably nocturnal, between 220 and 205 million years ago, with a jaw hinged directly on the skull and molars that met precisely enough to chew.

Mammaliaforms are a group wider than mammals. The first mammaliaforms were animals the size of a shrew, probably nocturnal. Their jaw had come to hinge directly on the skull at the dentary-squamosal joint, between the dentary, the bone of the lower jaw, and the squamosal, a bone of the skull. This joint defines the group. Their molars met precisely enough to chew. Mammaliaforms are usually described as generalist insectivores, animals that eat insects of any kind. Gill and colleagues (2014) measured bite force and tooth wear in two of the earliest and found that neither was a generalist. Morganucodon cracked hard prey, including beetles. Kuehneotherium, which lived alongside it, took soft-bodied insects such as moths.

Why it matters

Two thirds of mammalian history unfolded in the presence of dinosaurs. During that long interval the teeth of mammaliaforms were specialising from the outset, and the mammal lineage acquired its mode of chewing, of hearing and of producing energy. These features were in place well before mammals had the space to become large.

Dating & uncertainty

Researchers have moved the boundary between mammaliaforms and mammals with each revision of the family tree, so the first mammal has no fixed date. The jaw of earlier animals hinged at another joint. The bones left over from that old joint became the ossicles of the middle ear, the small bones that carry sound. Fossils show this transition step by step, which is unusual.

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