EVOLUTION × LIFE

The First Jawed Vertebrates

~439 to 419 million years ago

Phanerozoic · Paleozoic · Silurian

The jawed vertebrates are first known from two finds in South China whose ages differ by three million years, between 439 and 419 million years ago, and the older find is a set of tooth whorls, teeth set in paired rows.

Gnathostomes are the jawed vertebrates. Their beginning is known from two finds in South China whose ages differ by three million years. The older find is a set of tooth whorls. The teeth of a whorl were never shed: new teeth were added on the inner side as the animal grew, so that one whorl preserves the whole sequence of a lifetime. The younger find is an entire fauna of fishes preserved complete, each small enough to be held in the hand. It includes placoderms, which were armoured fishes, and early relatives of sharks. Jaws turn a filter feeder, an animal that strains small particles of food from the water, into a predator. Every jawed vertebrate since, humans included, descends from the first gnathostomes.

Why it matters

Three bones of the human jaw are the premaxilla and the maxilla, in the upper jaw, and the dentary, in the lower jaw. They were named before their antiquity was known. Zhu and colleagues (2013) described a placoderm 419 million years old, from rocks of the Silurian period in China, with the same three bones in the same positions. This fossil indicates that the three bones predate the separation between bony fishes and sharks. It also indicates that the ancestors of sharks had these bones and that sharks lost them.

Dating & uncertainty

Isolated tooth whorls about 439 million years old are the oldest direct evidence of jawed vertebrates. The oldest complete jawed fishes are 436 million years old. Molecular estimates, which date the origin of a group from the differences between the genes of living species, place the origin of jawed vertebrates earlier than both, in the Ordovician, the period before the Silurian.

Earth history as one day

21:40:452 h 19 min before midnight

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