LIFE × EVOLUTION

The Shell That Rolled Up

~409 to 393 million years ago

Phanerozoic · Paleozoic · Devonian

Cephalopods that had carried a straight cone rolled it into a spiral in the Early Devonian. Through the rest of the period the coil wound tighter, the shell each animal hatched with grew smaller, and adults in some lineages grew larger, a combination that probably reflects a shift towards producing many more young. The spiral is a buoyancy device: gas in the sealed chambers behind the animal holds it in mid-water without effort. The design lasted 340 million years and went out with the dinosaurs.

Why it matters

Ammonoids are among the best clocks in Palaeozoic and Mesozoic rock. Their species changed fast enough, and spread widely enough, that a single shell can date the layer it lies in to within a few hundred thousand years. Much of the timescale used elsewhere on this timeline was built on them.

Dating & uncertainty

Ammonoids derive from bactritoids with straight shells in the Early Devonian, but the first appearance moves with each new locality, and the embryonic shell closed its coil independently in at least three lineages. Any single origin date is a convention, not a discovery.

Sources

1. De Baets et al. (2012), Evolution · Early evolutionary trends in ammonoid embryonic development
2. Klug et al. (2015), Topics in Geobiology · Ancestry, origin and early evolution of ammonoids
3. Klug et al. (2010), Lethaia · The Devonian nekton revolution

Earth history as one day

21:50:162 h 10 min before midnight

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