LIFE × EVOLUTION

The Shell Crushers

~235 to 66 million years ago

Phanerozoic · Mesozoic · Triassic

Crabs, rays and bony fish able to crush a shell spread through the Mesozoic seas, and their prey answered with thicker walls, tighter coiling, spines and the habit of burying themselves. Sitting still on the seabed stopped being safe.

Why it matters

The shells record both sides of it, which is rare: the armour on one, the drill holes and the crush marks on the other. That makes predation readable as a force that shapes bodies, over spans no experiment can reach. Since the Middle Devonian at least, what a seabed animal looks like, and whether its lineage lasted, has depended in part on what could crack it open.

Dating & uncertainty

The escalation is read from shells: thicker walls, tighter coiling, spines, and drill holes left by predators. When it starts and how fast it goes are disputed, and so is the causal reading itself: predators and armoured prey rising together make a correlation before they make an arms race. The mid-Palaeozoic had a well-documented dress rehearsal, and counting broken shells suggests much of the increase is in fact Cenozoic, which would make the Mesozoic the middle of the story rather than the whole of it.

Sources

1. Signor & Brett (1984), Paleobiology · The mid-Paleozoic precursor to the Mesozoic marine revolution
2. Tackett (2016), Palaios · Late Triassic durophagy and the origin of the Mesozoic marine revolution
3. Oji, Ogaya & Sato (2003), Geology · Increase of shell-crushing predation recorded in fossil shell fragmentation

Earth history as one day

22:45:271 h 15 min before midnight

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