Squid remade the nautiloid shell into a chitin pen and jet
Squid in Myopsida, Oegopsida, and Bathyteuthida are soft-bodied cephalopods with eight arms, two prey tentacles, huge eyes, and Jurassic–Cretaceous radiation into open-water predator niches. Color change, ink clouds, and sometimes bioluminescent counter-illumination equip the chase and the escape.
A squid's body is folded around its hunting. Head and foot sit at the leading end of the long mantle; the ancestral foot became the ring of eight arms and two tentacles around the mouth and a funnel through which water jets out. Each arm is a muscular hydrostat lined with suckers whose chitin-stiffened rims may carry tiny teeth, and a small ganglion under each sucker gives it independent control. The retractable tentacles, far longer than the arms, carry suckers only on their flattened tips. In mature males, part of one left arm ends in a pad used to place a sperm packet inside the female's mantle.
The ancestral coleoid probably had a straight, chambered shell buried in its mantle for buoyancy. Squid kept only a feather-shaped chitinous pen that stiffens the mantle and anchors muscle, while the calcareous cuttlebone of cuttlefish seems to have evolved afresh much later. Their eyes rival those of vertebrates. The vampire squid, despite its name, is closer to octopuses, and bobtail and ram's horn squid belong to separate orders.
True squid split from other cephalopods in the Jurassic and flourished about 100 million years ago, radiating alongside teleost fish until the two resembled each other in size, habitat and behaviour. Because soft bodies decay and even beaks are fragile, Mesozoic squid fossils are scarce, and some studies wrongly concluded that squid were far younger. In 2025, researchers used grinding tomography to scan fragile beaks from Japan's Yezo Group, identifying both main squid orders from the earliest Cenomanian and suggesting squid were already abundant by the mid-Cretaceous.
Squid change colour, squirt ink, and sometimes glow to erase their silhouettes, and they are fished from Japan to the eastern Pacific for calamari.
Source: Squid
