The small holes in three fossil clams reveal that 75 million years ago, ancient octopuses burrowed into their prey. The discovery pushes the evidence for this behavior 25 million years ago, according to scientists, Feb. 22 in the Biological Journal of the Linnean Society.
The clams, Nymphalucina occidentalis, lived in what is now South Dakota, where an inland sea divided the west and east of North America. While examining the shells, now at the American Museum of Natural History in New York, paleontologists Adiël Klompmaker of the University of Alabama at Tuscaloosa and Neil Landman of AMNH saw holes in an oval shape. Each hole was between 0.5 and 1 millimeter in diameter, thinner than a strand of spaghetti.
A modern octopus uses a sharp ribbon of teeth called a radula on its tongue to drill a hole in the thick shell prey – useful for when the shell is too hard for the octopus to separate with its suction cups. The octopus then injects venom into the hole, paralyzing the prey and dissolving it slightly, which makes the food easier. Holes drilled with octopus were previously found in shells dating back 50 million years, but the new discovery suggests that this drilling habit evolved 25 million years earlier in its history.
These holes increase the poor fossil record of octopus evolution. The soft bodies of the eight-armed intelligent Einstein do not lend themselves well to fossilization, but tend to decay (SN: 8/12/15). That fossils exist – a handful of specimens dating back about 95 million years – suggest few changes in the basic body plane from ancient to modern octopuses.
The finding also places the evolution of octopus drilling in the Mesozoic Marine Revolution, an escalation in the ancient arms race between oceanic predators and prey (SN: 15/06/17). During the Mesozoic era, which spanned between 251 and 66 million years ago, predators lurking near the bottom of the sea became experts at crushing or piercing the shells of their prey.