What Is a Belemnite? Bullet-Shaped Fossils From an Animal Like a Squid

Belemnites were cephalopods related to modern squid and cuttlefish, with ten arms bearing hooks rather than suckers, large eyes, an ink sac and a streamlined body. Inside that body sat a hard structure in three parts: the guard, a solid calcite cylinder tapering to a point at the rear, which is what nearly all fossils are; the phragmocone, a chambered conical structure set into the front of the guard that functioned for buoyancy in the same way an ammonite shell did; and the pro-ostracum, a thin blade extending forward. The guard is dense, solid calcite and survives almost anything, which is why it dominates the fossil record of the group while the more delicate parts are rare. Its function was probably as a counterweight balancing the buoyant chambered section, allowing the animal to swim horizontally, which is a straightforward mechanical requirement that the arrangement satisfies neatly.

The chemistry of the guard has made these fossils unusually valuable:

• •The calcite was deposited in equilibrium with seawater, so its oxygen isotope ratios record the temperature at which it formed
• •That made belemnites a standard material for reconstructing Jurassic and Cretaceous ocean temperatures, and they were used in the foundational work establishing isotopic palaeothermometry
• •One specimen became the original reference standard for oxygen and carbon isotope measurements, with values reported relative to a belemnite from South Carolina, and that standard is still named after it despite the material being exhausted
• •Growth increments within the guard record seasonal variation, allowing temperature to be tracked within a single animal's lifetime
• •Trace element composition supplies further environmental information
• •Their abundance means large samples are available, which is what makes statistical work possible

Guards occur in enormous numbers in certain marine deposits, and some beds contain so many aligned in the same direction that they record current flow across the sea floor, which sedimentologists use as a directional indicator. Accumulations described as belemnite battlefields contain thousands of guards packed together, and the explanation has been argued over, with mass mortality after spawning proposed on the analogy of modern squid, which die after breeding, alongside predator accumulation and simple hydraulic sorting by currents. Guards are also found inside the fossilised gut contents of marine reptiles and sharks, which establishes them as prey and supplies direct evidence of Mesozoic food webs, and hooks from their arms turn up in the same places. The rarity of soft tissue means the few exceptional specimens preserving arms, ink sacs and hooks carry a great deal of the anatomical information, and several come from a small number of unusually preserving deposits.

Guards are conspicuous, durable and shaped unlike anything obviously biological, which produced a substantial body of interpretation before they were understood. They were called thunderbolts or thunderstones across much of Europe, on the belief that they were produced where lightning struck the ground, and they were collected and kept for protection. Devil's fingers, elf bolts and various local names attach to them. They were used medicinally, with powdered guard prescribed for eye complaints and for horses, and comparable uses are recorded for other fossils including the sea urchins called shepherd's crowns. That pattern, in which a durable and distinctive fossil acquires a supernatural explanation and a practical use, recurs with ammonites interpreted as coiled snakes, with fossil sea urchins, with sharks' teeth called tongue stones and with many others, and the folklore is well documented and is itself a subject of study.

Belemnites were abundant through the Jurassic and much of the Cretaceous and disappeared at the end of that period, alongside the ammonites and for what appear to be related reasons. Both groups had planktonic larvae dependent on a food web that collapsed after the asteroid impact, while the cephalopods that survived, including the ancestors of modern squid, octopus and cuttlefish and the nautiluses, had either different reproductive strategies or occupied habitats less affected. Their disappearance left the ecological role available, and the modern squid groups expanded afterwards. Fossils of their descendants and relatives are far rarer than belemnite guards because modern squid have reduced or lost the hard internal structure entirely, retaining only a flexible pen, which preserves badly. That is a useful illustration of how the fossil record's apparent abundance tracks preservability rather than actual numbers, since squid today are enormously abundant and will leave almost nothing.

The common bullet-shaped fossil is a solid calcite counterweight from inside a squid-like animal, balancing a chambered buoyant section further forward. Its calcite formed in equilibrium with seawater, so it records ocean temperature, and one specimen became the reference standard that isotope measurements are still reported against. They were called thunderstones and powdered for medicine before anyone knew what they were.