Endoceras

Endoceras comes from Ancient Greek words meaning "inside" and "horn". It was a type of large, straight-shelled sea creature called a cephalopod. These amazing animals lived a very long time ago, between 470 and 443 million years ago, during a time called the middle and upper Ordovician.

Endoceras belonged to a group of creatures known as Endocerida. One special feature of Endoceras was a big tube-like structure called a siphuncle along its shell. This siphuncle helped the creature stay balanced in the water. Scientists first gave this name, Endoceras, to these fossils in 1847.

Fossils of Endoceras have been found in many places, especially in North America and Europe, but also in Australia. Endoceras looked similar to another ancient sea creature called Cameroceras, but it had some differences. For example, the siphuncle in Endoceras was on the underside of the shell.

We believe that grown-up Endoceras were likely sneaky hunters, waiting on the sea floor to catch their food. Younger Endoceras with slimmer bodies might have moved around more to find food.

Size

A specimen of Endoceras giganteum at the Museum of Comparative Zoology measures 3 meters (9.8 feet) long as it was found. Scientists in 2015 think the full shell may have been about 5.73 meters (18.8 feet) long. This would make it the largest cephalopod by length that we know of from fossils.

Even though it was very big, the shell of Endoceras was narrow, so the animal would have been lighter than other large cephalopods. A study from 2025 guessed that the biggest shells weighed around 100 kilograms (220 pounds). For comparison, the heaviest ammonoid (Parapuzosia seppenradensis) may have weighed about 400 kilograms (880 pounds), and the living colossal squid could reach 495 kilograms (1,091 pounds).