|1st Species||Endoceras proteiforme|
|2nd Species||Endoceras primigenium|
|3rd Species||Endoceras perannulatum|
|4th Species||Endoceras deparcum|
|Time Range||470-445 mya|
|Name Meaning||Inner horn|
|Physical Dimensions||5.7 meters long|
Endoceras is an endoceratid endoceridan cephalopod mollusk from the Middle-Late Ordovician of the entire world. It was named in 1847 by James Hall. It was on par with the orthoceridan mollusk Orthoceras in terms of distribution, and it was one of the largest orthoconic cephalopods, only beaten in size by Cameroceras.
Like its cousin, Cameroceras, Endoceras had the generic orthoconic cephalopod body plan. It was a squid-like mollusk with tentacles long enough to capture food, a small mouth hiding between the circular row of tentacles, and a large, straight shell. Its body would have been covered in smooth skin, and mostly covered by its hard shell.
Endoceras was a planktivore, sucking up colonies of plankton drifting through the water. Its tentacles would have been used to trap plankton, while its mouth would suck them up.
Endoceras was near the top of the food chain, as it would have been immune to predation from Megalograptus; the only cephalopod that could be higher than it on the food chain would have been Cameroceras. Endoceras was once thought to be a large nektobenthic ambush predator, grabbing any unsuspecting fish, trilobites, eurypterids, and smaller cephalopods with its tentacles before killing them with a bite from its hard beak. However, no evidence of a hard beak has been found, and it is likely that its huge, straightened shell would have impeded its movement, proving to be inefficient when hunting free-swimming prey. Now, it is theorized that it swam near the surface and fed on plankton, possibly using its tentacles to trap plankton before sucking it up with its mouth.