|1st Species||Orthoceras regulare|
|Time Range||463-460.9 mya|
|Location||Estonia, Lithuania, Sweden, Ukraine, Belarus|
|Name Meaning||Straight horn|
|Physical Dimensions||37.5 centimeters-1.3 meters long|
Orthoceras is an orthoceratid orthoceridan cephalopod mollusk from the Middle Ordovician of Estonia, Lithuania, Sweden, Ukraine, and Belarus. It was named in 1820 by Ernst Friedrich, Baron von Schlotheim. It was the most common orthoconic cephalopod during the start of the Paleozoic era, and is often confused with straight-shelled ammonites.
Orthoceras represented the generic orthoconic cephalopod. It was a squid-like mollusk with It was a squid-like mollusk with tentacles long enough to capture food, a small mouth hiding between its circular row of tentacles, and a 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.
Despite its small size, Orthoceras was once thought to be a nektobenthic ambush predator, feeding on small invertebrates; in order to hunt, it would have swam up to its prey, right before proceeding to grab them with its tentacles and tear them apart with its beak. However, no evidence of a hard beak has been found, and it is likely that its large shell would have impeded its movement, making for an incompetent predator of free-swimming prey. Now, it is theorized that it fed on plankton drifting near the surface, possibly using its tentacles to trap prey before sucking it up with its mouth. As well as this, when attacked by predators, it may have been able to hide by withdrawing into its shell.