Friday, April 20, 2012


Gonioceras was a benthic actinocerid orthocone of the Ordovician. Its distribution included the eastern half of North America, including Canada and the North Pole area next to Greenland. This would not have meant that it was a polar animal, it just means that the continents have shifted a lot, and that the climate has also changed quite a bit. In the Ordovician, the whole world was tropical, even the poles.

Gonioceras had a convex top of the shell and the bottom was flat. This was ideal for living on the sea floor, because that meant Gonioceras would not sink into the muck. This is the same principle as the spines of many trilobite, which helped the trilobites keep themselves from sinking into soft mud. Trace fossils show that tubular-shelled nautiloids did sometimes rest on the bottom, but they did not live their whole life there as Gonioceras did. Tubular-shelled nautiloids such as Cameroceras also probably sometimes dragged across the bottom to catch trilobites and other benthic prey.

Gonioceras chasing a trilobite

Gonioceras was a unique nautiloid because it was flat. Unlike other nautiloids, it had a triangular form. The name Gonioceras, meaning "angle horn," well suits this animal, because few other nautiloids, except for other actinocerids, were flat and triangular like this.

Gonioceras grew up to about one foot long. I hypothesize that it probably had little or no need for a complex balancing system because it almost always stayed touching the bottom, and it probably never ventured into midwater. For a creature this shape, hatched on the bottom of the ocean, it would take quite a long time for it to get its flat shape into the water. The shell could be compared to a one-foot-long flat rock, and it would have been very hard for such a small animal to lift such a heavy object up into the water. The shell would have been heavy in the first place, and considering the weights Gonioceras would have needed to keep the gas in its shell from slowing making it float up to the surface, it would have been very heavy. So it would have been hard for Gonioceras to lift itself up more than a few inches off the sea floor.

Top and side view of Goniceras

Gonioceras was actually smaller than its shell, because only a small part of the shell houses the live animal, which would have been a couple of inches long. Its bottom-dwelling habits must have meant that it preyed on bottom-dwelling animals like trilobites or worms. Rays and flounders may have a similar place in the food chain today as Gonioceras did in the Ordovician.

The only living relatives of Gonioceras are of the genera nautilus or allonautilus. Actinocerids like Gonioceras only lived in the Ordovician, but other orthocones lived to the Triassic, and orthocone-like ammonites such as Baculites lived in the Cretaceous.

Gonioceras was probably preyed on by eurypterids and larger nautiloids. Like all cephalopods, they had many tentacles surrounding a beak-like mouth, a syphon propelling them through the water, and a mantle behind their head. Nautiloids and aminoids are the only shelled cephalopods, besides the modern genus argonauta, a shelled octopus. Members of this genus can leave their shells at any time, and only the females have shells. The shells of Gonioceras were probably more delicate than those of other orthocones, because they were flatter and thinner. The whole shell is very rarely preserved in a fossil.

Gonioceras could probably partially bury itself in sand with backward shovel-like motions of its shell being propelled by the syphon,and its tentacles throwing sand on top of its body, similar to living rays and flounders, who do this with their fins. Some living cephalopods sometimes bury themselves by throwing sand on top of their body with their tentacles.

Gonioceras resting on the sea floor

Although nautiloids like Gonioceras and the modern nautilus do not have suckers on their tentacles, they have a very strong grip. Modern nautiloids can hardly ever be pulled off of their prey without ripping off their tentacles because the grip is so strong. Nautiloids also have more tentacles than other cephalopods.

Because of its flat shape, Gonioceras probably would have been very hydrodynamic on the sea floor, jetting itself quickly just above the bottom. Since the ventral side of its shell was flat, it would have been much easier for Gonioceras to rest on a flat surface such as sand or mud than on rocks, which meant it probably lived closer to sandy shores. Orthocones could not have lived in the deep sea because their shells would have cracked due to the pressure. Coiled nautiloids could have easily gone into deep water because their tightly-packed shells would have offered more protection.


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