Lifestyle of the Trocholitidae

Trocholitids are coiled nautiloid cephalopods belonging to the Tarphycerida, from the latter part of the Ordovician Period. are distinguished by the presence of a dorsal siphuncle, and as often the case, some what anisotrophic coiling such that one side is more concave, or less convex, than the other. In the jargon of cephalopod morphology this asymmetry is known as torticonic, as opposed to trochoidal as used to describe the anisotrophic coiling in gastropods, resulting a a spire. As to why these nautiloids aren't called the Torticonidae, your guess is as good as mine. So much for the vagaries of nomenclature..

In straight or moderately curved forms, dorsal siphuncles are located anywhere from slightly above the center to along the upper, or dorsal margin. In vertically oriented coiled forms, where the whorls at the top are upside down, dorsal siphuncles run through the inner part of the coils to along the inner margin. In vertically oriented forms they indicate that the last formed septa are high in the shell where the siphuncle is actually low and most efficiently placed to remove the seawater from the last formed chamber.

Vertically oriented trocholitids, more concave, or less convex, on the right side would tend to veer to the right going ahead or to the rear, like a ship with the rudder shifted right. These with the left side more concave, or less convex, would likewise veer to the left, like a ship with the rudder shifted left. This seems to be more of a disadvantage to coiled forms capable of swimming off the sea floor than an advantage. So what was its purpose other than perhaps to confused predator or prey. Perhaps the answer lies in a different orientation during life.

Could it be that at least some trocholitids lived out most of their lives lying on the sea floor on one side, like flounders. The dorsal siphuncle doesn't seem to be any particular advantage, or disadvantage here, but even a slightly trochoidal coiling does. One side, probably the upper side, being more convex or domed than the other might have helped bottom currents hold the shell to the sea floor. In this scenario these trocholitids would have spent their time relaxed on the sea floor, ready to ambush the unexpected passer by, only to used there tentacles to help shift position on occasion.

A siphunclde found

Shown here is part of the siphuncle of the Late Paleozoic nautiloid cephalopod Domatoceras that was found in a specimen at the New Mexico Museum of Natural History and Science. This may be a one-of-a-kind find that adds to our understanding of this genus.


Descriptions of Domatoceras so far have been limited to the evolutely coiled shell with a subquadrate whorl section.
The siphuncle is only mentioned in passing as being subcentral and probably orthochoanitic. Subcentral it is but orthochoanitic, at least in this case, it is not.



The picture to the left shows the specimen in which the siphuncle was found, exposed in the second whorl near the bottom.