Pachydiscus or Parapuzosia?


Haliphron Atlanticus
Staff member
Moderator (Staff)
Mar 17, 2003
Every day when I walk down the hall to my office, I pass a cast of a giant ammonoid labeled Pachydiscus seppenradensis. It is a little over 2.2 m in diameter and .4 m thick. Is the genus Pachydiscus or Parapuzosia? When I search for this species, I find both references.

By the way, are there any reasonable estimates of the mass of this thing. I has to be one of the largest cephalopods ever.

Both seem to be valid genera, someone must have split Parapuzosia from Pachydiscus. Or maybe it was erroneously refered to Pachydiscus before someone revised it. Older reports will refer it to Pachydiscus (or it's the other way around, i'm not sure). The reconstructions in This Thread show both, but they look very similar, at least the shells do.

I dont know about any mass calcs, but the above reconstructions have someones idea of a massive animal.

After reading thru the revised cretaceous treatise, if the specimen has ribs that are continuous from the umbilicus to the venter it is Parapuzosia. Both may have smooth body chambers.

his is Parapuzosia

From the Treatise:
Parapuzosia: Very Large; Moderately involute; compressed with flat sides to rather inflated with convex sides; early whorls constricted, sooner or later replaced by strong major ribs with short secondaries or intercalatories on outer 1/3 of side.

Pachydiscus: Compressed high whorl; flat or convex sides; ribs tending to differentiate into short umbilicals and seperate ventrolaterals, the later tending to be interupted on venter or to disappear.
I think the specimen was originally described as Pachydiscus back in 1895 on its discovery, I have a very old photo somewhere of a cast of the ammonite in a packing crate being shipped to the LA County Museum from an article in Geotimes labelled as such. As you say Kevin, maybe the Parapuzosia was assigned at a later date.

I've seen this monster labelled under both genera and have been confused about this too!
Following Kevins lead in a different thread, I thought this might help in visualising the size of Parapuzosia. :wink:


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