Devonian Horn Corals: How do you tell Streptelasma ungula from Stereolasma rectum?

[Mediospirifer]

I've been going through the fossils I collected over the summer, trying to identify them. I have a lot of horn corals, several of which look like they could be either Streptelasma ungula or Stereolasma rectum:

Several of them have matrix filling the calyx and obscuring the detail. With or without that, though, I don't know how to tell them apart! Except that my fossil guide lists Stereolasma as potentially being a little larger. This only rules out a couple of my specimens, though.

Can anyone tell me what the diagnostic features of both species are?

[FossilDAWG]

The genera are differentiated based on internal structures. Unfortunately this means that if they co-occur you have to make thin sections to distinguish them, they can't be differentiated based on external features, especially if you also can't see the interior of the calyx.

As far as I know the biggest difference is in the major septa. In Stereolasma during most of the growth of the coral the major septa extend to the center of the corallite, where they meet and secondary deposits form that make a central solid columella. In the mature coral the septa become shorter and stop forming a columella just below the base of the calyx. On other hand, in Streptelasma the major septa never reach the center and form a solid columella.

There may be other differences as well, if I get time I'll try to look it up.

For Stereolasma, you may find this paper helpful.

Don

[Mediospirifer]

Well, that would explain why I can't tell them apart!

I was hoping the difference was something I could see. In the "Palaeontology of New York" volume on corals and bryozoan (by James Hall), there's a detailed description of Streptelasma strictum from the lower Devonian, including mention of how many vertical areas on the corallum show emergence of additional striations.

I was hoping to include a couple in the New York fossils and minerals display I'm putting together for our Mineral Club open house next month. I'll still include them, but I guess it'll have a note with it explaining that genus-level differences can be extremely hard to distinguish!

The ones I'm planning to display are the second from left in the two bottom rows. The one at bottom has a nice crinoid holdfast on it (visible in the photo), the one above it has a pair of encrusting brachiopods I still need to identify.

Thanks for the paper. It does look useful.