A new paper on a cryptic crustoid graptolite from the Middle Ordovician of Estonia

I have long enjoyed exploring the Ordovician and Silurian rocks of Estonia with my Estonian friend Olev Vinn. We have done a lot of work together, and Estonia continues to provide fascinating fossils for our studies. Our circle of paleontologists has expanded continually over the years in Estonia, including other Estonians, Brits and Americans.

This latest paper, Vinn et al. (2019), is from a project Olev, Ursula Toom, and I pursued with a single specimen from the Middle Ordovician (Darriwilian) of Estonia. It analyzes the unusual preservation of crusted graptolite (rather rare in its own right) preserved inside the gloomy hollow off a nautiloid shell (its conch). Here is the abstract:

“A light grey nautiloid conch has a dark brown colony attached to its internal surface. This colonial fossil resembles hederellids and bryozoans, but is in fact a crustoid graptolite (Hormograptus? sp.). The colony has been lithoimmured inside this nautiloid conch by early cementation. Crustoid graptolites were a part of the encrusting communities in the Middle Ordovician of Baltica, but their abundance among encrusters of biogenic substrates reached a peak in the middle Sandbian. The cryptic mode of life appeared very early in the evolution of the crustoids. The discovery of this crustoid graptolite in a nautiloid conch indicates that the Baltic Middle Ordovician cryptic communities were taxonomically more diverse than was known previously. The nautiloid conch studied is sparsely encrusted with an encrustation density that is similar to those of other Middle Ordovician cryptic surfaces described from Estonia.”

From figure 2: Hormograptid graptolites from the Ordovician of Estonia. A–C. Hormograptus? sp., attached to the internal surface of a nautiloid conch; Harku Quarry, Kunda Regional Stage (lowermost Darriwilian) (GIT 494-41-1). [Image C is at the top of this post.]

The unusual taphonomic pathway of this specimen was through lithoimmuration, in which early calcite cement essentially entombed the crustoid graptolite colony against the internal nautiloid shell surface. That shell was made of aragonite which quickly dissolved, leaving the base of the graptolite exposed for us. That was enough to make the identification and show a bit of cryptic niche space occupied in the Middle Ordovician.

Reference:

Vinn, O., Wilson, M.A. & Toom, U., 2019. A crustoid graptolite lithoimmured inside a Middle Ordovician nautiloid conch from northern Estonia. Annales Societatis Geologorum Poloniae, 89: doi: https://doi.org/10.14241/asgp.2019.17