ASGP (2019), vol. 89: 285–290

A CRUSTOID GRAPTOLITE LITHOIMMURED INSIDE A MIDDLE ORDOVICIAN NAUTILOID CONCH FROM NORTHERN ESTONIA

Olev Vinn (1*), Mark A. Wilson (2) & Ursula Toom (3)

1) Department of Geology, University of Tartu, Ravila 14A, 50411 Tartu, Estonia; e-mail: olev.vinn@ut.ee
2) Department of Earth Sciences, The College of Wooster, Wooster, OH 44691, USA; e-mail: mwilson@wooster.edu
3) Department of Geology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia; e-mail: ursula.toom@ttu.ee
*Corresponding author

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: 285– 290.

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.

Manuscript received 18 June 2019, accepted 10 September 2019