Revision of Ordovician chitinozoan Lagenochitina esthonica sensu lato: morphometrics, biostratigraphy and paleobiogeography

Lagenochitina esthonica is a globally distributed chitinozoan in Early to Middle Ordovician rocks. It is regarded as an index species for the early Floian in North America and has a stratigraphically constrained range in other regions. Lagenochitina esthonica is distinguished from other chitinozoans by a distinct flexure, a nearly rounded-square chamber, and a cylindrical neck with a flaring collar. However, since the first description of the species in the 1950s, it has included two varieties: a relatively short form with a test length ~400 μm, and a slender form usually longer than 600 μm. In order to revise the taxonomy of the L. esthonica group, we carried out a statistical morphometric study of a large collection of well-preserved specimens from the Baltic region where the taxon was first established. Additionally, the stratigraphic and geographic distribution of both forms was analyzed based on available occurrence data. The results show that the short form occurs in the upper Tremadocian to lower Dapingian, whereas the slender form is mostly reported from the lower and middle Darriwilian. Both forms are identified on Baltica; the short form has also been reported from Laurentia and South China, whereas the other is known also from Avalonia and Gondwana. The morphological distinction, together with differences in stratigraphic and spatial ranges, suggest that the two forms represent separate species: the original stout L. esthonica, based on the morphology of the holotype, and the slender L. megaesthonica n. sp., described herein. The updated taxonomy enhances the stratigraphic and biogeographic usefulness of lagenochitinids globally.