Resolving the confusion between two fossil freshwater diatoms from Otago, New Zealand: Encyonema jordanii and Encyonema jordaniforme (Cymbellaceae, Bacillariophyta)

Encyonema jordanii and E. jordaniforme are two freshwater pennate diatoms from Otago, New Zealand which have been misidentified in the past. The first is of uncertain age, while E. jordaniforme , dominant in a Konservat-Lagerstatte diatomite from Foulden Maar near Middlemarch, South Island, New Zealand, is dated as earliest Miocene in age (23 Ma). The two species are similar in appearance as both are swollen at their centres. However, E. jordanii is constricted near its wide ends, which are ventrally deflected near their tips, whereas E. jordaniforme has more acute apices and less strongly radiating central striae. Both species are illustrated with light and scanning electron microscopy observations which highlight the main morphological features that can be used to separate the two species. A brief history of the taxonomy and description of both species is added.