A specimen of Psammolepis in the Gauja Formation (Estonia)

Abstract

This contribution summarizes various aspects related to the discovery of a Psammolepis in the Gauja Formation in Estonia. The fragmentary material was identified after assembly as Psammolepis undulata. This identification is based on the observations and the process of elimination, as follows:  the size and shape of the shield suggested that it is a psammosteid;  the plate lacks pleromin and this suggests it to be a dorsal shield;  the sensory system closely matches that of a previously known dorsal shield of P. undulata, with only minimal differences;  considering the species previously known in the Gauja Formation, the other characters fit the best P. undulata. A psammosteid shield with well-preserved lateral line system is very rare and fact that the current specimen has preserved an extensive system of canals makes it exceptional. This specimen also shows that the sensory system of Psammolepis was larger than previously thought. Features of the visceral surface suggest that P. undulata could have a double sensory system like amphiaspids, but these preliminary observations need further approval. Current specimen bears a partly regenerated bite mark, likely created in course of an attack by a sarcopterygian of the genus Laccognathus. As the bite mark and related cracks are situated in anterior part of the plate, it suggests a frontal attack. Vertebrate fossils in the Kalmetumägi locality are of unique preservation being only weakly deformed. The new plate represents the first and only specimen of P. undulata found in the Gauja Formation in Estonia. The new record makes the distribution interval of P. undulata similar to that in Latvia and in the Devonian of Scotland. This