Frontal auxiliary impressions in the Ordovician trilobite Dalmanitina Reed, 1905 from the Barrandian area, Czech Republic
Abstract
Fossils preserved in Cambrian to Devonian sediments of the Barrandian area (Czech Republic) have contributed significantly to our knowledge of numerous invertebrate groups. With respect to trilobites, important data has been discovered on the construction of eyes, healing of exoskeletal injuries, as well as on the morphology of soft parts.
The generally rarely preserved frontal auxiliary impressions (FAN) on the glabellar surface of trilobites were first described in Devonian examples from this area in the mid-nineteenth century by Barrande. Such impressions have only rarely been documented in a few trilobite species in the Lower Palaeozoic of the Barrandian area.
Here we describe twelve exceptionally preserved holaspid cephalic shields of Dalmanitina socialis Barrande, 1846. These specimens are internal moulds and were collected at three localities in the Upper Ordovician Letna Formation.
This material documents a high level of variability in the disposition of FAIs within the glabella. However, the FAIs show a common pattern at the anterior glabellar margin and arc arranged in two pairs around a medial impression and are also associated with a third pair situated more posterolaterally.
This current study is the first to focus on the distribution of FAIs within the Dahnanitidae. Dalmanitina socialis specimens with FAIs from the Letna Formation indicate that the depositional environment at the several localities in that unit was favourable to exceptional preservation.
Excellently preserved cephalic shields of Dalmanitina demonstrate the presence of the posterior median impression (pmi) of Eldredge (1972) and enable new terminology to be proposed for other FAIs.