Middle and Late Triassic thylacocephalans from Slovenia: implications for their diversification

Thylacocephalans are enigmatic fossil marine euarthropods, often associated with pancrustaceans. Known from at least the Silurian to the Cretaceous, they have a patchy fossil record stratigraphically. They were particularly diversified during the Triassic, which is considered as the period with their highest taxonomic richness. Unknown until recently in the fossil record of Slovenia, many specimens of thylacocephalans have been collected from the Middle Triassic beds (Anisian) of the Velika planina member and Strelovec Formation in the Kamnik–Savinja Alps, as well as in the Upper Triassic beds (Carnian) of the Martuljek Formation in the Julian Alps. In this study, we report eight thylacocephalan taxa, including three new species. Atropicaris lintveri sp. nov. is described from the Velika planina member and represents the oldest mention of Atropicaris . In the Strelovec Formation, common Middle and Late Triassic taxa, including the oldest mention of Microcaris , are documented ( Microcaris sp., Atropicaris aff. lintveri ) and two new morphotypes are also identified, including Kamnikaris cemazevkaensis gen. et sp. nov . Finally , Austriocaris sp., Atropicaris sp. and Microcaris ajdovskae sp. nov. are described from the Martuljek Limestone Formation, in which accumulations of specimens probably resulting from numerous mass mortality of swarm of thylacocephalans are observed. Based on those new findings, we compile diversity data for Triassic thylacocephalans and investigate their diversification. We show that the thylacocephalan diversity is relatively high as early as the Early Triassic. •