The Triassic thylacocephalans from Slovenia: implications for their evolution and diversification
Abstract
Thylacocephala is an enigmatic fossil euarthropod ingroup, often associated with
Pancrustacea. Despite the remaining questions on their anatomy, thylacocephalans are
characterized by key anatomical features: a folded shield enveloping most of the body,
hypertrophied compound eyes, three pairs of raptorial appendages, a trunk made of eight
to 22 segments bearing appendages, and eight pairs of gills. Known from at least the
Silurian to the Cretaceous, it was during the Triassic that thylacocephalans were the most
taxonomically diversified. Unknown until recently in the fossil record of Slovenia, many
specimens of thylacocephalans have now been collected from various localities in the Julian
and Kamnik-Savinja Alps. Dating back from the Anisian up to the Carnian, these localities
display well-diversified thylacocephalan assemblages. Taxa already known from the
Middle and Late triassic of Austria and Italy (Microcaris Pinna, 1974; Atropicaris Arduini
and Brasca, 1984) have been recorded while new morphologies have also been identified.
The new morphologies present typical characters of Jurassic representatives, especially a
posterior notch and posterior spines. These new findings provide important insight into the
diversity and geographic distribution of thylacocephalans during the Triassic but also into
the morphological evolution of this puzzling group.