Thylacocephalans are an extinct group of arthropods of an uncertain systematic position. Originally considered phyllocarid crustaceans, they have since been classified within their own class, the Thylacocephala Pinna, Arduini, Pesarini...
moreThylacocephalans are an extinct group of arthropods of an uncertain systematic position. Originally considered phyllocarid crustaceans, they have since been classified within their own class, the Thylacocephala Pinna, Arduini, Pesarini and Teruzzi, 1982 on the basis of the exceptionally preserved Lower Jurassic (Sinemurian) species Ostenocaris cypriformis from Osteno, Italy. Since that time, the membership of the Thylacocephala has only grown as a number of new species have been discovered, as well as previously known species moved into the group.
Thylacocephalans are characterized by a body almost entirely enclosed within a bivalved-carapace, often with prominent anterior rostrum-notch complex. Typically, thylacocephalans also have large, compound eyes; three pairs of large, subchelate appendages on the anterior part of the body; at least eight pairs of lamellate gills on the middle part of the body, and a posterior series of small, paddle-like limbs. The central problem in Thylacocephalan taxonomy is the affinity of carapace-only fossils that lack the key synapomorphies that reside in the soft-parts. The oldest known putative representative (Zhenghecaris) is known from the Lower Cambrian Maotianshan shales of China, while the oldest unequivocal thylacocephalan (Thylacares brandonensis) comes from the Silurian of Wisconsin, USA. The last of the thylacocephalans occur in the Upper Cretaceous limestones of Lebanon. Although common in many different localities of different ages in the world (e.g., Devonian of Europe and Australia), the best-known and preserved species come from the Triassic and Jurassic of Italy and France. Over 20 genera of thylacocephalans have been recognized so far.
