Thylacocephala

Thylacocephala Temporal range: Sandbian to Late Cretaceous (possible early Cambrian record) PreꞒ Ꞓ O S D C P T J K Pg N
Restoration of Clausocaris, a Concavicarida
Reconstruction of Thylacares, once considered to be the earliest known thylacocephalan
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Clade:	Mandibulata
Class:	†Thylacocephala Pinna et al., 1982
Orders
Concavicarida Conchyliocarida

The Thylacocephala (from the Greek θύλακος or thylakos, meaning "pouch", and κεφαλή or cephalon meaning "head") are group of extinct probable mandibulate arthropods,^[1] that have been considered by some researchers as having possible crustacean affinities. As a class they have a short research history, having been erected in the early 1980s.^[2][3][4]

They typically possess a large, laterally flattened carapace that encompasses the entire body. The compound eyes tend to be large and bulbous, and occupy a frontal notch on the carapace. They possess three pairs of large raptorial limbs, and the abdomen bears a battery of small swimming limbs.

Inconclusive claims of thylacocephalans have been reported from the lower lower Cambrian (Zhenghecaris),^[5] but later study considered that genus as radiodont or arthropod with uncertain systematic position.^[6] The oldest unequivocal fossils are Upper Ordovician and Lower Silurian in age.^[7][8] As a group, the Thylacocephala survived to the Santonian stage of the Upper Cretaceous, around 84 million years ago.^[9][10]

Beyond this, there remains much uncertainty concerning fundamental aspects of the thylacocephalan anatomy, mode of life, and relationship to the Crustacea, with whom they have always been cautiously aligned.