Thylacocephala
| Thylacocephala Temporal range: Late Ordovician (Sandbian) to Late Cretaceous (Santonian) (possible early Cambrian record)
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| Restoration of Clausocaris, a Concavicarida | |
| Reconstruction of Thylacares, once considered to be the earliest known thylacocephalan | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Clade: | Mandibulata |
| Class: | †Thylacocephala Pinna et al., 1982 |
| Orders | |
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Thylacocephala (from the Greek θύλακος or thylakos, meaning "pouch", and κεφαλή or cephalon meaning "head") is an extinct group of mandibulate arthropods, that are generally regarded as a kind of crustacean, though their exact position within this group is uncertain. As a class they have a short research history, having been erected in the early 1980s.
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. Their size ranges from ~15 mm to potentially up to 250 mm.
Inconclusive claims of thylacocephalans have been reported from the lower lower Cambrian (Zhenghecaris), but later study considered that genus as radiodont or arthropod with uncertain systematic position. The oldest unequivocal fossils are Upper Ordovician (Sandbian) in age, around 460-450 million years old. Thylacocephala survived at least until the Santonian stage of the Upper Cretaceous, around 84 million years ago.
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.