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      The Thylacocephala (from the Greek thylakos, meaning "pouch", and cephalon meaning "head") are a unique group of extinct arthropods, with possible crustacean affinities. As a class they have a short research history, having been erected in the early 1980s.[1][2][3] 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. The earliest thylacocephalan fossil is thought to date from the lower Cambrian,[4] while the class has a definite presence in Lower Silurian marine communities.[5] As a group, the Thylacocephala survived to the Upper Cretaceous.[6] 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.
<I>Clausocaris lithographica</I> - a Jurrassic representative of Thylacocephala
Clausocaris lithographica - a Jurrassic representative of Thylacocephala
An enigmatic animal <I>Ainiktozoon loganense</I> probably also belongs to the Thylacocephala
An enigmatic animal Ainiktozoon loganense probably also belongs to the Thylacocephala

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Virtual museum of the Czech Geological Survey, www.geology.cz, (C) Czech Geological Survey, 2011, v.0.99 [13.12.2011]