Techniques of trilobite exuviation

Abstract
Examples of the [Cambrian and Ordovician] trilobites Toxochasmops extensus (Boeck, 1838), Asaphiscus wheeleri Meek, 1874, Encrinurus mitchelli Foerste, 1888, Ogygopsis klotzi (Rominger, 1887), Paradoxides davidis Salter, 1863 and Oryctocephalus spp., which are interpreted as exuvial configurations, are described. Four specimens of T. extensus are known in which the pygidium rests either directly behind the cephalon, or with only 3 intervening thoracic segments. During exuviation the old pygidium became wedged behind the cephalon. This facilitated its removal. An ecdysial configuration of A. wheeleri is described which possesses inverted and partially rotated free cheeks. Part of the thorax of the specimen is wedged obliquely behind the cephalon. This is considered to have aided withdrawal of the trilobite from its exoskeleton. Two specimens of E. mitchelli are described which possess free cheeks inverted beneath the cranidium by lateral rotation, in a manner similar to that of A. wheeleri. Three examples of Ogygopsis klotzi are described, one a failed exuvia and two in which the free cheeks were inverted and rotated through 180.degree. with respect to their original position and came to rest beneath the thorax. An identical exuvia of P. trapezopyge is also described. Two specimens of Oryctocephalus exhibiting 2 different arrangements of inverted free cheeks are recorded. Possible mechanisms for each of these free cheek inversions are proposed.

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