Techniques of trilobite exuviation

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Abstract

Examples of the 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 Toxochasmops extensus arc known in which the pygidium rests either directly behind the eephalon, or with only three intervening thoracic segments. It is considered that during exuviation the old pygidium became wedged behind the cephalon. This facilitated its removal. An ecdysial configuration of Asaphiscus wheeleri is described which possesses inverted and partially rotated free cheeks. In addition, 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 old exoskeleton. Two specimens of Encrinurus 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° with respect to their original position and came to rest beneath the thorax. An identical exuvia of Paradoxides davidis trapezopyge is also described. Two specimens of Oryctocephalus exhibiting two different arrangements of inverted free cheeks are recorded. Possible mechanisms for each of these free cheek inversions are proposed.

Ancillary