PAIRED CHAETAE ASSOCIATED WITH SPIRAL SHELLS OF THE LATE EARLY CAMBRIAN MOLLUSC PELAGIELLA FROM THE KINZERS FORMATION: TAPHONOMY, FUNCTIONAL MORPHOLOGY, AND POTENTIAL EVOLUTIONARY RELATIONSHIPS

About 45 specimens of Pelagiella appear in varied states of preservation on a small slab of shale (part and counterpart) from the Emigsville Member of the Kinzers Formation, near Lancaster, Pennsylvania. Associated with the apertures of most of these shells are two sets of fine, straight or gently curved, bristle-like structures that must have been relatively stiff. Among a cluster of shell molds on another shale fragment, a set of these structures can be seen to fan out in three dimensions from their point of common origin. The consistent forms of these clusters and their regular association with the same parts of the shells indicate that they were organs of the makers of the shells, not epibionts. We interpret these structures as chaetae and seek to distinguish among their potential functions in relation to the mode of life of the organism. Chaetae are employed by annelids in locomotion, also as sensors, and in feeding as filters or sweeps. In brachiopods they typically constitute sensory grilles. The dimensions and inferred physical properties of the chaetae of Pelagiella imply that they were actively manipulated, as sweeps. In regard to the animal’s pattern of development and mode of life, none of the following issues is firmly resolved for Pelagiella. Was the shell endogastric or exogastric? Was it orthostrophic or hyperstrophic? Was the animal torted (hence a gastropod), partially torted (as suggested by some reports of muscle scars but not others), or untorted (hence presumably a monoplacophoran)? Did the animal carry its shell on its back, either erect or reclining at a low angle, or was it sessile, reclining on the sea floor? The occurrence and disposition of chaetae does not immediately resolve any of these issues, but it does significantly reduce the range of possibilities, as we will show.