A Middle Devonian symbiotic relationship involving a gastropod, a trepostomatous bryozoan, and an inferred secondary occupant

Abstract

Morris, P. J., Linsley, R. M. & Cottrell. J. F. 1991 01 15: A Middle Devonian symbiotic relationship involving a gastropod, a trepostomatous bryozoan. and an inferred sceondary occupant. Lethaia, Vol. 24. pp. 55–67. Oslo. ISSN 0024–1164. The high-spired gastropod Palaeozygopleuru (Loxonematacea) of the Hamilton Group (Middle Devonian) in New York State is often entrusted by the trepostomatous bryozoan Leptotrypella (Hetero-trypidae). The form of this enerustation leads us to infer a secondary occupant that dwelled in empty shells of Palaeozygopleura while the bryozoan grew upon them. Encrusted specimens usually have an open aperture, thin or no enerustation on the apertural side of the gastropod shell. and thick encrustation on the abapertural side. While the aperture is invariably open. the columellar and parictal lips of the aperture, and the apertural face of the first whorl are usually encrusted. While the gastropod is alive, this area rests upon the dorsal surface of the foot and the remainder of the shell rests upon the substratc. These encrustation patterns suggest that an occupant of the shell. other than the gastropexl. prevented overgrowth of the aperture and oriented the shell aperture-down. This allowed ahapertural growth of the bryozoan. The presence of thin encrustation on the apertural side of the shell is problematical. It requires either the presence of a secondary occupant capable of holding a thinly encrusted shell off the substrate, or the ability of the juvenile bryozoan colony to extend onto portions of the shell that were in contact with the substrate. The presence of a secondary occupant. such as a sipunculan worm. capable of lifting the shell, but usually resting it aperture-down on the substrate seems the most suitable explanation for the nature of the encrustation. Gastropoda. bryozoa. symbiosis. Devoniun. hermit crab, Sipuncula