LARVAL SHELL REPAIR SCAR ON A LINGULID BRACHIOPOD: EVIDENCE OF SHELL-BREAKING PREDATION IN THE CAMBRIAN PELAGIC REALM?

A dorsal valve of the Upper Cambrian lingulid brachiopod Westonia? notchensis (Walcott, 1908) exhibits a repair scar on the anterior lateral edge of its larval shell. The brachiopod is from the Hellnmaria Member of the Notch Peak Formation in the House Range area of Millard County, Utah, and was found in a relatively deep-water facies deposited in the House Range Embayment. The growth lines of the larval and postlarval shell were distorted until the broken area was filled in, then normal growth resumed. Additionally, fractures in the shell radiate from the point of breakage and are interpreted to have been caused by the same event. This species is characterized by an abrupt change in ornamentation from larval to postlarval growth. Shell secretion in the malformed area was with ornamentation characteristic of postlarval growth, even though the brachiopod was apparently at a late larval stage at the time of the injury.

The eventual normal secretion of the shell would appear to rule out a pathological cause for the malformation. There is no evidence that the malformed growth was in response to an encrusting epibiont, and the fractures in the shell suggest one or more point injuries. Modern lingulid brachiopod larvae are planktotrophic, and are interpreted to have been so throughout their history. Therefore, an environmental cause of shell damage seems unlikely, unless damage was sustained during a storm event. The observed injury is interpreted to have been caused by an unknown shell-breaking (durophagous) predator. Repaired victims of benthic durophagy have previously been reported from the Cambrian, but the trophic structure of the Cambrian pelagic realm is not well understood. The repair scar indicates that lingulid brachiopod larvae were able to survive shell breakage, and this specimen may represent evidence of failed durophagous predation on plankton in the Late Cambrian pelagic realm. This suggests that the pressure of predation, intentional or accidental, may have played a role in the evolution of the brachiopod larval shell morphology.