Caedichnus, a New Ichnogenus Representing Predatory Attack on the Gastropod Shell Aperture

Abstract

Predatory traces, in which the tracemaker has damaged the prey animal's skeleton to kill and consume it, have a deep fossil history and have received much scientific attention. Several types of predatory traces have been assigned to ichnotaxa, but one of the most studied predatory traces, the wedge-shaped excision produced as a result of attacks mainly by crustaceans on the apertures of gastropod shells, has yet to be described as an ichnotaxon. We propose the ichnogenus Caedichnus to describe the shell damage produced by aperture peeling behavior. Caedichnus is produced by predators that are unable to crush their prey's shells outright. Depending on the predator's peeling ability and the prey's withdrawal depth within the shell, the trace can extend through several whorls of the shell. Aperture peel attacks may fail, allowing such damage to be repaired by surviving gastropods. Thus, the types of attacks that produce Caedichnus may exert selective pressure on prey to evolve betterdefended shells (in the case of gastropods) or to inhabit betterdefended shells (in the case of hermit crabs). The identification of these trace fossils will enhance our understanding of how predation influences the morphological, and even behavioral, evolution of prey organisms.

Gastropod shell repair tracks predator abundance

“…Repair scars manifest as jagged disruptions in the shell surface that are not parallel to the growth margin (Alexander & Dietl ; Stafford et al . ; Fig. B).…”

“…Grubich ; Herrel & Holanova ; see Stafford et al . ) for examples involving aperture‐peeling predators on gastropod‐shelled prey). Unfortunately, inferences may be limited in application because predator or prey behavior can be influenced by captivity.…”

“…Laboratory-based observations of predatory interactions are utilized when the hypotheses require direct observations or control over physical conditions (e.g. Grubich 2000;Herrel & Holanova 2008; see Stafford et al 2015) for examples involving aperture-peeling predators on gastropod-shelled prey). Unfortunately, inferences may be limited in application because predator or prey behavior can be influenced by captivity.…”

Perforations of Freshwater Snail Shells from the Miocene of Germany:Nihilichnus covichin. isp.

“…The accumulation of the holes on the apertural and the abapertural sides of shells can be explained by the manipulation of the shells within the fish's mouth, because the shell would be easier to crush open when situated with the aperture in a horizontal position (see also Norton, 1988). Otherwise, the teeth can only crush the apertural lip ("apertural lip chipping" sensu Stafford et al, 2015) instead of the whole shell. The shell could also be passively oriented horizontally in the mouth, until it has reached a stable position.…”

“…Freshwater snails are preyed by various vertebrates, such as amphibians and fish that crush the shells (or tear off parts of them) or swallow them completely (e.g., Norton, 1988). Amongst invertebrate predators, crayfish, crabs, and insects may enter the shell through the aperture, pull the snail's soft body out of the shell, crush it completely, or cut wedge-shaped excisions into the last whorls (Stafford et al, 2015). None of them is known to produce such regular holes as Nihilichnus covichi.…”