Table of Contents
- Summary & Introduction
- Fatally Bitten Ammonites – Results and Interpretation
- Fatally Bitten Ammonites – Predatory Behavior
- Fatally Bitten Ammonites – Identity of the Predator
- Fatally Bitten Ammonites – Discussion and References
Identity of the Predator
The most likely candidate predators are crustaceans, nautiloids, dibranch cephalopods (squids and cuttlefish) and ammonites themselves. We discuss the pros and cons of each as the cause of ventral bite marks below.
Crustacea. Both crabs and lobsters have calcified pincers quite capable of a powerful enough bite to cause the damage. However, only lobsters have the swimming ability necessary to catch and damage ammonites in the water column. Even so, the most vigorous swimming action of lobsters is backwards, whereas to catch an ammonite would require rapid forward motion. It is also questionable whether or not lobsters could hold an ammonite and rotate it to the required position for such precise damage as we report herein, and Roll (1935) and Klompmaker et al. (2009) also described. This makes scavenging is more probable than active predation. Even so, damage typical of both crabs and lobsters usually involves breaking pieces of shell away from the aperture (Warner 1977; Mapes & Chaffin 2003; Schweitzer & Feldmann 2010). Thus, we think crustaceans are unlikely to have been the predator.
Nautiloids. Among modern cephalopods, only Nautilus has heavily calcified tips to its upper and lower jaws (Saunders et al. 1987) quite capable of inflicting the ventral damage observed on ammonites reported by Roll (1935), Klompmaker et al. (2009) and ourselves. Modern Nautilus has been observed to take bites out of the aperture margin of other caged Nautili (Mapes & Chaffin 2003). Saunders et al. (1978) reported fragments of chicken bones in the crops of Nautilus captured using chicken as bait, which implies modern Nautilus is capable of fragmenting such bones. Modern Nautilus is known to search for food with its digital tentacles extended in a ‘cone of search’ and then to use its buccal tentacles to hold food while biting (Bidder 1962). Bidder also described the ridged tentacles as highly adhesive and some were used to attach specimens of Nautilus macromphalus Sowerby to the sides of the aquarium when at rest. All these points support the idea of nautiloids as a possible predator. However, modern Nautilus has poor vision, is primarily a scavenger and has never been observed to catch active prey when they can escape, as opposed to live prey in aquaria or cages from which they cannot escape (see Basil et al. 2000 and references therein). If nautiloids, which occur at the stratigraphic levels from which the bitten ammonites derive, caused the damage scavenging would seem more likely than predation.
Ammonites. Some ammonites had uncalcified upper jaws and anaptychi (Lehmann 1979). Others had calcified beaks (called rhynchtoteuthids) and calcified anaptychi. Rhychoteuthids are thought to derive from the upper jaws of phylloceratids and/or lytoceratids (Riegraf and Luterbacher, 1989), whereas both aptychi and anaptychi can be heavily calcified and are now usually interpreted as lower jaws of ammonites. Lukeneder and Harzhauser (2002) report small ammonite jaws preserved in the crops or stomachs of larger ammonites. Although there are various explanations for the presence of hard parts within ammonite body chambers other than that these are the remains of animals eaten by the ammonites, such occurrences are suggestive that ammonites were cannibalistic. The possibility of larger ammonites being the predator largely depends on whether or not uncalcified jaws were capable of a sufficiently strong bite. It is also unknown if ammonites had the speed and manoeuvrability or the type of tentacles required to catch and manipulate small ammonites.
Other cephalopods. Modern squids and cuttlefish are active swimmers and excellent predators. Those with suckered arms are quite capable of manipulating prey in the way we suggest was necessary to inflict the damage described herein. Indeed, Chichery & Chichery (1988) recorded that the cuttlefish Sepia officinalis reorientates crabs it has captured to inflict a paralysing bite in a constant position on the prey, irrespective of the direction of initial capture. A video available at http://www.arkive.org/common-cuttlefish/sepia-officinalis/video-08a.html shows the cuttlefish capturing a crab and manipulating it as described by Chichery & Chichery (1988) and finally releasing the empty dorsal carapace and severed legs of the crab, as well as numerous small fragments of exoskeleton. Modern squids and cuttlefish have jaws entirely composed of conchiolin and presumably a weaker biting action (see review by Tanabe & Fukuda 1999) than, for example, the calcified jaws of nautiloids. Again, the possibility of dibranchs having been the predator depends on whether or not uncalcified jaws were capable of a sufficiently strong bite. Certainly, Sepia is capable of biting through the softer ventral skeleton of modern crabs. It is also pertinent that some birds, which also have unmineralized beaks, are capable of cracking nuts as resistant as ammonite shells.
In summary, we think predation is more likely than scavenging and the most likely predators were ammonites or, even more likely, cuttlefish with suckered arms.