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
Apparently fatal damage has been recorded in six genera of ammonites from limestone nodules, Arnioceras, Asteroceras (Fig. 6), Caenisites, Cymbites (Fig. 7), Promicroceras (Figs 8, 9) and Xipheroceras (Fig. 10). Damaged ammonites from limestone nodules occur through much of the stratigraphic range of Promicroceras, from the Yellowstones to the Coinstone (Lang’s beds 83-89; see Lang & Spath 1926; Simms 2004, fig. 2.8) (Fig. 11). Almost all are relatively small specimens even when apparently adult, ranging from 11 mm diameter (Cymbites) up to 39 mm diameter (Asteroceras margaritoides), but we have seen two damaged examples of Asteroceras obtusum, 52 mm (Fig. 6) and 105 mm in diameter (Fig. 12).
Pyritic moulds include Promicroceras, Crucilobiceras, Cymbites, Eoderoceras, and Paltechioceras (Fig. 13), and we have a single damaged example of Protogrammoceras from the Eype Nodule Bed (LYMPH 2010/2; Figs 14, 15). Most of the pyritic ammonites other than Promicroceras almost certainly derived from the Stonebarrow Pyritic Member (Lang’s beds 90-102; Lang & Spath 1926; see Simms 2004, fig. 2.8, p. 67). We have one pyritized Promicroceras (LYMPH 2010/1; Figs 3, 4) collected in situ from Lang’s bed 81 (Lang & Spath 1926) Thus, fatally damaged ammonites are not uncommon through most of the Black Ven Marl and Stonebarrow Pyritic members of the Charmouth Mudstone Formation. Interestingly, pyritized ammonites are very common near the top of the Belemnite Marls Member and so far not one ventrally damaged example has been found. Above that level we have not searched extensively.
Description of damage. The most consistent aspect of the damage is the position (Fig. 5). The damage usually takes the form of an irregular-shaped patch of shell missing from the rear of the body chamber, which invariably extends onto both sides of the shell when these can be seen. The rear margin sometimes impinges on the last suture, but it may start as much as 30 % of the body chamber length in front of the last suture and not uncommonly extends back to affect the last one or two chambers of the phragmocone. The least variable feature of the damage is the position of the mid-point of the damage expressed as a percentage of the body chamber length (mean 78 %). In strongly ribbed ammonites, such as Promicroceras and Xipheroceras, the damage usually affects two or three ribs.
Damage usually extends down the flanks to the umbilical seam on at least one side. The relatively few examples seen where both sides can be examined suggest that damage is asymmetrical in its extent, reaching the umbilical seam on one side, but stopping 1-2 mm short of it on the other side. There is apparently no preference as to which side is more extensively damaged. On two examples (LYMPH 2009/58 and 2009/60) the damage extends onto the penultimate whorl (Fig. 16), but this is relatively rare. One example of Promicroceras (LYMPH 2009/59) was bitten twice (Fig. 17), but this is the only example showing more than one bite. Shell chips from the damaged shells are not preserved adjacent to ammonites. We have two examples among the pyritized ammonites, one small, smooth pyritized nucleus (LYMPH 2010/15) and one Promicroceras (LYMPH 2010/16), where the damage affects the phragmocone, not the body chamber. Furthermore, we have a single Caloceras?
(LYMPH 2010/19) from Lang’s bed H40 (Lang 1924) in the Blue Lias Formation, in Pinhay Bay, where the damage affects the last three chambers of the phragmocone, but not the body chamber. Again, damage to the phragmocone is rare and presumably records unsuccessful attacks from the point of view of the predator, although still lethal to the ammonite.
Taphonomy. In concretions the ammonites occur unevenly. Many concretions contain no ammonites at all; others may be crowded with them. At least 28 separate ammonites occur in concretion LYMPH 2009/60 (Fig. 18), which measures approximately 170 by 140 mm. The ammonites are commonly randomly orientated with respect to bedding. The vast majority give every indication that the shells were complete when buried (except for the missing portion in the bitten ammonites). Almost without exception the phragmocones are filled with calcite (e.g., Figs 2, 6, 8, 10, 12, 16-18). Cope (1994) interpreted the ammonites from the stone band concretions as predominantly juveniles and suggested that they were buried rapidly by sudden influxes of sediment.
The body chamber has more variable fill. Typically in bitten examples, it is largely filled with the surrounding sedimentary rock. However, and uncommonly, examples occur where the sediment only penetrates a few mm from the site of damage and in from the aperture (e.g., Fig. 7), with the remainder of the body chamber filled with calcite. Undamaged ammonites occur with body chambers almost completely filled with diagenetic calcite, sediment only penetrating 1-2 mm into the aperture. Other undamaged ammonites appear to have the body chamber completely filled with sedimentary rock. Cope (1994) and Cope and Sole (2000) concluded that ammonites with calcite-filled body chambers probably had the body tissue preserved and the latter authors were able to recover jaws from some examples.
Damage to the ammonites discussed herein is very specific, both in location and extent. Wani et al. (2004) performed experiments on modern Nautilus shells to determine the sort of damage that occurred when they were rolled with sediment to simulate wave action on shorelines; crushed as sediment compacted; and caged together to simulate impact damage as they floated. The results showed that the different types of experimental environment produced recognizably different styles of damage, yet none of the experimental damage matches that on our Liassic ammonites. The damage to the Liassic ammonites is so specific that we think accidental damage during transport and burial can be ruled out (as did both Roll 1935 and Klompmaker et al. 2009). Furthermore, the Liassic ammonites were buried rapidly and in clays, which are unlikely to produce abrasion or impact damage.
The main features in common between the examples described and figured by Roll (1935) and Klompmaker et al. (2009), and our Liassic examples, are that the damage is common and that it occurs immediately in front of the last suture. Roll (1935) and Klompmaker et al. (2009) independently argued from both the morphology and the restricted position of the damage that it must have arisen from deliberate predation, and we agree. If anything, the damage to our Liassic ammonites is even more restricted in both morphology and occurrence.