By Chris Andrew, Paddy Howe, Chris Paul & Steve Donovan
Full text in Proceedings of the Geologists’ Association
doi: 10.1016/j.pgeola.2010.09.002 – Available at www.elsevier.com/locate/pgeola
Table of Contents
- Summary & Introduction
- Epifaunal Worm Tubes on Lower Lias Ammonites – Results
- Epifaunal Worm Tubes on Lower Lias Ammonites – Detailed Observations and interpretation
- Epifaunal Worm Tubes on Lower Lias Ammonites – Discussion
- Epifaunal Worm Tubes on Lower Lias Ammonites – Conclusions and References
Over forty ammonites, mostly Promicroceras, with epifaunal worm tubes are discussed from the Lower Jurassic, Charmouth Mudstone Formation of Dorset. Serpulids that were overgrown by the ammonites or responded to the ammonites’ growth attached to juvenile, living ammonites. Some epifaunal serpulids attached post-mortem, indicating oxygenated bottom water. The commonest pattern of growth for serpulids on live juvenile ammonites was attachment in the umbilical seam, with later growth onto, and finally around, the venter in the same direction as the ammonite grew. Reconstructing this pattern suggests that serpulids kept their aperture at 6 o’clock with respect to the orientation of the living ammonite (105-115o behind the ammonite aperture) throughout life. The 6 o’clock position of the aperture enabled feeding currents generated by the worms to parallel currents generated by swimming ammonites, thus maximizing food gathering and confirming that ammonites swam backwards. The mid-ventral position enabled the worm to deploy its branchia (feeding structures) on both sides of the ammonite. Reorientation of growth lines in serpulid worms just before the aperture suggests some worm tubes were fully grown. Growth on ammonites was beneficial to the worms, but parasitic to the ammonites. Promicroceras with epifaunal worm tubes died at smaller sizes than unencumbered examples and size at death correlates inversely to extra weight of worm tubes. Comparisons with living serpulids suggest fossil worms grew in one season and that Promicroceras reached full size in 2-3 years.
‘Epifauna’ as used here simply refers to animals that live on a surface, as opposed to infauna, which live within the sediment. Epifauna that attached to living ammonites were harmful because they increased weight and drag, adversely affecting the ammonite’s buoyancy and hindering swimming. However, they can reveal aspects of ammonite biology, such as life orientation and pelagic versus benthonic life style (e.g., Seilacher, 1960; Maeda and Seilacher, 1996, p. 548). Ammonite shells with attached epifauna are fairly common, but not necessarily evidence of attachment to a living animal. Seilacher (1982) and more recently Paul and Simms (2011) have discussed criteria for distinguishing pre- and post-mortem attachment. Epifauna confined to one side of an ammonite shell, especially if it is the preserved upper surface, implies post-mortem attachment. The ammonite shell formed a solid substrate in an inhospitable soft or unstable sea floor. Epifauna attached to the inside of an ammonite body chamber equally clearly attached after death of the ammonite. On the other hand, epifauna present on both sides of an ammonite shell suggests the attachment took place either when the ammonite shell was vertical in the water column or post-mortem as a result of the shell being turned over by currents. Even in the former case, however, some ammonite shells floated after death, as do shells of modern Nautilus (see Saunders and Spinosa, 1979, Wani et al. 2005), so attachment to both sides of an ammonite shell is suggestive, but not conclusive evidence of attachment to a living ammonite. The latter also seems unlikely in the present context as the ammonites are preserved in mudstones with little evidence of currents strong enough to overturn ammonite shells.
Wani et al. (2005) have shown that modern Nautilus shells less than 200 mm in diameter do not float after death. The chambered part of the shell (the phragmocone) contains insufficient gas to compensate for the weight of the shell. They argued that a similar size limit probably obtained for ammonite shells. In all the examples we describe the ammonites were far too small for their shells to have floated after death.
Ammonites that grew over attached epifauna were alive while the epifauna were attached. Equally, if the epifauna reacted to the growth and reorientation of the ammonite shell, both were alive simultaneously. Lange (1932) and Schindewolf (1934) were the first to document and illustrate such interactions. For example, Schindewolf (1934, pl. 2, figs 1-5) illustrated several Lower Jurassic ammonites with up to six worm tubes growing along the periphery of their shells, which were overgrown by the ammonites. Merkt (1966) has also described serpulids growing on ammonites, whereas Buys (1973) described serpulids growing on Promicroceras from Charmouth in a paper that has been largely overlooked. In addition, Seilacher (1960) and Meischner (1968) have documented cases where attached epifauna reorientated themselves to compensate for the rotation of the host ammonite shells as they grew.
Despite these examples, some of which involve repeated attachment, evidence of interactions between living ammonites and epifauna (usually bivalves or worm tubes) remains rare. Herein, we distinguish two orders of evidence that worm tubes were attached to living ammonites. Reactions of the ammonite to the attachment, such as overgrowth of the worm tube, are regarded as first order evidence, since no other interpretation is possible. Reactions of the worm tubes, such as specific orientation in relation to the ammonite shell, require additional assumptions (outlined in section 4 below) and are therefore regarded as second order evidence. However, we believe that in most cases described herein the best interpretation is that the worm tubes were growing on living ammonites. In this paper we describe epifaunal worm tubes on ammonite shells, mostly Promicroceras, from the Charmouth Mudstone Formation (Cox et al., 1999), of Charmouth, Dorset and discuss the evidence for attachment and growth before or after death of the ammonites. Here we do not discuss the ambiguous cases. Please see the full paper for their interpretation.
2. Material and methods
Almost all the ammonites described herein were collected loose from beneath Stonebarrow Hill, east of Charmouth (Fig. 1). We do not know the original orientation of the ammonites in the sedimentary rocks, nor the precise stratigraphical horizons (bed number) from which they originated. However, the ammonite Promicroceras ranges from the Birchi Tabular bed to the Coinstone (beds 76a to 89 of Lang and Spath, 1926; see Simms et al., 2004, figs 1.4-1.5, pp. 8-9; fig. 2.6a, p. 64; fig. 2.8, p. 67). In addition, almost all the specimens are pyritized internal moulds and many have ‘beef’ (fibrous calcite) coatings, suggesting that beef-coated examples came from Lang’s beds 81i or possibly 81j (Lang and Spath, 1926). We are confident that all but two of the examples we describe came from the Charmouth Mudstone Formation and, probably, from the Black Ven Marl Member. The sole exceptions are one Polymorphites (LYMPH 2010/66) from bed 118 (of Lang et al., 1928) in the Belemnite Marls, collected in situ on the foreshore just west of Golden Cap, and an Echioceras, the precise stratigraphic origin of which is unknown. Both show post-mortem encrustation (see below section 4.5). All occurrences are Lower Lias following Page (2010). All described specimens are preserved in the Lyme Regis Philpot Museum (LYMPH 2010/25-66).