FOSSIL INVERTEBRATES/Bivalves 375 Figure Morphology and mode of life of bivalves living on or boring into substrates (A) Spengleria rostrata (B) Spondylus americanus (C) Isognomon ephippium (D) Mytilus edulis (E) Placuna placenta (F) Gryphaea arcuata Not to scale Whereas byssate attachment is flexible and also renewable, some bivalves permanently attach to hard substrata by a calcareous cement The cemented habit always succeeds a byssate phase and it is clear that it has evolved independently in a number of different clades (e.g., oysters, rudists, and a number of pectinoids including Spondylus) Cemented bivalves are often easily recognizable from their irregular morphology, developed because of the requirement to conform to substratal irregularities, and are markedly inequivalve As they tended to evolve from pleurothetic byssate stock, most are also monomyarian (Figure 7B) Most cement mainly by the right valve, but a major group, the oysters, so by the left valve The size of the attachment scar varies and the substrate may be instantly recognizable; for example, oysters were often attached to ammonite shells, even if the scar and substrate are no longer attached Most cementers have thick, robust shells and may be extravagantly ornamented with spines or flanges (e.g., Spondylus; Figure 7B) Free Living A number of different taxa have independently abandoned attachment to become free living on softer sediments Here, the challenge is not to sink into the substrate, and this has been solved in two ways The first is by adopting a ‘snow-shoe’-type morphology, i.e., resting on a large surface area, epitomized by the wafer-thin window pane shell Placuna (Figure 7E) Alternatively, they may be semi-submerged in the soft sediment like an iceberg This strategy is inferred for the thick-shelled devil’s toenail Gryphaea, common in Mesozoic clay facies (Figure 7F) These ‘aberrant’ oysters clearly had a cemented phase, marked by a small attachment scar at the umbo A few free-living bivalves (notably several groups of scallops) also have the ability to swim short distances if they are threatened These have smooth hydrodynamic shells and a well-developed posterior adductor muscle whose vigorous contraction provides the propulsion Boring A number of groups, once again polyphyletically, have evolved to excavate ‘burrows’ in hard substrates by boring The most successful of these, the mytilid lithophagids, so principally by acidic secretions (which presents its own challenge of not dissolving its own shell), whilst others, e.g., Pholas, bore at least partly by physically rasping the substrate with small projections on the outside of the shell Some of the most bizarre borers are the teredinids which excavate long cylindrical boreholes in wood and have the enzymatic capability of digesting the cellulose These ‘shipworms’ are thought to have been part of the undoing of the ships of the Spanish Armada Members of the boring group as a whole have a very varied morphology, but may be easily recognized because they are almost invariably fossilized within their characteristic boreholes (Figure 7A)