FOSSIL INVERTEBRATES/Bivalves 373 Figure Various aspects of the internal morphology of bivalve shells (A) The left valve of a generalized burrowing dimyarian (B) The right valve of a generalized byssate scallop (C) The rela tionship between the two valves and their associated mantle lobes, adductor musculature and ligament but others attach to or bore into hard substrates by a variety of means; others have become free living, some with the ability to swim Most bivalves are marine, exploiting niches from the intertidal zone down into the abyssal depths, but successful groups (including the oysters) have invaded more brackish conditions and even freshwater, where modern unionid mussels cause enormous damage as biofoulers It is clear that the most primitive bivalves were marine shallow burrowers and that other life styles evolved later It is also apparent that many of the more specialized life habits have evolved separately in a number of different lineages (i.e., polyphyletically) Seminal work by S M Stanley firmly established how different aspects of the morphology of living bivalves could be related to their life habits, such that it is possible to use these characteristics of extinct taxa to reconstruct the life habits of fossil groups Burrowing A large proportion of all bivalves (around 50% of all modern families) burrow into soft sediments using the foot Most are equivalve and are isomyarian (i.e., the posterior and anterior adductor muscles are of equal size) The depth to which they burrow varies between taxa, from those which lie just under the surface with the edge of the shell virtually level with the sediment– water interface (e.g., Cerastoderma; Figure 6C), to depths of several centimetres (e.g., Mya; Figure 6B), with Panopea reaching spectacular depths of up to m The key to successful burrowing is maintaining contact with the seawater in order to continue both feeding and respiration This is achieved by the siphons, snorkel-like extensions of the posterior mantle The length of the siphons, and therefore the depth of burrowing, can be inferred from the shells by the size of the pallial sinus; deeper burrowers have more indented pallial sinuses, whereas very shallow burrowers have no sinus at all (see Figure 6) Very deep burrowers, such as Mya, have siphons so long that they are unable to withdraw them fully into the shell when it shuts, and have a permanent posterior siphonal gape through which they protrude Shallow burrowers generally have strong, robust shells, often with a pronounced radial or concentric ornament that may assist the burrowing process or help the animal remain ‘locked’ into the sediment Deeper burrowers tend to have thinner shells and are often smooth shelled Although the foot is never preserved, its presence may be inferred from the pedal musculature on the inside of the valves and, in cases where the animal is a rapid and deep burrower (such as the razor shell Ensis), the foot may be so well developed as to require an anterior pedal gape It is clear from studies of the siphons of living bivalves that they are constructed in a number of different ways, suggesting that the deep burrowing habit has evolved independently in several clades Attachment Almost all larval bivalves attach to the substrate, if only briefly, with tanned protein threads (the byssus) secreted by a gland at the base of the foot In a large number of taxa, this habit has been neotenously retained into adulthood, and again it is clear that this has happened repeatedly in different groups Byssate bivalves fall into two categories: those like Pinna (Figure 6A) and Modiolus that are orthothetic and live attached to clasts within the sediment in which they are partially buried (endobyssate), and those that are attached to the surface of hard substrates (epibyssate), either in an orthothetic (e.g., Mytilus; Figure 7D) or a pleurothetic (e.g., Isognomon ephippium; Figure 7C) orientation Orthothetic byssate bivalves tend to be equivalve and have much reduced anteriors This anterior reduction is reflected