FOSSIL INVERTEBRATES/Bivalves 377 to play in providing the temporal data that will help to discriminate between putative hypotheses, and provides a time-frame for establishing branching points on suggested trees Evolutionary History Figure shows the changing familial diversity of bivalves over the Phanerozoic Aside from fluctuations, largely associated with mass extinction events, the overriding impression is one of almost exponential increase in diversity towards the present day This taxonomic explosion has been accompanied by a movement away from the primitive shallow burrowing and byssate habits into more specialist habits (detailed above) in a series of adaptive radiations Bivalves are generally thought to have evolved from the extinct molluscan class Rostroconchia, although this is not universally accepted The first fossils widely accepted (but again not universally) as bivalves are from Early Cambrian rocks Fordilla troyensis is known from a large number of specimens from Tommotian localities in North America, Greenland, and Western Europe, whilst Pojetaia runnegari is known from Australia Both Fordilla and Pojetaia are small, only a few millimetres in length, and are generally thought to have been shallow burrowers Despite this early appearance, there is then a gap in the bivalve fossil record throughout the Middle and Upper Cambrian before their reappearance in the Early Ordovician (Tremadoc) This gap is both perplexing and frustrating; it spans some 4% of the evolutionary history of the class and covers an interval when bivalves apparently became larger and more diverse in terms of both taxa and the life habits Figure Familial diversity of bivalves over the Phanerozoic employed There have been several claims for Middle and Upper Cambrian bivalves, but most of these have been dismissed as misidentified inarticulate brachiopods It is difficult to explain this gap in the bivalve record, but one plausible explanation is that the earliest bivalves may have lived in nearshore silty facies which are seldom preserved By the close of the Ordovician, bivalves were more common and abundant and all the subclasses recognized in Table had appeared There is evidence of shallow burrowers and both endo- and epibyssate forms and signs of different feeding strategies (deposit feeders, suspension feeders) Bivalve diversity continued to increase throughout the Palaeozoic, although there are small decreases associated with the Ordovician and the Devonian (Frasnian/Famennian) mass extinctions Throughout the era most bivalves remained in ‘primitive’ life habit, with only one genus each of borers (Coralliodomus) and cementers (Pseudomonotis) being recognized (neither of which is widespread) Very few Palaeozoic bivalves (e.g., Lyrodesma) have pallial sinuses Although the effect of the endPermian mass extinction is clearly evident in Figure 8, losses were small in contrast with those sustained by other shelly benthos, for example the brachiopods The beginning of the Mesozoic saw a further combined increase in the number of taxa, with an ever increasing repertoire of life habits Many shallow burrowers moved deeper, and large numbers of byssate taxa adopted more specialized habits, such as cementing, boring, and free living The cementing habit is exemplified by the successful oysters and rudists The first evidence of the adaptations for swimming in scallops are recognized in Jurassic taxa, and free-reclining gryphaeid oysters became abundant and widespread in this period This major adaptive radiation is widely thought to have been linked to the Mesozoic Marine Revolution – the profound restructuring of shallow marine communities and, in particular, a massive increase in predation pressure at that time Bivalves are extremely vulnerable to predators and are attacked by a wide range of important predatory groups (e.g., fish, gastropods, starfish, crustaceans, mammals, and birds) and by a variety of different methods (swallowing whole, crushing, drilling, or prising apart) It has been suggested that different groups of bivalves responded in a variety of different ways, for example by living deeper within the sediment, encasing themselves in hard substrates and becoming more robust, and, in some cases, growing formidable spines Although the endCretaceous extinctions had a marked effect on the bivalves, with major groups such as the inoceramids and rudists being lost, their diversity continued to increase during the Cenozoic The number of groups