Spirals in Time: The Secret Life and Curious Afterlife of Seashells Read Online Free PDF Page B

unanswered question.
    Which came first, the mollusc or the shell?
    By the Late Cambrian, most of the major mollusc groups had evolved. There were indisputable bivalves, gastropods,cephalopods and chitons; scaphopods came along a while later. All of them became more abundant and diverse in the following geological period, the Ordovician. A few other mollusc groups came and went through the eons, including the now-extinct rudists; back in the Jurassic and Cretaceous these twin-shelled molluscs formed the foundations of teeming tropical reefs, similar to the coral reefs of today.
    All things considered, the mighty mollusc lineage has been going for at least half a billion years, and in all that time these super-abundant, super-diverse animals have kept some secrets to themselves. We still don’t really know how the different groups – the bivalves, cephalopods, chitons and so on – are related to each other, and we don’t know for sure which of them came first.
    Following years of research, including comparisons between living animals and more recently the arrival of genetic techniques, experts are still wrangling over molluscs. Like a pack of playing cards, the mollusc groups keep being shuffled around; should we put all the red cards together, the kings and queens in one place, should diamonds go next to hearts because they’re the same colour or do they belong with the spades because they have a point at the top? Scientists keep grabbing the pack of mollusc cards from each other and moving things around.
    The wobbliness of the mollusc family tree (or phylogeny) and the fact that it keeps changing shape has important implications for the way we understand evolution and the variety of life on Earth. It matters, for example, to people studying the evolution of complex brains whether cephalopods and gastropods are closely related or not, because both these groups have well-developed nervous systems; did these systems evolve twice, independently, or just once in a shared ancestor?
    These questions, and many more, are tackled by a recent trio of studies that delve deep into the mollusc phylogeny. The three studies involved large research teams led by Kevin Kocot from Auburn University in Alabama, Stephen Smith, now at University of Michigan, Ann Arbor, and Jakob Vinther, now at Bristol University in the UK. The methods they all used were incredibly complex, with the outcomes depending on many things, from the choice of mollusc species and out-group (the non-mollusc species used as a comparison) to the way the data are analysed. All three teams used similar DNA sequencing techniques (using nuclear protein-coding genes, not ribosomal genes as in earlier studies), but the results they throw up don’t all agree.
    One conclusion that all three studies do settle on is the identity of the aculifera; they all confidently proclaim that chitons, solenogastres and caudofoveates do indeed belong together on the same branch of the mollusc family tree.
    A radical outcome from one of these studies is the relationship between cephalopods and gastropods. Traditionally, these two classes were clustered together as sisters, offshoots from the same junction on the mollusc family tree. But rather than bringing them together, some of the latest genetic findings have separated the octopuses from the snails. Cephalopods could instead be more closely allied with the mysterious monoplacophorans, the deep-sea molluscs that were thought to be long extinct. Morphological studies in the past had linked these two groups, based on their fossils having a similar arrangement of internal organs, and now genetic studies have breathed new life into this idea. The gastropods are bundled, quite confidently, in with the bivalves and the scaphopods (although the scaphopods continue to be a pain in the neck to identify; we simply don’t know enough about them to be sure where exactly they fit in). If this is correct then it suggests that molluscs evolved complex