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

Caron at the Royal Ontario Museum published a paper proudly claiming Odontogriphus for the molluscs. Caron and his colleaguesalso drew a close connection between these and another, even older fossil called Kimberella . Discovered in the 1960s in the Ediacara Hills in South Australia, the flattened egg-shaped fossils of Kimberella were first thought to be jellyfish. Then trace fossils were found that suggested they spent their lives not pulsing through open water but creeping backwards across the seabed, scraping up food with tiny teeth. But Kimberella ’s teeth have never been found, so no one knows whether their snail-like scuff-marks really were made by a radula.
    Some striking recent advances in our understanding of molluscan ancestry came from looking at these ancient fossils in a completely new way. For his Ph.D, Martin Smith put fossils inside a scanning electron microscope and captured images of electrons bouncing off atoms deep inside the specimens. This revealed their inner structure in unrivalled detail and convinced him that Wiwaxia and Odontogriphus were not worms. Smith worked out that both of them shed their teeth and grew new ones throughout their lives, and occasionally they would swallow them; a few fossils have teeth lodged in their guts. The bigger the animal, the more teeth it had, and each tooth swivelled relative to its neighbours. All of this, and more besides, lent weight to the idea that these fossils had molluscan kinship.
    In a 2014 paper, Smith provided more support for the idea that Wiwaxia was an early mollusc. He studied a handful of Wiwaxia fossils that seemed to have a single foot, like modern slugs and snails. But part of the puzzle remains unsolved. Smith hasn’t yet been able to decipher exactly where to place Wiwaxia on the tree of life, although he has at least narrowed things down. One possibility is that it belongs among the molluscs that don’t have a single shell, the aculifera (including the chitons, solenogastres and caudofoveates). These weren’t the earliest molluscs to evolve, so it would mean Wiwaxia wasn’t a mollusc ancestor. Alternatively, Wiwaxia could be placed on a lower branch, asa stem group to all the molluscs. This would make it a precursor to the mollusc phylum, closer to molluscs than to any other modern group, but not quite a mollusc.
    The concept of stem and crown groups has gained interest in palaeontological circles over the last 15 years. Crown groups are living species that share key characteristics (along with an ancestor that they all have in common, plus any extinct species that also evolved from that same ancestor). Stem groups are extinct species that have some but not all of those characteristics of the crown group. They are aunts and uncles to the crown group, taxonomically speaking.
    This approach is helping palaeontologists to make sense of the jumble of strange animals that emerged around the time of the Burgess Shale. Many of these in-betweenie fossils could be stem groups to living phyla rather than members of fully formed phyla themselves, living or extinct. It underscores the fact that key characteristics defining a particular group of living things didn’t all evolve at once but rather gradually, step-by-step, over time. It’s the difference between going to a department store to buy a whole outfit compared to assembling a look from a mixture of vintage hand-me-downs, old favourites and new shoes.
    Contemplating stem groups in the deep past reveals that the boundaries drawn between phyla are perhaps somewhat arbitrary. Looking at living species, it is plain to see that molluscs are very different from, say, annelids or echinoderms. But as palaeontologists peer further back through time and in greater detail, those boundaries become blurred.
    If Wiwaxia is a stem-group mollusc, it would suggest that the radula, sclerites and a single foot were among the earlier characteristics to appear in the mollusc lineage. But it leaves an important