FEATURE ARTICLE
Virtual Fossils from 425 Million-year-old Volcanic Ash
A set of exceptionally preserved but difficult-to-extract fossils reveals the diverse creatures from a Silurian sea-floor community
Derek E. Briggs, Derek J. Siveter, David J. Siveter, Mark D. Sutton
The Worm that Wasn't
It is difficult for us to predict what one of these animals looks like based on random fractures through a few concretions, so we tend to refer to them by informal names until we can work out their shapes in detail. One of the first species we tackled in this way was Acaenoplax hayae, which we called just "spiny worm."
After much work, we determined that the under-surface of this animal was smooth but had a number of flexible lobes, arranged in a pattern of overlapping chevrons, which probably helped it to gain purchase on the sediment. The body was wrinkled and carried an array of sharp spines behind the head that must have served for defense. The largest spines projected from fleshy ridges on its back, which also had seven plate-like shells along its length, most of which were also spiny. At the rear, two shells enclosed a respiratory cavity from which fleshy gills protruded.
It was hard to know what to make of such a strange creature. The presence of seven shells, together with an obvious space where an eighth seems to have been lost in evolution, suggested to us that Acaenoplax is related to the living chitons, which are eight-shelled polyplacophoran mollusks. Other investigators have interpreted almost identical shells (preserved without any evidence of soft parts) from Silurian rocks in Gotland, Sweden, as belonging to this group. Our reconstruction made it clear, however, that Acaenoplax was no chiton: it lacked a foot, a feature that is characteristic of this group.
The body plan of Acaenoplax is closer to aplacophorans, simple wormlike mollusks that lack a foot and have a rear respiratory cavity. However, no aplacophoran is known with shells. Acaenoplax appears to be a kind of molluscan 'missing link'—neither a polyplacophoran nor an aplacophoran, although more closely related to the latter.
This conclusion implies that today's aplacophorans are secondarily simplified, having lost the shells present in their Acaenoplax-like ancestors. So, contrary to orthodox thinking, the earliest mollusks were not simple wormlike forms. Acaenoplax revamped our understanding of how the mollusks fit into the evolutionary tree of life.
Even though we had grown accustomed to calling it spiny worm, in the end it became clear that Acaenoplax was a mollusk rather than a real worm. Kenostrychus clementsi, however, the second most common species in the Herefordshire fauna, is a polychaete or bristle worm, an early representative of the most diverse worm group in the modern oceans.
Kenostrychus is neither the oldest nor the most unusual fossil polychaete, but it is by far the best preserved of any age. Although it appears normal in most respects, its gills take the form of coiled tentacles attached to an unusual part of its trunk appendages. This configuration has implications for the early evolution of polychaete respiratory structures. Kenostrychus provides an example of how even relatively routine animals of this antiquity, when preserved with sufficient fidelity, can also inform our understanding of evolution.
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