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
Other Creatures Great and Small
Our Herefordshire finds shed light on a variety of animals besides crustaceans, including the pycnogonids, or sea "spiders," which are a group of marine invertebrates that are widespread in the oceans today, with almost 1,200 living species. But their delicate form has resulted in an extremely sparse fossil record of just a few tens of specimens belonging to perhaps nine species.
We discovered the most completely preserved fossil sea spider in the Herefordshire deposit, older by about 35 million years than the previous record holder, at least for an adult animal. (Tiny specimens from the Cambrian of Sweden have been interpreted as sea-spider larvae, but this identification is disputed.)
The relationship of sea spiders to other arthropods has been a subject of debate for two centuries. There are two main views on the position of sea spiders on the tree of life: Either they belong with the chelicerates, or they represent a group separate from all other arthropods. Well-developed pincers on the large first appendage of the Silurian sea spider we discovered, Haliestes dasos, support the former view, and a comparison with other fossil and living species shows that the features typical of today's sea spiders had arisen by Silurian times.
Our studies also improved the understanding of the Marrellomorpha, a poorly known group of fossil arthropods that contains five genera and species. Marrellomorphs are important because they branched off the arthropod lineage earlier than did the trilobites, chelicerates and crustaceans and their various relatives. Our discovery of Xylokorys chledophilia from Herefordshire, provides the most complete three-dimensionally preserved marrellomorph, and the first from the Silurian.
The Herefordshire fauna includes other ancient animals that, unlike Xylokorys or Haliestes, were already known to science—but only from their fossilized skeletal remains. Our finds have added information on their soft tissues.
The Herefordshire specimens of the sea-star Bdellacoma, for example, preserve not only the mineralized plates of the skeleton, but also soft tissues including tube feet and papulae (respiratory organs). We have also described a gastropod mollusk that uniquely preserves internal structures such as a coiled gut, previously unknown in a fossil.
Another example is provided by the brachiopods or "lamp shells," a type of shellfish unrelated to the mollusks. Brachiopod shells are among the most common fossils, and many paleontologists have spent their entire careers studying them, examining countless thousands of specimens—not one of which contained any internal soft tissues. We have so far reconstructed just one brachiopod specimen from Herefordshire, and the amount of information it provides is so great that it almost mocks the normal fossil record of brachiopods, which consists only of shells.
The fossil we studied contains three tiny post-larval brachiopods, representing at least two different species, which are attached to a larger brachiopod, Bethia serraticulma, a third species. All three species preserve the fleshy pedicle (attachment stalk), and one also preserves sensory hairs. The largest brachiopod fossil shows internal structures including the mantle that secretes the shell, and the filaments used for filter feeding.
In spite of its spectacular state of preservation, however, Bethia has proved difficult to place, ironically because the soft tissues obscure parts of the shell that are critical for classification. Bethia, nonetheless, carries an important message: Paleontologists who use living forms as models for fossil animals can easily be misled, particularly for organisms like brachiopods, which are much less diverse today than they were in ancient times.
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