Paleontologists Discover Well-Preserved Silurian Crustacean

An international team of paleontologists has made an extremely rare discovery of a 425-million-year-old fossil ostracod crustacean with body, limbs, eyes, gills and alimentary system preserved.

Left: reconstruction of the 425-million-year-old ostracod crustacean found in Herefordshire, England. Right: specimen of Pauline avibella in rock: the exact boundary between structures such as body and limbs, as indicated by color changes, is somewhat arbitrary; the gap in data marks the line of split of the specimen in the nodule (David Siveter et al)

Fossils of the 10 mm long shrimp-like animal, named Pauline avibella, were found in Herefordshire, Welsh Borderland, in old rocks dating to the Silurian period of geological time, when southern Britain was a sea area on a small continent situated in warm, southerly subtropical latitudes. The ostracods and associated marine animals living there were covered by a fall of volcanic ash that preserved them frozen in time.

Pauline avibella is special because it is exceptionally well preserved, complete with not only the shell but also the soft parts – its body, limbs, eyes, gills and alimentary system. Such discoveries are extremely rare in the fossil record.

Prof David Siveter of the University of Leicester, lead author of a paper in the Proceedings of The Royal Society B, said: “the two ostracod specimens discovered represent a genus and species new to science, named Pauline avibella. The genus is named in honor of a special person [Pauline Siveter, late wife of the lead author] and avibella means ‘beautiful bird’, so-named because of the fancied resemblance of a prominent feature of the shell to the wing of a bird.”

“Ostracods are the most abundant fossil arthropods, occurring ubiquitously as bivalved shells in rocks of the last 490 million years, and are common in most water environments today. The find is important because it is one of only a handful preserving the fossilized soft-tissues of ostracods. Its assignment to a particular group of ostracods based on knowledge of its biology is at odds with its shell form, thus urging caution in interpreting the classification of fossil ostracods based on shell characters alone.”

“The preservation of soft-parts of animals is a very rare occurrence in the fossil record and allows unparalleled insight into the ancient biology, community structure and evolution of animals – key facts that that would otherwise be lost to science. The fossils known from the Herefordshire site show soft-part preservation and are of global importance.”

The fossils were reconstructed ‘virtually’, by using a technique that involves grinding each specimen down, layer by layer, and photographing it at each stage. 10 mm is relatively tiny, but at an incremental level of 20 micrometers that yields 500 slices, which can then be pieced together in a computer to provide a full, three-dimensional image of each fossil, outside and in.

“Fossil discoveries in general help elucidate our own place in the tree of life. This discovery adds another piece of knowledge in the jigsaw of understanding the diversity and evolution of animals.”

“It is exciting to discover that a common group of fossils that we thought we knew a lot about may well have been hood-winking us as to their true identity, which we now realize because we have their beautifully fossilized soft-parts. A case of a ‘wolf in sheep’s clothing,” Prof Siveter said.

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Bibliographic information: Siveter DJ et al. 2012. A Silurian myodocope with preserved soft-parts: cautioning the interpretation of the shell-based ostracod record. Proc R Soc B, vol. 280, no. 1752; doi: 10.1098/rspb.2012.2664