Friday, 25 April 2014

Three dimensional soft tissue preservation in a Myodocope Ostracod from the Herefordshire Lagerstätte.

Ostracods are small Crustaceans which conceal their bodies between two large valves in a bodyplan convergent with the unrelated Bivalve Molluscs and Brachiopods. They have an extensive fossil record, beginning in the Late Cambrian, and are probably the most abundant fossil Crustaceans, largely due to their small size and abundant natures. The largest Ostracods reach about 3 cm in length, but most species are under 3 mm. Due to their ubiquitous nature in marine and many freshwater ecosystems, and the distinctive patternation on the shells of many Ostracods, which makes species identification relatively easy, Ostracods are frequently used in biostratigraphy (the use of fossils to date rocks and establish sequences in rock formations).

In a paper published in the Proceedings of the Royal Society Series B: Biological Sciences on 12 December 2012, a team of palaeontologists led by David Siveter of the Department of Geology at the University of Leicester describe a new species of Myodocope Ostracod from the Herefordshire Lagerstätte in southwest England.

The Herefordshire Lagerstätte comprises a large number of small (at most centimetres) organisms from the Middle Silurian (about 425 million years ago). The organisms are preserved in three dimensions within calcareous nodules within a layer of volcaniclastic sediments (i.e. a volcanic ashfall in a marine environment), and can only be accessed using computerised tomography scanning techniques. Brachioods, Polychaete Worms, Gastropods, Aplacophorans, Chelicerates, Marrellomorphs, Mandibulates, Barnacles, Phyllocarids, Ostracods, Starfish and Sponges have all been found in the Herefordshire Lagerstätte; together these are known as the Herefordshire Biota.

The new Myodocope Ostracod is named Pauline avibella, where ‘Pauline’ honours the late Pauline Siveter and ‘avibella’ means ‘beautiful bird’ in reference to the posterodorsal lobal structure of the Ostracod (part of the shell ornamentation) which resembles the wing of a Bird. The species is described from two specimens, one 6.4 mm in length and the other 10.2 mm.

Pauline avibella, 10.2  mm specimen, carapace with soft-parts: (a–k, n–t) ‘virtual’ reconstructions; (l) specimen in rock. The exact boundary between structures such as body and limbs, as indicated by colour changes, is somewhat arbitrary. (a) External right lateral view (stereo-pair). (b) Right lateral view (stereo-pair), valves omitted. (c) Anterior view. (d) Right lateral view (stereo-pair) of gut system. (e) Oblique ventral view of labrum and atrium oris. (f) Anteriorview (stereo-pair), valves omitted. (g) Ventral view. (h) Dorsal view. (i) Ventral view (stereo-pair), valves omitted. (j) Dorsal view (stereo-pair), valves omitted. (k) Posterior view. (l ) Lateral oblique section. (n–t) Oblique posterior approximately medial view of left limbs: (n) first antenna, (o) second antenna, (p) mandible, (q) first maxilla. (r) second maxilla. (s) sixth limb. (t) seventh limb. (m) Halocyprid myodocope Discoconchoecia pseudodiscophora, carapace, right lateral view; Recent, Sea of Japan, depth 320 m. All scale bars 1 mm. a1, first antenna; a2ba, a2en, a2ex, basipod, endopod and exopod of second antenna; a6, sixth limb; a7, seventh limb; alo, anterior lobe; an, anus; ao, atrium oris; as, adductorial sulcus; co, costa(e); cm?, contact margin structure?; ec?, epibranchial canal?; fs, finger-like structure; fu, furca; gi, gills; hl, hinge line; is, isthmus; la, labrum; le, lateral eye; lv, left valve; maba, maen, maex, basipod, endopod and exopod of mandible; mr, marginal ridge; mx1, first maxilla; mx1ba, mx1en, basipod and endopod of first maxilla; mx2, second maxilla; mx2ba, mx2ep, mx2r, basipod, epipod and ramus of second maxilla; no, node; oe, oesophagus; pg, posterior gape; ri, rostral incisure; ro, rostrum; rv, right valve; s-s, line of section through specimen (h,l ); st, stomach. Siveter et al. (2012).

Myodocope Ostracods have weekly calcified shells and form part of the plankton; unlike other Ostracods which have more strongly calcified shells and live on the sea floor. As a consequence they are poorly represented in the fossil record, with only a handful of Palaeozoic specimens known, predominantly from the Herefordshire Biota. However, while it is clear from its soft tissue anatomy that Pauline avibella is a Myodocope, were only its shell available it would have probably been classified as a Palaeocopid, considered to be the most diverse and abundant Ostracod group in the Palaeozoic, with over 500 described genera. From this Siveter et al. conclude that considerable caution is needed in the classification of Palaeozoic Ostracods, and that the group may well be in need of review.

Silurian Myodocopida. (a–j) Pauline avibella, 6.4 mm specimen carapace with soft-parts: (a, c–j) ‘virtual’ reconstructions; (b) specimen in rock. The exact boundary between structures such as body and limbs, as indicated by colour changes, is somewhat arbitrary; the gap in data marks the line of split of the specimen in the nodule. (a) External right lateral view (stereo-pair). (b) Oblique section. (c) Dorsal view. (d) Lateral view (stereo-pair) of stomach, gills, seventh limb and finger-like projections (other soft-parts and valves omitted). (e) Anterior view. (f) Anterior view (stereo-pair), valves omitted. (g) Ventral view. (h) Right lateral view, valves omitted. (i) Posterior view. (j) Ventral view, valves omitted. (k–m) Herefordshire Lagerstätte myodocopids; ‘virtual’ reconstructions of carapaces with soft-parts: (k) Nymphatelina gravida, left lateral view; (l) Nasunaris inflata, right lateral view; (m) Colymbosathon ecplecticos, left lateral view. All scale bars 1 mm. Abbreviations: as for figure above; s-s, line of section through specimen (b,c). Siveter et al. (2012).

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