Friday 23 November 2018

Spiricopia aurita: A Cylindroleberidid Ostracod from the Silurian Herefordshire Lagerstätte with well-preserved gills.

Ostracods are small Crustaceans with a bivalved body plan; their body is sandwiched laterally between two large valves, with the animal using its legs to generate a current through the shell, enabling it to feed, and in many cases swim (check). Ostracods are small (seldom much over a millimetre) and can be very abundant, making them common fossils in many deposits. They also often have distinctive shell ornamentation, enabling the identification of species from valves alone, and are both fast-evolving and sensitive to a range of environmental conditions, making them useful in both biostratigraphy (dating rocks using fossils) and palaeoenvironmental reconstruction. 

Due to their small size most Ostracods can satisfy their oxygen needs by simple diffusion from their surface to their tissues, with bigger species (larger than about 3 mm) supplementing this with a simple circulatory system. Only a single group of Ostracods, the Cylindroleberidids have taken this a step further and developed gills to aid the acquisition of oxygen, typically having seven pairs. Ostracods in general have a good fossil record, but this is largely of their hard shells only, with very few examples known with their soft tissues preserved, and correspondingly few with gills, all from the Silurian Herefordshire Lagerstätte.

In a paper published in the journal Biology Letters on 7 November 2018, David Siveter of the School of Geography, Geology and the Environment at the University of Leicester, Derek Briggs of the Department of Geology and Geophysics, and Yale Peabody Museum of Natural History at Yale University, Derek Siveter of the University Museum of Natural History, and the Department of Earth Sciences at the University of Oxford, and Mark Sutton of the Department of Earth Sciences and Engineering at Imperial College London describe a new Cylindroleberidid Ostracod from the Herefordshire Lagerstätte with well-preserved gills.

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 specimen is described as a new species under the name Spiricopia aurita, where ‘Spiricopia’ means ‘abundance of the breath of life’, in reference to its gills, and ‘aurita’ means ‘having ears’ in reference to the shape of its posterodorsal lobes. The specimen is 7500 μm in length, 2950 μm high and 3200 μm wide, with two pairs of antennae, a broad, flat mandible, seven pairs of limbs and five pairs of gills.
(a)–(g), (i)–(l) Spiricopia aurita: ‘virtual’ reconstructions (a), (c)–(g): stereo-pairs). (a) Right lateral view. (b) Anterior view. (c) Right lateral view, valves omitted. (d)–(g) Inner right gill lamellae. (i) Posterior view. (j) Ventral view. (k) Valve ornament. (l) Dorsal view. (h) Holocene Cylindroleberidid Leuroleberis surugaensis, gill lamella. Scale bars: (a)–(i), (l) 2.5 mm; (k) 1.3 mm. a1, first antenna; a2ba, a2ex, basipod and exopod of second antenna; a7, seventh limb; as, adductorial sulcus; cm, contact margin; ec, epibranchial canal; fu, furca; g1–5, gill lamellae; hc, hypobranchial canal; is, isthmus; le, lateral eye; li, ligament; lv, left valve; maba, limb base of mandible; mr, marginal ridge; mx1en, endopod of first maxilla; mx2ep, epipod of second maxilla; pg, posterior gape; pl, posterodorsal lobe; ri, rostral incisure; ro, rostrum; rv, right valve. Siviter et al. (2018). 

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