A Silurian myodocope with preserved soft-parts: cautioning the interpretation of the shell-based ostracod record

Abstract

Ostracod crustaceans are the most abundant fossil arthropods. The Silurian Pauline avibella gen. et sp. nov., from the Herefordshire Lagerstätte, UK, is an extremely rare Palaeozoic example with soft-part preservation. Based on its soft-part morphology, especially the exceptionally preserved limbs and presence of lateral eyes, it is assigned to the myodocopid myodocopes. The ostracod is very large, with an epipod on the fifth limb pair, as well as gills implying the presence of a heart and an integrated respiratory–circulatory system as in living cylindroleberidid myodocopids. Features of its shell morphology, however, recall halocyprid myodocopes and palaeocopes, encouraging caution in classifying ostracods based on the carapace alone and querying the interpretation of their shell-based fossil record, especially for the Palaeozoic, where some 500 genera are presently assigned to the Palaeocopida.

Fossil Calibrations for the Arthropod Tree of Life

“…Older fossils, from the Tremadocian (~40 Myr older) have been reported from numerous localities across the current and Ordovician world: Argentina, Australia, China, Iran, Norway, Sweden, and the UK (Williams et al, 2008). However, all of these fossils are known solely from carapaces, which are known to be highly homoplastic (Siveter et al, 2013). The affinities of Tremadocian ostracods are therefore ambiguous.…”

The fossil record and palaeoenvironmental significance of marine arthropod zooplankton

“…However, usually the crucial soft anatomy is 520 missing, in part a result of post-mortem drifting of carcasses, making it more 521 circumspect to interpret the lifestyle of these organisms. The interpretation of a 522 pelagic lifestyle is then derived from skeletal morphology (which is not always 523 reliable; see Siveter et al, 2003Siveter et al, , 2013 and from a set of indirect inferences that 524 include palaeogeographic distribution, depositional setting and associated faunas 525 (Fortey, 1974(Fortey, , 1975(Fortey, , 1985Siveter et al, 1991). 526…”

Vision in a Middle Ordovician trilobite eye

“…Using these methods, exceptionally preserved soft-parts, including internal organs, can be examined, and this aids in the determination of morphological stability between recent and extinct taxa (Sutton et al, 2001;Siveter et al, 2003;Briggs et al, 2004;Siveter et al, 2004Siveter et al, , 2007aSiveter et al, , 2007bSutton, 2008;Matzke-Karasz et al, 2009;Siveter et al, 2010Siveter et al, , 2013. Moreover, 3D reconstructed digital images can be used to calculate three coordinate points for each phenotypic character of fossilized specimens to determine the original position of each fossilized lens in the compound eyes of extinct arthropods.…”