The arthropodOffacolus kingi(Chelicerata) from the Silurian of Herefordshire, England: computer based morphological reconstructions and phylogenetic affinities

Abstract

The small, non-biomineralized, three-dimensionally preserved arthropod Offacolus kingi Orr et al. from the Wenlock Series (Silurian) of Herefordshire, England, is re-evaluated, and the new family Offacolidae erected. This new study is based on specimens which have been serially ground, reconstructed by computer and rendered in the round as coloured models. Offacolus possesses a prosomal appendage array similar to that of Limulus, but also bears robust and setose exopods on appendages II-V which are unlike those found in any other arthropods. Opisthosomal appendages are similar in number and morphology to the book-gills of Limulus. Cladistic analysis places Offacolus basally within the Chelicerata, as a sister taxon to the eurypterids and extant chelicerates, but more derived than the Devonian Weinbergina.

Silurian horseshoe crab illuminates the evolution of arthropod limbs

“…Horseshoe crabs, such as living Limulus, are characterized by two homonomous series of limbs (in the prosoma and opisthosoma), their biphasic development presumably under the control of Hox genes (14). Offacolus (7,8) has a strikingly similar prosomal appendage arrangement to that in Dibasterium. In the light of its position stemward of Dibasterium and Weinbergina (Fig.…”

“…S1); Xiphosurida are characterized by a fused opisthosoma (4). The interrelationships of synziphosurines are uncertain: limbs are only known in Offacolus kingi (7,8), Venustulus waukeshaensis (9), and Weinbergina opitzi (10). The generic identity of synziphosurines is therefore based on the morphology of the dorsal exoskeleton.…”

“…Surfaces were imaged digitally and image stacks used to generate a three-dimensional "virtual fossil" using the custom SPIERS software suite (www.spiers-software.org) (8,24). The virtual fossil (Movie S1) was studied on-screen using the manipulation, virtual dissection, and stereoscopic-viewing capabilities of SPIERS.…”

“…Synziphosurines also first appear as an undescribed taxon in the Lower Ordovician of Morocco, although they likely originated even earlier (5), and range into the Lower Carboniferous (Namurian) (6). The interrelationships of synziphosurines are uncertain: limbs are only known in the Silurian Offacolus kingi from Herefordshire, England (7,8), here shown to be a synziphosurine, and Venustulus waukeshaensis from Waukesha, Wisconsin, USA (9), and the Devonian Weinbergina opitzi from Hunsrück, Germany (10).…”

A starfish with three-dimensionally preserved soft parts from the Silurian of England

“…Fossils from this deposit are preserved as three-dimensional calcite in-fills in nodules in a volcaniclastic ash (Orr et al 2000), and preserve high-fidelity details of soft tissues that can be studied through serial grinding and computer reconstruction (Sutton et al 2001b). The deposit has yielded a diversity of sponges; radiolarians ; an aplacophoran-like mollusc, Acaenoplax hayae (Sutton et al 2001a; several arthropods including Offacolus kingi (a stem group chelicerate; Orr et al 2000;Sutton et al 2002), Colymbosathon ecplecticos (a myodocopid ostracod crustacean; Siveter et al 2003), Cinerocaris magnifica (a phyllocarid crustacean; Briggs et al 2004), and Haliestes dasos (a pycnogonid; Siveter et al 2004); a polychaete worm, Kenostrychus clementsi (Sutton et al 2001c); and an array of as yet undescribed species, including several more echinoderms.…”