Offacolus

Its only species, O. kingi, has been found in deposits from the Silurian period (Homerian epoch) in the Wenlock Series Lagerstätte of Herefordshire, England.The species name honors Robert Joseph King, a British mineralogist who found the fossils of Offacolus.[1] Similar to Dibasterium, Offacolus possess limb-like exopods (outer limb branches) on appendage II to V,[2][3] a character suggest to be plesiomorphic (observable in the putative stem-chelicerate taxon Habeliida[4]) and lost within the prosomapod clade.[2] A phylogenetic analysis (the results presented in a cladogram below) conducted by James Lamsdell in 2013 on the relationships within the Xiphosura and the relations to other closely related groups concluded that the Xiphosura, as presently understood, was paraphyletic (a group sharing a last common ancestor but not including all descendants of this ancestor) and thus not a valid phylogenetic group.This family currently includes along with O. kingi, Dibasterium and Setapedites and represent the sister group of Prosomapoda.
Ventral appendages of Offacolus kingi , showing limb-like exopods (Ex2-5)
HomerianPreꞒScientific classificationEukaryotaAnimaliaArthropodaChelicerataEuchelicerataOffacolidaeType speciesextincteucheliceratecheliceratearthropodsSilurianperiodWenlock SeriesLagerstätte of HerefordshireEnglandMerciaOffa's DykemineralogistplesiomorphicHabeliidaarthropodparaphyleticlast common ancestorsister taxonmonotypicfamilyDibasteriumSetapeditesProsomapodaFuxianhuiaAntennulataEmeraldellaTrilobitomorphaSidneyiaMegacheiraYohoiaAlalcomenaeusLeanchoiliaPycnogonidaPalaeoisopusPycnogonumHaliestesWeinberginaVenustulusCamanchiaLegrandellaXiphosuraKasibelinurusWillwerathiaXiphosuridaLunataspisPlanatergaPseudoniscusCyamocephalusPasternakeviaBunodesLimuloidesBembicosomaDekatriataChasmataspididaSclerophorataArachnidaEurypteridaBriggs, Derek E. G.BibcodeWikidataPaleobiology Database