Offacolus

{{Automatic taxobox

| name = Offacolus

| fossil_range = Homerian, ~{{Fossil range|425}}

| image = 20200810 Offacolus kingi.png

| image_caption = Reconstruction of Offacolus kingi

| display_parents = 2

| parent_authority = Sutton et al., 2002

| taxon = Offacolus

| authority = Orr et al., 2000

| type_species = †Offacolus kingi

| type_species_authority = Orr et al., 2000

}}

Offacolus is an extinct genus of euchelicerate, a group of chelicerate arthropods. 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 genus is named after Offa, a king from the ancient kingdom of Mercia, and colus, a person who dwelled among (this time referring to) the Offa's Dyke. The species name honors Robert Joseph King, a British mineralogist who found the fossils of Offacolus.{{cite journal|title=A new arthropod from the Silurian Konservat–Lagerstätte of Herefordshire, UK|first1=Patrick J.|last1=Orr|first2=Derek J.|last2=Siveter|first3=Derek E. G.|last3=Briggs|authorlink3=Derek Briggs|first4=David J.|last4=Siveter|first5=Mark D.|last5=Sutton|journal= Proceedings of the Royal Society of London. Series B: Biological Sciences|volume=267|issue=1452|pages=1497–1504|year=2000|doi=10.1098/rspb.2000.1170|pmid=11007324|pmc=1690702|issn=1471-2954}}

File:20201122 Offacolus kingi ventral appendages.png

Similar to Dibasterium, Offacolus possess limb-like exopods (outer limb branches) on appendage II to V,{{Cite journal|last1=Sutton|first1=Mark D.|last2=Briggs|first2=Derek E. G.|last3=Siveter|first3=David J.|last4=Siveter|first4=Derek J.|last5=Orr|first5=Patrick J.|date=2002-06-22|title=The arthropod Offacolus kingi (Chelicerata) from the Silurian of Herefordshire, England: computer based morphological reconstructions and phylogenetic affinities|url= |journal=Proceedings of the Royal Society of London. Series B: Biological Sciences|volume=269|issue=1497|pages=1195–1203|doi=10.1098/rspb.2002.1986|pmc=1691018|pmid=12065034}}{{Cite journal|last1=Dunlop|first1=Jason A.|last2=Lamsdell|first2=James C.|title=Segmentation and tagmosis in Chelicerata|url=https://www.academia.edu/28212892|journal=Arthropod Structure & Development|year=2017|language=en|volume=46|issue=3|pages=395–418|doi=10.1016/j.asd.2016.05.002|pmid=27240897|bibcode=2017ArtSD..46..395D |issn=1467-8039}} a character suggest to be plesiomorphic (observable in the putative stem-chelicerate taxon Habeliida{{Cite journal|last1=Aria|first1=Cédric|last2=Caron|first2=Jean-Bernard|date=2017-12-21|title=Mandibulate convergence in an armoured Cambrian stem chelicerate|url= |journal=BMC Evolutionary Biology|volume=17|issue=1|pages=261|doi=10.1186/s12862-017-1088-7|issn=1471-2148|pmc=5738823|pmid=29262772 |doi-access=free |bibcode=2017BMCEE..17..261A }}) and lost within the prosomapod clade.

Classification

Offacolus was originally described as an arthropod with chelicerate affinities, with detailed redescription done by Sutton et al. 2002 further suggested it to be unambiguously a chelicerate arthropod.

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. Offacolus was recovered as the sister taxon (closest relative) of Prosomapoda.{{Cite journal|title=Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura|last=Lamsdell|first=James C.|year=2012|journal=Zoological Journal of the Linnean Society|volume=167|issue=1|pages=1–27|doi=10.1111/j.1096-3642.2012.00874.x|issn=0024-4082|doi-access=free}}

O. kingi was once considered the only mamber of the monotypic family Offacolidae. This family currently includes along with O. kingi, Dibasterium and Setapedites and represent the sister group of Prosomapoda.{{cite journal |last1=Lustri |first1=Lorenzo |last2=Gueriau |first2=Pierre |last3=Daley |first3=Allison C. |title=Lower Ordovician synziphosurine reveals early euchelicerate diversity and evolution |journal=Nature Communications |date=7 May 2024 |volume=15 |issue=1 |pages=3808 |doi=10.1038/s41467-024-48013-w |pmid=38714651 |pmc=11076625 |bibcode=2024NatCo..15.3808L }}

{{clade|{{clade

|1=†Fuxianhuia

|2={{clade

|label1=†Antennulata

| 1={{clade

| 1=†Emeraldella

| 2=†Trilobitomorpha

| 3=†Sidneyia

}}

| 2={{clade

|label1=†Megacheira

| 1={{clade