Offacolus

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. It is the only member of the monotypic family Offacolidae, and classified as a basal ("primitive") genus in the clade Euchelicerata, along with Dibasterium and Prosomapoda. 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.[1]

Ventral appendages of Offacolus kingi, showing limb-like exopods (Ex2-5)

Offacolus Temporal range: Homerian, ~425 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Reconstruction of Offacolus kingi
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Chelicerata
Clade:	Euchelicerata
Family:	†Offacolidae Sutton et al., 2002
Genus:	†Offacolus Orr et al., 2000
Type species
†Offacolus kingi Orr et al., 2000

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.[5]

Classification

Offacolus was originally described as an arthropod with chelicerate affinities,[1] with detailed redescription done by Sutton et al. 2002 further suggested it to be unambiguously a chelicerate arthropod.[2] Offacolus is the only genus in the monotypic family Offacolidae,[2] classified in the clade Euchelicerata together with the genus Dibasterium and the clade Prosomapoda.[5]

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.[5]

Arachnomorpha

†Fuxianhuia     †Antennulata   †Emeraldella     †Trilobitomorpha     †Sidneyia       †Megacheira   †Yohoia       †Alalcomenaeus     †Leanchoilia       Chelicerata Pycnogonida   †Palaeoisopus       Pycnogonum     †Haliestes       Euchelicerata   †Offacolus   Prosomapoda   †Weinbergina         †Venustulus     †Camanchia         †Legrandella     Xiphosura   †Kasibelinurus       †Willwerathia   Xiphosurida   †Lunataspis       †Bellinurina     Limulina           Planaterga     †Pseudoniscus     †Cyamocephalus       †Pasternakevia       †Bunodes     †Limuloides       †Bembicosoma   Dekatriata   †Chasmataspidida   Sclerophorata   Arachnida     †Eurypterida

References

1. Orr, Patrick J.; Siveter, Derek J.; Briggs, Derek E. G.; Siveter, David J.; Sutton, Mark D. (2000). "A new arthropod from the Silurian Konservat–Lagerstätte of Herefordshire, UK". Proceedings of the Royal Society of London. Series B: Biological Sciences. 267 (1452): 1497–1504. doi:10.1098/rspb.2000.1170. ISSN 1471-2954. PMC 1690702. PMID 11007324.
2. Sutton, Mark D.; Briggs, Derek E. G.; Siveter, David J.; Siveter, Derek J.; Orr, Patrick J. (2002-06-22). "The arthropod Offacolus kingi (Chelicerata) from the Silurian of Herefordshire, England: computer based morphological reconstructions and phylogenetic affinities". Proceedings of the Royal Society of London. Series B: Biological Sciences. 269 (1497): 1195–1203. doi:10.1098/rspb.2002.1986. PMC 1691018. PMID 12065034.
3. Dunlop, Jason A.; Lamsdell, James C. "Segmentation and tagmosis in Chelicerata". Arthropod Structure & Development. 46 (3): 395. ISSN 1467-8039.
4. Aria, Cédric; Caron, Jean-Bernard (2017-12-21). "Mandibulate convergence in an armoured Cambrian stem chelicerate". BMC Evolutionary Biology. 17 (1): 261. doi:10.1186/s12862-017-1088-7. ISSN 1471-2148. PMC 5738823. PMID 29262772.
5. Lamsdell, James C. (2012). "Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura". Zoological Journal of the Linnean Society. 167 (1): 1–27. doi:10.1111/j.1096-3642.2012.00874.x. ISSN 0024-4082.