Offacolidae
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The overall appearance is similar to other [[Synziphosurina|synziphosurines]] and [[Xiphosura|horseshoe crabs]], featuring a dome-shaped prosomal (head) shield and an opisthosoma (trunk) divided by [[Tergum|tergites]] (dorsal exoskeleton). The prosoma bearing a pair of [[chelicerae]] ([[Chelae|pincer-like]] first appendage) and 5 pairs of [[endopod]]s (leg-like lower limb branches). [[Arthropod eye|Eyes]] are generally absent, except ''[[Bunaia]]'' which might have ventral ocelli. The opisthosoma has around 9 to 11 segments in total, with 6 pairs of [[Book gill|gill]] bearing flap-like appendage (opercula) located underneath its second to seventh segments. The last 3 segments may of may not forming a narrow postabdomen. Similar to many euchelicerates, the first opisthosomal tergite is variably reduced. |
The overall appearance is similar to other [[Synziphosurina|synziphosurines]] and [[Xiphosura|horseshoe crabs]], featuring a dome-shaped prosomal (head) shield and an opisthosoma (trunk) divided by [[Tergum|tergites]] (dorsal exoskeleton). The prosoma bearing a pair of [[chelicerae]] ([[Chelae|pincer-like]] first appendage) and 5 pairs of [[endopod]]s (leg-like lower limb branches). [[Arthropod eye|Eyes]] are generally absent, except ''[[Bunaia]]'' which might have ventral ocelli. The opisthosoma has around 9 to 11 segments in total, with 6 pairs of [[Book gill|gill]] bearing flap-like appendage (opercula) located underneath its second to seventh segments. The last 3 segments may of may not forming a narrow postabdomen. Similar to many euchelicerates, the first opisthosomal tergite is variably reduced. |
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Nonetheless, the family is united by several features, notably the |
Nonetheless, the family is united by several features, notably the 6-segmented prosomal [[exopod]]s (upper limb branches) originated from the second to (at least) fifth prosomal appendages, fringed by radially arranged terminal setae. The base of each exopod appears to be separated from their corresponding endopods,{{Cite journal |last=Briggs |first=Derek E. G. |last2=Siveter |first2=Derek J. |last3=Siveter |first3=David J. |last4=Sutton |first4=Mark D. |last5=Garwood |first5=Russell J. |last6=Legg |first6=David |date=2012-09-25 |title=Silurian horseshoe crab illuminates the evolution of arthropod limbs |url=https://www.pnas.org/doi/full/10.1073/pnas.1205875109 |journal=Proceedings of the National Academy of Sciences |volume=109 |issue=39 |pages=15702–15705 |doi=10.1073/pnas.1205875109 |pmc=3465403 |pmid=22967511}} but some are suggest to be united by a basal segment (basipodite). Other diagnostic features including elongated chelicerae, reduced prosomal [[gnathobase]]s , paddle-like seventh (first opisthosomal) appendages fringed by marginal setae, as well as a terminally bifurcated [[telson]]. Some also bearing a plate-like structures below their posterior body segment, a feature which rather resembling ''[[Habelia]]'' and [[Vicissicaudata]]n [[artiopod]]s. |
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== Paleoecology == |
== Paleoecology == |
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[[File:Lustri et al. 2025 f02.png|thumb|Comparison between offacolid prosomal appendages (C, D, E, F) and suspension feeding crustaceans (A, B, G, H).]] |
[[File:Lustri et al. 2025 f02.png|thumb|Comparison between offacolid prosomal appendages (C, D, E, F) and suspension feeding crustaceans (A, B, G, H).]] |
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Offacolids are [[Marine life|marine]] animals, possibly using their endopods for walking while using their gill opercula for both breathing and swimming.{{Cite journal |last=Orr |first=Patrick J. |last2=Siveter |first2=Derek J. |last3=Briggs |first3=Derek E. G. |last4=Siveter |first4=David J. |last5=Sutton |first5=Mark D. |date=2000-08-07 |title=A new arthropod from the Silurian Konservat–Lagerstätte of Herefordshire, UK |url=https://royalsocietypublishing.org/doi/10.1098/rspb.2000.1170 |journal=Proceedings of the Royal Society of London. Series B: Biological Sciences |language=en |volume=267 |issue=1452 |pages=1497–1504 |doi=10.1098/rspb.2000.1170 |issn=0962-8452 |pmc=1690702 |pmid=11007324}} The feeding ecology of offacolids had been largely unexplored, with only ''[[Offacolus]]'' initially though to be a bottom-dwelling [[Predation|predator]] or [[scavenger]]. However, a restudy in 2025 suggest offacolids might be a group of [[suspension feeder]], an ecological niche that was explored by [[crustacean]]s but unique within the dominantly [[carnivore]] chelicerates. This hypothesis is supported by the morphology of their prosomal exopods (shape and orientation unsuitable for locomotion, setae density suitable for collecting [[mesoplankton]]), elongated chelicerae (long and slender shape ineffective for immobilizing prey, but have wider range of movement for manipulating tiny food particles) and reduced gnathobases (unsuitable to crush and manipulate hardened food item on the ventral side).{{Cite journal |last=Lustri |first=Lorenzo |last2=Collantes |first2=Luis |last3=Esteves |first3=Cristiana J. P. |last4=O’Flynn |first4=Robert J. |last5=Saleh |first5=Farid |last6=Liu |first6=Yu |date=2025-06-11 |title=A Hypothesis on Suspension Feeding in Early Chelicerates (Offacolidae) |url=https://www.mdpi.com/1424-2818/17/6/412 |journal=Diversity |language=en |volume=17 |issue=6 |pages=412 |doi=10.3390/d17060412 |issn=1424-2818}} |
Offacolids are [[Marine life|marine]] animals, possibly using their prosomal endopods for walking while using their gill opercula for both breathing and swimming.{{Cite journal |last=Orr |first=Patrick J. |last2=Siveter |first2=Derek J. |last3=Briggs |first3=Derek E. G. |last4=Siveter |first4=David J. |last5=Sutton |first5=Mark D. |date=2000-08-07 |title=A new arthropod from the Silurian Konservat–Lagerstätte of Herefordshire, UK |url=https://royalsocietypublishing.org/doi/10.1098/rspb.2000.1170 |journal=Proceedings of the Royal Society of London. Series B: Biological Sciences |language=en |volume=267 |issue=1452 |pages=1497–1504 |doi=10.1098/rspb.2000.1170 |issn=0962-8452 |pmc=1690702 |pmid=11007324}} The feeding ecology of offacolids had been largely unexplored, with only ''[[Offacolus]]'' initially though to be a bottom-dwelling [[Predation|predator]] or [[scavenger]]. Under this scenario, the prosomal exopods are purported to be sensory organs. However, a restudy in 2025 suggest offacolids might be a group of [[suspension feeder]], an ecological niche that was explored by [[crustacean]]s but unique within the dominantly [[carnivore]] chelicerates. This hypothesis is supported by the morphology of their prosomal exopods (shape and orientation unsuitable for locomotion, setae density suitable for collecting [[mesoplankton]]), elongated chelicerae (long and slender shape ineffective for immobilizing prey, but have wider range of movement for manipulating tiny food particles) and reduced gnathobases (unsuitable to crush and manipulate hardened food item on the ventral side).{{Cite journal |last=Lustri |first=Lorenzo |last2=Collantes |first2=Luis |last3=Esteves |first3=Cristiana J. P. |last4=O’Flynn |first4=Robert J. |last5=Saleh |first5=Farid |last6=Liu |first6=Yu |date=2025-06-11 |title=A Hypothesis on Suspension Feeding in Early Chelicerates (Offacolidae) |url=https://www.mdpi.com/1424-2818/17/6/412 |journal=Diversity |language=en |volume=17 |issue=6 |pages=412 |doi=10.3390/d17060412 |issn=1424-2818}} |
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== Taxonomy == |
== Taxonomy == |
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Since its original description,{{Cite journal |last=Sutton |first=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=https://royalsocietypublishing.org/doi/10.1098/rspb.2002.1986 |journal=Proceedings of the Royal Society of London. Series B: Biological Sciences |language=en |volume=269 |issue=1497 |pages=1195–1203 |doi=10.1098/rspb.2002.1986 |issn=0962-8452}} the family was included only the type genus ''Offacolus'' for decades. In 2024, ''[[Setapedites]]'' was described and the family was redefined, following by the re-classification of both ''[[Dibasterium]]'' and ''[[Bunaia]]'' as offacolids. They are part of the [[paraphyletic]] grade of basal euchelicerates known as [[Synziphosurina|synziphosurines]].{{cite journal |last1=Lamsdell |first1=James C. |title=Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura: Re-evaluating the Monophyly of Xiphosura |journal=Zoological Journal of the Linnean Society |date=January 2013 |volume=167 |issue=1 |pages=1–27 |doi=10.1111/j.1096-3642.2012.00874.x}} |
Since its original description,{{Cite journal |last=Sutton |first=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=https://royalsocietypublishing.org/doi/10.1098/rspb.2002.1986 |journal=Proceedings of the Royal Society of London. Series B: Biological Sciences |language=en |volume=269 |issue=1497 |pages=1195–1203 |doi=10.1098/rspb.2002.1986 |issn=0962-8452}} the family was included only the type genus ''Offacolus'' for decades. In 2024, ''[[Setapedites]]'' was described and the family was redefined, following by the re-classification of both ''[[Dibasterium]]'' and ''[[Bunaia]]'' as offacolids. They are part of the [[paraphyletic]] grade of basal euchelicerates known as [[Synziphosurina|synziphosurines]].{{cite journal |last1=Lamsdell |first1=James C. |title=Revised systematics of Palaeozoic 'horseshoe crabs' and the myth of monophyletic Xiphosura: Re-evaluating the Monophyly of Xiphosura |journal=Zoological Journal of the Linnean Society |date=January 2013 |volume=167 |issue=1 |pages=1–27 |doi=10.1111/j.1096-3642.2012.00874.x}} |
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It is an intermediate taxon between the [[Cambrian]] basal chelicerates (e.g. [[Habeliida]], ''[[Megachelicerax]]'') and other post-Cambrian euchelicerates ([[Prosomapoda]]), indicating by their well developed prosomal exopods which possibly retained from their Cambrian ancestors and lost in Prosomapoda.{{Cite journal |last=Lerosey-Aubril |first=Rudy |last2=Ortega-Hernández |first2=Javier |date=2026-04-01 |title=A chelicera-bearing arthropod reveals the Cambrian origin of chelicerates |url=https://www.nature.com/articles/s41586-026-10284-2 |journal=Nature |language=en |pages=1–7 |doi=10.1038/s41586-026-10284-2 |issn=1476-4687}} Phylogenetic results suggest it is either a [[monophyletic]] clade sister to Prosomapoda or a |
It is an intermediate taxon between the [[Cambrian]] basal chelicerates (e.g. [[Habeliida]], ''[[Megachelicerax]]'') and other post-Cambrian euchelicerates ([[Prosomapoda]]), indicating by their well developed prosomal exopods which possibly retained from their Cambrian ancestors and lost in Prosomapoda.{{Cite journal |last=Lerosey-Aubril |first=Rudy |last2=Ortega-Hernández |first2=Javier |date=2026-04-01 |title=A chelicera-bearing arthropod reveals the Cambrian origin of chelicerates |url=https://www.nature.com/articles/s41586-026-10284-2 |journal=Nature |language=en |pages=1–7 |doi=10.1038/s41586-026-10284-2 |issn=1476-4687}} Phylogenetic results suggest it is either a [[monophyletic]] clade sister to Prosomapoda or a paraphyletic grade where Prosomapoda arose. |
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{{clade |
{{clade |
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|1={{clade |
|1={{clade |
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|state1=double|1=[[Megacheira]] |
|state1=double|1=†[[Megacheira]] |
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|2={{clade |
|2={{clade |
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|1=[[Artiopoda]] |
|1=†[[Artiopoda]] |
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|label2=[[Chelicerata]]|2={{clade |
|label2=[[Chelicerata]]|2={{clade |
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|1=[[Pycnogonida]] (sea spiders) |
|1=[[Pycnogonida]] (sea spiders) |
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|2={{clade |
|2={{clade |
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|1={{clade |
|1={{clade |
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|1=''[[Habelia]]'' |
|1=†''[[Habelia]]'' |
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|2=''[[Mollisonia]]'' |
|2=†''[[Mollisonia]]'' |
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}} |
}} |
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|label2=[[Euchelicerata]]|2={{clade |
|label2=[[Euchelicerata]]|2={{clade |
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|label1='''Offacolidae'''|1={{clade |
|label1=†'''Offacolidae'''|1={{clade |
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|1=''[[Dibasterium]]'' |
|1=†''[[Dibasterium]]'' |
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|2={{clade |
|2={{clade |
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|1=''[[Offacolus]]'' |
|1=†''[[Offacolus]]'' |
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|2={{clade |
|2={{clade |
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|1=''[[Bunaia]]'' |
|1=†''[[Bunaia]]'' |
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|2=''[[Setapedites]]'' |
|2=†''[[Setapedites]]'' |
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}} }} }} |
}} }} }} |
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|label2=[[Prosomapoda]]|2={{clade |
|label2=[[Prosomapoda]]|2={{clade |
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|1=''[[Weinbergina]]'' |
|1=†''[[Weinbergina]]'' |
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|2={{clade |
|2={{clade |
||
|1={{clade |
|1={{clade |
||
|1=''[[Camanchia]]'' |
|1=†''[[Camanchia]]'' |
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|2=''[[Venustulus]]'' |
|2=†''[[Venustulus]]'' |
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}} |
}} |
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|2={{clade |
|2={{clade |
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|1=''[[Legrandella]]'' |
|1=†''[[Legrandella]]'' |
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|2={{clade |
|2={{clade |
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|1=[[Xiphosura]] (horseshoe crabs) |
|1=[[Xiphosura]] (horseshoe crabs) |
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|2={{clade |
|2={{clade |
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|1={{clade |
|1={{clade |
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|1=''[[Pasternakevia]]'' |
|1=†''[[Pasternakevia]]'' |
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|2={{clade |
|2={{clade |
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|1=''[[Cyamocephalus]]'' |
|1=†''[[Cyamocephalus]]'' |
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|2=''[[Pseudoniscus]]'' |
|2=†''[[Pseudoniscus]]'' |
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}} }} |
}} }} |
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|2={{clade |
|2={{clade |
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|1=[[Chasmataspidida]] |
|1=†[[Chasmataspidida]] |
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|2={{clade |
|2={{clade |
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|1=[[Eurypterida]] (sea scorpions) |
|1=†[[Eurypterida]] (sea scorpions) |
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|2=[[Arachnida]] |
|2=[[Arachnida]] |
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}} }} }} }} }} }} }} }} }} }} }} }} }} |
}} }} }} }} }} }} }} }} }} }} }} }} }} |
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