Offacolidae
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| Offacolidae Temporal range: | |
|---|---|
 | |
| Specimen of Dibasterium (A), Setapedites (B), Offacolus (C) and Bunaia (D). | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Chelicerata |
| Clade: | Euchelicerata |
| Family: | †Offacolidae Sutton et al., 2002 |
| Type species | |
| †Offacolus kingi Orr et al., 2000[1] | |
| Genera | |
Offacolidae is a family of basal euchelicerate arthropods from the Ordovician and Silurian of Europe, Morocco and North America.[2][3]
The overall appearance is similar to other synziphosurines and horseshoe crabs, featuring a dome-shaped prosomal (head) shield and an opisthosoma (trunk) divided by tergites (dorsal exoskeleton). The prosoma bearing a pair of chelicerae (pincer-like first appendage) and 5 pairs of endopods (leg-like lower limb branches). Eyes are generally absent, except Bunaia which might have ventral ocelli.[2][4] The opisthosoma has around 9 to 11 segments in total, with 6 pairs of 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.[1][2][3]
Nonetheless, the family is united by several features, notably the 6-segmented prosomal exopods (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,[5] but some are suggest to be united by a basal segment (basipodite).[1][2] Other diagnostic features including elongated chelicerae, reduced prosomal gnathobases , 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 Vicissicaudatan artiopods.[2][3]
