Fossil specimens of Yohoia range in size from 7 to 30mm,[1] they have a head shield which is followed by 13 flexible trunk segments, each represented by an arch-like tergite (dorsal plate). On both sides, the bottom side of the first 10 of these ended in backward-pointing tergopleurae (lateral extensions), where the tips are pointed in Y. tenuis and blunt in Y. utahana.[1] The last three segments were complete tubes, circling the entire trunk. At the end of the trunk was a paddle-like telson (tail).[3]
The head possess a pair of large eyes and a pair of great appendages at the front. The great appendages had a pronounced "elbow" and 4 distal segments ended in four long spines, looking rather like fingers. In the case of Y. tenuis, the appendages showing a "slender" and a "stout" morphotypes. Serrated fingers similar to Parapeytoia and Fortiforceps are evident in some specimens as well.[4] There were another 13 pairs of subequal appendages on the bottom of the body, 3 pairs below the head shield and 10 pairs below all but the last 3 trunk segments. Each of these appendages compose of a leg-like endopod and a flap-like exopods fringed with setae.[5][3]
Ecology
The suggested movement of the great appendage of Y. tenuis
Yohoia is assumed to have been a mainly benthic (bottom-dwelling) creature that swam just above the muddy ocean floor, using its great appendages to scavenge or capture prey in a method similar to modern mantis shrimp.[3] The exopods are probably used for swimming and respiration.[6]
Classification
Fossils described by Walcott (1912)
The genus and type species was first described in 1912 by Walcott, who considered it an anostracancrustacean.[7] Upon late 1990s, Yohoia is re-classify under Megacheira (great appendage arthropods),[8] a class of extinct arthropod which has controversial phylogenetic position either as stem chelicerates or a distinct lineage basal than both chelicerates and mandibulates.[9] Within megachierans, Yohoia is generally considered to be part of a clade including Haikoucaris and leanchoiliids, collectively known as Cheiromorpha.[10][11][12][13][14] Some recent studies suggest alternative positions, for example closer to other megacheirans[4][15] or Yohoia itself represent a distinct lineage[16][17].
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