Dzik,
J. 2008. Gill structure and relationships of the Triassic cycloid crustaceans.
Journal of Morphology (in press)
Unusually
well-preserved fossils of a Halicyne-like cycloid crustacean frequently occur in the early Late
Triassic lacustrine clay bed at Krasiejów in Opole Silesia, southern Poland.
Its gill-like structures form a horseshoe-shaped pair of units composed of
numerous calcified blades with reverse U-shaped cross-section. Originally,
these were parallel slits opening on the ventral surface of the carapace.
Lobation of the posterior margin of the carapace, of unusually large mature
size for the group, make the animal different from other members of
Halicynidae, and the new name Opolanka
decorosa gen. et sp. nov. is proposed for it. More completely preserved
specimens of cycloids from Vosges, France, and Madagascar show that the slit
openings were located above radially arranged coxae of the walking appendages
and a reduced abdomen. The disposition and arrangement of the cycloid gills
suggest at least close analogy, and possibly homology, with the “respiratory
areas” of the Branchiura, serving mostly as ion-exchange organs. It is
proposed that they originated, in connection with the body size increase and
adaptation to fresh-water environment, as radially arranged infoldings of the
respiratory areas cuticle, with strongly calcified rigid dorsal parts
suspended from the carapace. At least three ecologically and anatomically
distinct lineages were represented in the order Cyclida, which was probably
initially confined to marine environments and gradually adapted to life in
continental waters. New taxa Schraminidae fam. nov. (with Schramine
gen. nov.) and Americlidae fam. nov. (with Americlus gen. nov.) are
proposed.
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