The Cambrian origin of the circulatory system of crustaceans

The carapace of Recent crustaceans such as myodocope ostracodes and phyllocarids is pervaded with well-developed anastomosing sinuses conveying hemolymph from the metabolizing organs to the dorsal heart. The inner lamella cuticle, which separates the sinuses from seawater, is thin enough to allow gaseous diffusion (e.g., O₂ uptake) over its surface. Comparable radiating and/or anastomosing features, of possible vascular origin, are herein recognized in several possible Crustacea from the Cambrian: cambriid, svealutiid, hipponicharionid and beyrichonid Bradoriida and in Carnarvonia from the middle Cambrian Burgess Shale. The vascular network is basically the same in these groups, consisting of sinuses radiating from supposed adductorial areas or from inferred areas of dorsal attachment of the body. The integumental (carapace sinuses) and branchial (gills) systems of respiration in crustaceans and crustacean-like animals were probably already differentiated by the middle Cambrian. The oldest record of probable integumental circulation is in the bradoriid Petrianna from the early Cambrian of Greenland. Similar circulatory systems may be represented by radiating ridges on the cephalon of other Cambrian arthropod groups such as the arachnomorphs ( Burgessia ) and trilobites ( Naraioa ) and may also be manifest in the carapaces of Ordovician-Devonian leperditicope ostracodes. Organs on the thoracopods of Cambrian supposed crustaceans, such as Canadaspis , resemble the foliaceous thoracic gills of Recent nebaliid phyllocarids and therefore may have served the same (respiratory) function.