Phylogenetic patterns in the heterostracan families Cyathaspidae, Ariaspidae and Ctenaspidae

• The order Heterostraci includes early armoured vertebrates that are defined by a pair of common external branchial openings on either side of the head. The position of these openings and their arrangement in relation to the plates of the dermal armour has been shown to vary in different species of the Cyathaspidae, Ariaspidae and Ctenaspidae (previously grouped in the family Cyathaspididae). In order to explore evolutionary patterns in the branchial region of heterostracans, a phylogenetic analysis comprising 55 skeletal characters and 37 species-level taxa belonging to the Cyathaspidae, Ariaspidae and Ctenaspidae was carried out; this is the first cladistic appraisal of these heterostracan families. The resulting trees place Asketaspis interstincta in a basal position with all other taxa forming a discrete monophyletic clade. This infers aprogressional shift of the branchial opening and rearrangement of the adjacent dermal plates in Alainaspis, Allocryptaspis, and culminating in Ctenaspidae where the branchial opening is relocated to the posterolatertal corner of the dorsal shield and coincides with the loss of the superficial layer of the dermal armour.

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