A review of the lateral line sensory system in psammosteid heterostracans

The heterostracan lateral line system consists of a network of closed canals within the dermal plates that open to the surface by pores. This system is well known in some heterostracans, where it consists of a symmetrical arrangement of canals, but is known in only a few psammosteids, where the pattern is more irregular. A new obrucheviid from the Late Devonian of arctic Canada shows a typical heterostracan pattern, but the canals were held in open surface grooves, generally considered a derived feature in heterostracans. A second new arctic species similar to Psammosteus features a reticulate dorsal canal system with the canals delineating polygonal areas and the normal pattern of longitudinal and transverse canals lost. This pattern is unique within heterostracans and may be a derived feature related to a need for increased sensory function. The canals were situated below the surface tesserae and are thus only exposed by abrasion. Species with small tesserae have canals enclosed in tubes and their position may be indicated on the surface as ridges formed by the presence of small, elongated tesserae. Those with large tesserae have canals enclosed within the spongy layer of the dermal plate. Some species of Psammosteus may have had a double sensory canal system in the dorsal plate, with one system deeper in the spongy layer, the other close to the surface of the plate but still under the ornament. Behind the carapace the sensory canal system is situated directly below the scales and connects to the surface with short branches. These penetrate the scales, which are thin and have no basal layer.