This tail structure clearly differs from that of heterostracans, which are currently grouped with arandaspids and astraspids in the clade Pteraspidomorphi (Gagnier 1993, 1995; Donoghue & Smith 2001; Sansom et al. 2005), in which the caudal fin looks diphycercal (i.e. symmetrical) and is strengthened by a few large radials (Janvier 1996).

[5] Sacabambaspis is named after the village of Sacabamba, Cochabamba Department, Bolivia, where the first fossils of the genus were found.

[6] There are 30 known specimens of this Bolivian species, all crammed into a very confined area, believed to be the result of a fish kill, probably due a sudden inflow of freshwater from a large storm.

The Oman discoveries showed that the fish were present all around the periphery of the ancient continent of Gondwana and not just in the southern regions as had previously been shown by the findings from South America and Australia.

This is a line of pores within each of which are open nerve endings that can detect slight movements in the water, produced for example by predators.

The arrangement of these organs in regular lines allows the fish to detect the direction and distance from which a disturbance in the water is coming.