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ICHTHYOLITES OF THE OLD RED SANDSTONE

HETEROSTRACI

Early Heterostracan fossils, known only from isolated plates of head armor, were thought to be from crustaceans until Huxley and later Lankester suggested they originated from fish. In 1868 Lankester was also the first to note the pair of common external branchial openings which are today considered to be the unique characteristic of this group. There are two major Heterostracan groups, the Pteraspidiformes (which include Rhinopteraspis and Drepanaspis) and the Cyathaspidiformes (which include Poraspis and Anglaspis). The first Cyathaspids were described in 1856 and included in the Pteraspidiformes but it was Jaekel in 1911 that subdivided Heterostraci into Palaeaspidi and Pteraspidi, the former corresponding to the Cyathaspididae. This group is first seen in the late Silurian (although some fossils with heterostracan features date from the early Silurian) and the fossils are not observed beyond the Frasnian of the late Devonian. The distribution of heterostracans is limited to the Northern Hemisphere, with some excellent sites in the UK .

Pteraspis sp.

Although some of the early scientific literature considered Heterostracans to have been limited to fresh water, it is now thought that they lived in shallow marine systems as well as river deltas.

The Heterostracan head is covered with several armoured plates, the body with large scales (see image above) and the tail is most often diphycercal. This group does not have paired fins but large median dorsal and ventral ridge scales are always present.

Heterostracans in my collection (click on thumbnails to see larger images)

Pteraspidiformes

Protaspis Cottonwood Canyon, Wyoming, USA