NEW INFORMATION ON PORASPIDS (AGNATHA, HETEROSTRACI) FROM THE EARLY DEVONIAN OF THE WESTERN UNITED STATES

Heterostracans are a widespread group of armored jawless vertebrates that are particularly associated with Late Silurian and Early Devonian non-marine and proximal marine environments of the Old Red Sandstone continent. Although stratigraphically useful in Western Europe the value of heterostracan faunas has been restricted in the western United States by endemism and a lack of independent biostratigraphic indicators. It is therefore important that for the first time a genus well known from regions outside the western United States has been recognized there. A new species of the cyathaspid Poraspis is reported from the Emsian (Early Devonian) Lippincott member of the Lost Burro Formation in Death Valley, California. This species extends the geographic range of the genus, which had previously been reported from Western Europe, Spitsbergen, the Canadian Arctic Archipelago, and District of Mackenzie. In addition the temporal range is extended as the previously described species range from the Late Silurian into the Pridolian.

The unusually complete preservation of this material provides new insights into the morphology and growth of cyathaspids. Articulated specimens show that the rarely preserved oral area consisted of a single rectangular oral plate, in contrast to the accepted view that it was formed by an arrangement of numerous, finger-like plates. The external armor was previously shown to have been initiated at a juvenile stage at growth centers on the plates and scales and to have proceeded anteriorly in the anterior portion of the animal but posteriorly on the trunk and tail. One ventral shield of the new species shows growth lines indicating that growth occurred in a posterior direction, opposite to that previously described. An additional poraspid is reported from a single specimen from the Grassy Flat member of the Water Canyon Formation, northern Utah, which is also Emsian in age. This specimen represents a new genus and also shows a rare example of predation on an agnathan by a gnathostome. In this case the bite marks suggest attack by an acanthodian. These new heterostracans represent the first true cyathaspids to be reported from the Devonian of the western United States.