THE HABITAT OF EARLY PTERASPIDOMORPH AGNATHANS: UPPER ORDOVICIAN HARDING SANDSTONE, COLORADO

Since initial reports of pteraspidomorphs from the Harding Sandstone by Charles Walcott in 1892, the habitat of these primitive vertebrates has been disputed. Even among recent studies, interpretations range from fluvial to estuarine to marine. We reevaluated the depositional environments of the Harding Sandstone, based on a sequence stratigraphic analysis of sixteen outcrops in central Colorado, and have documented occurrences of pteraspidomorph remains to establish the habitat of these primitive vertebrates.

The Harding Sandstone contains open-marine facies deposited in transition zone (between fair-weather and average storm wave base) and shoreface environments throughout the study area. These open marine sediments are highly bioturbated, but include uncommon wave-ripple lamination and hummocky cross-stratification, as typical of modern low wave-energy coasts. In the Cañon City area, the middle, shale-rich portion of the Harding consists of bay-lagoon, bayhead delta, and probable washover fan facies. Waltherian and non-Waltherian (flooding surfaces and sequence boundaries) facies contacts abound and explain difficulties in previous attempts to interpret the Harding as either a single depositional environment or a simple Waltherian succession. Harding flooding surfaces display abrupt shifts from proximal to distal facies, and evidence of stratigraphic condensation, such as burrowed horizons, cemented horizons, and fossil accumulations. Sequence boundaries exhibit abrupt basinward shifts in facies, evidence of subaerial exposure, and localized gravel lags. The Harding contains two depositional sequences, each lacking a lowstand systems tract.

Disarticulated pteraspidomorph remains are most strongly associated with shoreface facies (55 of 62 occurrences), are less common in bay-lagoon and washover fan facies, and are absent from bayhead delta facies. Two of the four known articulated pteraspidomorphs have good locality data and both were found in shoreface facies. Collectively, these observations suggest that pteraspidomorphs lived in a shoreface environment. Other primitive vertebrates found in the Harding may have had different habitat preferences.