SUBSTRATE REVOLUTION IN THE EARLY PALEOZOIC OF UTAH: THE OLDEST EVIDENCE OF MACROBORING ADAPTIVE STRATEGIES IN WESTERN NORTH AMERICA

The major intervals of biotic diversification in the Cambrian and Early Ordovician were times when new adaptive strategies for occupying sea floor habitats were adopted by benthic communities. While deep-burrowing lifestyles in unconsolidated sediment, as evidenced by Skolithos and other long vertical burrows, had been adopted by some marine animals as early as the beginning of the Cambrian, the innovation of a macroboring lifestyle in hard (rock or shell) substrates did not occur until considerably later. The ability to create a domicile by boring into a lithified or mineralized substrate was a revolutionary development that opened up new habitat space for benthic organisms and contributed to the development of new ecological niches. Such a capability required anatomical and/or physiological adaptations for penetrating the hard substrate by either mechanical drilling or chemical dissolution (or a combination of the two).

The oldest undoubted macroborings reported in western North America occur in the Lower Ordovician Fillmore Formation in Utah's House Range. These bioerosion trace fossils apparently were excavated as dwelling structures in limestone hardgrounds by an unknown organism. The macroborings are referred to Gastrochaenolites, and they closely resemble similar macroborings of approximately the same age in Sweden and Norway. Recently, some older trace fossils have been discovered in the Middle Cambrian Wheeler Formation in Utah's Drum Mountains. Because of the sharp boundaries of the trace fossils and their cross-cutting relationships with ooids and diagenetic calcite seams, it appears that these structures may have been excavated in a lithified limestone substrate rather than in unconsolidated lime mud. If this was the case, this discovery extends the earliest record of bioerosion trace fossils in western North America to the Middle Cambrian. But even if not, Utah still can boast that its Ordovician macroborings are the oldest bioerosion trace fossils on this side of the continent.