Hadrophycus immanus
Middle Cambrian
Marjum Formation, Utah

Coming from the Marjum Formation, Milliard County, Utah, these stromatolites date to the Middle Cambrian some 525 million years ago when stromatolite was no longer as prevalent and abundant as during the Proterozoic. In particular, by the Cambrian, photosynthetic bacteria responsible biogenic formation of stromatolite structures no longer had the earth to itself. The oxygenated atmosphere had become toxic to some bacteria, and they had to compete with other organisms ranging from eukaryotic microorganisms (bacterial, algae, plants and animals), some of which might have been predaceous heterotrophs feeding on Eubacteria.

This specimen is intriguing in several ways. First, the rough side shown has very small dome structures (domes prototypical of some stromatolite), but these are tiny compared to the large domes often found in the Proterozoic forms. Second, The specimen exhibits pronounced (eye visible) filament (rod like) structure that may be indicative of a specific organism. Third, there are several dark, circular bodies that may be part of the stromatolite, though they also have some resemblance to other organisms (sponges come immediately to mind). The later interpretation is plausible since trilobites and Cambrian, soft-bodied organisms are sometimes found in association with the microbial mats in the Marjum Formation.