LATE ORDOVICIAN PHOSPHATIC MICROFOSSIL SPHERES FROM ROCHESTER, MINNESOTA

Middle to Late Ordovician carbonates across North America are marked by an increase in associated phosphate and chert. In the Upper Mississippi Valley, this increase occurs across a regionally recognized sequence stratigraphic boundary (the M4/M5) within the Decorah Formation and is manifest as an abundance of phosphatic microfossils (e.g. conodonts, gastropod steinkerns, small shelly fauna, and conulariid fragments). While many of these microfossils have known taxonomic affinities, others do not and some have simply been labeled as ‘weird things’. One such group of unidentified structures are small (100-200μm) phosphatic spheres. The purpose of this study is to document and describe a collection of these phosphatic spheres and the associated microfossil assemblage from Rochester, Minnesota.

Limestone samples were collected from exposures of the Platteville and Decorah formations in an abandoned quarry in Rochester, Minnesota. Samples were dissolved in a buffered acetic acid solution and phosphatic microfossils were concentrated using heavy liquid separation. The spherical fossils are extremely rare, averaging ~ 1 sphere per 1 kg of rock dissolved, and are found in association with abundant conodont elements and sponge spicules. Scanning electron microscope (SEM) images were used to examine sphere morphology and surface texture. Microfossils range from individual spheres to clusters of up to 5 spheres. The small size and inconsistency of size among connected spheres suggests that these microfossils are not metazoan embryos. Similarly, the absence of internal laminations indicates that they are not conodont pearls. While the taxonomic affinity is not known at present, this study represents the first attempt (to our knowledge) to document and describe these spherical structures and will aid with any future attempts at identification.