Middle Pennsylvanian fossils from the Buckhorn Asphalt quarry of Southern Oklahoma are unusual in that early sealing by tar has preserved many of their original textural and mineralogic features and those of the earliest stages of diagenesis. Echinoderm fragments collected from this locality axe well preserved, with original stereom morphology clearly visible optically. One- to five-micrometer anhedral calcian dolomite grains occupy as much as 30 percent of the original stererom of the fragments, as displayed by differential etching and MgKalpha X-my maps. The composition of skeletal fragments is 75 percent Ca (sub .98) Mg (sub .02) Co 3 , 25 percent Ca (sub .54) Mg (sub .46) Co 3 . Single crystal X-ray diffraction, XRD, EMPA, TEM, and STEM microanalysis were used to characterize the calcian dolomite phase. TEM micrographs revealed an approximate 100 A, "tweed structure" identical to that reported by Reeder and Wenk (1979). Our data imply that magnesium in the calcian dolomites was derived largely from the original magnesian calcite of the echinoderm stereom. Original biogenic high-magnesium calcite is altered via local microdissolution--reprecipitation to anhedral disordered calcian dolomite grains within low-magnesium calcite. The product phases (low-magnesium calcite and calcian dolomite) retain the original crystallographic orientation of the stereom. These products of early diagenesis are apparent precursors to well-defined, ordered euhedral microdolomites. The relationships between the low-magnesium calcite and calcian dolomite, and the chemistry and complex ultrastructure of the calcian dolomites support the kind of low-temperature mechanism of dolomite formation postulated by Gains (1977), Goldsmith and Graf (1958), and others on the basis of hydrothermal experiments.