SYSTEMATIC ANALYSIS OF PHYLLOCARID MANDIBLES FROM THE MISSISSIPPIAN MARSHALL FORMATION IN SOUTHERN MICHIGAN AND THE LATE DEVONIAN SILICA FORMATION IN NORTHWESTERN OHIO

Specimens currently identified as phyllocarid mandibles from the Late Devonian Silica Formation of Ohio have been subject to different interpretations since their original identification as the holocephalan fish, Pseudodontichthys whitei. Phyllocarid mandible specimens from the Silica Formation and previously undescribed specimens from the Mississippian Marshall Formation in southern Michigan were compared. Specimens display denticle morphology and the overall structure of the mandible, for which we provide a comprehensive breakdown of the structure and terminology. Comparisons between specimens are based on the number of cusps present and their morphology. Further morphological work utilizing SEM on architecture of preserved cuticle on specimens from the Silica Formation, along with morphological comparison with SEM processed fish teeth, indicates that these specimens are not from a fish. Chemical analysis of the cuticle and the interior of the denticles using EDS strengthens this hypothesis, showing that specimens from the Silica Shale share a similar chemical composition with other examples of preserved crustacean cuticle. The phyllocarid mandibles are differentiated into two distinct groups, right and left, based upon convexity differences, crown morphology, and number of cusps. This indicates that the structural differences are side-based and are not specimens from different taxa. Lithologic comparison of the Silica Shale and Marshall Sandstone confirms that taphonomic processes resulted in preservational differences between the two formations.