Mackenziurus emielityi new species: A new encrinurine (Trilobita) from the Silurian (Wenlock-Ludlow) of Wisconsin and Illinois

Extract

Tripp et al. (1977, p. 862) briefly described a small encrinurine from the Silurian of Wisconsin and Illinois. They refrained from naming this taxon, due to its being “as yet imperfectly known,” referring to the lack of testiferous material, the distortion and abrasion of most of the specimens, and the paucity of known cephalic sclerites. Lateral compression and abrasion of the only known cranidium (Tripp et al., 1997, pl. 114, figs. 8 and 9, from Brookfield, Wisconsin) were severe, but large, sparse tubercles were discernible. Gass et al. (1992) established Mackenziurus lauriae to include this and other material, all from various inter-reef Wenlock-Ludlow Racine Dolomite and Sugar Run Dolomite localities in Wisconsin and Illinois, respectively. All of this material is in the form of internal and external molds in dolomitized calcilutite, and most of the Wisconsin specimens are distorted to varying degrees. Among the material which Gass et al. (1992) introduced was a nonabraded, slightly-distorted cranidium with articulated librigenae (Gass et al., 1992, fig. 5.1-5.3) from the same locality in Wauwatosa, Wisconsin, which produced the majority of the other known material, and a complete, enrolled, nondistorted but weathered exoskeleton (Gass et al., 1992, fig. 5.4-5.7) from an unrecorded locality and formation in Grafton, Illinois. Those are only the second and third specimens known to preserve the cranidium. Differences in the rostral plate [inferred to be narrower (tr.) on the Wauwatosa specimen] and glabellar tuberculation (larger and fewer in number on the Grafton and Brookfield specimens) were attributed to intra-specific variation. Due to its superior preservation in contrast to the other cranidiumbearing specimens, and the more complete occurrence data, the Wauwatosa specimen was designated as holotype of Mackenziurus lauriae.