Biometric and taxonomic analysis of the genus Isotelus (Trilobita) from Cincinnatian (Upper Ordovician) rocks of Ohio, Kentucky, and Indiana
Creators:Hall, Cathleen Paige
Advisor:Babcock, Loren E.
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Publisher:The Ohio State University
Series/Report no.:The Ohio State University. Department of Geological Sciences Senior Theses; 1993
Three species of the trilobite genus Isotelus are commonly cited from Upper Ordovician (Cincinnatian) rocks of southwestern Ohio and adjacent areas of Kentucky and Indiana. They are: 1, I. gigas DeKay, 1824, which was first described from the Trenton Limestone of New York; 2, I. maximus Locke, 1838, which was first described from Cincinnatian rocks of Ohio; and 3, I. brachycephalus Foerste, 1910, which was first described from Cincinnatian rocks of Ohio. Holaspid specimens of I. gigas are easy to distinguish from the other two species by the lack of genal spines. However, I. maximus and I. brachycephalus are extremely similar morphologically and recently it has been hypothesized that they are conspecific. Foerste's (1910) criterion for distinguishing I. brachycephalus from I. maximus was that I. brachycephalus supposedly had a wider exoskeleton. Well-preserved specimens referable to I. maximus, I. brachycephalus, and I. gigas were statistically analyzed using regression analysis to determine the exact number of species in Ohio. Based on this quantitative analysis, I found there to be no consistent morphological differences between I. maximus and I. brachycephalus; I. brachycephalus is therefore considered to be a junior synonym of I. maximus. In light of this work, only two species of Isotelus, I. gigas and I. maximus, seem to be present in Cincinnatian rocks of Ohio, Kentucky, and Indiana.
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