COMPARISON OF BRACHIOPOD GENERA FROM THE MID-LATE PENNSYLVANIAN IN NORTH CENTRAL TEXAS.

Three Middle to Late Pennsylvanian brachiopod genera from the East Mountain Shale and the Salesville Shale in Palo Pinto County, Texas, were investigated to determine morphological relationships. The oldest species, Mesolobus (East Mountain Shale), goes extinct at the end of the Desmoinesian. Chonetina and Punctospirifer are found in the Missourian-aged Salesville Shale. Deposition in the field area was related to the prograding Perrin Delta. During high sea-level and in interlobe basins, marine shales with abundant fossil faunas were deposited. Both the East Mountain Shale and the Salesville Shale were formed in such a way. 363 specimens of Chonetina, 91 specimens of Mesolobus, and 59 specimens of Punctospirifer were photographed using a Plugable digital microscope. The scaled photos were used to take four measurements of valve structure (hinge length, valve width, and left and right lobe width), using ImageJ software. Valve characteristics and relationships were plotted in a series of scatter plots, histograms, and frequency curves to evaluate differences between species. Simple statistical analyses were used to evaluate relationships. The genera have overlapping form, though Punctospirifer demonstrated more variability. Punctospirifer has statistically longer hinge lengths and wider widths than its contemporaneous genus Chonetina. Both Chonetina and Punctospirifer exhibit similar length-to-width ratios. The earlier Mesolobus has a statistically smaller length-to-width ratio than the later Chonetina. Mesolobus and Chonetina likely occupied the same niche given their similar valve characteristics and their classification in the suborder Chonetidina, order Strophomenidina. Chonetina replaced Mesolobus and took over its niche following the extinction of Mesolobus. The unique valve characteristics of Punctospirifer indicate that while the genera have similar growth forms and lived in the same environment, they likely occupied different niches. Statistical differences in measurable valve characteristics are not adequate to aid in generic identification. The relationship of valve characteristics to these genera provides some insight into the relationship between the genera and their environment.