A MIDDLE EOCENE OPHIUROID FAUNA FROM THE WECHES FORMATION, TEXAS

Ophiuroid biodiversity over time is poorly understood due to the rarity of complete fossilized specimens. New methods have been developed that use disarticulated skeletal elements to identify different taxa based on morphology. These methods were used to document a Middle Eocene fauna from the coastal plain of Texas. Microfossil samples were collected from the “green sand” member of the Weches Formation, Nacogdoches Co., in Texas from a road cut north of TX Hwy 21, north of Chireno, Texas, and just west of East Loop 34. The poorly indurated green sand facies consisted of small bioclasts including brittle star plates, foraminifera, ostracods, gastropods, and glauconite peloids in clay matrix. To access the fossils, samples were soaked in a bath of hydrogen peroxide and water then washed through sieves to remove the fine material and to classify the remaining grains by size. Ophiuroid plates were picked from the sample under a microscope and imaged using an SEM. The stereom microstructure of the plates was well preserved and not infilled by syntaxial overgrowths; however, the surface of most specimens was dotted with scattered, small, 10-50 µm, iron oxide spheres that were difficult to completely remove without damaging the specimens. Three taxa were identified from lateral arm plates in the sample: Ophiura sp., Ophiodoris sp., and an amphiurid, all likely new species. The most abundant taxon by far is Ophiura sp. characterized by distally positioned tentacle notch, vertical striae that are prevalent in the genus, and a generally rectangular plate outline. The Ophiodoris species has up to four distally elongate, double-lobe spine bases, medium in size with a tooth between the lobes and strong ventral-distal projection to the outline. The amphiurid has three double-lobe spine bases that are large and lack a tooth between the lobes, and a triangular to distally bowed plate outline. The use of microfossils paints a more accurate picture of the prevalence and diversity of brittle stars through time. If it continues to be applied to more formations and time periods, ophiuroid biodiversity and environments can be better understood.