AN UNUSUAL ASSEMBLAGE OF OPHIUROIDS, CRINOIDS AND HOLOTHUROIDS FROM THE LOWER CRETACEOUS (ALBIAN) FORT WORTH MB. OF THE CADDO FM., TEXAS

Echinoderm microfossils can often change our perception of past diversity because the remains of many taxa are taphonomically fragile and rarely preserved as articulated skeletons. A sample of marl from the Lower Cretaceous (Albian) Fort Worth Member of the Caddo Fm. contains abundant crushed remains of a spatangoid echinoid, but no other obvious echinoderm fossils. This sample from 2 km NW of Krum, Denton Co., TX was processed for microfossils by soaking and screen washing through a 125 µm sieve. In addition to a wealth of ostracods, foraminifera, and fragmented remains of mollusks, washed residues contain a diverse echinoderm assemblage of ophiuroids, crinoids and holothuroids, and fragmented echinoids. Crinoids are represented by two genera of roveacrinids, Poecilocrinus and Roveacrinus including numerous cups and brachials, and holothuroids are represented by synaptid anchor ossicles. Echinoid material is represented by small test fragments and spines of a spatangoid, likely Macraster sp. Of particular interest are abundant lateral arm plates (LAPs) of four ophiuroid species. LAPs of an unidentified ophiodermatid genus are characterized by a trapezoidal outline, much higher proximally than distally, a well-defined tentacle notch, up to four double lobed spine articulations set into a scalloped distal margin and two weak spurs. LAPs of Ophiozonella have an elongate rectangular shape, a poorly defined tentacle notch, up to three double lobe spine articulations set into the distal plate edge and two strongly defined spurs. LAPs of Ophioplax are characterized by a hatchet-shaped plate with rounded and strongly concave distal and proximal margins respectively and an extended proximal ventral plate corner, weakly developed overlap scar lacking obvious spurs and four well-defined, double lobed spine articulations with a tooth along the distal plate margin. LAPs of Ophiactis have an arcuate shape with strongly concave and convex margins with proximally projecting extensions strongly developed along the dorsal and ventral margins, a well-developed overlap scar lacking obvious spurs and up to five double lobed spine articulations along the distal margin lacking a tooth. This fauna shows that the description of echinoderm faunas in the absence of microfossils is incomplete and omits the bulk of the diversity.