Crinoid

cf. Tholocrinus

• Mississippian
• Girkin Formation
• Russellvillle, Logan County, Kentucky, USA

Size: 2.5 cm crown

This crinoid is consistent with the genus Tholocrinus in having endotomous arm branching, axillary first primibrachials except for the anterior (A) ray with several primibrachials, an anal sac platform above the arms with multiple central plates surrounded by short spines, and a pentagonal stem. It is rather unusual for Tholocrinus in having a relatively tall and wide bowl-shaped cup, as well as uniserial arms that completely lack spines. Tholocrinus is described as having a low, "somewhat saucer-shaped" cup, and arms that are at least partially biserial typically with spinose axillaries (Chesnut & Ettensohn 1988). This specimen is in the size range of typical mature Tholocrinus, so these peculiarities are likely not attributable solely to immaturity.