Crinoid

Exochocrinus sp.

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

Size: 3.5 cm crown

Exochocrinus is a highly unusual crinoid which lacked a stem and presumably lived a benthic lifestyle on the seafloor, similar to the likewise stemless Agassizocrinus and Staphylocrinus. These crinoids fused their infrabasal plates into singular solid infrabasal disks/cones, and probably detached from their stems during development like modern free-swimming comatulid crinoids (Burdick & Strimple 1969). Notably, Exochocrinus has incredibly bulbous and tumid (i.e. swollen), almost spherical calyx plates giving it a "bubbly" appearance. The main trait that distinguishes this crinoid from the closely-related Staphylocrinus is its very small, flat, and nearly pentagonal infrabasal disk that lies within a deep basal concavity, as well as significantly larger and more bulbous basal plates on which it probably sat in life. Exochocrinus tumulosus is the only species of Exochocrinus described from North America. However, the genus is a relatively poorly-understood taxon and I lack the expertise to identify this specimen with certainty.

This crinoid is also exceptionally rare to the point of obscurity, especially in terms of articulated specimens. Two partial calyxes were known for the original description in 1969, and in 1981 a calyx with the axillary first primibrachials attached was described from Oklahoma (Strimple 1981). In that same 1981 paper, Strimple mentioned the existence of a more complete Exochocrinus crown with similar arms as the genus Staphylocrinus, i.e. "20 uniserial arms that branch on primibrachs 1 and commonly on secundibrachs 6", but this specimen has not been officially figured or described. From what I can see, there seems to have been no further description of more complete crowns since. With that context, the specimen shown here is quite special in that it preserves a significant length of arms, especially on the A and B rays which preserve up to secundibrachs 7 and 6 respectively. There is no indication of a second branching, which would otherwise occur at secundibrachs 8 and 7 at the earliest.

Addendum: To my surprise, upon closer examination there appears to be a significant length of extraordinarily thin, delicate stem still attached to the infrabasal disk of this specimen. All characterizations of E. tumulosus in the literature describe a complete lack of any stem, along with a columnar cicatrix (i.e. scar) on the infrabasal disk. I don't believe this peculiarity can simply be attributed to immaturity, because this specimen is well-within the size range of presumably mature Exochocrinus specimens in the literature. In any case, this stem was almost certainly too weak to elevate the crinoid up in the water, and probably worked more as a tether akin to a "ball and chain", if it was functional at all. In addition, in my opinion the infrabasal disk on this specimen is closer to a proper pentagon with rounded vertices, as opposed to typical E. tumulosus whose infrabasal disk appears nearly star-shaped due to its sharp, "pinched out" vertices. This specimen may be an atypical example of E. tumulosus or perhaps a new species of Exochocrinus altogether.