Crinoid

Exochocrinus sp.

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

Size: 3.5 cm crown

Exochocrinus is a highly unusual stemless crinoid similar to the likewise stemless Agassizocrinus and Staphylocrinus, though it appears to be more closely related to the latter. The main trait that distinguishes this crinoid is its small, flat, and nearly pentagonal infrabasal disk that lies within a basal concavity and is not visible from side view, as well as significantly larger and more tumid (i.e. swollen/bulbous) basal plates on which it probably sat in life. Its genus name derives from exochos (Greek) which roughly means "exquisite/extreme", and refers to the extremely tumid nature of the basal and radial plates (Burdick & Strimple 1969).

Exochocrinus is also exceptionally rare to the point of obscurity, especially in terms of articulated specimens, and like Staphylocrinus is limited to Gasperian Age strata. 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 arms similar to those of 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: Upon closer examination, there appears to be a significant length of extraordinarily thin, delicate stem still attached to the infrabasal disk of this specimen, with alternating narrow and wide columnals. Exochocrinus tumulosus is the only species of Exochocrinus described from North America, and all characterizations 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 and the infrabasals have fully fused into a singular disk. However, this taxon is poorly-represented and relatively poorly-understood. In the closely-related Staphylocrinus there does not seem to be a direct relationship between maturity and fusion of the detached stem into the infrabasals, as "infrabasal discs of various sizes are found in all states of fusion" and presumably "growth of the infrabasals over the stem depends on when the stem is broken off and how short it is broken" (Burdick & Strimple 1969). Even then, I can find no record of any specimens of either genus with such a definite stem still attached.

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.

Addendum 2: Though this specimen is somewhat crushed, I am not sure that its infrabasals would be completely concealed in a basal concavity as is typical for Exochocrinus, given the size of its infrabasals and the shapes and suture positions of its basals. I also notice this specimen bears some similarities to a crinoid that Strimple described as Staphylocrinus supratumulosus, which has strong calyx plate tumidity that starts not at the sutures but slightly away from them, as well as small nodes or granulations on the calyx plates and brachials (Burdick & Strimple 1969). That being said, this specimen's basals and especially anal plates are closer to typical Exochocrinus in their sheer tumidity, and its infrabasals are still notably smaller and flatter than its basals compared to the figured holotype of S. supratumulosus. Moreover, the single poorly-preserved holotype is the only known specimen of S. supratumulosus, and so I am not certain that it is even a valid taxon considering the relatively minor traits said to distinguish it from S. bulgeri.