Crinoid

1. Plaxocrinus aff. P. dornickensis
2. Parethelocrinus ellipticus

• Pennsylvanian
• Holdenville Formation
• Okmulgee County, Oklahoma, USA

Size: 4 cm crown for the Plaxocrinus

Here is an incredible crinoid double and arguably my best fossil representative of Pennsylvanian-aged crinoids and their late-Paleozoic adaptations. It excellently preserves two prevalent and iconic Pennsylvanian crinoid types: a Pirasocrinid (Plaxocrinus on the left) and a Cromyocrinid (Parethelocrinus on the right). The Pennsylvanian period saw a rise in the cladid crinoids, along with several adaptations presumably to defend against increasing predation from fishes newly-equipped with powerful "crushing" teeth. One strategy, exhibited by crinoids such as the Cromyocrinids, was to develop a large, globose calyx comprised of a small number of extremely thick plates. This would make it rigid and resistant to crushing forces. Another strategy, exhibited chiefly by the Pirasocrinids, was to threaten would-be predators with a formidable array of spines protruding from the arms and from a highly-specialized anal sac. Both were evidently quite successful given the prevalence of such forms throughout the Pennsylvanian; hence why this particular specimen is such a great representative of the period.

1. This crinoid is arguably the "flashier" of the two and a member of the family Pirasocrinidae. It boasts extraordinarily long primaxillary and anal sac spines, among the longest and most impressive I have seen among crinoids of the family. While fairly abundant throughout the Pennsylvanian, these Pirasocrinids are almost always found only as loose calyxes and brachial/anal spines. Especially noteworthy on this specimen is the magnificent umbrella-like anal sac: it is built from circlets of porous, hexagonal plates which transition to more stellate plates as they approach the summit. At the termination of the anal sac is a platform of several polygonal plates surrounded by a circlet of formidable defensive spines; a veritable "crown of thorns". It is postulated that the Pirasocrinid anal sac contained nutritious, high-value targets to predators such as the crinoid's viscera or gonads.

Strimple's original 1961 paper on the Holdenville crinoid fauna described about a dozen different Pirasocrinids, and in my opinion there is only one among them that resembles my specimen: Plaxocrinus aff. P. dornickensis which was represented by just a single loose calyx. Atypically for the genus, this form has a calyx with a very smooth outline owing to flat, non-bulbous basals and nonimpressed, almost imperceptible sutures. If this is what my specimen is, it may be one of if not the only known articulated crowns of its kind from the Holdenville.

Addendum: Peter Holterhoff has confirmed this crinoid as indeed comparable to P. dornickensis, hence Plaxocrinus aff. P. dornickensis.

Addendum 2: Plaxocrinus dornickensis was reassigned to Metaffinocrinus dornickensis by Knapp 1969. Plaxocrinus aff. P. dornickensis from the Holdenville Formation remains without an official reassessment or redescription. From examining specimens of M. dornickensis from the Pumpkin Creek Limestone of Oklahoma (the type locality), I believe this Holdenville specimen should likewise be reassigned to the genus Metaffinocrinus on account of its aforementioned smooth-contoured calyx with nontumid plates and nonimpressed sutures, which are consistent with the genus definition.

2. Here is the other half of my beautiful Holdenville double: a robust Cromyocrinid crinoid known as Parethelocrinus ellipticus. Though not as exotic-looking as the Pirasocrinid, it is no less impressive of a specimen. P. ellipticus is one of the most common constituents of the Holdenville crinoid fauna but not often this complete and well-articulated. As explained in my previous posts, Cromyocrinids such as this one exemplify several general adaptive trends in the late Paleozoic cladid crinoids. For example, their calyx plates thickened and reduced in number to become more rigid and armored in response to predation pressure. One way this was achieved was via a reduction in the number of anal plates; P. ellipticus has two anal plates, down from the primitive three, though this is far from the most extreme case. It also exhibits the advanced cladid trait of robust biserial pinnulate arms, compared to the more primitive uniserial pinnulate arrangement. Though P. ellipticus generally has a smooth, unornamented calyx, certain specimens such as this one exhibit teeth-like edges to the calyx plates that give their sutures a subtle "stitched" appearance.