The youngest carpoid: occurrence, affinities, and life mode of a Pennsylvanian (Morrowan) mitrate from Oklahoma

Abstract

Abundant, well-preserved specimens of a new peltocystidan mitrate carpoid,Jaekelocarpus oklahomensisn. gen. and sp., have been discovered in the Pennsylvanian (Morrowan Series) Gene Autry Shale Member, Golf Course Formation, in southern Oklahoma. The new carpoid postdates the youngest previously known carpoids (Middle Devonian) by about 65 million years. It is characterized by a small (to 4 mm in length) globose theca composed of six plates: two large adaulacophorals, two large marginals, and two relatively small plates that frame the main thecal orifice. One of the plates at the orifice bears a single stout spine. The theca and plates that comprise the theca display a high degree of bilateral symmetry. In contrast, the styloid has an asymmetrical array of spines and blades.We hypothesize that most mitrates, including this one, were adapted to an infaunal life mode. The streamlined, generally symmetrical body shape is an adaptation to moving on or through the substrate. Spines and blades on the lower surface of the styloid and proximal aulacophore are inferred to be adaptations for gripping the substrate as the animal pulled itself backwards (aulacophore first). The main thecal orifice opposite the aulacophore was the site of both mouth and anus. Evidence for a sluggish, epifaunal life mode in the cornute carpoids includes the typically flattened, asymmetrical body, the presence of downward projecting spines and knobs on the marginal thecal plates of many species, and the specialized thecal pores and slits that faced away from the substrate.

The pre‐radial history of echinoderms

“…Detailed anatomical studies have been undertaken of a large number of stylophoran taxa (e.g. Caster 1983;Cripps 1988Cripps , 1989Cripps , 1991Cripps and Daley 1994;Jefferies 1969Jefferies , 1973Jefferies , 1986Jefferies and Lewis 1978;Jefferies et al 1996;Kolata and Jollie 1982;Kolata et al 1991;Lefebvre 2000Lefebvre , 2001Lefebvre , 2003Lefebvre and Gutiérez-Marco 2003;Lefebvre and Vizcaino 1999;Martí Mus 2002;Parsley 1991Parsley , 2000Parsley et al 2000;Ruta 1997aRuta -c, 1999aRuta and Bartels 1998;Ruta and Jell 1999a-d;Ruta and Theron 1997;Ubaghs 1968aUbaghs , 1979Ubaghs , 1981Ubaghs , 1994Woods and Jefferies 1992). Yet there remains no agreement as to the orientation or anatomical structure of stylophorans.…”

“…Jefferies and co-workers (e.g. Jefferies et al 1996) contend that stylophorans represent stem-group urochordates, cephalochordates and craniates as well as stem-group chordates; Ubaghs (1975), Philip (1979), Parsley (1994), Kolata et al (1991) and Gee (1996) all place them as stem-group echinoderms; while Sumrall (1997) and David et al (2000) place them amongst the radiate echinoderms as derived crown-group echinoderms and sister group to the crinoids.…”

“…The groove running along the stylocone and distal ossicles marks the position of a notochord and dorsal nerve. Philip (1979), Kolata and Jollie (1982) and Kolata et al (1991) also thought that the stele was locomotory and not an ambulacrum, but believed that it was not homologous to either the crinoid stem or the chordate tail. Ruta (1999c) has the appendage as both feeding arm and locomotory appendage.…”

“…(1) As Philip (1979), Kolata et al (1991) and Jefferies (1986) have pointed out, the cover-plates of mitrates overlap with complex abutment that would prevent them from opening. Even in cornutes, where the overlap is less marked and cover-plates have been preserved in an open position, splayed out on either side of the stele (Ubaghs 1981;Lefebvre 2003), this could simply represent a taphonomic artefact.…”

“…Hence, they do not consider the possibility that the possession of ambulacra or pentamery may be restricted to crown-group echinoderms only, and carpoids are forced to have homologues of the ambulacra even where none are apparent. Some workers (Kolata et al 1991;Ruta 1999a;Lefebvre 2000) claim to have demolished the calcichordate theory by demonstrating that one particularly weak inference drawn by Jefferies (1986) about the homology of surfaces in mitrates and cornutes is false. As first pointed out by Philip (1979), cornutes and mitrates show a very similar morphological distinction between upper and lower surfaces of the theca and appendage that has traditionally been taken as evidence of their homology.…”