Solving the mystery of crinoid ancestry: new fossil evidence of arm origin and development

Abstract

Apektocrinus ubaghsinew genus and species is a monospecific taxon assigned to the new family Apektocrinidae based on additional preparation of a single previously studied specimen.Apektocrinusis among the oldest known crinoids (Early Tremadoc, Early Ordovician). Although expressing crinoid apomorphies, it is interpreted as retaining plesiomorphies in its arms reflecting early edrioasteroid rather than blastozoan (eocrinoid) ancestry. Apomorphies represent basal crinoid and cladid (crownward) levels of phylogeny.Restudy fortifies previous reports of the presence of a basal echinoderm plesiomorphy; floor plates above brachials in the arms ofApektocrinus, as well as in other approximately contemporary crinoids.Apektocrinusfurnishes the first record of podial basins in crinoid arms. Arms and calyx ofApektocrinusmerge gradually, facilitated by continuations of interbrachials (extraxial body plates) extending onto the arms and separating floor plates from brachials. These arm interbrachials, which diminish and pinch out distally as floor plates nestle into the brachial (adoral) groove, have not been recognized as such in crinoids.

Bayesian estimation of fossil phylogenies and the evolution of early to middle Paleozoic crinoids (Echinodermata)

“…Thus, the early stratigraphic position, mosaic distribution of plesiomorphic and apomorphic traits, and strong support from previous phylogenetic analyses all indicate Apektocrinus occupies a position near the base of the non-camerate tree (Guensburg and Sprinkle, 2009;Guensburg, 2012;Ausich et al, 2015).…”

“…Camerates were not included in the analysis because they diverged from non-camerate crinoids by at least the earliest Ordovician (Guensburg and Sprinkle, 2003;Guensburg, 2012;Ausich et al, 2015;Cole, 2017). Although tip-dating analyses do not per se require use of an outgroup (Ronquist et al, 2012), there are several reasons I used the Tremodocian species Apektocrinus ubaghsi Guensburg and Sprinkle, 2009 to assist rooting the tree. Apektocrinus was originally described as a tentative cladid that featured traits intermediate between protocrinoids and nominal cladids (Guensburg and Sprinkle, 2009).…”

“…Although tip-dating analyses do not per se require use of an outgroup (Ronquist et al, 2012), there are several reasons I used the Tremodocian species Apektocrinus ubaghsi Guensburg and Sprinkle, 2009 to assist rooting the tree. Apektocrinus was originally described as a tentative cladid that featured traits intermediate between protocrinoids and nominal cladids (Guensburg and Sprinkle, 2009). Protocrinoids were originally considered basal crinoids stemward of the divergence between camerates and non-camerates (Guensburg and Sprinkle, 2003).…”

Cambrian stalked echinoderms show unexpected plasticity of arm construction

“…The variety of different ways feeding appendages are constructed in the Cambrian shows that plating organization was highly flexible and only later came to be stereotypic. While dibrachicystids have arms that arise as direct outgrowths of the body as in crinoids, they are different in construction to those of protocrinids, where a biseries of flooring plates is largely internalized (see [9,10]). …”

“…Undisputed crinoids first appear at or close to the base of the Ordovician [9][10][11] and possess true arms; that is to say, appendages with a central lumen that are directly connected to the theca and presumably carried coelomic extensions of the body cavity, as in all living crinoids. Some workers have proposed that crinoids are related to Cambrian stalked echinoderms with a different form of feeding appendages (blastozoans [12]) while others have argued for their origin from a group lacking appendages (edrioasteroids [9,10]). For some, then, the origin of feeding appendages in crinoids and blastozoans is a classic example of convergent evolution [13].…”