Evolutionary significance of the blastozoanEumorphocystisand its pseudo-arms

Abstract

Twelve specimens of Eumorphocystis Branson and Peck, 1940 provide the basis for new findings and a more informed assessment of whether this blastozoan (a group including eocrinoids, blastoids, diploporites, rhombiferans) constitutes the sister taxon to crinoids, as has been recently proposed. Both Eumorphocystis and earliest-known crinoid feeding appendages express longitudinal canals, a demonstrable trait exclusive to these taxa. However, the specimen series studied here shows that Eumorphocystis canals constrict proximally and travel within ambulacrals above the thecal cavity. This relationship is congruent with a documented blastozoan pattern but very unlike earliest crinoid topology. Earliest crinoid arm cavities lie fully beneath floor plates; these expand and merge directly with the main thecal coelomic cavity at thecal shoulders. Other associated anatomical features echo this contrasting comparison. Feeding appendages of Eumorphocystis lack two-tiered cover plates, podial basins/pores, and lateral arm plating, all features of earliest crinoid ‘true arms.’ Eumorphocystis feeding appendages are buttressed by solid block-like plates added during ontogeny at a generative zone below floor plates, a pattern with no known parallel among crinoids. Eumorphocystis feeding appendages express brachioles, erect extensions of floor plates, also unknown among crinoids. These several distinctions point to nonhomology of most feeding appendage anatomy, including longitudinal canals, removing Eumorphocystis and other blastozoans from exclusive relationship with crinoids. Eumorphocystis further differs from crinoids in that thecal plates express diplopores, respiratory structures not present among crinoids, but ubiquitous among certain groups of blastozoans. Phylogenetic analysis places Eumorphocystis as a crownward blastozoan, far removed from crinoids.

The first record of floor plates in pinnules and the earliest record of an anitaxis in rhodocrinitid diplobathrid camerate crinoids

“…The finding of floor plates in pinnules fortifies previous evidence indicating that these constructs echo the pattern seen in arms where larger supporting skeletal structures, brachials, also derive from the extraxial body wall (Mooi ad David, 1997, 1998; Guensburg and Sprinkle, 2001; Guensburg et al, 2016, 2020b). Floor and cover plates in both locations represent conserved plesiomorphic morphology.…”

Crinoid calyx origin from stem radial echinoderms – CORRIGENDUM

“…This approach has been suggested to emphasize differences rather than similarities among these taxa (e.g., Ausich, et al, 2015a;Sheffield and Sumrall, 2019). Guensburg et al (2021) responded to this criticism by indicating specific ways in which the EAT provides homology criteria to unify entire regions of echinoderm body structure, resulting in a framework that can unite echinoderm clades despite their disparate body plans (David and Mooi, 1996;Mooi andDavid, 1997, 2008;David et al, 2000).…”

“…The most parsimonious interpretation for blastozoans is to recognize that brachioles Journal of Paleontology:1-24 are apomorphic, free ambulacra (Paul and Smith, 1984;David et al, 2000;Mooi et al, 2005), and that these lack extensions of the theca that could include anatomically internal coelomic extensions of the somatocoels. This is why there is no connectivity between the thecal cavity and blastozoan feeding appendages, including pseudo-arms (Guensburg et al, 2021). This anatomy is (2) exterior mid-calyx region indicated in Figure 1.1, ∼2 cm aboral to the calyx-tegmen juncture, primary plates at center and surrounding secondary rosette, adjoining secondaries with large dark calcitic gaps indicating epispire locations; larger secondaries beyond those contacting primary; (4) interior mid-calyx region ∼2 cm aboral to calyx-tegmen juncture, primary, surrounding rosette of secondaries, gaps visible between adjacent elements mark epispires, larger secondaries beyond.…”

The diploporite blastozoan Glyptosphaerites (Echinodermata: Blastozoa) and the origin of diplopores

“…9, p. 1098, but is otherwise identical to Protocrinites Eichwald, 1840. Eumorphocystis Branson & Peck, 1940 has initially mural ambulacra that conform to the definition above, but then they develop erect, pinnate 'arms' with triserial axes and alternating biserial brachioles (Parsley 1982;Sheffield & Sumrall 2019a;Guensburg et al 2021).…”