A re-interpretation of the ambulacral system of Eumorphocystis (Blastozoa: Echinodermata) and its bearing on the evolution of early crinoids

Abstract

Recent debates over the evolutionary relationships of early echinoderms have relied heavily on morphological evidence from the feeding ambulacral system. Eumorphocystis, a Late Ordovician diploporitan, has been a focus in these debates because it bears ambulacral features that show strong morphological similarity to early crinoid arms. Undescribed and well-preserved specimens of Eumorphocystis from the Bromide Formation (Oklahoma, USA) provide new data illustrating that composite arms supported by a radial plate that bear a triserial arrangement of axial and extraxial components encasing a coelomic extension can also be found in blastozoans. Previous reports have considered these arm structures to be restricted to only crinoids; these combined features have not been previously observed in blastozoan echinoderms. Phylogenetic analyses suggest that Eumorphocystis and crinoids are sister taxa and that shared derived features of these taxa are homologous. This evidence concerning Eumorphocystis’ arms suggests that crinoid arms were derived from a specialized blastozoan ambulacral system that lost feeding brachioles and strongly suggests that crinoids are nested within blastozoans.

Evolutionary significance of the blastozoanEumorphocystisand its pseudo-arms

“…In this paper, however, we add descriptive data and imagery that enhance our understanding of, and provide a basis for, interpretation of Eumorphocystis ‘arms’ that is in agreement with the original suggestion that Eumorphocystis is of strictly blastozoan affinity (Parsley, 1982). The present data confirm most of the basic ‘arm’ construction details, but reveal significant points of departure, particularly at the juncture of arms to theca (Sheffield and Sumrall, 2019b). This new information forms the core of our reappraisal of the evolutionary significance of Eumorphocystis .…”

“…Evidence central to this suggestion, largely derived from a single specimen, is the description of a longitudinal canal within each of this taxon's feeding appendages. These canals are stated to pierce the theca and connect with the thecal coelomic cavity in a manner similar to that known for crinoids (Sheffield and Sumrall, 2019b).…”

“…Despite doubts about this interpretation that arose during the mid-20th century (Ubaghs, 1953, 1968; Sprinkle, 1973), this canon was perpetuated in major reference works such as the Treatise on Invertebrate Paleontology and persists in many publications to the present day. Works seeking to separate blastozoans and crinoids as only distantly related pentaradiate forms or to reject the pelmatozoan hypotheses that crinoids and blastozoans share exclusive common ancestry have met with considerable resistance (e.g., Clausen et al, 2009; Zamora and Smith, 2012; Kammer et al, 2013; Sumrall, 2017; Sheffield and Sumrall, 2019a, b).…”

“…Recently, a proposal has been published that Eumorphocystis multiporata Branson and Peck, 1940 (hereafter Eumorphocystis Branson and Peck, 1940, because all specimens are conspecific topotypes), a diplopore-bearing blastozoan, represents the nearest-known sister group to crinoids (Sheffield and Sumrall, 2019b). Evidence central to this suggestion, largely derived from a single specimen, is the description of a longitudinal canal within each of this taxon's feeding appendages.…”

“…Initially, these were considered of blastozoan origin and only homoplastic on crinoid arms (Parsley, 1982). Recently, the discovery that early crinoids express what could also be considered a tripartite pattern (Guensburg and Sprinkle, 2009) formed the basis for a reinterpretation that Eumorphocystis ‘arm’ anatomy is homologous to that of early crinoids (Sheffield and Sumrall, 2019b). In this paper, however, we add descriptive data and imagery that enhance our understanding of, and provide a basis for, interpretation of Eumorphocystis ‘arms’ that is in agreement with the original suggestion that Eumorphocystis is of strictly blastozoan affinity (Parsley, 1982).…”