Athenacrinusn. gen. and other early echinoderm taxa inform crinoid origin and arm evolution

Abstract

Intermediate morphologies of a new fossil crinoid shed light on the pathway by which crinoids acquired their distinctive arms. Apomorphies originating deep in echinoderm history among early nonblastozoan pentaradiate echinoderms distinguish Tremadocian (earliest Ordovician) crinoid arms from later taxa. The brachial series is separated from the ambulacra, part of the axial skeleton, by lateral plate fields. Cover plates are arrayed in two tiers, and floor plates expressed podial basins and pores. Later during the Early Ordovician, floor plates contacted and nestled into brachials, then were unexpressed as stereom elements entirely and cover plates were reduced to a single tier. Incorporation of these events into a parsimony analysis supports crinoid origin deep in echinoderm history separate from blastozoans (eocrinoids, ‘cystoids’). Arm morphology is exceptionally well-preserved in the late Tremadocian to early Floian Athenacrinus broweri new genus new species. Character analysis supports a hypothesis that this taxon originated early within in the disparid clade. Athenacrinus n. gen. (in Athenacrinidae new family) is the earliest-known crinoid to express what is commonly referred to as ‘compound’ or ‘biradial’ morphology. This terminology is misleading in that no evidence for implied fusion or fission of radials exists, rather it is suggested that this condition arose through disproportionate growth.UUID: http://zoobank.org/b383e039-3298-4472-a7e3-e81684f87cfe

Origin and significance of Lovén's Law in echinoderms

“…Here again, however, the situation is more complicated. Some early crinoids do have biserial flooring plates (Guensburg et al, 2020), and we have detected one possible case of Lovén's Law in Hybocrinus nitidus Sinclair, 1945(see Guensburg et al, 2016 fig. 2F).…”

Evolutionary significance of the blastozoanEumorphocystisand its pseudo-arms

“…Recent workers have appropriately applied new or previously little used methodology to the issue of crinoid phylogeny (Ausich, 2015b;Wright et al, 2017), but such approaches should incorporate information from other well-founded methodologies including those utilized here and in other works (e.g. Guensburg et al, 2016;Guensburg, et al, 2020).…”

“…Adding uncertainty to this matter is the scarcity of comparative information for brachiolar and floor plate canals of blastozoans in general (see Sprinkle, 1973;Clausen et al, 2009, for examples). Among early crinoids, these data have only recently been extensively analyzed in a phylogenetic context, although the nature of these canals has long been understood from an anatomical standpoint (Heinzeller and Welsch, 1994; and summarized in Guensburg et al, 2020). Present evidence shows that longitudinal feeding appendage canals, otherwise termed median canals (Sprinkle, 1973), exist in a diversity of blastozoans (gogiids, rhipidocystids, rhombiferans, blastoids) (Fay, 1960;Sprinkle, 1973;1975;Clausen, et al, 2009;Sumrall and Sheffield, 2019a) (Figs 2-4, 6.1-6.4, 6.6, 6.8, 6.9).…”

“…The situation among early crinoids (see Guensburg et al, 2010;Guensburg et al, 2020, for detailed analysis) (Figs. 5, 6.5, 6.7) is not comparable to that of Eumorphocystis or any other blastozoan (see Guensburg et al, 2020, for detailed analysis) (Figs. 6.5, 6.7).…”

“…These data are then compared with an expanded update of early crinoid arm morphology recently made available through examination of earliest crinoids. These data have only recently been more fully explicated in the context of crinoid origins (Guensburg et al, 2020). A full understanding of the feeding appendages of Eumorphocystis has been a long-term process, and even now some details, such as their full length, remain unknown (Fig.…”