Abstract
AbstractIntermediate 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 FloianAthenacrinus browerinew genus new species. Character analysis supports a hypothesis that this taxon originated early within in the disparid clade.Athenacrinusn. 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
Highlights
The origins of crinoids remain debated in disparate approaches
Arm morphology is exceptionally well-preserved in the late Tremadocian to early Floian Athenacrinus broweri new genus new species
Arm data from Tremadocian crinoids, combined with data from younger (Floian, late Early Ordovician) crinoids documents the events preceding the acquisition of typical Paleozoic crinoid morphology
Summary
The origins of crinoids remain debated in disparate approaches (most recently Guensburg, 2012; Ausich et al, 2015a, b; Guensburg et al, 2016; Sheffield and Sumrall, 2019a, b) Bearing on this issue are the earliest-known (Tremadocian, 485.4– 477.7 Ma), crinoids, but the fossils are few and imperfect (Ubaghs, 1969; Guensburg and Sprinkle, 2003, 2009; Guensburg, 2012; Guensburg et al, 2016). We use new morphologic data, together with those from other Tremadocian crinoids, to characterize this skeletal construction of arms and to compare similar anatomy outside the crinoid clade with that of more archaic echinoderms. This information invites a reanalysis of crinoid origin using a new phylogenetic dataset. A broad picture of Tremadocian crinoid evolution based on these occurrences is characterized by standardization and reduction that accompanied emergence of camerate, cladid, and disparid clades
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
