Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes

Abstract

The extinct echinoderm clade Stylophora consists of some of the strangest known deuterostomes. Stylophorans are known from complete, fully articulated skeletal remains from the middle Cambrian to the Pennsylvanian, but remain difficult to interpret. Their bizarre morphology, with a single appendage extending from a main body, has spawned vigorous debate over the phylogenetic significance of stylophorans, which were long considered modified but bona fide echinoderms with a feeding appendage. More recent interpretation of this appendage as a posterior "tail-like" structure has literally turned the animal back to front, leading to consideration of stylophorans as ancestral chordates, or as hemichordate-like, early echinoderms. Until now, the data feeding the debate have been restricted to evaluations of skeletal anatomy. Here, we apply novel elemental mapping technologies to describe, for the first time, soft tissue traces in stylophorans in conjunction with skeletal molds. The single stylophoran appendage contains a longitudinal canal with perpendicular, elongate extensions projecting beyond hinged biserial plates. This pattern of soft tissues compares most favorably with the hydrocoel, including a water vascular canal and tube feet found in all typical echinoderms. Presence of both calcite stereom and now, an apparent water vascular system, support echinoderm and not hemichordate-like, affinities.

Arm waving in stylophoran echinoderms: three-dimensional mobility analysis illuminates cornute locomotion

“…The theca is subcircular in outline with an invagination where the aulacophore inserted. The posterior margin [ 9 : fig. 1 a ] echinoderm interpretation) of the theca is incomplete where it extends beyond the edge of the concretion.…”

“…The distal aulacophore is characterized by a continuation of the groove, flanked by two ridges. Lateral projections have been interpreted as movable cover plates [ 9 ]. The segments are wider than long and separated by relatively flat articulations orthogonal to the axis of the aulacophore.…”

“…Past interpretations of aulacophore function have framed a role in locomotion and feeding as contrasting hypotheses [ 8 , 9 , 12 , 13 , 15 ]. However, there is no basis for considering these functions as mutually exclusive: appendages in extant echinoderms perform both feeding and locomotion (i.e.…”

Cambrian Chordates and Vetulicolians

“…The five pairs of gill structures in vetulicolians could in fact represent mid-gut glands of some sort, although this seems unlikely. The bizarre carpoids, with (like vetulicolians) a bipartite body division are now considered to be regular echinoderms with no gill slits and a feeding arm with a normal water vascular system [72]. Still, the mythical chimaera-like nature of many of the early deuterostomes suggests that the group may very well require the erection of multiple extinct phylum-level taxa to avoid systematic shoehorning.…”

Biogenic Iron Preserves Structures during Fossilization: A Hypothesis

“…[ 4–10 ] Similarly, exceptionally preserved soft parts in fossils from the slightly younger Fezouata Shale (Ordovician, Morocco) were decisive in ending long‐standing debates on the systematic affinities of various enigmatic taxa (e.g., machaeridians, stylophorans). [ 11–13 ] The Chengjiang Biota (Cambrian, China) has also yielded a considerable number of soft arthropod taxa with complex nervous systems. [ 14–16 ] In most cases, nervous tissues from the Chengjiang Biota are pyritized (i.e., preserved in FeS 2 ) or show an association of pyrite and organic matter.…”