The first Ordovician cyclocystoid (Echinodermata) from Gondwana and its morphology, paleoecology, taphonomy, and paleogeography

Abstract

Abstract.-Moroccodiscus smithi represents a new cyclocystoid genus and species based on moldic specimens from the Middle Ordovician Taddrist Formation (Darriwilian) of SE Morocco. This represents the earliest articulated member of the Cyclocystoidea and is the first complete cyclocystoid described from the Ordovician of Gondwana, as well as the first cyclocystoid ever recorded from Africa. The anatomy and morphology of this new species were studied using a combination of conventional paleontological methods and nondestructive X-ray computed tomography. Because Moroccodiscus differs from other cyclocystoids, in particular by lacking cupules attached to the marginal ossicles, it is assigned to the new family Moroccodiscidae. This new taxon illustrates the relatively poorly known early diversification of these enigmatic extinct echinoderms and sheds light on the mode of life of cyclocystoids, including injuries to plate circlets during early ontogeny and folding of these disk-like specimens at the time of death. The overall thecal shape was very similar in cyclocystoids and many domal edrioasteroids, probably because they were both sessile or attached, benthic, suspension feeders. However, many oral surface, ambulacral, and marginal ring features had become very different, indicating that these two groups had either converged because of similar life modes or were only distantly related sister groups.

Perforocycloides nathalieae new genus and species, an unusual Silurian cyclocystoid (Echinodermata) from Anticosti Island, Québec, Canada

“…Nichols (1969) and Smith and Paul (1982) presented several pieces of evidence supporting an interpretation that the mouth (ventral surface) faced toward substrate and that these creatures were vagile, using tube feet located along the rays for locomotion and consumed small organic particles from the sediment via cilia located in the cupules. However, Sprinkle et al (2015) and Reich et al (2017) favoured an orientation with the annular plate surface (dorsal) facing toward the substrate and the mouth (ventral surface) facing toward the water column, the organisms being sessile (and/or hemisessile), and with the radial tube feet used for capturing suspended food particles (an orientation favoured by many previous workers including Sieverts-Doreck 1951, Kesling 1963, 1966and Kolata 1975. Furthermore, questions as to their origins and interrelationships remain speculative (e.g.…”

“…They are typically small, circular to pentagonal, flattened organisms, which have been divided into two valid families, 21 genera, and about 40 species. They are known both from articulated and disarticulated material from North America, Europe, Australia, North Africa (Reich et al 2017), and China (Reich et al unpubl. ), but with most articulated material known from Ordovician strata.…”

“…Cyclocystoids were extensively reviewed and monographed by Smith and Paul (1982), who erected the modern taxonomic and morphological framework within which all subsequent work has been based. Since this time, several new taxa and genera have been described, along with a new family lacking cupules (Reich et al 2017).…”

“…Furthermore, questions as to their origins and interrelationships remain speculative (e.g. Smith and Paul 1982;Berg-Madsen 1987;Rowe 1988;Reich et al 2017).…”

Progress in echinoderm paleobiology

“…Cole et al (2017) report a new diverse fauna of Ordovician crinoids (dominated by camerates) from Spain that fills an important gap in the history of this group in Gondwana. Reich et al (2017) report the first complete cyclocystoid from the Ordovician of Gondwana, describing its morphology in great detail with the aid of X-ray computed tomography. Sheffield and Sumrall (2017) revise the Holocystites fauna from the Silurian of North America, suggesting that the plating of the oral area is more informative for taxonomic purposes than thecal morphologies.…”