Fossil Weathering and Preparation Mimic Soft Tissues in Eocrinoid and Somasteroid Echinoderms from the Lower Ordovician of Morocco

Farid Saleh1, *, Bertrand Lefebvre1 , Aaron W. Hunter,2

Planètes, Environnement, F-69622 Villeurbanne, France 2

3Czech Geological Survey, Klárov 3, Praha 1, 118 21 Czech Republic *

Abstract: Investigation of the Fezouata Shale has added to our knowledge on the initial diversification of metazoans. These Lower Ordovician deposits yielded abundant and diverse remains of cuticu- larized to lightly sclerotized organisms, in addition to numerous soft tissues. Described fossilized soft parts recovered from the Fezouata Shale belong mainly to arthropods. Soft tissues in echinoderms, a main component of the Fezouata Biota, remain largely unexplored. Here, we show that soft tissue-like impressions previously reported in eocrinoid and somasteroid echinoderms from this formation, are the results of modern weathering and fossil preparation that involved the use of latex molds: they do not reflect any original (soft) anatomi- cal features of these organisms. These two examples suggest that reports of putative soft parts, especially in taxa with no current repre- sentatives, need to be thoroughly and critically evaluated.

Keywords: Echinoderms, Exceptional Preservation, Fezouata Lager- stätte, Morocco, Ordovician, SEM, X-ray fluorescence maps

Introduction Exceptional fossil preservation consists of the preserva-

tion of non-biomineralized soſt parts (for example, digestive and nervous systems of animals) in the geological record. Fos- siliferous localities showing this type of preservation are called Konservat-Lagerstätten. Te late Tremadocian Fezouata Shale in the Anti-Atlas of Morroco, deposited approximately 480 mil- lion years ago, is the only Ordovician period Konservat-Lager- stätte to yield a fully marine diverse exceptionally preserved fauna [1]. Te ∼900 meter thick siltstones of the Fezouata Shale have yielded over 200 taxa of marine invertebrates [2]. Most of them belong to arthropods and echinoderms [2,3]. In these deposits, exceptionally preserved soſt parts (for example, guts) are well-documented in various groups of arthropods (for example, anomalocaridids, trilobites) [1,2,4,5], annelids [6], hyolithids [7], molluscs [8], and palaeoscolecid worms [9,10]. In marked contrast, very few occurrences of exception-

ally preserved soſt parts have been reported from Fezouata echinoderms: a putative gut was mentioned in one specimen of solutan [11], and more abundant remains (water-vascular system, gut) were described in stylophorans [12]. Te water- vascular (or ambulacral) system is a complex, non-biomin- eralized, coelomic, hydraulic structure, which is unique to echinoderms. It consists of ambulacral canals leading to lat- eral sets of smaller tube feet, which are used for feeding, loco- motion, and respiration. Such a rarity of exceptionally preserved soſt parts in echi-

noderms from the Fezouata Shale is not surprising. In spite of some recent spectacular reports, for example, in the Silurian

24 doi:10.1017/S1551929519001238

Herefordshire Lagerstätte, UK [13,14,15] or the Devonian Hun- srück Slate, Germany [16], very few examples of soſt parts have been described in fossil echinoderms. Tis situation mainly results from the fast post-mortem disarticulation of


multi-element calcite skeleton and, hence, their low preserva- tion potential, [3,17]. Experimental taphonomy, the study of how organisms decay and become fossilized, on modern echi- noderms suggests that both skeletal elements and soſt parts disarticulate and degrade within a few days aſter death [18]. Consequently, due to taphonomic biases [19], the preserva- tion of complete echinoderm specimens yielding soſt tissues is extremely rare even within Lagerstätten. Because of these biases, understanding the preservational pathway of an organ- ism is vital prior to any paleontological description especially for animal groups with no current representatives. In this study, we aim at understanding the preservation of eocrinoids and somasteroids from the Fezouata Shale [20,21] and criti- cally evaluate the recent reports of putative soſt tissues (water- vascular system) in these two groups.

Material and Methods Te taphonomy of soſt parts was analyzed in the two

best-preserved specimens of eocrinoid and somasteroid echi- noderms from the Fezouata Shale [20,21]. Tis material


deposited in the paleontological collections of the Musée des Confluences, Lyon, France (acronym “ML”), and Claude Bernard Lyon 1 University, Villeurbanne, France (acronym “UCBL-FSL”), respectively. Te eocrinoid specimen (ML20- 269159, Reboul collection) was collected at Bou Izargane, about 18 km north of Zagora, Morocco. Tis locality is a well-known fossiliferous site, where abundant and diverse late Tremado- cian exceptionally preserved fossils have been collected (for example, anomalocaridids, marrellomorphs, palaeoscolecids), including trilobites and stylophoran echinoderms with soſt parts [5,12,22]. Te somasteroid specimen (UCBL-FSL 424962, Vizcaïno collection) is from an unknown locality, probably late Tremadocian in age, from the Ternata plain, north of Zagora, Morocco. Te two specimens were characterized using a FEI Quanta

250 scanning electron microscope (SEM) equipped with a backscattered electron detector in addition to an energy- dispersive X-ray analyzer (EDX) at accelerating voltages that varied from 5 to 15 kV. Te backscattered electron detec- tor allowed the acquisition of images with dark pixels corre- sponding to light elements (that is, small atomic number Z) • 2020 January and Martina Nohejlová1,3

1Univ. Lyon, Université Claude Bernard Lyon 1, ENS de Lyon, CNRS, UMR 5276 Laboratoire de Géologie de Lyon: Terre, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ UK

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