Non-technical Summary
Living echinoderms (e.g., sea stars, brittle stars, sea urchins, sea cucumbers, sea lilies, and feather stars) are a major component of all marine environments on the planet. Their oldest fossils, dating from the Stage 3 of the Cambrian (more than half a billion years ago), are already very diverse, with three different major groupings with wide ranges across the paleocontinents. Here, we describe a new genus of oldest known African echinoderms (Issafen Formation, Cambrian Series 2, Stage 3; Morocco). The discovery of articulated specimens of the oldest echinoderms from northeastern Spain clarifies the systematic position of ‘Rhopalocystis’ mesonesensis (Clausen, Reference Clausen2004), which is assigned to our new genus Atlaseocrinus, which belongs to the Gogiida, an order of echinoderms comprising a stem and erect feeding appendages called brachioles. Atlaseocrinus is unique among gogiids due to the presence of a single basal plate connecting the body to the stem. It currently comprises two species: Atlaseocrinus issafenensis new genus new species from Morocco, which has straight brachioles, and Atlaseocrinus mesonesensis new combination from Spain, with coiled brachioles.
Introduction
The earliest known fossil echinoderms appear in Cambrian Stage 3 strata and already include disparate forms belonging to three classes: Edrioasteroidea, Eocrinoidea, and Helicoplacoidea (Ubaghs, Reference Ubaghs1975; Zamora et al., Reference Zamora, Darroch and Rahman2013). These earliest echinoderm records have been documented from at least three different paleocontinents: Gondwana and peri-Gondwanan regions (e.g., Richter and Richter, Reference Richter and Richter1940; Donovan and Paul, Reference Donovan and Paul1982; Ubaghs and Vizcaïno, Reference Ubaghs and Vizcaïno1990; Elicki and Schneider, Reference Elicki and Schneider1992; Smith et al., Reference Smith, Zamora and Álvaro2013); Laurentia (e.g. Durham and Caster, Reference Durham and Caster1963; Durham, Reference Durham1967); and Siberia (e.g., Rozhnov et al., Reference Rozhnov, Fedorov and Sayutina1992; Kouchinsky et al., Reference Kouchinsky, Bengtson, Clausen, Gubanov, Malinky and Peel2011, Reference Kouchinsky, Bengtson, Clausen and Vendrasco2015). The combination of high initial taxonomic disparity with a global paleobiogeographic distribution, hints at an even earlier, but undocumented origin of the phylum (Mooi and David, Reference Mooi and David1998; Zamora and Rahman, Reference Zamora and Rahman2014).
Among those early occurrences, eocrinoids were the first to develop free (erect) feeding appendages: the brachioles (sensu Sprinkle, Reference Sprinkle1973). Historically, the oldest known eocrinoids were described in the Cambrian Series 2, Stage 3 of Laurentia (Sprinkle, Reference Sprinkle1973; Durham, Reference Durham1978; Wilbur, Reference Wilbur2005; Nardin et al., Reference Nardin, Lefebvre, David and Mooi2009). New occurrences have also been described from the Cambrian, Series 2, Stage 4 from China (Balang Biota) (Zhao et al., Reference Zhao, Parsley and Peng2007; Yang et al., Reference Yang, Lin, Zhang, Wu and Meng2015; Wang et al., Reference Wang, Nohejlová, Sun, Zeng, Lefebvre, Yang and Zhao2024). However, the oldest known eocrinoids date from Cambrian Series 2, Stage 3 (Zamora et al., Reference Zamora, Darroch and Rahman2013). These include isolated skeletal elements from Laurentia and Siberia (Sprinkle, Reference Sprinkle1973; Rozhnov et al., Reference Rozhnov, Fedorov and Sayutina1992; Kouchinsky et al., Reference Kouchinsky, Bengtson, Clausen and Vendrasco2015) and well-preserved articulated material from two Gondwanan occurrences: the gogiid Alanisicystis andalusiae Ubaghs and Vizcaïno, Reference Ubaghs and Vizcaïno1990 from southern Spain, and as yet undescribed specimens from the Issafen Formation, Morocco (e.g., Ubaghs and Vizcaïno, Reference Ubaghs and Vizcaïno1990; Zamora et al., Reference Zamora, Darroch and Rahman2013).
The Moroccan eocrinoid material was formerly assigned to the genus Alanisicystis Ubaghs and Vizcaïno, Reference Ubaghs and Vizcaïno1990, suggesting probable affinities with coeval Spanish gogiids (see, e.g., Nardin, Reference Nardin2007; Smith et al., Reference Smith, Zamora and Álvaro2013). However, the Issafen eocrinoid has never been formally described. Consequently, the aims of this study are to (1) describe one of the oldest echinoderms ever found in Gondwana; (2) discuss its systematic position within gogiid eocrinoids; and (3) compare it with more recent echinoderm remains from the Valdemiedes Formation (Cambrian Stage 4) in the Iberian Chains, Spain.
Geological setting
New specimens of eocrinoids from Cambrian Series 2, Stage 3, here assigned to Atlaseocrinus new genus, have been discovered in the Issafen Formation of the central Anti-Atlas (Morocco) and the Valdemiedes Formation of the eastern Iberian Chains (Spain). The Issafen Formation (Siegert, Reference Siegert1986), 25–400 m thick, consists of variegated shales with subsidiary nodular (‘calcaire scoriacé’), lenticular, and bedded carbonate intercalations, exhibiting nonreefal shelly and reefal microbial-archaeocyathan fabrics (Álvaro and Debrenne, Reference Álvaro and Debrenne2010; Álvaro et al., Reference Álvaro, Benziane, Thomas, Walsh and Yazidi2014). The fauna also includes brachiopods, chancelloriids, hyoliths, trilobites (Geyer and Landing, Reference Geyer and Landing1995), skeletonized microfossils, and echinoderms (Smith et al., Reference Smith, Zamora and Álvaro2013). The geological section that yielded the new specimens of Atlaseocrinus was first mentioned by Hupé (Reference Hupé1953, Reference Hupé1960), who reported fossiliferous shales rich in trilobites in the vicinity of Timkit (or Timguite). The trilobite assemblage allowed him to identify the Antatlasia (or IV) Biozone, although some of the trilobite species were also found in the overlying Bondonella and Neltneria (V), and Longianda and Gigantopygus (VI) biozones belonging to the Issafenian Stage. Geyer (Reference Geyer1990b) and Geyer and Landing (Reference Geyer and Landing1995, Reference Geyer and Landing2004) proposed a new chronostratigraphic framework and subdivided the same stratigraphic interval into the Antatlasia hollardi and Antatlasia guttapluviae biozones (Bani or Banian Stage), which correspond to the archaeocyath-based Polystillicidocyathus-Halysicyatus Biozone (Debrenne and Debrenne, Reference Debrenne and Debrenne1995; Álvaro et al., Reference Álvaro, Benziane, Thomas, Walsh and Yazidi2014). The co-occurrence of sclerites of Antatlasia sp. indet. and Atlaseocrinus, at ~335 m above the Amouslek/Issafen contact, allows the assignation of the eocrinoid remains to the Antatlasia hollardi-Antatlasia guttapluviae biozones of the Banian Stage (Fig. 1.1, 1.2, 1.5).
Simplified geological map reporting occurrences of Atlaseocrinus: (1) Cambrian outcrops of France, Morocco, Portugal, and Spain; (2) Cambrian outcrops in High Atlas and Anti-Atlas regions of Morocco; (3) Paleozoic outcrops of Iberian Chains in Spain; (4) Jarque locality (J), red star marks outcrop; (5) Issafen locality (Is). M = Mesones locality.
Figure 1. Long description
Panel 1 is a regional map of France, Spain, Portugal, and Morocco. Yellow shaded areas indicate Cambrian outcrops across the Armorican Massif, Central Massif, Montagne Noire, Pyrenees, and the Iberian Peninsula. A red box highlights the Iberian Chains in Spain, and another highlights the High Atlas and Anti-Atlas in Morocco.
Panel 2 focuses on Morocco. A legend identifies light purple for Adoudou, Tata, Feijas Internes, and Tabanite groups, and orange for Palaeoproterozoic and other units. Key features include the S A F (South Atlas Fault) and A A M F (Anti-Atlas Major Fault). A black circle labeled I s marks the Issafen locality in the Kerdous region.
Panel 3 is a detailed map of the Iberian Chains. The legend shows Devonian (dark brown), Silurian (pale yellow), Ordovician (orange), Cambrian (green), and Ediacaran (?) - Terreneuvian (black). A black circle labeled J marks the Jarque locality, and M marks the Mesones locality. The Datos Fault runs Northwest to Southeast.
Panel 4 is a landscape photo of the Jarque locality. A red star on a rocky hillside marks the specific outcrop. The foreground contains dry brush and trees.
Panel 5 is a landscape photo of the Issafen locality. It shows a wide valley with layered sedimentary rock formations sloping down from the right toward a small settlement in the distance.
On the other hand, silicified echinoderm holdfasts were etched from shelly limestone interbeds of the Valdemiedes Formation in the vicinity of Mesones de Isuela village, eastern Iberian Chains (Clausen, Reference Clausen2004). This formation, 20–150 m thick, is composed of shale/limestone alternations, the latter exhibiting shelly and stromatolitic fabrics (Álvaro and Vennin, Reference Álvaro and Vennin1997), and hydrothermally induced silicification processes in the Mesones sampling area (Martínez-Benítez et al., Reference Martínez-Benítez, Esteve and Álvaro2025). These partially silicified limestones have also yielded disarticulated hexactinellid spicules and the trilobite Alueva undulata Sdzuy, Reference Sdzuy1961, indicating the top of the regional Bilbilian Stage. The eocrinoid remains occur ~37 m below the first occurrence of Acadoparadoxides mureroensis (Sdzuy, Reference Sdzuy1958), which marks the regional Bilbilian–Leonian (formerly ‘lower–middle Cambrian’) boundary (Álvaro et al., Reference Álvaro, Gozalo, Liñán and Sdzuy1993). Due to their disarticulated preservation, eocrinoid remains were initially tentatively assigned to previously described taxa, e.g., ‘Rhopalocystis’ (Clausen, Reference Clausen2004) and Alanisicystis-like gogiids (Smith et al., Reference Smith, Zamora and Álvaro2013; Zamora et al., Reference Zamora, Álvaro, Arbizu, Colmenar, Esteve, Zamora and Rabano2015). Additional articulated specimens and disarticulated ossicles of the same species were also found in the vicinity of Jarque, another classical section of the eastern Iberian Chains (Fig. 1.1, 1.3, 1.4). Due to their co-occurrence with Protolenus jilocanus (Liñán and Gozalo, Reference Liñán and Gozalo1986), these remains can be placed within the coeval biozone, which marks the top of the Bilbilian Stage.
The regional ‘lower–middle Cambrian’ boundary is different in Iberia and Morocco: in the former, it is placed at the First Appearance Datum (FAD) of Acadoparadoxides mureroensis; and in Morocco, at that of the genus Hupeolenus Sepkoski, Reference Sepkoski2002 (Álvaro et al., Reference Álvaro, Gozalo, Liñán and Sdzuy1993; Geyer and Landing, Reference Geyer and Landing1995). However, the presence of common taxa allows the identification of chronostratigraphic links between the two regions, such as the FAD of the trilobite species Hupeolenus termierelloides (Geyer, Reference Geyer1990a), Onaraspis altus (Liñán and Gozalo, Reference Liñán and Gozalo1986), and Acadoparadoxides mureroensis (see Sundberg et al., Reference Sundberg, Geyer, Kruse, McCollum, Pegel, Żylińska and Zhuravlev2016; Álvaro et al., Reference Álvaro, Esteve and Zamora2018).
Material and methods
Material
Forty-three specimens from Morocco (Issafen Formation) and seven specimens from Spain (Valdemiedes Formation) were studied here. All specimens are fully to partially articulated and preserved as natural molds in mudstones.
Methods
Some specimens were carefully prepared using a Micro Jack 2 air scribe (Paleotools; Brigham, Utah, USA). Morphology was observed from latex casts, which were coated with ammonium chloride sublimate. Photographs were then taken with a Nikon D7100 camera (Nikon, Tokyo, Japan) equipped with a macro 60 mm lens. Some specimens were drawn using a camera lucida mounted on a Zeiss Discovery.V8 binocular microscope.
Repositories and institutional abbreviations
Natural History Museum, London, UK (BMNHUK); Museo Geominero IGME-CSIC, Madrid, Spain (MGM); Muséum d’Histoire Naturelle, Nantes, France (MHNN); Muséum d’Histoire Naturelle, Toulouse, France (MHNT); collections of the Museo de Ciencias Naturales, Universidad de Zaragoza, Spain (MPZ); and paleontological collections of the Université Claude Bernard Lyon 1, Villeurbanne, France (UCBL-FSL).
Systematic paleontology
Terminology and higher classification follow Sprinkle (Reference Sprinkle1973), although the monophyly of the Eocrinoidea and Gogiida has been questioned (e. g., Smith, Reference Smith1984; David et al., Reference David, Lefebvre, Mooi and Parsley2000; Zamora et al., Reference Zamora, Gozalo and Liñán2009; Zamora and Smith, Reference Zamora and Smith2012).
Phylum Echinodermata Klein, Reference Klein1778
Subphylum Blastozoa Sprinkle, Reference Sprinkle1973
Class Eocrinoidea Jaekel, Reference Jaekel1918
Order Gogiida Broadhead, Reference Broadhead and Lawrence1982
Remarks
Gogiids are loosely divided into monogeneric families of derived forms (i.e., Lichenoidae Jaekel, Reference Jaekel1918; Rhopalocystidae Ubaghs, Reference Ubaghs1967; Lyracystidae Sprinkle and Collins, Reference Sprinkle and Collins2006) and larger families grouping seemingly basal forms (Eocrinidae Jaekel, Reference Jaekel1918). The latter is defined with broad characteristics to encompass blastozoans that cannot be attributed to other families, and as such are considered paraphyletic (Parsley and Zhao, Reference Parsley and Zhao2006). Thus, family attribution is pending until further revision of the order Gogiida.
Family unassigned
Atlaseocrinus new genus
Type species
Atlaseocrinus issafenensis new genus new species.
Other species
Atlaseocrinus mesonesensis (Clausen, Reference Clausen2004), see below.
Diagnosis
A genus of gogiids with a single basal plate and a theca partly organized in regular circlets of tesselate polygonal plates.
Occurrence
The genus Atlaseocrinus occurs in the Issafen Formation (Morocco) and the Valdemiedes Formation (Spain); Cambrian Series 2, Stages 3 and 4.
Etymology
The prefix Atlas- is given both because of the location of the type locality in the Moroccan Anti-Atlas, and because of the presence of a single basal plate bearing the rest of the theca, evoking the support provided by the god Atlas in the ancient Greek myth. The suffix -eocrinus is used to indicate an eocrinoid genus.
Remarks
Articulated eocrinoid remains from the Issafen Formation have been repeatedly assigned to Alanisicystis (e.g., Nardin, Reference Nardin2007; Smith et al., Reference Smith, Zamora and Álvaro2013; Zamora et al., Reference Zamora, Darroch and Rahman2013; Zhao et al., Reference Zhao, Guo, Selden, Cong and Li2022; Rahman and Zamora, Reference Rahman and Zamora2024). Initially described as a subgenus of Gogia Walcott, Reference Walcott1917 (Ubaghs and Vizcaïno, Reference Ubaghs and Vizcaïno1990), Alanisicystis was later considered a separate genus (see, e.g., Zamora et al., Reference Zamora, Gozalo and Liñán2009, Reference Zamora, Darroch and Rahman2013). A major difference between Alanisicystis and the Issafen eocrinoid lies in their aboral region: Alanisicystis has a short, wide stem leading to a large polyplated anchor disc, whereas no fully preserved stem has yet been found for Atlaseocrinus. In the latter, the few remains of the stem suggest that it was extremely narrow. Moreover, the stem in Atlaseocrinus is attached to a single thecal basal plate, and the theca is organized in circlets. These two characters are unique to Atlaseocrinus among other gogiid echinoderms, thus justifying its assignment to a new genus. Finally, brachioles of Atlaseocrinus seem to be arranged in bundles of three and supported by spouts emerging from the oral face of the theca, unlike those observed in Alanisicystis. Although Atlaseocrinus mesonesensis was originally described by Clausen (Reference Clausen2004), its description was exclusively based on disarticulated basal plates. The new material of Atlaseocrinus mesonesensis described here is partially articulated. Therefore, Atlaseocrinus issafenensis, represented by more numerous and better-preserved specimens exhibiting most of the morphological traits of the genus, has been designated as the type species of the new genus.
Atlaseocrinus issafenensis new genus new species
Latex casts of Atlaseocrinus issafenensis: (1) MGM-7987X preserved in three dimensions; (2) holotype, BMNHUK EE 14823, showing brachioles and part of stem; (3) magnified view of brachiole of holotype, BMNHUK EE 14823; (4, 5) part and counterpart of mostly complete specimen, MGM-7986X; (6) BMNHUK EE 14829 showing partly complete theca and part of stem; (7) BMNHUK EE 14814 showing brachioles and part of stem; (8) magnified view of basal plate and partial stem of BMNHUK EE 14814. Arrows indicate basal plate. Br = brachiole; CP = cover plate; FG = food groove.
Figure 2. Long description
A multi-panel collection of eight photos showing fossil casts.
Panel 1: A three-dimensional bulbous theca with a textured surface. A line points to the top edge labeled F G for food groove. Scale bar is 2 millimeters.
Panel 2: The holotype specimen showing a conical theca with long, thin brachioles extending upward. A white arrow at the bottom points to the basal plate. Scale bar is 2 millimeters.
Panel 3: A high-magnification vertical view of a single brachiole. Labels C P for cover plate and B r for brachiole indicate the segmented structure. Scale bar is 1 millimeter.
Panel 4: A large specimen with a flattened theca composed of distinct plates. Multiple long, parallel brachioles extend vertically. Scale bar is 5 millimeters.
Panel 5: A counterpart view of the specimen in panel 4, focusing on the junction where the brachioles attach to the theca. Scale bar is 2 millimeters.
Panel 6: A partially complete theca showing the arrangement of individual ossicles. A white arrow points to the basal region. Scale bar is 2 millimeters.
Panel 7: A specimen showing the central body and a portion of the stem extending from the bottom. A white arrow points to the basal plate. Scale bar is 2 millimeters.
Panel 8: A magnified view of the basal plate and the proximal portion of the stem, showing the circular segments. Scale bar is 2 millimeters.
Interpretative drawings of Atlaseocrinus issafenensis: (1, 2) partial oral surface and brachiole insertion, respectively; (3) partial specimen BMNHUK EE 14813a showing development of epispires from lower half of theca; (4) transverse slice of brachiole; (5) simplified morphology of reconstructed specimen. green = axial region; light blue = perforate extraxial region; dark blue = imperforate extraxial region. Scale bars = 5 mm.
Figure 3. Long description
A multi-panel scientific illustration with five numbered sections.
* Panel 1 and 2: Detailed views of the oral surface and brachiole insertion. Green segments represent the axial region, while dark blue segments represent the imperforate extraxial region. The green brachioles extend upward from the blue base plates.
* Panel 3: A partial specimen labeled B M N H U K E E 14813a, showing a cluster of dark blue plates with small circular openings called epispires developing from the lower half of the theca.
* Panel 4: A transverse cross-section of a brachiole. It is a green circular structure with a central hole labeled Lumen. A gap at the top is labeled Putative ambulacral groove, flanked by two smaller green segments.
* Panel 5: A simplified reconstruction of the entire organism. At the top, long green brachioles are arranged in bundles of 3. These attach to a dark blue top section labeled Spouts. Below this is a bulbous body composed of hexagonal plates in alternating light blue (perforate extraxial region) and dark blue (imperforate extraxial region). The plates are organized into horizontal rows labeled Circlet 1 through Circlet 5. The body tapers down to a dark blue Basal plate which connects to a narrow, vertically textured Stem at the bottom.
Scale bars in panels 1, 2, and 3 represent 5 mm.
Paleoart reconstruction of Atlaseocrinus issafenensis by C. Dupichaud. Cover plates open at distal ends of each brachiole, revealing tube feet.
Reference Nardin2007 Alanisicystis; Nardin, figs. III-14, III-48.
Reference Álvaro, Zamora, Clausen, Vizcaïno and Smith2013 Alanisicystis; Álvaro et al., p. 76.
Reference Smith, Zamora and Álvaro2013 Alanisicystis; Smith et al., p. 3, figs. 3, 4d.
Reference Zamora, Darroch and Rahman2013 Alanisicystis n. sp.; Zamora et al., p. 162.
Reference Clausen, Álvaro and Zamora2014 Alanisicystis; Clausen et al., p. 83.
Reference Zhao, Guo, Selden, Cong and Li2022 Alanisicystis sp.; Zhao et al., p. 6, fig. 4.
Reference Rahman and Zamora2024 Alanisicystis sp.; Rahman and Zamora, p. 301.
Materials
BMNHUK EE 14813, EE 14814, EE 14818, EE 14821, EE 14825, EE 14834, and EE 18364; MGM-7986X–7991X; MHNN.P.04521–04530; MHNT.PAL.2005.0.186–191; UCBL-FSL 711749–711751, UCBL-FSL 711969–711976.
Diagnosis
A species of Atlaseocrinus with at least eight straight brachioles. Theca rotund, with five rings of plate crowned by elevated spouts supporting the brachioles. Thecal plates bearing fine granulations and simple, moderately deep, circular sutural pores on their shared side. Sutural pores rarely covered by a circular platelet. Single basal plate likely connected to a polyplated stem.
Occurrence
Atlaseocrinus issafenensis occurs in the Issafen Formation (Morocco), at the type locality of Issafen, ~50 km northwest of Tata, central Anti-Atlas, Morocco. The type stratigraphic level is the Issafen Formation, Antatlasia hollardi and Antatlasia guttapluviae trilobite zones, Tasousekhtian regional substage, Banian regional stage, Cambrian Series 2, Stage 3.
Description
Subspherical theca aborally enclosed by single basal plate and five weakly differentiated circlets; a highly constricted, oral region forming a very short spout. Thecal plates with very fine sinuous ridges and pustules radiating from plate centers. Simple circular epispires shared equally across plate sutures. Sutural pores prominent and raised. Lateral edges of thecal plates bearing from none to three halves of sutural pores. Epispires never occurring at plate corners. Epispire number and distribution apparently correlated with ontogeny (see Ontogeny and morphological implications subsection in the Discussion). Sutural pores open, except on two specimens (EE 14813a, EE 14829) preserving covering platelets inside several sutural pores (Figs. 2.6, 3.3). Adorally, single thecal basal plate bearing epispires. Longitudinal central lumen ~1 mm in diameter, piercing basal plate. Basal plate connected in some specimens to thin, polyplated stem. Stem plates small, polygonal, and seemingly arranged randomly. Neither fully preserved stem nor anchor structures have been found. Oral face situated on top of theca, bearing at least eight straight brachioles. Brachioles arrayed in clusters of three; four clusters visible. Each cluster supported by spout rising from theca. Each spout encased by two series of smooth, rectangular plates bent in a U shape and bearing no epispires. Transition between aboralmost thecal circlet and spout poorly preserved. Oral area unknown, but rays connected to base of each spout apparently converging with it. Each ray covered by series of small plates. Periproct and non-epispire pores unknown. Up to eight brachioles observed on single specimen. Brachioles straight, long, comprising two series of alternating floor plates and two series of cover plates, folding closed to cross portion of next cover plate. Each cover plate aligned with floor plate supporting it. Disconnected floor plates showing two grooves: one on top of plate, at base of insertion of cover plate; another in middle of floor plates, forming a circular lumen inside each brachiole, in suture between floor plates.
Etymology
After the type locality.
Remarks
Some of this material was already reported and assigned to Alanisicystis. Nardin (Reference Nardin2007) provided camera lucida drawings and latex casts of several specimens, and Smith et al. (Reference Smith, Zamora and Álvaro2013) figured another specimen and proposed a first tentative reconstruction. However, none of these works provided a complete description of the gogiid material from the Issafen area. The main difference from the Spanish material (see below) lies in the morphology of the brachioles: they are straight in Atlaseocrinus issafenensis and coiled in the Spanish specimens. Ornamentation also differs between species, with Atlaseocrinus issafenensis harboring a minute granulation, compared to the coarser pustulation in Atlaseocrinus mesonesensis.
Atlaseocrinus mesonesensis (Clausen, Reference Clausen2004) new combination
Latex casts of Atlaseocrinus mesonesensis: (1, 2) part and counterpart, respectively, of MPZ 2026/55 showing brachioles associated with partial theca; (3, 4) isolated basal plate of MPZ 2026/56; (5) isolated basal plate of MPZ 2026/57; (6, 7) part and counterpart, respectively, of isolated basal plate of MPZ 2026/58 showing several thecal plates associated with basal plate. Arrows indicate basal plate.
Figure 5. Long description
A multi-panel scientific photo containing seven numbered sections.
* Panel 1 (Top-Left): A large fossil cast showing a partial theca at the bottom with long, thin brachioles extending upward. A 2 mm scale bar is at the bottom right.
* Panel 2 (Top-Center): The counterpart to panel 1, showing the same thecal structure and brachioles from the opposite perspective. A 2 mm scale bar is at the bottom right.
* Panel 3 (Top-Right): A close-up of an isolated basal plate of M P Z 2026/56, showing a circular, segmented structure with a central hole. A 1 mm scale bar is at the bottom right.
* Panel 4 (Middle-Right): A side view of the same isolated basal plate from panel 3. A 1 mm scale bar is at the bottom right.
* Panel 5 (Below Panel 4): An isolated basal plate of M P Z 2026/57, appearing more rounded and smooth. A 1 mm scale bar is at the bottom right.
* Panel 6 (Bottom-Left): Part of an isolated basal plate of M P Z 2026/58 showing several clustered thecal plates. A white arrow at the bottom points to the primary basal plate. A 2 mm scale bar is at the bottom right.
* Panel 7 (Bottom-Right): The counterpart to panel 6, showing the same cluster of thecal plates. A white arrow at the bottom left points to the basal plate. A 2 mm scale bar is at the bottom right.
Reference Clausen2004 Rhopalocystis? mesonesensis Clausen, p. 373, fig. 3A, pl. 1, figs. 1–13.
Reference Clausen2004 Rhopalocystis? cf. mesonesensis; Clausen, p. 376, fig. 3B, pl. 1, figs. 14–16.
Reference Clausen2004 gen. et sp. indet.; Clausen, p. 376, fig. 3C, pl. 1, figs. 17–19.
Reference Zamora, Liñán, Domínguez Alonso, Gozalo and Gámez Vintaned2007 Rhopalocystis?; Zamora et al., p. 253, fig. 3.
Reference Zamora and Rahman2008 Rhopalocystis? mesonesensis; Zamora and Rahman, p. 302.
Reference Zamora and Rahman2008 Rhopalocystis? cf. mesonesensis; Zamora and Rahman, p. 302.
Reference Zamora and Rahman2008 gen. et sp. indet.; Zamora and Rahman, p. 302.
Reference Zamora, Gozalo and Liñán2009 Rhopalocystis? mesonesensis; Zamora et al., p. 253.
Reference Zamora2011 Rhopalocystis?; Zamora, p. 65.
Reference Zamora, Darroch and Rahman2013 isolated gogiid plates; Zamora et al., p. 162.
Reference Zamora, Álvaro, Arbizu, Colmenar, Esteve, Zamora and Rabano2015 gogiid eocrinoid closely related to Gogia; Zamora et al., p. 215.
Reference Zamora, Meléndez, Núñez and Tomás2016 Rhopalocystis? mesonesensis; Zamora, p. 11.
Reference Allaire, Lefebvre, Nardin, Martin, Vaucher and Escarguel2017 undescribed gogiid eocrinoid; Zamora in Allaire et al., p. 685.
Reference Jell and Sprinkle2021 Rhopalocystis mesonesensis; Jell and Sprinkle, p. 15.
Holotype
MPZ 17070 (Clausen, Reference Clausen2004), from the Valdemiedes Formation, near Mesones de Isuela village, Iberian Chains, northeastern Spain.
Emended diagnosis
A species of Atlaseocrinus with at least six left-hand coiled brachioles. Theca rotund and bearing numerous minute sutural pores. Thecal plates bearing relatively coarse granulations. Stem unknown.
Revised description
Seemingly rotund theca bearing at least six brachioles coiled in a left-handed arrangement. Disarticulated theca or isolated plates showing relatively coarse granulations and epispires located on both sides and corners. Single thecal basal plate bearing thin epispires adorally. Oral face, brachiole insertion, periproct, non-epispire pores, and stem unknown.
Remarks
Rhopalocystis? mesonesensis was originally described by Clausen (Reference Clausen2004), based on silicified, isolated sketelal elements that were identified as large basal thecal plates, similar to those present in the genus Rhopalocystis, previously known from Lower Ordovician strata of Morocco. Within eocrinoids (and blastozoans), the genus Rhopalocystis is one of the very few taxa possessing a single basal plate (Ubaghs, Reference Ubaghs1963; Chauvel, Reference Chauvel1971; Allaire et al., Reference Allaire, Lefebvre, Nardin, Martin, Vaucher and Escarguel2017). In recent years, additional material was collected from equivalent levels of the Valdemiedes Formation at a new locality close to Jarque de Moncayo village. The new specimens are preserved as dissolved skeletal elements, sometimes in partial anatomical connection. One specimen (MPZ 2026/58) shows the basal plate associated with other thecal plates, the morphology of which is closely similar to those found in Atlaseocrinus issafenensis or even Gogia (i.e., polygonal plates with sutural pores and a granular ornamentation; Figs. 5.6–7), hence the revised attribution to the genus Atlaseocrinus. One specimen, despite the lack of basal plate, comprises thecal plates associated with coiled brachioles, highlighting the difference with the straight brachioles of Atlaseocrinus issafenensis. Basal plates are also common both as dissolved residues from limestone or as disarticulated elements in shale beds. Such occurrences suggest that the disarticulated specimens described by Clausen (Reference Clausen2004) most likely belong to the same taxon.
Discussion
Ontogeny and morphological implications
The 43 studied specimens of Atlaseocrinus issafenensis exhibit large variation in the number and distribution of sutural pores on the theca. One specimen (BMNHUK EE 14813a; Fig. 3.3) bears only a few sutural pores around the two adoralmost circlets, where lie the largest thecal plates. Aborally, some plate junctions display pustules akin to closed sutural pores. Neither sutural pores nor pustules are present on the aboralmost circlet. This specimen is also one of the smallest (based on thecal dimensions). In some larger specimens (e.g., BMNHUK EE 14829; Fig. 2.6), the sutural pores are arranged around the theca, but with no more than one pore per plate side. The largest individuals (Figs. 2.4–5) display up to three sutural pores per plate side, with the number of pores being higher on the two adoralmost circlets. The presence of multiple sutural pores on a single plate edge seems to be more pronounced on the largest thecal plates, both in fully articulated specimens and in isolated plates. The variations observed in the distribution and density of epispires seem related to the thecal size, suggesting that this pattern could be controlled by ontogeny. It is difficult to establish a reliable correlation with size, because only a few specimens are preserved well enough to show the aboral and adoral ends of the theca, and even among these, taphonomic flattening biases length measurements.
Despite variation in thecal size and individual thecal plates, no new thecal plates seem to have been added during ontogeny (holoperipheric growth). Therefore, the thecal organization remains unchanged throughout growth.
Based on the above observations, the following postmetamorphic ontogenetic sequence can be described for the theca of Atlaseocrinus issafenensis: (1) an initial stage in which the theca lacks sutural pores, and consists of a single basal plate, five circlets, and spout plates; (2) an early stage, with the first sutural pores restricted to the two aboralmost circlets, and with a maximum of one pore per plate edge, pustules can also appear on upper circlets, upon which forthcoming epispires will occur; (3) a medium stage with sutural pores distributed all over the theca, from the basal plate to the lowermost spout elements, but with only one pore per plate edge; and (4) a late stage with additional sutural pores appearing alongside those already existing, starting from the two aboralmost circlets, and extending to the basal plate and upper circlets.
Comparing the ontogeny of Atlaseocrinus with that of Acanthocystites Barrande, Reference Barrande1887 (Nohejlová and Fatka, Reference Nohejlová and Fatka2016), it is remarkable that the theca of the former enlarges solely through the accretionary growth of individual plates, and not through the addition of new ones. Further material will be necessary to determine if the number of brachioles increases during ontogeny, as seen in Acanthocystites. Growth patterns for the brachioles and stem remain unknown. The former probably follows the Ocular Plate Rule (OPR), as is commonly the case for the axial region of echinoderm body wall (Mooi and David, Reference Mooi and David1998).
Atlaseocrinus is the oldest known example of an echinoderm with free feeding appendages, along with the co-occurring, undescribed lepidocystid from the Issafen Formation (Smith et al., Reference Smith, Zamora and Álvaro2013), and Alanisicystis andalusiae (see Ubaghs and Vizcaïno, Reference Ubaghs and Vizcaïno1990). Despite being one of the oldest known echinoderms, Atlaseocrinus exhibits a surprisingly organized thecal plate pattern, with one basal plate surmounted by five circlets of thecal plates. Furthermore, this thecal pattern appears to remain unchanged throughout ontogeny. Evolutionary trends in echinoderms usually tend toward a reduction in the number of plates and an increase in organisation, as in other blastozoans (Nardin, Reference Nardin2007), or more specifically, in solutans (Parsley, Reference Parsley1997) or stylophorans (Lefebvre, Reference Lefebvre2001). Compared to other gogiids, e.g., Alanisicystis or Gogia, which have unorganized thecal plating, the organised thecal pattern of Atlaseocrinus suggests a relatively derived morphology. This observation is further supported by the co-occurrence of an undescribed lepidocystid alongside Atlaseocrinus issafenensis (see Smith et al., Reference Smith, Zamora and Álvaro2013). Lepidocystids have been described as exhibiting a supposedly plesiomorphic body plan among blastozoan echinoderms, because they lack a theca (Lefebvre et al., Reference Lefebvre, Nardin, Fatka, Zamora, Rabano, Zamora and Rabano2015; Nardin et al., Reference Nardin, Lefebvre, Fatka, Nohejlová, Kašička, Šinágl and Szabad2017; Wang et al., Reference Wang, Nohejlová, Sun, Zeng, Lefebvre, Yang and Zhao2024). This suggests that diversification of body plans among the blastozoans could have occurred even before early Cambrian Stage 3.
Paleobiogeographic and paleoecologic implications
Atlaseocrinus issafenensis and Atlaseocrinus mesonesensis both occur in comparable settings, within two paleogeographically neighboring margins of West Gondwana (Álvaro et al., Reference Álvaro, Zamora, Clausen, Vizcaïno and Smith2013; Clausen et al., Reference Clausen, Álvaro and Zamora2014). Both the Valdemiedes and Issafen formations show meter-thick, shallowing-upward cycles, starting with offshore-dominant claystone substrata (remains from Issafen and Jarque del Moncayo localities), locally punctuated by shelly storm-induced carbonate interbeds (from Mesones de Isuela).
In these calm environments, Atlaseocrinus was very likely sessile, although the exact nature of its attachment to the substratum remains unknown. Its slender stem was probably fragile, which could explain why specimens with broken or missing stems have only been found so far. It is therefore possible that the appendage was more or less elongated and terminated by an anchoring structure, e.g., a disk, allowing attachment to hard substrata, e.g., trilobite sclerites or brachiopod valves (e.g., Lin, Reference Lin2009; Zamora et al., Reference Zamora, Sumrall, Zhu and Lefebvre2017). Atlaseocrinus appears to have adopted a ‘pelmatozoan’ lifestyle, comparable to that of stemmed crinoids and most gogiids.
Taphonomy
The samples from Issafen and Jarque de Mesones are preserved as natural molds encased in shale. In contrast, the isolated basals of Atlaseocrinus mesonensis described by Clausen (Reference Clausen2004) are preserved as three-dimensional calcite elements in packestone tempestites (Álvaro and Vennin, Reference Álvaro and Vennin1996). Atlaseocrinus issafenensis exhibits diverse states of preservation, ranging from isolated ossicles to nearly complete skeletal remains. Most specimens preserved in anatomical connection are laterally collapsed, rendering the oral face inaccessible. Brachioles are locally present, but rarely connected to the theca, and their tips are commonly disarticulated. In a few cases, part of the stem is preserved and connected to the basal plate. However, even in this case, the stem is invariably broken, and no distal end is preserved. Thus, the theca appears to represent the most taphonomically resistant part of the Atlaseocrinus skeleton, followed by the brachioles, and finally the stem.
A few rare specimens of Atlaseocrinus issafenensis (3 of 43) exhibit a theca preserved in three dimensions, mostly undeformed. In these specimens, the plate sutures are hardly discernible, giving the theca a smooth appearance, even though the epispires remain visible. This unusual preservation likely resulted from rapid burial in extremely fine clayey substrata that favored the three-dimensional preservation of the theca and leaving a smooth impression of the epidermis covering the skeleton. In one case (MHNN.P.045822), this type of preservation is also associated with the record of eight brachioles loosely disconnected from the theca.
Conclusions
Atlaseocrinus issafenensis is one of the oldest articulated echinoderm fossils known to date. Its theca displays unique traits among gogiid echinoderms (i.e., thecal plates organized in regular circlets and a single basal plate). These characteristics justify the erection of a new genus. The fossils can be interpreted as representing a constrained, ‘derived’ morphology relative to that of coeval and later Cambrian gogiids. This unique morphology is also present in gogiids from the Valdemiedes Formation, which led to the assignment of the material originally described there to Atlaseocrinus mesonesensis. The exceptionally well-preserved material from the Issafen Formation allows the proposal of an ontogenetic model for the growth of Atlaseocrinus issafenensis.
Acknowledgments
We would like to address our thanks to R. and V. Reboul and D. Vizcaïno for relating their rediscovery of Hupé’s locality in the Issafène Valley. T. Ewin (BMNHUK), Y. Laurent (MHNT), and S. Régnault (MHNN) are acknowledged for access to material in collections. I. Pérrez-Urresti (Zaragoza) and V. Perrier (Lyon) took the photographs. Financial support for fossil sampling in Morocco was provided by a French CNRS project led by S. Clausen (Lille). This work was supported by the Spanish Ministry of Science, Innovation and Universities (PID2021-125585NB-I00), cofinanced by the European Regional Development Fund, project ‘Aragosaurus: Recursos Geológicos y Paleoambientales’ (E18_17R) funded by the Government of Aragon. The authors are also grateful to the two reviewers, T.E. Guensburg and J.-P. Lin, and the associate editor, R. Mooi, for their constructive comments, which greatly improved the manuscript.
Competing interests
The authors declare none.
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