Thursday, 26 March 2020

Athenacrinus broweri: A new species of Crinoid from the Early Ordovician Fillmore Formation of Millard County, Utah.

Crinoids are stalked Echinoderms, related to Starfish and Sea Urchins, but with a very different morphology. They are attached to by either by a stalk with a holdfast cemented to some hard substrate, of by a flexible stalk able to wrap itself around some object, typically a Coral or a Sponge. Above this grows an animal with a cup-like central body known as the calyx, surrounded by five branching arms, covered in long, flexible, tube feet which catch plankton and marine detritus and pass them to the mouth, located in the middle of the top of the calyx. The origins of crinoids remain debated. The earliest known Crinoids come from the Tremadocian (Earliest Ordovician 485.4–477.7 million years ago), but the fossils are few and imperfect.

In a paper published in the Journal of Paleontology on 9 December 2020, Thomas Guensburg of the Field Museum, James Sprinkle of the Department of Geological Sciences at the University of Texas at Austin, Rich Mooi of the Department of Invertebrate Zoology at the California Academy of Sciences, Bertrand Lefebvre of the Université Claude Bernard, Bruno David of the Muséum National d’Histoire Naturelle, and Biogéosciences at the Université de Bourgogne Franche-Comté, Michel Roux of the Département Systématique et Évolution at the Muséum National d’Histoire Naturelle, and Kraig Derstler of the Department of Earth and Environmental Studies at the University of New Orleans, describe a new species of Crinoid from the Early Ordovician Fillmore Formation of Millard County, Utah.

The new species is named Athenacrinus broweri, where 'Athenacrinus' is a compounding of Athena, patron goddess of classical Athens, whose often lithe and rangy representation is reminiscent of this elegant taxon, and -crinus, the standard suffix applied to crinoid taxa, and 'broweri' honours the late James Brower, student of fossil Crinoids, who revealed much about early crinoids in his long and productive career.

Athenacrinus broweri, specimen PE 52742, small calyx with short anal sac and proximal stalk: (1) entire, immersed; (2)–(5) AB, C, A, and E views, respectively, all immersed except coated E view, E radial proportionately shorter than other radials; (6), (7) AB and AE interrays and tegmen, several small cover plates extending over peristome area, small rounded plates filling tegmen interrays, dry uncoated images. Guensberg et al. (2019).

The species is described from six specimens collected from the Slope-forming shaly siltstone member, of the Fillmore Formation, central Black Hills of Millard County, Utah. Approximately the lower three-quarters of the Slope-forming shaly siltstone member, are considered upper Tremadocian in age, whereas the upper remaining quarter above ranges into the lower Floian. This part of the section is less resistant to weathering and therefore less well exposed. In all but one case, finds were projected laterally to these marked sections to obtain close stratigraphic control. The stratigraphic position of the final specimen was estimated based on lithofacies associations and nearby exposure. 

Athenacrinus broweri is estimated to have had maximum complete length in excess of 25 cm; and an estimated maximum crown height at least 12 cm. The cup of these Crinoids was steep-sided, and had a zigzag juncture with stalk below; it was oblong to pentalobate in cross section. Expansion of the cup began on the upper part of the stalk and extended into the arms above. It has two-tiered cover plating, with cover plates stacked in rows, as distinct non- or little-intercalated layers. Presumably these formed a continuous but slightly flexible sheet that was hinged at the contact with floor plates. This trait is not unique to Crinoids, occurring elsewhere in certain Edrioasteroids, such as Paredriophus, and Pseudedriophus, more derived forms that retain axial anatomy similar to plesiomorphic nonblastozoan basal Pentaradiates, such as Camptostroma, Stromatocystites, and Kailidiscus. No standardized terminology exists for the two tiers of arm cover plating, and no terminology links cover plates of Edrioasteroid-like basal Pentaradiate Echinoderms with those of early Crinoids. ‘Primary’ was used for lateral, and ‘secondary’ for medial cover plates in previous work on the Cambrian basal pentaradiate Stromatocystites. ‘Inner’ and ‘outer’ were used for two-tiered Late Ordovician crinoids. The term ‘lappets’ was applied to Camerate Crinoids with a single biserial tier and, in some cases, to extant Crinoid cover plates. ‘Lateral’ and ‘medial,’ the term used by Guensberg et al., apply to early Crinoids and to cover plates of Edrioasteroids and Edriosteroid-like early taxa. Lateral cover plates can be considered homologous to ‘primary’ elements, and medial cover plates in part homologous to any other cover plate elements that occur more medially.

Athenacrinus broweri: (1), (2) Specimen PE 52750, large cup showing rays A, B, and E with proximal stalk: (1) entire specimen, coated; (2) cup and stalk, proximal two meres not reaching laterally adjacent meres, four primibrachials above short E radial, immersed; (3) specimen USNM 165237, flattened cup with short stalk segment, CD view, irregular posterior plating, small gaps between adjacent primibrachials accentuated by air abrasion, immersed; (4)–(8) specimen PE 52751, disarticulated cup with articulated arm, arm segments, and stalk: (4) entire specimen, coated; (5) disarticulated cup, cover plating showing tegmen to arm transition, immersed; (6) similar view with cup plates and proximal stalk pentamere ring in cross section (left) showing large round lumen and fine crenulae; (7) proximal stalk, slightly weathered showing marginal crenulae, coated; (8) two views of separate medial-distal stalk segment, above with short projections centered on pentameres, accessory ossicular covering, below weathered on right showing pentameres beneath ossicular layer, coated. Guensberg et al. (2019).

Even the small sample size is sufficient to indicate considerable intraspecific variation in Athenacrinus broweri. Variable arm and cup construction have been reported in other Disparid Crinoids including Cincinnaticrinus, Anomalocrinus, and Ristnacrinus. The two-tiered cover plate pattern of Athenacrinus broweri continues onto the tegmen where it transitions into a single-tiered double biseries. This thecal cover plate pattern is common among Pentaradiate Echinoderms (i.e., certain Blastozoans, Crinoids, Edrioasteroids). The question as to whether a two-tiered earliest Crinoid arm cover plate pattern is a class-level apomorphy is problematic given that no immediate Crinoid ancestor is known. The distal stalk of specimen PE 52751 is covered with secondary ossicles. This expression is documented elsewhere only in Musivocrinus from the Permian of Timor and its phylogenetic significance is unknown.

Three characters provide collective evidence that indicates that Athenacrinus is an early DIsparid by traditional practice: (1) An anibrachial plate and subsequent uniserial anal column arising from the C ray, (2) a monocyclic cup extending only a short distance above radials, and (3) disproportionate ray plating in C and E rays. This third feature characterizes a diversity of disparids including Anomalocrinids, Cincinnaticrinids, and Othneiocrinids. Othneiocrinus is closest to Athenacrinus in terms of age, geographic occurrence, and morphology. Similarities include the wide, expanding cup, wide stalk with proximal pentameres, and the fact that the third C ray plate, primibrachial two, is the anibrachial. This last trait is an uncommon expression, known elsewhere only in the Late Ordovician Peniculocrinus, an Eustenocrinid cup, the plating of which is otherwise unlike Athenacrinus in lacking interradials and with radials (first ray plates) equal in width to the first primibrachials above. Othneiocrinus expresses much shorter primibrachials, larger first cup-like primibrachials, and small armlets, in contrast to the more robust branches of Athenacrinus.

Athenacrinus broweri: (1)–(4) Specimen PE 52751: (1) adjacent ramules or armlets, midarm, medial cover plates in regular alternating double biseries, immersed; (2), (4) well-preserved arm segment: (2) brachials just visible at bottom, then floor plates (small rectangular elements), large rectangular lateral cover plates, and alternating double or triple series along perradial suture above, coated; (4) specimen turned to accentuate cover plates, dry, raking light; (3) arm trunk with two branchings, irregular plating below; (5) specimem PE 52753, well-preserved collapsed proximal cup, BC view, anibrachial apparently primibrachial one, attached proximal stalk segment and detached medial stalk with small, about 1 cm missing interval, pentameres with spinose processes, coated; (6)–(8) specimen PE 52752, associated arm and stalk segments: (6) medial stalk and adjacent arm segment, meres with attachment pits, coated; (7) distal stalk transitioning below to plate mosaic, coated; (8) much of surface with specimen segments showing articulated long arm segment with two branchings below, then long section without branchings, dry uncoated. Guensberg et al. (2019).

Crinoid arms are not derived from Blastozoan feeding appendages. Crinoid and Blastozoan feeding appendages are constructed in fundamentally distinct ways, with significantly different topologic relationships among only partly homologous body walls and body cavity systems. This interpretation is further supported by character evidence provided by Athenacrinus broweri. Briefly, Blastozoan and Crinoid arms express ambulacral floor and cover plates; these are found in other major groups of Echinoderms as well. Floor plates in Blastozoans can form exposed primary appendage and brachiole structures. However, in Crinoids, floor plates are thin, largely internal structures that the most crownward Crinoids do not express at all as stereom; these are instead represented by the tissue shelf that lies adoral to the extensions of the right and left somatocoels, but still supports the water vascular system. Crinoid appendages extend from the ambulacral rays on the tegmen, and are supported by brachials derived from the extraxial thecal wall. Crinoid appendages always house extensions of the left and right somatocoels entering the arm at the thecal shoulders, emanating from these coeloms from within the theca. Blastozoan appendages lack these thecal openings and therefore also lack left and right somatocoel components, although presumably they were present as coeloms within the theca.

Athenacrinus is the oldest Crinoid expressing disproportionate cup-ray plating, and evidence from this taxon provides a hypothesis for the origin of this morphology. This unusual trait of many early Disparids is a tendency to express disparate uneven cup plating in which some rays possess one or two offset plates forming interlocking patterns with adjacent rays. A compound radial (‘biradial’) terminology for those rays was created in the 1940s, with two cup plates; proximal elements were termed ‘inferradials,’ and distal elements ‘superradials.’ This terminology has become entrenched in family-level taxonomy in Disparids and remains in common usage today, yet there is no evidence that this interpretation is correct. Initial ray plates in Athenacrinus are all large, similar to those of Iocrinid radials. The second, more distal, C ray plate is smaller than the radial but still cup-like, carrying both an arm and anal column above. If multiple fixed-ray plating constitutes an emergent Crinoid apomorphy as is now supported by the Tremadocian Crinoid record, then this plate should be termed the first fixed primibrachial, with its cup-like appearance inherited from deeper in crinoid phylogeny. The same reasoning can be used for the E ray of Athenacrinus.

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