Ammonicrinus leunisseni, Bohatý, 2011

Bohatý, Jan, 2011, Revision of the flexible crinoid genus Ammonicrinus and a new hypothesis on its life mode, Acta Palaeontologica Polonica 56 (3), pp. 615-639 : 628-630

Ammonicrinus leunisseni sp. nov.

Figs. 2B, D , 7 , 8 , 12, 14C .

1926 Ammonicrinus wanneri Springer, 1926: 22–25 , pl. 6: 5−5b. 1938a Ammonicrinus wanneri Springer, 1926 ; Wolburg 1938a: pl. 18,

fig. 10.

1978 Ammonicrinus wanneri Springer, 1926 ; Moore 1978: T787, fig. 526: 5d, e.

pars 2003 Ammonicrinus wanneri Springer, 1926 ; Webster 2003: GSAwebpage, A. wanneri Springer 1926 , pl. 6: 5−5b, only.

Etymology: After Robert Leunissen (Nideggen−Wollersheim), for his tremendous help in sampling of material for the present publication.

Type material.— Holotype: USNM−S2115 ( Springer 1926: 105, pl. 6: 5−5b, only) ( Fig. 2B, D ); also see colour photos of the Springer−original on the webpage−search of the USNM Department of Paleobiology collection.

Type locality (assumed): “Prüm”, within the Prüm Syncline , in the surrounding of locality 3 ( Eifel , Rhenish Massif, Rhineland Palatinate, Germany) .

Type horizon (assumed): Uppermost part of the Freilingen Formation ( upper Eifelian ) or, more probable, superposed Ahbach Formation ( Eifelian / Givetian threshold, Middle Devonian) .

Diagnosis.—An Ammonicrinus , distinguished by fine tubercles on the surface of the cup ossicles; dististele either long and composed of numerous columnals (“encased runner−type”), short and composed of only few columnals, or nearly reduced (“settler−type”); dististele may develop radices, distal−most dististele connected with a substrate−controlled holdfast (attachment disc or variously formed holdfasts); LCEE of the mesistele interconnected with several columnals with broadened extensions and combined with smaller, “regular” columnals; connection between disti− and mesistele variously formed; axial canal pentalobate; shape of coiled stem oblate spheroidal; cup completely covered by the mesistele; mesistele, dististele and attachment spined.

Description.—The crown is relatively small and distinguished by the short arms with short and wide brachials and the small cup, which is characterised by irregularly arranged, fine tubercles on the surface of all ossicles ( Fig. 12F). The short and narrow proxistele causes distinct impressions of columnals on the cup and spine−tubercles are developed on the external flanks, obviously loosing spines throughout the ontogeny. These tubercles are well developed on the surface of the lateral and external flanks of the mesistele and have movable, articulated spines ( Figs. 7 , 8 , 12I, J 1 , L, 14C ) that allowed coiling over the spined columnals. The LCEE of the mesistele are interconnected with several columnals with broadened extensions that could interlock in a coiled position and are combined with smaller, “regular” columnals ( Fig. 2D ). Columnals of the mesistele one barrel−shaped columnal showing short LCEE (arrow on the right); the specimen shows numerous preserved spines on the mesistele; one radial plate is visible (arrow on the left); aboral view of proxistele and base of cup (J 2). K. Specimen GIK−2132. Isolated holdfast (K 1) attached to a fenestrate bryozoan (arrow); view of the exterior columnal flanks of uncoiled mesistele (K 2) on matrix (“runner−type”). L. Coiled specimen (GIK−2103), attached on a brachiopod brachial valve ( Schizophoria sp. ) (compare to reconstruction, figured in Fig. 8 ); the specimen strongly reduced the dististele and settled with an attachment disc on the brachiopod (“settler−type”). Scale bars 10 mm.

doi:10.4202/app.2010.0020

are less curved external flanks and medium long extensions. The connection between the dististele and the mesistele is variously formed, with floating transitions between those individuals with none or one to several columnals with LCEE on the proximal−most, barrel−like dististele (compare Fig. 12A and 12C) and the following mesistele, which is solely distinguished by these extensions. Dististele is either long and composed of numerous barrel−like columnals, developing the “runner−type” ( Figs. 7 , 14C ), or short and composed of only few or nearly reduced columnals, characterizing the “settler−type” ( Figs. 8 , 12L). Several examples with developed radices on the columnals of the dististele are known ( Figs. 7B , 12I). The distal−most dististele is connected with an attachment disc (rare) or, typically, with a variously formed holdfast composed of radices ( Figs. 7B , 12K 1 ). Columnal axial canal pentalobate. The shape of the coiled proximal “stem globe” (proxistele and proximal to middle or nearly complete mesistele), that completely covers the crown, is oblate spheroidal ( Figs. 2D , 12A, B).

For dimensions of the studied material, see indication of size within the figure descriptions.

Differential diagnosis.— Ammonicrinus leunisseni sp. nov. differs from A. wanneri by the wider columnals of the mesistele, which have shorter LCEE in comparison with A. wanneri . The LCEE of the spined A. leunisseni sp. nov. are interconnected with several columnals with broadened extensions and combined with smaller, “regular” columnals. The unspined A. wanneri developed very long and fine extensions that protrude nearly orthogonally from both sides of the narrow columnals, forming a narrow discoidal coiled proximal column in closed position, which is oblate spheroidal in A. leunisseni sp. nov.

Stratigraphic and geographic range.—Freilingen Formation (Eifelian) to Loogh Formation (lowermost Lower Givetian) of the Prüm and Hillesheim and (?)Gerolstein synclines (Eifel, Rhenish Massif, Rhineland Palatinate, Germany). The species has its maximum distribution within the Olifant and Zerberus members of the Müllert Subformation (Ahbach Formation, lowermost Lower Givetian).

Figures

Fig. 2. The first figures of Ammonicrinus from Springer (1926) and Krause (1927). A. Ammonicrinus wanneri Springer, 1926 (from Springer 1926: pl. 6: 4a, b). A1, view of the extetrnal flanks of the coiled mesistele; A2, coiled mesistele in lateral view. B. Ammonicrinus leunisseni sp. nov. (= “A. wanneri” in Springer 1926: pl. 6: 5, 5b). B1, view of the extetrnal flanks of the coiled mesistele; B2, coiled mesistele in lateral view. C. Photograph of the holotype of Ammonicrinus wanneri Springer, 1926, USNM−S2115; lateral view of coiled mesistele; connection between mesi− and dististele, dististele and attachment missing (see fracture surface at distal mesistele). D. Photograph of USNM−S2115, the Springer (1926) original of “Ammonicrinus wanneri” (= A. leunisseni sp. nov. herein); lateral view of coiled mesistele; connection between mesi− and dististele, dististele and attachment missing (see fracture surface at distal mesistele). E. Ammonicrinus doliiformis Wolburg, 1938a (for 1937) (= “A. wanneri” in Krause 1927: pl. 8: 4, 2). E1, view of the extetrnal flanks of the coiled mesistele; E2, coiled mesistele in lateral view. A–D from the Middle Devonian, Eifel Limestone; Prüm, Eifel, Germany (Springer 1926: 25); E from the Middle Devonian of Sötenich, Eifel (Krause 1927: 456). A, B, E not to scale; C, D scale bars 10 mm.

Fig. 7. Ammonicrinus leunisseni sp. nov. A. Reconstruction of a “encased runner−type” of A. leunisseni sp. nov. attached to a tabulate coral (model); the spined specimen dwelled enrolled on the muddy seafloor. B. The original (GIK−2102) from the Eifel (locality 6, Appendix 1), Germany, Lower Givetian (Middle Devonian); showing slightly compressed proximal mesistele. A not to scale, B scale bar 10 mm.

Fig. 8. Reconstruction of a spined “settler−type” of Ammonicrinus leunisseni sp. nov., attached to a brachiopod brachial valve (Schizophoria sp.); the original (GIK−2103) from locality 6 is figured in Fig. 12L. Not to scale.

Fig. 14. A. Schematic reconstruction of Ammonicrinus (strongly modified after Piotrowski 1977: 209, fig. 3) of a feeding Ammonicrinus within low−intensity current water. Alternating water pressure was possibly generated in the interior of the enrolled proximal stem by slow, bellow−like partial opening and closing (red arrows) of the base of the central mass; due to the synarthrial (bifacial) articulation of the ammonicrinid mesistele that developed two huge ligamentary facets (orange), separated by the fulcrum, bellow−like partial opening could possibly enabled by stiffening of the outer ligaments (see orange bars in A1); closing could be controlled by stiffening of the inner ligaments (see orange bars in A2). A1, suction during opening may result from low−pressure (P−) and create an ingesting water flow (blue arrow); A, ejection during closure (red arrow) resulted from overpressure (P+); to minimise faecal recycling, 2 the water ejection may have occurred laterally (blue arrows), feasibly at both lateral centres, which have “openings”. B. Lobe−like enlarged appendages (framed in red) could possibly support the lateral water faecal−ejection (modified from Ubaghs 1952: pl. 1: 4). C. Reconstruction of a feeding “encased runner−type” of A. leunisseni sp. nov., attached to a tabulate coral (model); the spined specimen dwelled enrolled on the muddy seafloor; alternating water pressure was obviously generated in the interior of the enrolled proximal stem globe by non−muscular, probably MCT−controlled, slow, bellow−like partial opening and closing of the oblate sphere at its bottom (dashed arrow); suction during opening created an ingesting water flow (see arrow on the left), which was funnelled in a “canal”, formed by the unspined interior of the columnals of the mesistele, whose U−shaped LCEE additionally formed a protection against immersive sediment; ejection during closure resulted from overpressure; to minimise faecal recycling, the water ejection occurred supposably laterally, feasibly at both lateral centres, which accordingly show “openings” (see arrows on the right). Not to scale.

Fig. 1. A. Reconstruction of the Ammonicrinus life time position (modified after Piotrowski 1977:208, fig. 2). B. Ammonicrinus plate diagram (not to scale).