Protaxocrinus salteri? ( Angelin, 1878 )

Ausich, William I., Wilson, Mark A. & Vinn, Olev, 2012, Crinoids from the Silurian of Western Estonia, Acta Palaeontologica Polonica 57 (3), pp. 613-631 : 625-626

Protaxocrinus salteri? ( Angelin, 1878)

Figs. 4C , 5E .

Material.—A single specimen (TUG 1395−4) from the Ninase Member, Jaani Formation (Wenlock) at Suuriku Cliff, Saaremaa Island, Estonia is questionably assigned to this taxon.

Description.—The crown is large and expanding ( Fig. 5E 1 ). The aboral cup has a low cone shape, the width to height ratio is approximately 1.6 ( Fig. 5E 2), and external plate sculpturing is not preserved. Infrabasal plates are visible in side view, and the infrabasal circlet is approximately 13% of aboral cup height. The CD basal is the only basal plate fully known; and it is hexagonal, wider than high, and smaller than radial plates. Parts of the C and D radial plates are visible, and they are probably hexagonal and are wider than high. Radial facets are plenary and straight. Three anal plates are in aboral cup ( Fig. 4C ). The radianal is immediately below the full width of the C radial and smaller than the C radial plate. The anal X is sutured below to CD basal and radianal and sutured laterally to the C and D radials. This fills the entire gap between the C and D radials. One additional anal plate is immediately above the anal X and is sutured laterally with the C radial.

Arms are completely separate above the radial plates and branch. The third primibrachial is axillary on the D−ray arm, and the second primibrachial is axillary on the C−ray arm. The first two or first two and one−half secundibrachials in a ray are sutured medially to the adjacent secundibrachial. On the D−ray arm, the third secundibrachial is axillary and the eighth tertibrachial is axillary. Arm divisions are slightly heterotomous. Brachials are uniserial rectangular, wider than high, with straight sutures. Pinnules are absent.

The column is xenomorphic. The proxistele narrows distally, and proxistele columnals range from approximately 6 to 15 times wider than high. Mesistele columnals are approximately 5 times wider than high.

Discussion.—TUG 1395−4 is an unusual crinoid that is difficult to place systematically. With the radianal occupying the full width beneath the C radial plate, this specimen is most similar to Protaxocrinus , and it probably represents a new species with a distinctive CD basal plate. However, the holotype and only known specimen is severely beach−worn, so that plate sculpturing is not preserved and other aspects of its morphology must be inferred with some question.

The exposed portion of the specimen clearly has a column with a proxistele, a dicyclic aboral cup, and a posterior interray, which are all very similar to Silurian Protaxocrinus . Because this specimen is so deeply weathered, it is impossible to verify whether the unusual anal plate arrangement is portrayed as it would be on a well−preserved specimen. At least one more, well−preserved specimen is needed to confidently describe this taxon.

The unique aspect of this specimen, which needs verification, is that the distal suture of the CD basal plate does not have an indentation to hold the anal X adjacent to the C radial plate and radianal. Instead, the anal X occupies the full width between the C and D radials ( Fig. 4C ). If this Estonian specimen is a new species, it would be characterized by this unusual plating in the posterior interray.

Protaxocrinus occurs on Gotland. Franzén−Bengtson (1983) listed four species: P. distensis ( Angelin, 1878) , P. interbrachiatus ( Angelin, 1878) , P. ovalis ( Angelin, 1878) ,

http://dx.doi.org/10.4202/app.2010.0094

and P. salteri ( Angelin, 1878) . Protaxocrinus distensis was unlocalized. Protaxocrinus salteri occurs in the Högklint and Slite formations, both Wenlock; P. interbrachiatus is only from the Wenlockian Slite Formation; and P. ovalis is from the Slite and Eke formations, Wenlock and Ludlow, respectively.

TUG 1395−4 is questionably placed in P. salteri because one specimen illustrated by Angelin (1878: pl. 22: 1) appears to have a CD interray quite similar to the Estonian specimen. However, Angelin’s (1878) illustration is not definitive, especially because a certain artistic license was commonly employed on these illustrations.

Figures

Fig. 4. Plate diagrams of Estonian crinoids. A. Saaremaacrinus estoniensis gen. et sp. nov., TUG 1395−3, middle Äigu Beds, Kaugatuma Formation (Pridoli), Kaugatuma Cliff. Normal interray plating (A1), C−ray plating (A2), CD−interray plating(A3), and lateral outline of theca (A4). B. Calceocrinus baliticensis sp. nov, GIT 405−212, Juuru Stage (Llandovery), Heltermaa. D−ray view of crown. C. Protaxocrinus salteri? (Angelin, 1878), TUG 1395−4, Ninase Member, Jaani Formation (Wenlock), Suuriku Cliff. Lateral outline. Plate designations: black, radial plate; ruled, inferradial plate; cross−ruled, radianal plate; stippled, interradial plates or anal plates; P, primanal; X, anal X plate; grey, openings into theca. Scale bars 1 mm.

Fig. 5. Camerate, disparid, and flexible crinoids from the Silurian of Estonia. A. Myelodactylus? sp., TUG 1395−7, Ohesaare Formation (Pridoli), Ohesaare Cliff. Articular facet of columnal. B. Saaremaacrinus estoniensis gen. et sp. nov., holotype, TUG 1395−3, middle Äigu Beds, Kaugatuma Formation (Pridoli), Kaugatuma Cliff. B1, lateral view of theca, interpretation of plates in Fig. 4; B2, tegmen view of theca. C, D. Cicerocrinus osiliensis (Jaekel, 1900), Ohesaare Formation (Pridoli), Ohesaare Cliff. C. TUG 1376−1. C−ray lateral view of calyx. D. GIT 405−242. C−ray lateral view of calyx. E. Protaxocrinus salteri? (Angelin, 1878), TUG 1395−4, Ninase Member, Jaani Formation (Wenlock), Suuriku Cliff. E1, CD interray view of crown and proximal column; E2, CD interray view of calyx, proximal arms and column, note badly beach−worn preservation. F. Caleocrinus balticensis sp. nov., GIT 405−12, Juuru Stage (Llandovery), Heltermaa. D−Ray lateral view of crown. G, H. Desmidocrinus laevigatus sp. nov., middle Äigu Beds, Kaugatuma Formation (Pridoli), Kaugatuma Cliff. G. TUG 1395−1, holotype. G1, oblique D−ray lateral view of calyx; G2, CD−interray lateral view of calyx; G3, D−ray lateral view of incomplete crown. H. TUG 1395−2, paratype. D−ray lateral view of calyx.

Fig. 1. Silurian stratigraphy of Estonia. The majority of the new fossil material is from the West−Estonian islands, represented in the middle of the chart of lithostratigraphic units (based on Hints 2008). D1, Lower Devonian; O, Ordovician; O3, Upper Ordovician.