Protaster, Glass, 2006

Glass, Alexander, 2006, Pyritized tube feet in a protasterid ophiuroid from the Upper Ordovician of Kentucky, U. S. A., Acta Palaeontologica Polonica 51 (1), pp. 171-184 : 177-179

Genus Protasterina Ulrich, 1878

not 1849 Protaster gen. nov.; Forbes 1849: 1–2, pl. 4: 1–4.

not 1858 Taeniaster gen. nov.; Billings 1858: 81, pl. 10: 3a–d.

not 1872 Alepidaster gen. nov.; Meek 1872: 275.

1878 Protasterina gen. nov.; Ulrich 1878: 95–96, pl. 9: 9, 9a–c.

Type species: Protasterina fimbriata Ulrich, 1878 ; by monotypy; Edenian (Upper Ordovician), Cincinnati region .

Age and geographic distribution: Kope Formation, Edenian, Cincinnatian, Upper Ordovician , near Covington, Kentucky and Cincinnati, Ohio, central United States; also possibly Maysvillian aged rocks (for details see Appendix 1) .

Note. —Terminology for the ventral outlines of the ambulacrals follows Glass and Blake (2004).

Revised diagnosis.— Protasterid ophiuroid with wide dorsal interambulacral muscle gaps. The dorsal surface of the ambulacral is trapezoidal with raised proximal and distal ridges. The dorsal surface of the disk is covered by ossicles of uncertain shape although some of these are distinctly star−shaped in outline. The ventral surface of the disk is covered by ossicles of uncertain shape. Both the dorsal and ventral surfaces of these ossicles are overlain by small irregularly distributed granules. Thin, pointed, articulated spines are present on both sides of the disk. A small, round, flattened madreporite with a wavy channel running around its circumference is situated near the second ambulacral. Dorsal arm covering not preserved. Arms taper evenly but distal arm tips are not preserved. The ventral surface of the ambulacrals is boot−shaped with flattened distal and proximal edges. The abradial edge of the toe is straight. The foot is nearly twice as wide as the distal fitting. In the first two ambulacrals the foot is as wide as the leg is long, and this ratio is retained well into the proximal portions of the arms, although foot width decreases distally. The width of the central leg is narrower than the width of the distal fitting giving the leg of the boot a distinctive hour−glass appearance, especially distally where the width of the foot decreases. The median suture is strongly sinuous, almost sharply angular. Ventrally, laterals are straight to slightly crescent−shaped. The proximal, free laterals bear at least five slightly petaloid groove spines and at least four evenly tapering vertical spines.

Comparison.—Because protasterid ophiuroids are in need of revision, modern phylogenetic treatment and consensus on generic diagnoses are unavailable (but see Hammann and Schmincke 1986). Ventral outline of ambulacral plates traditionally has been found to be useful in differentiating among Paleozoic ophiuroids (e.g., Ulrich 1887; Gregory 1897; Spencer 1934), although there has been no effort to develop comprehensive comparative analysis. As a first step toward such analysis, Glass and Blake (2004) provided six different ambulacral ventral outlines suggested to be representative of nine protasterid ophiuroids. Ambulacral outline varies among and within specimens; the reconstructions are based primarily on those ambulacrals immediately distal to the disk of specimens of average size. The six shapes of Glass and Blake (2004) are slightly modified here and reproduced in sets of four to aid orientation ( Fig. 4A 3 –F ). The shape of Protaster ambulacrals is re−assigned. Based on drawings by Spencer (1934: text−fig. 297) and notes by Hammann and Schmincke (1986), Glass and Blake (2004) suggested that the ambulacrals of Protaster are similar to those of Bundenbachia . Study of the type material of Protaster sedgwickii (BMNH E6374b) could detect no indication of the prominently hooked toe and concave proximal fittings depicted by Spencer (1934: text−fig. 297). Protaster is now aligned with Bohemura although phylogenetic implications of all suggested shape groupings must be left to future work. Glass and Blake (2004) developed a terminology for different parts of the ambulacral; this terminology is retained unaltered. Comparison will focus on characters found here to differentiate Protasterina Ulrich, 1878 from other protasterids.

The wide dorsal interambulacral muscle gaps in Protasterina are shared by only five other protasterid genera. These are Palaeophiomyxa Stürtz, 1899 , Strataster Kesling and Le Vasseur, 1971 , Drepanaster Whidborne, 1896 , and perhaps Eugasterella Schuchert, 1914 . The dorsal ambulacrals of Chattaster Hahn and Brauckmann, 1981 have been described as trapezoidal but their rod−like shape with an almost undifferentiated ventral toe are unlike those of any other protasterid genus. Haude (1982) and Haude and Thomas (1994) discussed the atypical nature of the morphology of Chattaster .

The ventral shape of the ambulacrals of Protasterina , with their flattened distal and proximal articulations, the distally hour glass shaped leg, and the great length and width of the toe proximally is different from those of all other protasterid ophiuroids with large dorsal muscle gaps.

Eugasterella logani ( Hall, 1867) , the type species of Eugasterella , is known only from the ventral surface. However, additional material assigned to this species by Harper (1985) exhibits wide interambulacral muscle gaps. Eugasterella and Strataster are very similar, especially in the ventral shape of their ambulacrals. Indeed, Harper (1985) saw Strataster as a junior synonym of Eugasterella , but Hotchkiss (1993) continued to recognize Strataster . In both genera, the foot is not significantly wider than the width of the distal fitting and the central width of the ambulacral is only slightly less than the total width of the leg. The abradial edge of the ambulacral is nearly straight and distal and proximal fittings are usually slightly to distinctly concave. The overall ventral shape of the ambulacrals of both Strataster and Eugasterella ( Fig. 4A, B ) easily distinguish them from Protasterina . In addition, Strataster has a well−developed carinal row of spines on the arms, and Eugasterella lacks spines on the disk.

Palaeophiomyxa is known from a single specimen from the Lower Devonian Hunsrück Slate of Germany ( Glass and Blake 2004). Palaeophioymxa differs from Protasterina in the lack of spines on the disk, the ventral shape of its ambulacrals ( Fig. 4D ), which have distinctly concave distal and proximal fitting, a wider, much more rounded toe, and possibly a greater number of groove spines.

Little is known about Drepanaster . The specimens figured by Whidborne (1898: pl. 29: 1, 2) show little detail and are highly stylized. Hammann and Schmincke (1986) pointed out that Spencer (1934, 1940) discussed the genus but based much of his interpretations on Drepanaster grayae Spencer, 1943 rather than the type species Drepanaster scabrosus (Whidborne, 1896) . His reconstruction of the dorsal surface of D. scabrosus ( Spencer 1940: text−fig. 326C) is based on an additional specimen that was not part of the original type series. This specimen is figured with large interambulacral muscle gaps. Much of the discussion of Drepanaster in Spencer (1940) is based on the species Drepanaster grayae Spencer, 1934 . Until the type material of Drepanaster is redescribed, full comparison to other protasterids must be deferred.

Hammann and Schmincke (1986) placed Mastigophiura Lehmann, 1957 alongside protasterids with wide muscle gaps. Re−examination of all of the Mastigophiura type material listed in Lehmann (1957: 51) and an additional specimen figured by Opitz (1931: fig. 57, Forschungsinstitut und Naturmuseum Senckenberg, SNF 5a) demonstrated that the ambulacrals are better described as quadrate (Glass in press). Because the ambulacrals are wider than long the muscle gaps can appear relatively large, especially where ossicles have shifted during preservation. Mastigophiura further differs from Protasterina in its highly variable groove and vertical spines and the presence of a well−developed carinal row of spines on the arms.

Taeniaster Billings, 1858 , Bundenbachia Stürtz, 1886 (see Glass and Blake 2004), Protaster Forbes, 1849 , Bohemura Jaekel, 1903 , and Klarasterina Petr, 1989 , all have narrow dorsal interambulacral muscle gaps.

The protasterid genera Astutuaster Boczarowski, 2001 and Weigeltura Boczarowski, 2001 are only known from suites of disarticulated ossicles. Unfortunately, it is not clear how Boczarowski (2001) grouped different disarticulated ossicle types into genera and species. It is possible that his suites of ossicles represent a mixture of different taxa. The holotypes of the type species of both Astutuaster and Weigeltura consist of a single ambulacral ( Boczarowski 2001: figs. 5A and 7A respectively) both of which are quadrate in dorsal outline and can therefore be distinguished from Protasterina .

Too little is known about the protasterid genera Palaeophiura Stürtz, 1890 (Lower Devonian, Hunsrück Slate, Germany), and Inyoaster Phleger, 1936 (Middle Ordovician, Inyo Mountains, California) for effective comparison. However, Inyoaster is likely a member of the Palaeuridae instead of the Protasteridae (Frederick H.C. Hotchkiss, personal communication, 2005). Glass (in press) argued that Palaeophiura is a juvenile specimen of Bundenbachia . The figured specimens of Inyoaster bradleyi ( Phleger 1936: pl. 20: 1, 2) are too poorly preserved to allow comparison. Indeed, although Spencer and Wright (1966) listed this genus under the family Protasteridae , they described the specimens as unrecognizable.

Discussion.— Hotchkiss (1970) tentatively placed one of the paralectotypes of Palaeocoma spinosa Billings, 1857 (GSC 1404a), as well as Protaster whiteavesianus Parks, 1908 , Taeniaster maximus Willard, 1937 , Drepanaster grayae Spencer, 1934 , and the fossils described as Taeniaster spinosus ( Billings, 1858) by Cramer (1957) into the genus Protasterina Ulrich, 1878 because all of these specimens exhibit wide dorsal interambulacral muscle gaps. However, this character is not restricted to Protasterina and is therefore not diagnostic by itself. Revision of these taxa is beyond the scope of the present study, and because the dorsal gap is not a unifying apomorphy, they are all allowed to stand pending reevaluation. Cramer (1957) found that the type and only known specimen of Taeniaster maximus had been lost but F. Hotchkiss located the specimen (USNM 111699) at the Smithsonian National Museum of Natural History (Frederick H.C. Hotchkiss, personal communication, 2005).

Hotchkiss (1970) also noted that a specimen described by Schuchert (1915) as Alepidaster sp. from the Trenton Limestone (Middle Ordovician; MCZ 108076) of New York has a mouthframe and ambulacrals that resemble those of Protasterina . This specimen is only partially exposed but restudy of the material revealed that the shape of the ambulacrals is quite different from that of Protasterina and it is not included in Protasterina here.

Figures

Fig. 4. Shape of the ventral surface of ambulacrals. The leg is parallel to the median suture; the foot articulates via the toes to the lateral. Proximal is top. A. Protasterina flexuosa (Miller and Dyer, 1878); A1, ambulacrals inside disk and proximal portions of free arms; A2, typical hour−glass shape of ambulacrals beyond the proximal portion of the disk; A3, four proximal ambulacrals for comparison with other taxa, median suture clearly sinuous*. B. Strataster ohioensis Kesling and LeVasseur, 1971 and Eugasterella logani (Hall, 1867), median suture straight to slightly sinuous. C. Bundenbachia beneckei* Stürtz, 1886; median suture straight to slightly sinuous. D. Palaeophiomyxa grandis*(Stürtz, 1886); median suture sinuous. E. Taeniaster spinosus (Billings, 1858); median suture straight to slightly sinuous. F. Bohemura jahni (Jaekel, 1903), Mastigophiura grandis* Lehmann, 1957, and Protaster sedgwickii* Forbes, 1849; median suture straight to slightly sinuous. Taxa marked “*” were reconstructed based on a study of the type material. All other reconstructions are based on published photographs.

Fig. 3. Protasterina flexuosa (Miller and Dyer, 1878), Edenian (Upper Ordovician), Cincinnati region. Lectotype, MCZ 108078 (A), and paralectotype, MCZ 108079 (B). A1. Lectotype (dorsal, at left) and additional specimen (ventral, at right). B1. Paralectotype, ventral. A2. Lectotype, dorsal surface of ambulacrals with large interambulacral muscle gaps. B2. Paralectotype, disk spines, and small madreporite (arrow). B3. Section of arm inside disk showing shape of proximalmost ventral ambulacrals.