Rhenechinus hopstaetteri Dehm, 1953

Smith, Andrew B., Reich, Mike & Zamora, Samuel, 2013, Morphology and ecological setting of the basal echinoid genus Rhenechinus from the early Devonian of Spain and Germany, Acta Palaeontologica Polonica 58 (4), pp. 751-762 : 758

Rhenechinus hopstaetteri Dehm, 1953

Figs. 6 , 7 .

1952 “Ein Seeigel aus dem Hunsrückschiefer”; Hopstätter 1952: 33.

1953 Rhenechinus hopstätteri ; Dehm 1953: 93, pl. 5: 1–4.

1961 Rhenechinus hopstätteri Dehm, 1953 ; Kuhn 1961: 33, figs. 15, 16.

1966 Rhenechinus hopstatteri Dehm, 1953 ; Kier 1966: U303, fig.224.2.

1970 Rhenechinus hopstätteri Dehm, 1953 ; Kutscher 1970a: 40.

1970 Rhenechinus hopstätteri Dehm, 1953 ; Kutscher 1970b: 96.

1980 Rhenechinus hopstaetteri Dehm, 1953 ; Mittmeyer 1980: 38.

1990 Rhenechinus hopstätteri Dehm, 1953 ; Bartels and Brassel 1990: 181, fig. 169.

1997 “Seeigel Rhenechinus hopstätteri mit erhaltenen Stacheln“; Bartels et al. 1997: 49, fig. 61.

1998 Rhenechinus hopstaetteri Dehm, 1953 ; Bartels et al. 1998: 210, fig. 188.

1999 Rhenechinus hopstätteri Dehm, 1953 ; Jahnke and Bartels 1999: 43.

2000 Rhenechinus hopstätteri Dehm, 1953 ; Jahnke and Bartels 2000: 43.

Holotype: BSPG. 1955 I 585.

Type horizon: Hunsrück Slate, Lower Emsian, Lower Devonian.

Type locality: Bundenbach, Rhineland−Palatinate, Germany.

Material.— One specimen, DBM.HS.285, Eschenbach−Bocksberg mine, Bundenbach, Rhineland−Palatinate, Germany .

Diagnosis.—A species of Rhenechinus with up to 8 interambulacral columns in a zone with plates of rather irregular shape and size; surface of plates lacking pitted ornamentation. Description.—The holotype ( Fig. 6 ) shows an articulated test in oral view and the new specimen ( Fig. 7 ) is a complete test squashed in lateral profile. The test must have been subglobular in life and taller than wide, with a diameter up to approximately 10 cm. Apical disc unknown but relatively small, as the ambulacra converge adapically ( Fig. 7A ).

Ambulacral zones are narrow and biserial with a primary element alternating with a small demiplate in each half ambulacrum ( Fig. 6B ). Pore−pairs are prominent and offset forming a double series in each column. The pore−pairs are about 1 mm in width with an obvious periporal rim; the inner pore pierces the plate rather than forming a marginal notch. There are small granules scattered on the adradial and perradial sides of the zone of pore−pairs. Towards the peristome plates become narrower and wider, and pore−pairs become smaller. Ambulacral plating appears to extend further onto the peristome than interambulacral plates ( Fig. 6C ).

Interambulacral plates are up to 6 mm in width and irregularly polygonal in outline. At the widest point there are some nine plates abreast. Only the adradial series of plates form a regular column and these may bear slightly larger tubercles than other plates. Plates are relatively thin—not much more than 0.6 mm in thickness, and have bevelled edges. A single plate forms the adoral boundary to the peristome and this is followed by two plates that just touch interradially and then by three plates, a large central hexagonal plate and two adradial pentagonal plates ( Fig. 6C ).

A membrane of small platelets covers the peristome ( Fig. 6C ), which is about 20 mm in diameter. While ambulacral plates extend a little way onto this membranous region, they are confined to the outer region.

Spines are up to 4 mm in length and are concentrated in the peristomial and ambulacral zones ( Figs. 6A , 7B ). There are several slender spines to each ambulacral plate, with somewhat longer spines being found adradially and shorter spines perradially. Small spines are also found on interambulacral plates but are not nearly as dense nor as long. Small spinate tridactylous pedicellariae are present on interambulacral plates (Figs. 5E, 7B). These are approximately 1 mm long and have a swollen base and long spine−like blades that are in contact along most of their length. The pedicellarial head rests on a small element some 0.2 mm in width that is either a stem element (as described by Haude 1998) or a small tubercle (preservation is not adequate to distinguish).

The lantern is present but internal. Hemipyramid tips can be seen ( Fig. 7C ) and show a deep abaxial groove for retractor and protractor muscle insertion. The teeth that protrude have a flat axial face and are simple oligolamellar in strucure with four or five laths abreast on each half. They end at a simple point.

Remarks.—The original description given by Dehm (1953) is detailed and largely correct. The new specimen provides additional information about the test shape and about the distribution of spines and pedicellariae. This is a species that attains almost twice the size of the Spanish species but we do not think the diagnostic differences are simply size related. Although R. ibericus may increase the number of interambulacral columns in each zone as it grows, the surface ornament in the two species is quite distinct with the Spanish species having a strongly pitted surface and the German species a smooth, unpitted surface.

One other echinoid, Porechinus porosus is known from the same formation ( Dehm 1961) and this does have a strongly pitted ornament to its plate surfaces. However, it differs in having distinctly thicker plating and ambulacral plates that are uniserially arranged. All the other echinoid specimens of the Lower Emsian Hunsrück Slate ( Lepidocentrus spp. ; Table 1) are based on fragmentary material and need to be revised. This is made especially difficult by the incomplete nature of the type material of Lepidocentrus ( Müller 1857) .

Stratigraphic and geographic range.—Hunsrück Slate, Lower Emsian, Lower Devonian; Gemünden and Bundenbach, Rhineland−Palatinate, Germany.

Figures

Fig. 6. Echinocystitid echinoid Rhenechinus hopstaetteri Dehm, 1953, BSPG.1955 I 585 (holotype), Hunsrück Slate, Lower Emsian, Lower Devonian; Gemünden, Rhineland−Palatinate, Germany. A. General view. B. Detail of ambulacral plating. C. Detail of oral region. Abbreviations: ps, peristomial membrane; t, tooth; 1, 2a, b, 3a–c, first three rows of interambulacral plates.

Fig. 7. Echinocystitid echinoid Rhenechinus hopstaetteri Dehm, 1953, DBM.HS 285, Hunsrück Slate, Lower Emsian, Lower Devonian; Eschenbach− Bocksberg mine, Bundenbach, Rhineland−Palatinate, Germany. A. General view. B. Detail of adoral plating showing pedicellariae and spines. C. Detail of adoral ambulacral plating. Abbreviations: amb, ambulacral zone; ap, apical disc region; p, pedicellaria; ps, pedicellarial stalk element; pst, peristome.