Saccocoma tenella (Goldfuss, 1831)

Brodacki, Michał, 2006, Functional anatomy and mode of life of the latest Jurassic crinoid Saccocoma, Acta Palaeontologica Polonica 51 (2), pp. 261-270 : 264-267

publication ID	https://doi.org/ 10.5281/zenodo.13643883
persistent identifier	https://treatment.plazi.org/id/038F7D2E-FF89-FFFE-CA13-F6343FECF81D
treatment provided by	Felipe
scientific name	Saccocoma tenella (Goldfuss, 1831)
status

Treatment

Figs. 3A–E View Fig , 4A, B View Fig , 5A–D, H, I View Fig , 6A, B, E, F, H, I View Fig .

1831 Comatula tenella sp. nov.; Goldfuss 1831: 204, pl. 62: 1. 1831 Comatula pectinata View in CoL sp. nov.; Goldfuss 1831: 205, pl. 62: 2. 1831 Comatula filiformis sp. nov.; Goldfuss 1831: 205, pl. 62: 3. 1892 Saccocoma tenella Goldfuss ; Jaekel 1892:659–694, pl. 29: 6, pl. 30. 1960 Saccocoma tenella Goldfuss ; Verniory 1960: 250–257, figs. 1–9. 1979 Saccocoma tenella Goldfuss, 1862 ; Pisera and Dzik 1979: 810–

811, fig. 3, pl. 1: 8, 9, pl. 2: 1–7, pl. 3: 1–3. 1980 Saccocoma tenella (Goldfuss, 1831) ; Holzer and Poltnig 1980:

207, fig. 2, pls. 1–3. 1987 Saccocoma tenella Goldfuss, 1862 ; Głuchowski 1987: 39–40, pl.

17: 1, 4–6, pl. 18: 1–5, pl. 19: 1–6, fig. 13: 7–10. 2002 Saccocoma tenella (Goldfuss, 1831) ; Hess 2002: 19, figs. 12, 13. Material.—More than 100 radials, mostly broken, and more than 400 brachials.

Description.—Radial plates ( Fig. 3A–E View Fig ) resembling an arrow−head in outline, gently curved, thin. External (aboral) surface with pronounced median ridge and a more or less developed reticulate sculpture. Edges vary from straight to serrate. The bulbous articulation facet with well−developed ligament fossa bears no spines. Inner (oral) side rather smooth, with a thin median ridge terminating at the base of the articulation facet, where openings of nerve canals may be seen. At the upper end, two ridges form a V−shaped surface for the attachment of muscles.

The first primibrachial (IBr1; Fig. 4A View Fig ) is cylindrical, has distinct ligament fossae on the proximal end and two well−developed, low, distally elongate muscle fields on the oral side. The aboral side shows a large, sloping, non−muscular (cryptosynarthial or synostosial) distal articulation facet that in side view forms an angle of about 20 ° with the long axis of the ossicle.

The second primibrachial or primaxillary (IBr2 = IAx; Fig. 4B View Fig ) bears two large, symmetrical, rounded, dish−like lateral expansions or wings (“Schwimmplatten”). Their proximal margins extend beyond the proximal articulation surface, which is non−muscular, sloping at an angle of about 20–30 °, and well visible in oral view. The two distal muscular articulation facets are comparatively low in distal view, their ligament fossae can be seen aborally. On the oral side, there are two weak ridge−like oral processes and a median processus (usually broken).

The first secundibrachial (IIBr1) is probably more similar to the IBr1 than to the IIBr3 (see the chapter on functional anatomy). The second secundibrachial (IIBr2; Fig. 5B–D View Fig ) generally resembles the IBr2 (IAx), except that it has only one muscular distal articulation facet, that it lacks the median processus, and that the proximal facet (sloping non−muscular) may be slightly bent to one side making the wing−like expansions more or less asymmetrical. The third secundibrachial (IIBr3; Fig. 5A View Fig ) is again short, with the proximal articulation surface developed as in the IBr1, but with distinctly higher muscle fields. Its distal articulation surface is non−muscular and almost perpendicular (60–80 °) to the element’s long axis. The fourth secundibrachial (IIBr4; Fig. 5H, I View Fig ) has more or less asymmetric wing−like expansions, a non−muscular, non−sloping (almost perpendicular) proximal articulation surface, a muscular distal facet, and low, paired oral processes that are more pronounced than in the IBr2 or IIBr2. Secundibrachials of higher order (IIBr5–IIBr7; Fig. 6A, B, E, F View Fig ) have progressively shorter wings, muscular articulation facets on both ends, and low but distinct oral processes. Even more distal secundibrachials (IIBr8 and higher; Fig. 6H, I View Fig ) have high and flat (often more or less broken) oral processes and no or very short lateral wings.

Remarks.—Most of the investigated ossicles probably represent subadult individuals. The radial plates do not reach 7–8 mm, as reported in other studies ( Głuchowski 1987; Holzer and Poltnig 1980), but only 2.5–3.5 mm at the most. The variability of their sculpture, first noted by Verniory (1960), reflects an ontogenetic change, as shown in detail by Holzer and Poltnig (1980): the smooth surfaces of small (below 2–2.5 mm) radials of young individuals became covered with a network of ridges and ribs as the animal grew.

The arrangement of the proximal brachials adopted here follows that of Jaekel (1892) who in his famous reconstruction illustrated the animal with five arms, each splitting at the second primibrachial (primaxillary) into two secondary arms, whose proximal parts consists of two wingless (IIBr1 and IIBr3) and five winged (IIBr2, IIBr4–IIBr7) secundibrachials. However, this is not necessarily the case, as Hess (1999: fig. 219) has shown in an articulated specimen with wings on third secundibrachials and, in one or two of the arms, with a second primibrachial that is not an axillary. This may be explained by pathological development or imperfect regeneration.

Kingdom	Animalia
Phylum	Echinodermata
Class	Crinoidea
Order	Roveacrinida
Family	Saccocomidae
Genus	Saccocoma