Rhyncopygus? janchrisorum, Holmes, 2004

Holmes, Francis C., 2004, A new Late Eocene cassiduloid (Echinoidea) from Yorke Peninsula, South Australia, Memoirs of Museum Victoria 61 (2), pp. 209-216 : 211-213

Rhyncopygus? janchrisorum sp. nov.

Figures 2A–F , 3A, B , 4A–C , 5 , 6A

Type material. Holotype, NMV P145616 from Late Eocene (Aldingan, Priabonian) Muloowurtie Formation, “Sliding Rocks”, Yorke Peninsula , South Australia . Paratypes. NMV P312113 to P312115 from the uppermost bed of the same formation, NNE of “Sliding Rocks” at approximately 34°34.17'S, 137°53.40'E ( Fig. 1 ) .

Description. Test moderately small, oval in outline at the ambitus, anterior and posterior evenly rounded, widest point central. Aboral surface moderately inflated, except for a depression posterior to the periproct, with the apex just anteri- or of the apical disk at the proximal end of slightly swollen ambulacrum III. Adoral surface slightly depressed around peristome and along the posterior paired ambulacra I and V.

Aboral tubercules very small, about 0.15mm diameter, and closely spaced with a density of about 12–15 tubercles per mm2. Adoral tubercles, where visible near the margin, are also closely spaced but larger, about 0.6 mm diameter. Naked granular zone in ambulacrum III and interambulacrum 5.

Apical system monobasal, centre of disk 42–45%TL from anterior margin, with 4 gonopores in contact with the apical disk but extending into the first pair of interambulacral plates, anterior pair closer together than posterior pair. Ocular plates relatively small and about equal in size. Approximately 80 hydropores ( Fig. 3 ).

Petals moderately short, broad and unequal in length. Longest in ambulacrum III (approx. 62% radius with about 30 pores per tract) and shortest in anterior pair II and IV (approx. 50% radius with about 20 pores per tract). Inner pores of pore pairs oval, outer pores slot like, with alignment noticeably oblique in ambulacra II and IV. At widest point interporiferous zone in ambulacra II, III and IV about equal in width to zones of pore pairs, parallel sided and open ended in ambulcrum III and narrowing distally in II and IV. Posterior pair of petals have narrower poriferous zones curving outwards distally with poriferous zones Ib and Va noticeably wider than Ia and Vb. Poriferous zones in individual petals equal in length. Anterior pair of petals diverge between 134° and 140°, posterior pair between 306° and 310°. The aboral swelling in ambulacrum III forms a low ridge along line of perradial suture for full length of petal.

Periproct supramarginal, transverse, situated at the anterior end of a pronounced posterior anal depression in interambulacrum 5, beneath a wide semi-circular projecting lip (about 19%TW) extending to nearly 20%TL from the posterior margin. The anal depression diminishes posteriorly and barely reaches the margin.

Peristome small, pentagonal, centre of opening situated 37-40%TL from the anterior margin, Floscelle well developed with very pronounced pointed and inflated bourrelets, anterior pair wide and wedge-shaped, posterior single and pair elongated and near parallel sided. Phyllodes deeply sunken proximally but with pronounced ridge between bourrelets at edge of peristomal opening. Detail of phyllode pores unknown.

Etymology. For Janice Krause and Christopher Ah Yee of Hamilton, Victoria.

Remarks. The description is based on four specimens, all of which have been subject to diagenetic compression resulting in radial cracks along adradial and interradial sutures. In the case of the holotype these cracks extend between half and two-thirds distance between ambitus and distal end of petals. Compared with a similarly compressed specimen of Australanthus longianus from the same locality, it is estimated that the holotype of Rhyncopygus? janchrisorum sp. nov. would have been about 24.0 mm long, 20.5 mm (85.5%TL) wide and a minimum of 10.5 mm (44%TL) high compared with the 26.8 mm long, 22.9 mm wide and 7.8 mm high dimensions of the compressed fossil test ( Fig. 4 ). Rhyncopygus? janchrisorum is easily distinguished from the type species of the genus, R. marmini , in having a larger, less elongated and less inflated test, a monobasal apical disk, far more pronounced and broader petals, and prominent bourrelets. It differs from R. donetzensis , based on the illustrations in Smith and Jeffery (2000), primarily in being far less tumid and having a longer and broader petal in ambulacrum III, shorter posterior petals and considerably wider ambulacral plates at the ambitus ( Fig. 5 ). The test of R.? janchrisorum is longer and proportionately wider than the other two species and has a more anterior periproct and projecting lip.

The occurrence of a similar but much less pronounced canopy above the periproct is also present in Hardouinia (Fauraster) priscus Lambert , in Lambert and Thiéry, 1924. Kier (1962: 143), in his description of the poorly preserved and compressed test, noted that the periproct is very wide and low; however, Smith and Jeffery (2000: 205) referred to this as an external feature that funnels into an opening only a little wider than tall and list Hardouinia waageni Holland and Feldman, 1967, and Hardouinia nuratensis Moskvin, 1984 , as synonyms of this species. Apart from this feature, H. (F.) priscus differs from R.? janchrisorum in its near circular outline with slightly truncated posterior, its tetrabasal apical system and smaller and equal sized petals with narrower poriferous tracts and much wider interporiferous zone

Figures

Figure 1. A and B, general location maps; C, map of St Vincent Basin in relation to Yorke Peninsula, Kangaroo Island and the Fleurieu Peninsula (Maslin Bay), South Australia; D, location of the northernmost exposure (1), and southernmost exposure (2), of the Muloowurtie Formation, west side of Gulf St Vincent.

Figure 2. Rhynchopygus? janchrisorum sp. nov. A–C, adapical, right lateral and posterior views of holotype NMV P145616; D, adoral view of paratype NMV P312115; E, adoral view of paratype NMV P312114; F, adapical view of paratype NMV P312113. All specimens form the Late Eocene Muloowurtie Formation, Yorke Peninsula, South Australia. Scale bar 10 mm.

Figure 3. Apical plate structure. A and B, Rhynchopygus? janchrisorum sp. nov., Late Eocene holotype NMV P145616; C, R. marmini, Late Cretaceous USNM 19559 from Port Brechay, La Manche, France (drawing adapted from Kier, 1962). Scale bars 1mm.

Figure 4. Rhynchopygus? janchrisorum sp. nov. A–C, reconstruction of adapical, right lateral and posterior characteristics of holotype NMV P145616 (actual specimen outline shown with broken line). Scale bar 10 mm.

Figure 5. Rhynchopygus? janchrisorum sp. nov. Drawing of adapical surface of NMV P312113 showing plate structure distal to petals. Scale bar 10 mm.

Figure 6. Comparative drawings of A, Late Eocene Rhynchopygus? janchrisorum sp. nov. from Yorke Peninsula, South Australia; B, Late Cretaceous (Late Maastrichtian) R. donetzensis from the Severny Donetz Basin, Ukraine (adapted from Smith and Jeffery, 2000); C1, Late Cretaceous (Late? Santonian) R. marmini from La Richardière, Dissay, France; C2, posterior profile of Late Cretaceous (Late Campanian) R. marmini from near Royan and St Palais-sur-Mer, France. Based on published illustrations, latter profile typical of Maastrichtian R. marmini specimens in general. Scale bars 5 mm.