Peribrissus janiceae, Holmes, 2011

Holmes, Francis C., 2011, A new species of Peribrissus (Echinoidea, Spatangoida) from the middle Miocene of South Australia, Memoirs of Museum Victoria 68, pp. 29-35 : 31-34

Peribrissus janiceae sp. nov.

Figures 2A–E , 3A–I , Table 1

Type material. Holotype and only known specimen, NMV P316528 , from the early middle Miocene Glenforslan Formation (Batesfordian, Langian), Morgan Group, 7 km north-northeast of Murray River Lock 1, Blanchetown , South Australia [ NMV locality PL3203 ].

Description. Test moderately large, ovate in outline with wellformed anterior sulcus; only known specimen 58.0 mm long, with maximum width of 52.0 mm (89.7%TL) occurring posterior of centre at 54.3%TL from anterior ambitus. Maximum test height 38.5 mm (66.4%TL) anterior of centre, but posterior of apical disk at 44.8%TL from anterior ambitus.

Adapical surface inflated with high, vertically convex anterior, gently curved ridge along interradial suture of interambulacrum 5 and prominent vertically truncated posterior. Laterally, sides gently curved at approximately 40° to the horizontal between dorsal ridge and well-rounded ambitus situated about one-third test height above the underside. Adoral surface posterior of peristome flat along centre line of labrum and plaston (fig. 2C–E).

Small, very closely spaced tubercles cover nearly all the test; smallest around ambitus and largest towards peristome. Tubercles in ambulacra II and IV first appear on plates 3a and b, and in I and V on plates 4a and b. By plates 5a and b, the size and spacing generally matches that of adjacent interambulacra. Largest tubercles with an approximate areole diameter of 1.0 mm occur on interambulacra 1 and 4 adjacent to adoral edge of plates 2a and b, aborally on plate 1, and along the adradial suture line between ambulacrum III and interambulacra 2 and 3 from the marginal fasciole to the apical disk. These tubercles have a perforate mamelon and crenulate platform but appear to lack a scrobular ring. Because of the very close spacing of these tubercles, miliary granules occur mainly towards the peristome and between the periproct and marginal fasciole in interambulacrum 5 where the spacing between the larger tubercles increases. They also occur around the apical disk.

A well-defined marginal fasciole occurs just above the sloping ambitus, dipping sharply below the periproct posteriorly but crossing ambulacrum III anteriorly slightly below the ambitus at about 25%TH (see fig. 2). The semipetalous fasciole is only marginally indented between the posterior paired petals and crosses interambulacra 1 and 4 on plates 8/9 before descending transversely to join the marginal fasciole at right angles, posterior to the angle of the anterior paired petals (see fig. 3I). Although continuous, fasciole widths vary but maintain a fine tubercule (granule) density of about 100–120 per mm 2.

Apical system situated well anterior of centre at 21.0% TL from anterior ambitus to centre of disk and is level with proximal end of paired petals. Ethymolitic with three gonopores, no gonopore in plate G2, and approximately 60 hydropores fairly evenly spaced over the latter’s length.

Paired petals straight, parallel sided, sunken, open distally and devoid of tubercles. Anterior paired petals 138% longer than posterior pair, extending 50% of the radius (28.0%TL) measured along the surface of the perradial suture from centre of ocular to ambitus. Anterior paired petals diverge at 175° and contain 23/24 pore pairs, posterior petals 315° and 20/21 pairs. Outer pores elliptical, inner pores slightly smaller and more tear shaped. Zone between inner and outer pores approximately equal in width to outer pores, pairs not conjugate.Interporiferous zone marginally narrower than poriferous zones.

Ambulacrurum III depressed for its full length below adjacent interambulacra, reaching a maximum depth of 3 mm (5.2%TL) below the anterior ambitus. Pore pairs are visible adapically between the ocular plate and approximately one-third of the radius to the anterior ambitus. Adapically, the pore pairs are angled inwards at approximately 45° to the perradial suture but gradually become monoserial halfway towards the anterior ambitus. The ambulacrum is covered with closely spaced small tubercles and miliary granules, the former gradually increasing in diameter adorally.

Peristome reniform and slightly sunken, longitudinal dimension 4 mm (6.9%TL), width 8.6 mm (14.8%TL), anterior edge situated 12.4 mm (21.4% TL) from ambitus. Phyllodes unipored with periporal areas protuberant. Basicoronal plates amphiplaceous.

Labrum small, wider than long, covered with small tubercles and flared anteriorly where bordered by a smooth raised rim (fig. 3H). Curved anterior edge projects over the peristome for about one-third of the latter’s length. Posterior edge does not extend beyond the first adjacent ambulacraI plates. Plastron wide, long, and covered with rows of closely spaced angular tubercles without interstices. Maximum width of plastron (45%TW) occurs about three-quarters of the test length from the anterior ambitus.

Periproct elliptical shaped with slightly pointed upper and lower junction with interradial suture, height 8.0 mm (13.8%TL), width 5.0 mm (5.6%TL). Underside of vertical opening situated high above base of test (44.2%TH) on truncated posterior surface. Subanal surface slightly depressed.

Etymology. Named for Janice Krause of Hamilton, Victoria, an exceptionally dedicated fossil echinoid collector.

Remarks. Comparison of Peribrissus janiceae sp. nov. with the type species P. saheliensis from Algeria and P. sotgiai from Sardinia is complicated by the lack of detailed descriptions, comparative measurements and illustrations of many of the important diagnostic features of the latter two species. The difficulty is compounded by the excellent preservation of detail found on the single specimen of P. janiceae and the large difference in size between specimens of the three species, with P. saheliensis approximately twice the length and width of P. janiceae and four times that of P. sotgiai . Where possible, diagnostic features of the three species are compared in table 1, based on the descriptions of Pomel (1887), Giorgio (1923) and Stefanini (1911), together with approximate measurements taken from their illustrations of the partial and poorly preserved type specimens.

Figures

Figure 2. Peribrissus janiceae sp. nov. A–E, outline drawings of adapical, adoral, left lateral, anterior and posterior views of holotype NMV P316528, showing paths followed by marginal (mf) and semipetalous (spf) fascioles, and position of peristome (pse) and periproct (ppt).

Figure 3. Peribrissus janiceae sp. nov. holotype NMV P316528. A–D and F, anterior, right lateral, posterior, adapical and adoral views; E, detail of apical disk; G, oblique left lateral view of semipetalious fasciole crossing interambulacrum 4, plates 7a, 8a, 9a and 9b; H, detail of peristome, labrum and phylodal plates; I, detail of junction between marginal and semipetalous fascioles on interambulacrum 1, plate 4b. Scale bars = 10 mm unless otherwise shown.