Spatagobrissus dermodyorum, Holmes & Yee & Krause, 2005

Holmes, Francis C., Yee, Christopher Ah & Krause, Janice, 2005, Two new Middle Miocene spatangoids (Echinoidea) from the Murray Basin, South Australia, Memoirs of Museum Victoria 62 (1), pp. 91-99 : 94-95

Spatagobrissus dermodyorum sp. nov

Figures 6A–E , 7A–C , 8A–E

Type Material. Holotype. NMV P312570 from early Middle Miocene Glenforslan Formation (Batesfordian), Morgan Group, in the vicinity of NMV locality PL3203 , 7 km NNE of Murray River Lock 1, Blanchetown , South Australia.

Paratypes, NMV P312571 – P312373 from the same general area. Other material used for statistical purposes is held in Museum Victoria and private collections.

Description. Test small, subcircular in outline with minimal posterior truncation and no anterior sulcus. Specimens range 30.0– 45.5 mm in length with maximum width 82–90%TL occurring 51–58 %TL from anterior ambitus. Maximum height 49.5–60%TL at 52.6–65.5%TL from anterior ambitus. Adapical surface moderately inflated, evenly curved transversely above well-rounded margin with ambitus situated at about 30%TH. Adoral surface very mildly convex but with prominent posterior keel caused by sharp rise of ambulacra I and V posterior to centre. In lateral view, posterior truncation covers about 35%TH.

Apical system ethmolytic with 4 genital pores, centre 31.3–38.6%TL from anterior ambitus ( Fig. 7A ). Paired petals short (26.5–32.5%TL measured along surface of perradial suture from centre of apical disk), narrow (7.0–9.0%TL), lanceolate, closing to closed, posterior pair marginally longer than anterior pair. Anterior pair diverge at about 135°, posterior pair 310°. Pore pairs conjugate, inner pores oval, outer-tear shaped. Anterior row of pore pairs in anterior paired petals distinctly narrower than posterior row and atrophied adapically. Interporiferous zone up to 1.5 times width of poriferous zone at widest point. Ambulacrum III flush adapially, with 2 rows of indistinct longitudinally orientated pore pairs and interporiferous zone containing few secondary tubercles and numerous miliaries.

Peripetalous fasciole subcircular, not indented. Numerous small, randomly spaced perforate, crenulate, primary tubercles occur in interambulacra within fasciole. Outside fasciole, tubercles in posterior half of test very small, but anterior to centre, gradually increasing in size towards interambulacra 2 and 3.

Peristome reniform, mildly sunken, width 16.0–19.3%TL, length 8.9–10.3%TL with anterior border 23.4–26.8%TL from anterior ambitus. Phyllodes moderately developed with slotshaped pores in circular depressions. Labrum short and wedgeshaped extending only to centre of second pair of adjacent ambulacral plates. Anterior edge raised above surrounding ambulacra and slightly projecting over peristome. Miliary tubercles present with few secondary tubercles in anterior half ( Fig. 7B ).

Plastron long, width about 55–60% length, with ambulacral plates indenting posteriorly. Subanal fasciole circular to transversely oval, enclosing 3 pore pairs each side of interradial suture, and with slight anterior projection at posterior end of prominent plastronal keel. Posterior edge of fasciole marginally clear of periproct opening. Adorally, ambulacra I and V relatively wide, covered with very fine, randomly spaced miliary tubercles up to sixth plate, then by tubercles of similar size to rest of adoral surface.

Periproct, tear-drop shaped, generally positioned vertically on truncated posterior margin but slightly visible from above on some specimens; height 17.1–19.1%TL, width 13.2–16.0%TL.

Etymology. Named for Michael and Marie Dermody, owners of Glenforslan Station.

Remarks. Spatagobrissus dermodyorum sp. nov. differs primarily from the Middle Miocene Port Campbell Limestone species, Spatagobrissus laubei ( Duncan, 1877) , in having narrower test with more posterior maximum width ( Fig. 8A ), markedly larger peristome and periproct ( Fig. 8D, E ), and very much shorter labrum ( Fig. 8B ). In addition, anterior paired petals are shorter and posterior petals longer ( Fig. 8C ), with divergent angle of latter greater than S. laubei . Aboral primary tubercles larger within peripetalous fasciole and interambulacra 2 and 3, while fine tubercles outside fasciole on posterior half of test are much smaller in diameter. Adorally, plastron wider and longer, and ambulacra I and V narrower.

The extant type species Spatagobrissus mirabilis is characterised by a larger (up to 110 mm long) and less inflated test, more posteriorly located apical system in line with maximum width, greater area enclosed by peripetalous fasciole, shorter peristome, and periproct situated on an obliquely truncated surface below the ambitus. Primary tubercles within the peripetalous fasciole of S. mirabilis are also larger and more closely spaced than in S. dermodyorum .

Spatagobrissus incus Baker and Rowe, 1990 , an extant species endemic to southeast Australian waters, particularly between Flinders Island (Tasmania) and western Spencer Gulf, South Australia, has a larger and more rounded test (up to 80 mm long) and, similar to S. mirabilis , more posteriorly located apical system and greater area enclosed by peripetalous fasciole.

Compared with S. dermodyorum , it also has a much wider and longer plastron and narrower adoral ambulacra I and V. Miskelly (2002: 156) noted two pairs of pore pairs occur in each side of the subanal fasciole, a feature also recorded for S. laubei ( McNamara et al., 1986: 80) . This contrasts with the three pairs found on S. dermodyorum ( Fig. 7C ).

Figures

Figure 6. Spatagobrissus dermodyorum sp. nov. A, B, D, adapical, adoral, and left lateral views of holotype, NMV P312570; C, adoral view of paratype, NMV P312571; E, posterior view of paratype, NMV P312572 All specimens from the early Middle Miocene Glenforslan Formation in the vicinity of locality NMV PL3203, north of Blanchetown, South Australia. Scale bars 10 mm.

Figure 7. Spatagobrissus dermodyorum sp. nov. A, detail of petals and apical disk of holotype, NMV P312570; B, peristome and labrum tuberculation of paratype, NMV P312571; C, subanal fasciole and pore pairs of paratype, NMV P312573. Scale bars 1mm.

Figure 8. Comparative biometric data on specimens of Spatagobrissus laubei (Duncan, 1877) from the Middle Miocene Port Campbell Limestone, Port Campbell, Victoria (•), and S. dermodyorum sp. nov. from the early Middle Miocene Glenforslan Formation, Blanchetown district, South Australia (o).