Podosphaeraster pulvinatus Rowe & Nichols, 1980

Gale, Andrew Scott, 2021, Taxonomy and phylogeny of the ‘ football stars’ (Asteroidea, Sphaerasteridae), Journal of Systematic Palaeontology 19 (10), pp. 691-741 : 735-736

Podosphaeraster pulvinatus Rowe & Nichols, 1980

( Figs 1A–E , 2E , 6E, P, Q , 7O, S, T , 8J, L , 25A–F, H, J–L)

$1980 Podosphaeraster pulvinatus Rowe & Nichols : 290, figs 1–3.

2002 Podosphaeraster pulvinatus Rowe & Nichols ; Fujita & Rowe: 327.

Material. This species is perhaps the most abundant of the genus, and a number of specimens were obtained from the Philippines for the study of ossicular morphology (MhnhL OPH 174–176). The overall morphology has been described by Rowe & Nichols (1980), and here the focus will be on the homologies of the primary ossicles and the morphology of the ambulacral groove and mouth frame ossicles.

Description. The undifferentiated primary ossicles of the body wall are polygonal and vary significantly in size, shape and arrangement ( Fig. 25A–C ; see also Fujita & Rowe 2002). However, all have a similar construction and ultrastructure. The external surface is made up of smooth perforate stereom ( Smith 1980), which displays irregular low ridges and grooves and resembles a rippled surface ( Fig. 25F, H ). Irregularly sized pores are scattered across the surface. In broken profile, the perforate stereom can be seen to form a superficial layer less than 80 M m in thickness ( Fig. 25J ). Beneath it is a thick zone of fine rectilinear stereom (200 M m in the centre of the plate, thickening to 500 M m towards the margins). A central internal zone is made up of coarse labyrinthic stereom. These zones are broadly similar to those described in Sphaeraster by Blake (1984). The abactinal ossicles articulate by means of vertical ridges that separate the papular pores ( Fig. 25H ). Between three and eight ridges are present on any one side of a primary ossicle, and these are between 200 and 400 M m across. Each ridge carries a circular or oval concavity, less than 100 M m in diameter, placed approximately halfway between the internal and external margins

The homologies of the abactinal, putative marginals and actinals of Podosphaeraster were discussed by Rowe (1985) and Fujita & Rowe (2002). The ‘apical system’ described by these authors is an inappropriate term (see above and Fig. 1D, E ).

The arrangement of the ico around the ce and periproct in Podosphaeraster ( Rowe et al. 1982, fig 2A) is closely similar in detail to those developed in Sphaeraster and Echinosphaeraster gen. nov. ( Figs 1 , 2 ). The ce is small and pentagonal, and the first interradials are somewhat elongated and occlude the first radials in some specimens, exactly as in the two Jurassic genera. Similarly, the twin interradial and ce also surround the periproct. However, the arrangement and number of ossicles of the ico is highly variable within and between species of Podosphaeraster ( Rowe 1985, fig. 2; Fujita & Rowe 2002) with the intercalation of up to 10 small ossicles with the 11 standard plates.

The primary interradials in Podosphaeraster can be identified using the Madreporic Plating Rule ( Gale 2011). The madreporite is enclosed within a large interradial ossicle, which articulates distally with a pair of tall ossicles ( Fig. 25E ). Equivalent ossicles are found in each interradius, and the large ossicles are therefore the pir. The pr are slightly enlarged ossicles in a radial position, separated from the pir by a pair of intercalated ossicles ( Fig. 1 ).

The madreporite is highly modified in Podosphaeraster . The plate is fused with, and situated centrally within, a large interradial ossicle. The surface of the madreporite is slightly raised and very porous, and carries about 15 narrow, bifurcating grooves that radiate from the centre of the ossicle ( Fig. 25E ). Fusion of the madreporite with the primary interradial and the two distal ossicles is seen in echinasterid and asteriid species ( Gale 2011). Whether this has happened in P. pulvinatus , or whether the madreporite has become incorporated within the primary interradial ossicle, is unknown. However, the consistent association of the madreporite with the primary interradial in the C-D interradius in all neoasteroids permits identification of the pir as the slightly larger, interradial ossicles ( Figs 1 , 2 ).

The adambulacrals of P. pulvinatus are all of similar size and shape ( Fig. 7O, S, T ). In proximal/distal profile, the ossicles are rhombic, and a low adradial face is set at right angles to the abactinal surface that contacts the ambulacrals. The external face is gently undulating, rectangular, twice as broad as long, and has weakly sigmoidal proximal and distal margins. The surface is made up of circular and oval rugosities of approximately even size (50 M m), composed of perforate stereom, surrounded by small pores. The proximal and distal faces are nearly flat, and a sharply defined oval depression for the adadm muscle is present. On the abactinal face, poorly defined articulation surfaces (ada1a, ada1b, ada2, ada3) can be identified as smooth areas of slightly coarser stereom, but areas for muscle insertion (padam, dadam) cannot be identified.

The ambulacrals are short, proportionately tall and rectangular in actinal aspect ( Fig. 6E, P, Q ). Head, waist and base can be distinguished, but the waist is not significantly narrow. Dentition and abtam are present, and the actam inserts within a tall, hollow conical process. The base broadens abradially, and the abradial margin is straight. In proximal/distal aspect, the ossicle expands abradially towards the base, which is the the tallest part of the ossicle. The articular surface for the adambulacral is set at 100 Ǫ to the actinal face, and articulation surfaces (ada1a, ada1b, ada2, ada3) are clearly visible as smooth areas of thicker trabeculae. The adradial surfaces (ada1a, ada2) are closely spaced, positioned on the actinal margin and well separated from the abradial ones (ada1b, ada3), which are more widely separated. No muscle insertion sites for padam, dadam and lim are visible, but a large surface of smooth stereom for the lia is visible.

The oral ossicles of P. pulvinatus are unusual ( Fig. 8J, L ). The body of the oral is oval, with a rounded proximal margin and a pointed distal one. On the radial face, a small oval insertion site for adadm and a round, diffuse adada are present. The dcoa forms a short, conical, radially directed projection, and an oval swelling; the pp is present centrally on the abactinal border of the oral body. The apophyse is small and ‘Y’-shaped, and a shallow rvg runs through the arms of the ‘Y’. The proximal arm of the apophyse is thin and striated, and the riom insertion is present on its surface. The inner (interradial) face has a concave surface for articulation of the odontophore, and an elongated, shallow insertion site for aciim on the actinal, proximal border. The abiim on the apophyse is well marked.

In summary, the morphology of the ambulacral groove ossicles in P. pulvinatus is highly modified compared with that known from any other valvatidan. The apparent absence of padam, dadam and lim is unique, as is the presence of the conical process on the apophyse in which the actam is inserted. However, P. pulvinatus shares certain features of the ambulacrals and adambulacrals with other members of the Sphaerasteridae , including the presence of a small, oval insertion site for the adadm on the proximal and distal faces of the adambulacrals. The relationship between Valettaster and Podosphaeraster is discussed above.

Figures

Figure 1. Homology of abactinal plating in Podosphaeraster pulvinatus Rowe & Nichols, 1980. A–C, homology adopted in the present paper, which identifies the primary ossicles (interradials, radials), inner calycinal ossicles and radials. Marginals cannot be identified, and unpaired interradial plates (I-plate, V-plate) are indicated. D, E, plating scheme of Fujita & Rowe (2002, fig. 1), who identified an ‘apical system’ and marginals. See text for discussion.

Figure 2. Homology of primary abactinal plates in valvatidan asteroids. A, Manfredaster bulbiferus (Forbes, 1848), Stauranderasteridae; B, Sphaeraster tabulatus (Goldfuss, 1833), Sphaerasteridae; C, Echinosphaeraster (gen. nov.) scutatus (Goldfuss, 1833), Sphaerasteridae; D, Nymphaster arenatus (Perrier, 1881), Goniasteridae; E, Podosphaeraster pulvinatus Rowe & Nichols, 1980, Podosphaerasteridae. Colours as in Figure 1.

Figure 6. Ambulacrals of sphaerasterids. A–D, Sphaeraster tabulatus (Goldfuss, 1833); A, B, proximal ambulacral in actinal and proximal/distal aspects, respectively (NHMUK EE 17633); C, D, actinal views of ambulacrals from mid-radius (NHMUK EE 17623a, 17634). E, P, Q, Podosphaeraster pulvinatus Rowe & Nichols, 1980; E, enlargement of actinal view of base of ambulacral; P, Q, actinal and proximal/distal views of ambulacral, respectively (Mnhnl OPH176). F, G, Eosphaeraster amellagensis gen. et sp. nov., ambulacral in actinal and proximal/distal views, respectively (NHMUK EE 17635). H–K, N, O, Valettaster argus (Spencer, 1907). H–K, proximal/distal views of ambulacral, note cavity replacing articulation facet (I); L, M, Bulbosphaeraster valettei gen. et sp. nov., ambulacral in actinal and proximal/distal views, respectively (NHMUK EE 17636); N, O, actinal views of ambulacrals (NHMUK EE 17637a, b). Provenance: A–D are from the upper Oxfordian, Hypsilum ammonite Zone, Plettenberg, near Balingen, Germany. F, G are from the upper Pliensbachian, outcrop on south side of wadi immediately to west of road, 0.5km south of the Kasbah of Amellago, Atlas Mountains, Morocco. L, M are from the lower Bathonian, Zigzagiceras zigzag ammonite Zone at La Pouza, near La Voulte-sur-Rhone, Ard̀eche, France. H–K, N–O are from the coranguinum Zone, Santonian, Broadstairs, Kent, UK. E, P, Q are Recent, Philippines. Scale bars: A, B, D, F, G, J–Q = 0.5mm; C = 1 mm; E, H, I = 0.2 mm.

Figure 7. Adambulacrals of sphaerasterids. A, B, Stauraster primaevus (von Zittel, 1880), adambulacrals in abactinal and proximal/ distal views, respectively (NHMUK EE 17638, 17632). C–H, L, N, Q, Sphaeraster tabulatus (Goldfuss, 1831); C, G, L, N, abactinal views (NHMUK EE 17623h–k); D, F, proximal/distal views (NHMUK EE 17623l, m); H, internal, Q, external views (NHMUK EE 17623n). I–K, M, Eosphaeraster amellagensis gen. et sp. nov.; I, J, proximal/distal and internal views (NHMUK EE 17639); K, M, proximal/distal views (NHMNUK EE 17640, 17641). O, S, T, Podosphaeraster pulvinatus Rowe & Nichols, 1980, adambulacrals in proximal/distal, internal and actinal views, respectively (MhnhL OPH174–176). P, R, Valettaster argus (Spencer, 1907), adambulacrals in abactinal and actinal views, respectively (NHMUK EE 17637f, g). Provenance: A, B are from the lower Oxfordian, Cardioceras bukowskii ammonite Subzone, basal Couches de Passage, Andelot Morval, Jura, France. C–H, L, N, Q are from the upper Oxfordian, hypsilum ammonite Zone, Plettenberg, near Balingen, Germany. I–K, M are from the upper Pliensbachian, outcrop on south side of wadi immediately to west of road, 0.5km south of the Kasbah of Amellago, Atlas Mountains, Morocco. P, R are from the coranguinum Zone, Santonian, Broadstairs, Kent, UK. O, S, T are Recent, Philippines. Scale bars: A, C–E, L–O, Q–T = 0.5 mm; B, F–K = 1 mm; P = 0.2 mm.

Figure 8. Oral ossicles of sphaerasterids. A–E, Sphaeraster tabulatus (Goldfuss, 1833); A, D, interradial view of orals of smaller individuals (NHMUK EE 17642, 17643); B, E, interradial (B) and radial (E) views of small oral (NHMUK EE 17644); C, large oral, interradial view (NHMUK EE 17623b). F, I, Eosphaeraster amellagensis gen. et sp. nov., oral in interradial and radial views, respectively (NHMUK EE 17645). G, oral ossicle of possible stauranderasterid or sphaerasterid, interradial view (MnhnL OPH173). H, oral ossicle, probably from Bulbosphaeraster valettei gen. et sp. nov., interradial view (NHMUK EE 17646). J, L, oral ossicle of Podosphaeraster pulvinatus Rowe & Nichols, 1980, in interradial and radial views, respectively (MhnhL OPH174–176). K, M, oral ossicle of Valettaster argus (Spencer, 1907), in interradial and radial views, respectively (NHMUK EE 17637h). N, interradial view of oral ossicle of Manfredaster bulbiferus (Forbes, 1848) (NHMUK EE 17647a). Provenance: A–E are from the upper Oxfordian, Hypsilum ammonite Zone, Plettenberg, near Balingen, Germany. F, I are from the upper Pliensbachian, outcrop on south side of wadi immediately to west of road, 0.5 km south of the Kasbah of Amellago, Atlas Mountains, Morocco. G is from the Carnian (Upper Triassic), Cortina d’Ampezzo, Italy. H is from the lower Bathonian, Zigzagiceras zigzag ammonite Zone at La Pouza, near La Voulte-sur-Rhone, Ard̀eche, France. K, M, N are from the coranguinum Zone, Santonian, Broadstairs, Kent, UK. J, L are Recent, Philippines. Scale bars: A, D, G, H, K, M = 0.5mm; B, E = 0.2 mm; C, F, I, J, L, N = 1 mm.

Figure 25. A–F, H, J–L, Podosphaeraster pulvinatus Rowe & Nichols, 1980; A–C, entire individual, in actinal, lateral and abactinal views, respectively (MnhnL OPH174); D, interior of individual to show papular pores and interradial structures (MnhnL OPH175); E, madreporite; F, external view of isolated abactinal ossicle; H, abactinal ossicle in lateral view to show papular pores and articulation structures; J, broken surface of abactinal plate, to show separate layers of stereom; dense perforate stereom (top); dense labyrinthic stereom (middle); open reticular stereom (bottom); K, L, abactinal spines (E, F, H, J–L, MnhnL OPH176). G, Valettaster ocellatus (Forbes, 1848), abactinal ossicle to show similarities to P. pulvillus (NHMUK EE 17697). I, Valettaster sp., oral ossicle, radial view (SNSB-BSGP 2020 XLV3). Provenance: A–F, H, J–L are Recent, Philippines. G is from the Santonian, east Kent, UK. I is from the lower Maastrichtian of Rugen, Germany. Scale bars: A–D = 10mm; E–H = 1 mm; I, K, L = 0.5mm; J = 0.2 mm.