Podosphaeraster pulvinatus Rowe & Nichols 1980
Creators: name: Gale, Andrew Scott
Publisher: name: Zenodo
Year: 2021
Subjects: Biodiversity
Taxonomy
Animalia
Echinodermata
Asteroidea
Valvatida
Podosphaerasteridae
Podosphaeraster
Podosphaeraster pulvinatus

description: 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.
description: Published as part of Gale, Andrew Scott, 2021, Taxonomy and phylogeny of the ' football stars' (Asteroidea, Sphaerasteridae), pp. 691-741 in Journal of Systematic Palaeontology 19 (10) on pages 735-736, DOI: 10.1080/14772019.2021.1960911, http://zenodo.org/record/10949919