Valettaster Lambert 1914
Creators: name: Gale, Andrew Scott
Publisher: name: Zenodo
Year: 2021
Subjects: Biodiversity
Taxonomy
Animalia
Echinodermata
Asteroidea
Valvatida
Sphaerasteridae
Valettaster

description: Genus Valettaster Lambert, 1914
Type species. Oreaster ocellatus Forbes, 1848, by subsequent designation of Wienberg Rasmussen (1950, p. 93).
Diagnosis. Sphaerasteridae in which the undifferentiated primary body ossicles have the form of a truncated cone. The flat top of the cone forms the external face of the ossicle, and carries beads of imperforate stereom. The interstices between the faces of the abactinal ossicles are packed with smaller ossicles which also carry a beaded sculpture. Together, the primaries and the interstitial ossicles form a flush outer surface. Tiny papular pores are formed by embayments in two or three interstitial ossicles.
Included species. Oreaster argus Forbes, 1848, O. ocellatus Forbes, 1848, Sphaerites digitatus Quenstedt, 1858, Valettaster granulatus Brunnich Nielsen, 1943, V. stipes Villier, 2010, V. thuyi sp. nov. Valettaster dangeardi Mercier, 1935 (1935, 38, pl. 2, fig. 7) from the Bathonian of Calvados, Normandy, was based upon a single, conical abactinal ossicle, with some resemblance to those of V. argus. Breton (1985) noted that the illustration is stylized, the material was lost and the species must therefore be considered as doubtful.
Reconstruction and description of Valettaster. Although material of Valettaster has been known since the middle of the nineteenth century, most descriptions have concentrated on the highly distinctive primary ossicles (undifferentiated actinals/marginals/abactinals). The ambulacral and adambulacral morphology of the Late Cretaceous species V. ocellatus was described briefly by Breton (1985), who also commented on the construction of the external surface and possible life habits.
The morphology of the genus can be reconstructed on the basis of well-preserved material of V. ocellatus from the Upper Cretaceous (Coniacian–Santonian) Chalk of southern England (NHMUK E 5012, E 5019). Additionally, a specimen of V. argus from the Santonian coranguinum Zone of Thanet, Kent (NHMUK EE 17637) comprises fragments of the external surface with all ossicles in situ, and abundant dissociated ossicles, including ambulacrals, adambulacrals and an oral. Several specimens of V. argus (e.g. NHMUK E 4344, E 20294) show the overall form to be spherical to sub-spherical but provide few further details.
The individual of V. ocellatus from the upper Santonian Marsupites Zone of Brighton (NHMUK E 5012), previously figured by Spencer (1907, pl. 25, fig. 4, 4a) and Breton (1985, fig. 5) shows the form and ossicle arrangement of the actinal portion of this species (compare Figs 23A, 24A, B). The abactinal surface is crushed, but can be seen to be made up of relatively few, large ossicles. The abactinal ossicles have the form of truncated cones, with an embayed polygonal outline. The actinal surface is largely undistorted, and has the form of a slightly prolate hemisphere, in which the ambulacral grooves extend up to the ambitus. The arrangement of the primary actinal plates can be precisely reconstructed in NHMUK E 5012 (Fig. 24C, D). The ossicles closest to the peristome form a double chevron, abactinal to which a single enlarged interradial ossicle is present.
The arrangement of abactinal ossicles is shown by a single specimen of V. ocellatus (NHMUK E 5019) from the coranguinum Zone (upper Coniacian–lower Santonian) of Micheldever, Hampshire. Although the specimen is fragmentary, and some ossicles are displaced, landmark features can be identified and thus allow reconstruction of the plating of the abactinal surface of Valettaster for the first time (Fig. 24D). Importantly, the pir can be identified, because these are abactinal to and articulate with an abactinally-actinally elongated pair of symmetrical ossicles that share a straight interradial margin. The pr are identified from their radial position and possess large, oval, external faces. The more distal radials have rounded-rectangular external faces and are proportionately broad. Four radials are present between the pr and the abactinal termination of the ambulacral groove. The terminal is not visible, but the most distal adambulacrals are more or less in place. The interstital ossicles that infill the spaces between the larger conical primaries are very variably sized and irregular in outline, and a number are still in their original place.
A most distinctive feature of Valettaster is the morphology of the (undifferentiated) abactinal ossicles, which have the form of truncated cones in V. digitatus, V. ocellatus and V. granulatus (Brunnich Nielsen, 1943) and cones in V. argus (see below). In the abactinal hemisphere the primary ossicles have rounded, irregularly embayed outlines and lack clear articulation surfaces with each other. The actinal ossicles immediately adjacent to the peristome and the ambulacral groove articulate with each other by means of flattened surfaces. The external faces of the primary ossicles possess a variably sized flattened exterior face (always smaller than the dimension of the inner face of the ossicles) which has a sculpture (constructed of perforate stereom) of granules (V. granulatus) or radiating rows of beadlike structures (V. ocellatus, V. argus).
The bevelled sides of the abactinal ossicles of Valettaster carry numerous irregularly sized and -shaped small articulation surfaces, each of which carries a central depression, which possibly housed a ligamentary structure. The spaces between the external faces are infilled with smaller secondary ossicles, which articulate with the primary ossicles by means of the small articulation structures with central pits (Fig. 23H, I, J). In V. argus, a few abactinal ossicles have an external face, but much of the surface is made up of smaller infilling secondary ossicles which possess a reticulate sculpture with poorly defined radial elements.
Breton (1985, fig. 4) reconstructed Valettaster with a peripheral zone of external ossicles which were enclosed in a soft dermal tissue, and decreased in size towards the exterior. An exceptionally preserved specimen of V. argus (Fig. 23H) demonstrates that this was not the case, supported by the presence of in situ smaller ossicles in a specimen of V. ocellatus (NHMUK E 5012; see Fig. 23A). The smaller infilling ossicles possess small, flattened external surfaces, with similar sculpture to that of the primaries, and together, the external faces of the primary and secondary ossicles form a flush external surface (Fig. 23H). The larger surfaces of the primary ossicles are set in a background of smaller secondary plates.
Very small papular pores are present on the external surface of V. argus (Fig. 23J), as rounded triangular or slit-like openings between secondary ossicles, 200–300 M m in diameter, defined by two or three concave faces. The madreporite is small and irregularly triangular in outline in V. argus. The surface bears coarse radial ridges. The ossicle is surrounded by the pir and two distal ossicles (Fig. 23K).
Because the morphology of Valettaster ocellatus has now been reconstructed, it is possible to relate individual morphologies of the (undifferentiated) abactinal/actinal primary ossicles to different positions on surface of the animal:
1. Oval to sub-rectangular, proportionately flat ossicles with a large external face and a narrow marginal, bevelled rim characterize the upper abactinal region and include the pr and pir.
2. Ambital ossicles are thick, elongated and irregular in outline, with a small oval external face. On the ambitus, this face is placed centrally, but immediately beneath the ambitus the face is positioned eccentrically, on the actinal portion of the ossicles.
3. Ossicles of the actinal interradius are moderately thick, rounded pentagonal to rectangular in outline, with a moderately sized external face and a broad, steep bevelled margin.
4. The unpaired actinal ossicle adjacent to the orals is rhombic in outline, symmetrical, with a moderately large external face.
The ambulacrals and adambulacrals of Valettaster have been described and figured for V. ocellatus by Breton (1985), and the description is here extended to a specimen of V. argus. The adambulacrals are tall and curved, the adradial face convex, the abradial concave (Fig. 7P, R). A depressed, oval insertion site for adadm is present on the proximal and distal faces. The abactinal surface (for contact with ambulacrals) is nearly flat and squarish in abactinal view. Articulation surfaces ada1a, ada1b, ada2 and ada3 are rounded to oval in outline and smooth, and define the corners of the abactinal face. Muscle sites padam and dadam are positioned centrally. Dadam is oval and transversely oriented, and padam is obliquely set to the long axis of the ambulacral. This arrangement is typical of valvatidan asteroids (Gale 2011). However, on a number of adambulacrals, the centres of the articulation surfaces are concave, and in one, the proximal facets are reduced to a single narrow transverse strip of perforate stereom and a short crescent representing the proximal margin of ada3 (Fig. 7P). The concavities may possibly have acted as sites for ligaments to strengthen the ambulacral-adambulacral connection, and are comparable with those developed in Sphaeraster tabulatus (Fig. 7C). The actinal face of the adambulacrals bears a row of small spine bases (seven to nine) for the furrow spines on the adradial margin. The remainder of the actinal face comprises irregular rounded and oval, variably conjoined patches of raised perforate stereom.
The ambulacrals are short, compressed and broad and very variable in shape in the proximal/distal aspect. In actinal view, the abtam and actam insertion sites are well defined, and dentition is clearly well developed as a series of coarse oblique ridges and grooves (Fig. 6H–K). The ambulacral head, shaft and base are of equal length and poorly defined. A transverse ridge, separating proximal and distal podial basins, is present. The articulation area with the adambulacral is set nearly at right angles (90–120 Ǫ) to the actinal face of the ossicle, and articulation surfaces ada1a and ada2 (adradially) and ada1b and ada3 (abradially) are separated by a concave region. The abradial articulations are borne on an actinal flattened area which has a sharply defined, straight abradial margin. On some ossicles, the articulation surfaces for the adambulacrals (ada2, ada3, ada1a, b) possess central concavities. Small muscle facets for padam and dadam are present on proximal and distal faces. In proximal/distal view, small facets for lim are present on the head of the ambulacrals. An abactinal-lateral process is present on the base of all ossicles, but varies considerably in shape and size. On some ossicles of material of V. argus studied (and all the ambulacrals of V. ocellatus in NHMUK E 5012), a large globular, actinally directed process is present actinal to the actam (Fig. 23F, G). Proximally, the surface of this structure is rounded and distally has a central hollow, contiguous with the podial notch of the ambulacral shaft. The tissues associated with this structure, and its function, are unknown. The ambulacrals of Valettaster are among the most bizarre encountered among all asteroids.
Two oral ossicles of V. argus are known (Fig. 8K, M). The body of the oral is oval and twice as long as deep, the radial face is convex, the interradial face slightly concave. The facet for insertion of the oradm is sub-parallel with the axis of the body. An oval swelling on the abactinal proximal margin of the ossicle is called a proximal protuberance (pp). A radial projection of the circumoral articulation (dcoa) is present. The apophyse is small and ‘Y’-shaped, comprising a short distal pcoa, and thin, wing-like striated proximal process for insertion of the riom. The ring vessel groove (rvg) is very shallow. The interradial face of the oral displays a region for insertion of the abiim on the proximal region of the apophyse. A narrow strip for insertion of the aciim on the proximal, actinal margin of the oral is present. The odontophore and circumoral are unknown in Valettaster.
Affinity and taxonomic position of Valettaster. The new description of articulated specimens of V. ocellatus forms the basis for the reconstructions made here (Fig. 24E, G), and enables the detailed arrangement of the primary ossicles to be compared with those developed in Podosphaeraster (e.g. Fujita & Rowe 2002, fig. 1). On the actinal surface, a single rhombic interradial ossicle is present adjacent to the oral ossicles, and the more distal interradial ossicles are arranged in a double (inset) chevron pattern in both genera (Fig. 23A). The primary abactinal ossicles adjacent to the groove have long, flat articulations with the adambulacrals in both genera; in V. ocellatus, seven of these ossicles are present, but only five are present in P. pulvinatus. On the abactinal surface, both possess oval, slightly elongated pir, which articulate distally with a pair of interradially symmetrical elongated ossicles. The pr are slighly larger in V. ocellatus. The number and arrangement of the other ossicles of the abactinal surface appear to be identical, although the ossicles adjacent to the periproct are unknown in V. ocellatus.
Valettaster is morphologically close to Podosphaeraster in terms of body shape (oblate spheroid) and overall construction, and the detailed arrangement and numbers of the relatively few, undifferentiated primary ossicles are identical. For example, only four radial ossicles distal to the pr are present in both genera (V. ocellatus, P. pulvinatus); the number and position of the ico between the pr and pir are identical; the ossicle pair adjacent to the distal margin of the pir are symmetrical and elongated in both genera; and the actinal primary ossicles are of similar arrangement and outline, with a single, rhombic ossicle adjacent to the orals, and an inset double chevron of plates present in the actinal interradius (Fig. 24E).
The articular faces of some of the abactinal ossicles in P. pulvinatus display a bevelled rim made up of irregular processes and embayments carrying rounded ligament pits that are closely similar to those developed in V. ocellatus (Fig. 25, compare F, G). Valettaster differs most importantly in the conical form of the primary ossicles, the external interstices of which are packed with small ossicles, forming a flush surface from which papulae protruded through special openings. It also differs in the presence of a free madreporite that articulates with three ossicles. In Podsphaeraster, the highly distinctive, small madreporite is enclosed within the pir ossicle or a group of fused ossicles (Fig. 23K).
All other Sphaerasteridae have tabulate primary ossicles, notched along the margin for papulae. Valettaster also differs significantly from other Sphaerasteridae (except Eosphaeraster gen. nov.) in the compressed, highly modified ambulacral ossicles. The ambulacrals compare well with those of Podosphaeraster in the shape of the ambulacral base and the orientation of the adambulacral articulation structures. The adambulacrals of Valettaster are very similar to those of Sphaeraster in their shape, articular surfaces and muscularization. However, they compare with those of Podosphaeraster in the development of a rugose external actinal face made up of patches of perforate stereom.
The close relationship between Valettaster and Podosphaeraster suggested by the overall constructional similarities is strongly supported by the detailed similarities between the peculiar and highly specialized oral ossicles of the two genera (Fig. 8J–M). The shape of the oral body and apophyse are similar, and the presence of numerous unique shared structures, including the proximal protuberance, the radially directed dcoa, and the striated insertion for the riom strengthen the relationship between the two genera. The presence of Valettaster in the Pliensbachian (Early Jurassic) indicates that the division between the two genera must predate this, and may well have occurred in the Triassic or earliest Jurassic (Fig. 4). It is also important to mention that Valettaster grew to considerably larger dimensions than any known species of Podosphaeraster. The specimen of V. ocellatus described here has a major radius (R) of between 2 and 2.5 cm and, judging from the dimensions of isolated ossicles of this species, it could probably achieve even larger sizes.
A number of features of Valettaster are unique amongst the Asteroidea. These include the cavities in the ambulacral-adambulacral articulation surfaces, which possibly housed ligaments conjoining the ossicles, and the curious globular processes on the actinal surfaces of some ambulacrals, of unknown functional significance.
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 725-731, DOI: 10.1080/14772019.2021.1960911, http://zenodo.org/record/10949919