Stromatocystitidae Bassler, 1935

Zamora, Samuel, Sumrall, Colin D. & Vizcaïno, Daniel, 2013, Morphology and ontogeny of the Cambrian edrioasteroid echinoderm Cambraster cannati from western Gondwana, Acta Palaeontologica Polonica 58 (3), pp. 545-559 : 547-548

Family Stromatocystitidae Bassler, 1935 Genus Cambraster Cabibel, Termier, and Termier, 1959

Type species: Trochocystites cannati Miquel, 1894 from the Languedocian (middle Cambrian) of Montagne Noire ( France) .

http://dx.doi.org/10.4202/app.2011.0152

Diagnosis.—Stromatocystitid with discoidal theca, bearing a marginal ring around the thecal ambitus bordered by peripheral skirt; aboral disk large with radially elongate plates centrally.

Discussion.— Miquel (1894) was the first to report specimens assignable to the genus Cambraster . These specimens were thought to be thecae of a cinctan and named Trochocystites cannati . Cabibel et al. (1959) determined that T. cannati was an edrioasteroid and erected the genus Cambraster to accept this species. They also described the edrioasteroid genus Eikosacystis with two species, E. couloumanensis and E.? ferralsensis. Termier and Termier (1969) described three additional species: Cambraster elegans , Eikosacystis miqueli , and E. courtessolei all based on very poorly preserved specimens.

Ubaghs (1971) concluded that the two species of Cambraster were valid, but synonymised Eikosacystis ( Cabibel et al. 1959) with Cambraster , because the only difference between the two forms was their state of preservation. Subsequently, Smith (1985) determined that C. cannati and C. elegans were synonyms, arguing that the main difference between the two forms (i.e., presence of a larger portion of the aboral surface plating around the margin in C. elegans ) was very likely a post−mortem preservational artefact.

A second valid species, Cambraster tastudorum was described by Jell et al. (1985) from the Cambrian of Tasmania, Australia. Cambraster tastudorum is represented by many well−preserved specimens, and is readily distinguished from C. cannati by the distribution of epispires on interambulacral plates, and by the plate pattern of the aboral disk surface ( Jell et al. 1985).

Although Cambraster is similar in gross morphology to isorophids, these similarities are strictly superficial. The oral frame construction is formed from radial elements in isorophids whereas the elements are interradial in Cambraster . These elements are homologues of the integrated interradial plates of Kailidiscus ( Zhao et al. 2010) and may be homologues of oral plates in other pentaradiate echinoderms (sensu Sumrall 2010a). Isorophids have uniserial floor plates that lack sutural pores in all groups, and in pyrgocystids a second series of outer biserial floor plates bearing pores are present ( Sumrall and Zamora 2011). Cambraster bears biserial floor plates that bear double ambulacral pores along the sutures. Cover plates of isorophids are biserial or in multibiseries, whereas in Cambraster they are multitiered series. Interambulacral plates of isorophids lack epispires and are added along the margins of the interambulacral areas, whereas they bear epispires and are added throughout the interambulacral areas in Cambraster .

The greatest difference is seen in the construction of the thecal ambitus and aboral surface. In isorophids, a peripheral rim is present, consisting of a series of irregularly plated circlets that decrease in size distally. All of the plates are highly imbricate and are in contact with the substrate ( Sumrall and Parsley 2003). The bottom surface is scored with a series of radial spur and groove structures. Interambulacral plates or plates of the pedunculate zone tuck under the edge of the peripheral rim rather than bearing a vertical suture ( Sumrall 1993). The bottom surface of the theca is uniformly unplated ( Bell 1976a).

In Cambraster , the marginal frame is constructed entirely different to that of a peripheral rim. The stout marginals are plates of the oral surface and are not in contact with the substrate. The interambulacral plates do not articulate with an imbricate suture beneath the plates, but abut with the upper surface of the plate with a vertical suture. Plates of the peripheral skirt, rather than being imbricate over one another, are adjacent and form a flexible portion of the peripheral wall of the theca along the ambitus. The bottom is fully plated in Cambraster rather than being unplated.

Several features of Cambraster are also similar to the early edrioasteroid−grade echinoderm Kailidiscus chinensis ( Zhao et al. 2010) . The mouth frame of both taxa bears six integrated interradial plates. In Kailidiscus these plates are bordered by a series of plates that correspond to the adradial series of floor plates, whereas in Cambraster this plate series is absent. Although both taxa bear multitiered cover plate series, they are more fully developed in Kailidiscus than in Cambraster . In both taxa the periproct is very large, offset to the left of the midline, and similar in construction with numerous small very narrow lathe shaped plates.

The poorly known early edrioasteroid−grade echinoderm Camptostroma is similar bearing numerous epispires in the interambulacral plates, but here the nature of the plating is much more complex in the arrangement of primary and secondary interambulacral plates to the epispires ( Smith 1985). The nature of the ambulacral system is poorly constrained, but it is clear that the ambulacra are curved. The side walls of the theca are corrugated and complexly plated. Although the bottom of the theca is plated, there is no differentiation of the plates into a peripheral series and a central disc.

A final taxon similar to Cambraster is Edriodiscus Jell et al. (1985) , which also bears a marginal ring, but constructed with many more medium−sized plates. Although the ambulacra are poorly known, they do bear biserial flooring plates. Edriodiscus is further differentiated from Cambraster by the plating of the aboral surface that bears more irregular plating with plates of approximately equal size. We tentatively assign Edriodiscus to Stromatocystitidae .

Geographic and stratigraphic range.— Cambraster is known from the Borobia and Coulouma Formations of West Gondwana ( Spain and France) and in the Cateena Group of East Gondwana ( Australia). The former corresponds to the Cambrian Series 3, Stage 5 and the Australian specimens occur in slightly younger strata from the Drumian stage of Series 3.