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Discussion
Until the discovery in August. 1952. of this problematic fossil no Jurassic asteroid was know from New Zealand. Although identification appeared at first sight almost hopeless, the unusual character of the five pairs of radially-placed ridged plates around the mouth seemed to offer a chance of success. It was first assumed that the mould was a negative impression of the adoral lower surface, and that the grooved plates were therefore oral plates. A few genera, such as Archaster, have very large oral plates, but their detailed sculpture differs, and it was difficult to understand why the plates in the fossil were grouped in radial pairs instead of interradial pais, as is universally the case Furthermore, the lack of any impression of inferomarginals was puzzling, and seemed to point rather to some non-phanerozonian genus, in which case the large size of the oral plates seemed anomalous. The alternative interpretation was therefore tired, namely that the specimen is a negative impression of the internal adoral surface, such as would result from the percolation of sand and mud into the coelomic cavity after decay of the aboral surface of the disc. On this assumption the grooved plates would be some peculiar development of the ambulacral series of ossicles This would account for their radial position and for their relatively greater distance from the month impression than should be the case with oral plates. It would also explain the lack of any impress of marginal plates. Since internal skeletal structures of asteroids are seldom mentioned in.
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Figs 1 and 2—Odontaster priscus sp. nov. holotype. Fig. 3—ambulacral ossicles in Odontaster meridonalis (Smith), seen from above in a specimen of R 40 mm. 1 AMB, enlarged first ambulacral. Figs. 1 and 2 are drawn from positive plasticine presses made from the original negative natural mould.
systematic diagnoses, it was necessary to dissect a series of representative genera in order to discover whether skeletal plates of the type noted occur in any of them. Analogous structures were found in a specimen of Odontaster meridionalis (Smith), from Heard Island. In this species the first pair of ambulacral ossicles is much enlarged, and each plate canres on its upper surface several transverse grooves and ridges which traverse a concave depression; Fig. 3 illustrates the condition in a specimen of major radius 40 mm. Reference to Fisher (1911) shows that he had noted similar structures in Odontaster crassus. He writes (p. 157): “The enlargement of the first pair of ambulacrals with the accompanying development of the dorsal transverse muscles is undoubtedly connected with the movement of the combined mouth plates. These, being angular, almost completely close the actinostome, and the small marginal spines completely close the opening … The enlargement of the first pair of ambulacrals is abrupt, and so noticeable as to form a good generic character. The same enlargement occurs, but in a leaser degree, in Asterodon singularis (Mueller and Troschel).”
Evidently the grooved and ridged radial plates in the Onewhero fossil are similarly enlarged ambulacral plates Dissected specimens of Asterodon miliaris, A. robustus and A dilatatus were found to show a weak development of the same type but, as Fisher found in A. singularis, the ambulacrals are not nearly so strongly enlarged as in Odontaster. Both Asterodon and Odontaster are placed in the family Odontasteridae. It is therefore concluded
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that some tendency towards enlarged and ridged first ambulacrals is a familial character of the odontasteirdae, but is fully expressed in Odontaster only. Accordingly the Onewhero fossil is referred to that genus, sensu lato.
A second fragmentary impression of a starfish of the same size as the holotype occurs on the same slab. It is an external mould, and the impressions of marginal plates can be seen on one arm. There is no reason to doubt that it is the same species. Odontaster, like Asterodon, is a southern circum-polar genus in existing seas, and is restricted to colder temperate and sub-antactic waters. However, before Odontaster priscus can be utilized in zoogeographic studies one will need to know whether it exhibits the other diagnostic features of odontaster sensu stricto.