Alkaidia sumralli Blake & Reid, 1998

Figs 16A–C, 17B–E, H, K

Alkaidia sumralli Blake & Reid, 1998: 529, fig. 9/1–14.

Alkaidia sumralli – Ewin & Gale 2020: 13, figs 10.1–10.2, 13.3, 13.8, 13.11–13.12.

Diagnosis

Alkaidia in which the primary radial ossicles are elongated and the terminal ossicle is not deeply notched on its proximal margin.

Material examined

The holotype (Texas Memorial Museum, number 1786 TX1) is from the Grayson Formation (lower Cenomanian) at the Waco shale pit (Waco, Mclennan County, Texas, USA).Additional material comprises a magnificent individual (NHMUK PI EE 15225), collected by Frank Holterhoff from the Grayson Formation (lower Cenomanian) of Dottie Lynn, Fort Worth, Texas and illustrated here (Fig. 16A–C), as well as numerous dissociated ossicles from the same locality (NHMUK PI EE 18005–18007, 18009) .

Remarks

The affinities of A. sumralli have recently been discussed in some detail by Ewin & Gale (2020) and the evidence for its inclusion in the Forcipulatida (Zorocallina) and the family Terminasteridae can be summarised briefly as follows: the presence of abundant, straight ‘duck-billed’ forcipulate pedicellariae is a characteristic of the Zorocallina (Fig. 16D); the construction of the abactinal surface is closely similar to that of zoroasterids and terminasterids, which also have large, Y-shaped first superomarginals and a row of robust, lobed, quadrangular radial ossicles which imbricate proximally and each carry a centrally placed conical spine (Ewin & Gale 2020).

Additionally, the morphology of the adambulacrals and ambulacrals, and the nature of their articulation is similar in zoroasterids, Terminaster and Alkaidia (Fig. 17). Ada1a and ada2 are concave on the adamulacrals (Fig. 17A–C) and positioned on a process on the ambulacrals (Fig. 17G–H, J–K). The dadam and padam facets are subequal in size, broad and short (Fig. 17A–C).

The abactinal construction is never seen in extant benthopectinids, in which the abactinal ossicles in the arms are small and parapaxilliform or very small, and never imbricate. Additionally, in benthopectinids the abactinal surface is invariably flat, and the arm section is not subcylindrical.