Holocystitidae Miller, 1889

Family Holocystitidae Miller, 1889

Emended diagnosis.—Anambulacralians with four or five ambulacra each with a single large facet; with humatipores (simplified from Paul 1971).

Remarks.—Sphaeronitids and holocystitids are united by the presence of six inter-radial oral cover plates called palatals ( Fig. 4 View Fig ). Holocystitids have pore-structures called humatipores ( Fig. 5 View Fig ), whereas sphaeronitids bear diplopores and are characterized by a mouth frame of six orals, five of which are directly radial ( Paul 2017: 587, fig. 5.1–3). Holocystitids have variable oral frame plating. Parasphaeronitids include two genera, both with an oral frame of eight plates, four of which are directly radial ( Bockelie 1984). Aristocystitids have an elongate mouth surrounded by eight plates and covered by a double series of cover plates, larger outer cover plates and smaller central cover plates ( Paul and Gutiérrez-Marco 2022: 154, fig. 1). Nevertheless, Oretanocalix Gutiérrez-Marco, 2000 , the only aristocystitid known with five ambulacra, also has six oral cover plates that reach all the way from the edge of the peristome to the median suture. These are thought to be homologous with palatals, suggesting that the palate of holocystitids and sphaeronitids is a derived paedomorphic character. Furthermore, Oretanocalix and some other aristocystitid and sphaeronitid genera have diplopores that were covered by a thin epistereom in life. Thus, their diplopores were closer to the structure of the humatipores of holocystitids.

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Frest (1983: 576, table 26) and Frest and Strimple in Frest et al. (2011: 69) subdivided the Holocystitidae into three subfamilies: the Holocystitinae, Pentacystinae, and Trematocystinae. These need re-assessment in cladistic terms ( Sheffield and Sumrall 2017: 759, fig. 5) and are not considered further here, although the adjective “pentacystine” is useful in discussing relevant genera (see below). Frest’s (1983) subgenera of Holocystites Hall, 1864 , are also not discussed further here.