Callocystitids: Ambulacra Advancement and Rhomb Reduction

The Upper Silurian callocystitid Staurocystis quadrifasciata, from Museum Victoria.

The Palaeozoic echinoderms included many distinctive groups that have no close relatives among the modern fauna: blastoids, cornutes, solutes, ctenocystoids... to name just a few. From the Ordovician to the Devonian, this diverse fauna also included a hodge-podge assemblage known as cystoids. Cystoids are a grouping of mostly stalked echinoderms in which certain plates in the theca are perforated by regular arrangements of pores that probably functioned in respiration. Cystoids were not always regularly pentamerous like other echinoderms, and some were notably asymmetrical. The ambulacra were recumbent on the theca, and the feeding appendages were brachioles rather than arms (for the difference between brachioles found in many fossil echinoderms and arms found in crinoids, see the post on blastoids). Cystoids would have been filter-feeders and were probably largely sedentary. Cystoids include some very disparate forms, and many researchers have suggested that they may represent a polyphyletic assemblage. Various authors have suggested cystoid ancestry for other echinoderm groups, such as blastoids or crinoids, but this remains controversial.

The Upper Silurian Schizocystis armata, from Kesling (1967). The two pore rhombs of this species are visible just above the center and at the lower right of the theca.

The Callocystitidae were a family of cystoids that persisted over most of the total cystoid time range. Callocystitids belonged to the major cystoid subgroup called the Rhombifera, in which the diagnostic pore groups were arranged as paired assemblies, commonly called pore rhombs, that spanned the border between two thecal plates (as opposed to the remaining cystoids, the Diploporita, in which pore assemblies each occupied a single plate). Broadhead & Strimple (1978) diagnosed the Callocystitidae based on the arrangement and position of the pore rhombs, together with their possession of a relatively small periproct (the circle of plates that indicates the position of the anus) and the number of radial plates in the theca. All callocystitids possessed a stalk, often divided into a flexible proximal section and a more rigid distal section. Broadhead & Strimple (1978) recognised four subfamilies of callosystitids, but one of these, the Apiocystitinae was explicitly suggested to be paraphyletic to the Callocystitinae and Staurocystinae. This was supported by the numerical phylogenetic analysis of Sumrall & Brett (2002), who furthermore suggested that the Callocystitinae was polyphyletic.

Theca of the Upper Silurian apiocystitine Lovenicystis angelini, from Kesling (1967).

The fourth of Broadhead & Strimple's subfamilies, the Scoliocystinae, was suggested to lie outside the clade formed by the other three; Sumrall & Brett's analysis only included Scoliocystis, but does not contradict this. Scoliocystines have the ambulacra relatively short, restricted to the summit of the theca, and would have had only a small number of brachioles. The most extreme example was the Lower Silurian Osculocystis, which had only a single extremely long brachiole (Paul & Donovan 2011). Another scoliocystine, Schizocystis, had one side of the theca relatively flat and the pore rhombs reduced in number and restricted to the other side, and may have lain on its side in life rather than standing upright.

Reconstruction of Pseudocrinites together with a number of individuals of the discosorid Phragmoceras by Alison Carey.

The remaining three subfamilies had more extensive ambulacra, extending right down to the base of the theca in some species. Apiocystitines and callocystitines had four or five ambulacra, usually branched in callocystitines and unbranched in apiocystitines, that did not strongly protrude above the surface of the theca and had widely spaced brachioles. The more distinctive Staurocystinae had two to four stongly protruding ambulacra that carried tightly packed brachioles. In the staurocystine Pseudocrinites, the theca was discus-shaped with its two ambulacra running around the outer rim of the disc (Kesling 1967).

REFERENCES

Broadhead, T. W., & H. L. Strimple. 1978. Systematics and distribution of the Callocystitidae (Echinodermata, Rhombifera). Journal of Paleontology 52 (1): 164-177.

Kesling, R. V. 1967. Cystoids. In Treatise on Invertebrate Paleontology pt. S. Echinodermata 1. General characters. Homalozoa-Crinozoa (except Crinoidea) (R. C. Moore, ed.) vol. 1 pp. S85-S267. The Geological Society of America, Inc., and The University of Kansas: Lawrence (Kansas).

Paul, C. R. C., & S. K. Donovan. 2011. A review of the British Silurian cystoids. Geological Journal 46: 434-450.

Sumrall, C. D., & C. E. Brett. 2002. A revision of Novacystis hawkesi Paul and Bolton 1991 (Middle Silurian: Glyptocystitida, Echinodermata) and the phylogeny of early callocystitids. Journal of Paleontology 76 (4): 733-740.