A review of the British Silurian cystoids

Abstract

New interpretations of morphology, new occurrences and new taxa of British Silurian cystoids are summarized. Two ambulacral patterns occur in the Callocystitidae. In one the first two brachiole facets branch to the left in ambulacra B and D, but only the first one in ambulacra A, C and E (the B þ D arrangement). In the alternative pattern all ambulacra are the same with only the first brachiole branching to the left. The B þ D pattern is plesiomorphic. Among callocystitids with four ambulacra the type species of Tetracystis has the B þ D arrangement as does 'Apiocystites' elegans; both are referred to Tetracystis. The British species 'T.' oblongus has all ambulacra the same and becomes type species of the new genus Troosticystis. New records include Homocystites brenchleyi sp. nov. from the Rhuddanian of South Wales, which demonstrates that the family Cheirocrinidae did not become extinct at the end of the Ordovician as previously suspected. The British Silurian cystoid Prunocystites fletcheri has been recorded from Norway, the only species known to occur outside Britain. Schizocystis armata (Forbes, 1848) occurs at a new locality near Buildwas, Shropshire. Glansicystis glans sp. nov. is described for 'the common form' of G. baccata. It differs from G. baccata in having a larger oral area, weaker thecal ornament, shallower plate sutures and a more cylindrical theca.

Callocystites fresti sp. nov., and the significance of ambulacral branching in the Callocystitidae (Echinodermata, Glyptocystitoida)

“…In addition, where the ambulacral structure is known, all callocystitines have the first two brachiole facets branching to the left in ambulacra B and D, but only the first on the left in the other three ambulacra (see Paul and Donovan, , pp. 436–7).…”

“…This pattern, known as the B D different pattern (Figs. and ), is plesiomorphic within the Callocystitidae and occurs in the ‘ancestral’ families Cheirocrinidae (Paul, ) and Macrocystellidae (Paul, ; Paul and Donovan, ), as well as in the Ordovician callocystitid Lepadocystis (Carpenter, , see Fig. ).…”

“…Broadhead and Strimple () transferred the genera Schizocystis Jaekel, , Glansicystis Paul, , and Osculocystis Paul, , from the family Echinoencrinitidae to the Callocystitidae because they possessed pectinirhombs with vestibules, unlike other echinoencrinitids, but like all callocystitids. They also added the new genus Tyrridiocystis Broadhead and Strimple.…”

“…This suggests the reduction is potentially significant. However, both three and four plates are present within the genus Staurocystis (see Paul, , figs 8, 12) and Pseudocrinites (compare Paul, fig. 15 with Kesling, , fig.…”

Testing for homologies in the axial skeleton of primitive echinoderms

“…14) illustrated two examples of the same rhombiferan species, Glansicystis glans Paul (in Paul and Donovan, 2011), which fall into both groups (see Fig. 12.1, 12.2).…”