Regulaecystis Dehm, 1932

Nardin, Elise & Bohatý, Jan, 2013, A new pleurocystitid blastozoan from the Middle Devonian of the Eifel (Germany) and its phylogenetic importance, Acta Palaeontologica Polonica 58 (3), pp. 533-544 : 535

Genus Regulaecystis Dehm, 1932

Type species: Regulaecystis pleurocystoides Dehm, 1932 , Bundenbach ( Germany), Hunsrückschiefer (Emsian) .

Species included: Regulaecystis pleurocystoides Dehm, 1932 (type species); R. testudineus sp. nov.

Emended diagnosis.—Theca subtrapezoidal in outline lacking posterior lobes; abanal face ornamented with a reticulate network; large angular periproct; single conjunct pectinirhomb relatively compressed (L3/L4); uniserial feeding appendages (emended from Paul 1974: 349).

Description.— Regulaecystis is distinguished by an angular outline. Six plates form the periproctal frame. The single pectinirhomb is asymmetrical and compressed with few slits (5–20). B1 is relatively small. The circlets of the thecal plates are organised in large arcs across the abanal face. The ornamentation of the theca is composed of granules and radiating narrow ridges. Robust brachiolar plates with an apparent uniserial arrangement are often ornamented with one thick granule on the opposite of the food groove.

Discussion.— Regulaecystis clearly differs from Amecystis Ulrich and Kirk, 1921 , Deltacystis Sprinkle, 1974 , Plethoschisma Sprinkle, 1974 , Pleurocystites Billings, 1854 , Praepleurocystis Paul, 1967 and Pygecystis Paul, 1984 on the number of pectinirhombs. Within the forms with a single pectinirhomb, Regulaecystis contrasts with Turgidacystis Parsley and Sumrall, 2007 by having uniserial brachioles, angular thecal outline, different thecal plating, and wider pectinirhomb with more slits. It possesses neither the typical features of Coopericystis : more roundish thecal outline, strong peripheral ornamentation, and more symmetrical pectinirhomb with less slits ( USNM 114198a), nor those of Henicocystis : subquadratic thecal outline and compressed symmetrical pectinirhomb with less slits ( Jell 1983). Paul (1974, 1984) proposed the genus Coopericystis as synonym of the genus Regulaecystis , based on the close resemblance in the thecal outline and the presence of a single pectinirhomb at the suture between L3 and L4. However, the morphology of the new specimens and the re−examination of the deposited specimens of R. pleurocystoides suggest that this synonymy should be rejected. Coopericystis retains the typical features of the Henicocystinae , but shows specifically a subpyriform theca with a roundish posterior outline (vs. a straight angular outline for Regulaecystis ), a localised raised area on the dorsal face surrounded by lateral ridges, and different plating in the basal and infralateral circlets. The generic attribution of R. devonica Paul, 1974 from the early Middle Devonian of Devon ( Great Britain) was already questioned by Sumrall and Sprinkle (1995). The re−examination of this form suggests an assignment to Coopericystis , regarding the subpyriform thecal outline, the crossing position of B1 and the similar ornamentation (notably the sinuous margins of the raised area; see Paul 1974: pl. 1), and the thecal plating as shown in C. pyriformis Parsley, 1970 . The main difference between those two species is concentrated on the shape of the pectinirhomb and the size of the laterals.

Stratigraphic and geographic range.—Emsian (Early Devonian) and Eifelian (Middle Devonian) of Germany.