Septocrinidae Mironov 2000

Family Septocrinidae Mironov, 2000

Diagnosis. Aboral cup inverted conical, much broader than high. All sutures between basals and radials conspicuous. Basal and radial circlets almost equal in height or radials higher than basals. Tegmen not inflated, with prominent anal sac. IBr1+2 ax, primibrachials without inner knobby processes. Maximum number of consecutive muscular articulations in free arms from 6 to16; brachial pair IIBr1+2 united by transverse synathry (not known for Zeuctocrinus); beyond IIBr2, flat and smooth synostoses, the first at IIBr3+4, the second usually from 7+8 to 9+10 (7+8 to 13+ 14 in Rouxicrinus), the others irregularly placed. First pinnule on Br6 to Br12. Pinnules without cover plates, lateral plates and x-like spicules on tube-feet; ambulacral grooves with two rows of curved rod-like spicules and sacculi; pinnulars crescentic in crosssection (not V-shaped). Proximal pinnules free, not attached to tegmen. Discoid proximal columnals single to numerous, proximalmost columnals one-pieced or compound. Distal synathries circular to ovoid with irregular relief on fulcral ridge. Distal stalk attachment by root-like radix

Included genera: Septocrinus Mironov, 2000, Zeuctocrinus A.M. Clark, 1973, Rouxicrinus gen. nov.

Remarks: The following features have been used as the main diagnostic characters for Septocrinus (the main differences from Zeuctocrinus gisleni and Z. spiculifer): (1) high distal interradial projections of radials, (2) long median prolongation in IBr2 and (3) compound structure of most proximal columnals (Mironov, 2000).

Extant ten-armed Bourgueticrinida exhibit six types of pinnule structure (Mironov, 2008), one of which is unique to the Septocrinidae. However, pinnule form and structure in Septocrinidae is closely similar to that of some comatulids, for example Antedon tenella, A. bifida, (Grieg, 1903, figs. 3A, 3C), Trichometra cubensis, Hypalometra defecta, Hathrometra prolixa (A.H. Clark, 1921, figs. 766, 768, 775), Phrixometra nutrix, Isometra vivipara, I. flavescens (John, 1938, figs.11, 15, 16). These comatulid species and septocrinids all have pinnulars crescentic in cross section, with rod-like spicules, and sacculi in the ambulacral grooves. Cover (ambulacral) and x-like tube feet plates are common in living Bathycrinidae (cover plates are lacking only in Discolocrinus Mironov, 2008) are absent in septocrinids.

Distolateral corners of adjacent RR in Septocrinus disjunctus form narrow triangular interradial projections separating bases of adjacent rays (Mironov, 2000, fig. 7. 4). The long interradial projections of the radials are known also in many comatulids and some extinct cyrtocrinids (Phyllocrinidae Jaekel, Eugeniacrinitidae Roemer, Nerocrinidae Manni and Nicosia). The interradial ridges have wide intraspecific variation in the Eocene genus Tormocrinus Jaekel, and they are weakly developed in fossil Conocrinus Orbigny (Bourgueticrinidae) (Roux, 1978). Another unusual feature in Septocrinus is a large median prolongation of IBr2ax, located between the two following elements. Similar prolongation of IBr2ax is known also in some extinct cyrtocrinids (Eugeniacrinites Miller, Lonchocrinus Jaekel). The compound proximal columnals is a stable feature of Septocrinus disjunctus, being well developed in all three known specimens. Fragmentation of the proximal columnals is otherwise unknown in the Bourgueticrinida, but is particularly well developed in Vityazicrinus petrachenkoi Mironov and Sorokina (Vityazicrinidae), Dumetocrinus antarcticus (Bather) and Feracrinus aculeatus Mironov and Sorokina (Hyocrinidae). The proxisteles of Septocrinus, Vityazicrinus, Dumetocrinus and Feracrinus with compound proximal columnals probably resulted from long evolutionary changes (See below “Notes on ecology and morphological adaptations”).

Septocrinidae also differ from the Bathycrinidae (sensu stricto) in the following characters: (1) no inner knobby processes on IBr ossicles (SEM micrographs of these processes in Bathycrininus aldrichianus can be found in Macurda and Meyer, 1976, Pl. 3. 5 and in B. mendeleevi by Mironov, 2008, Figure 4 C), (2) another proximal free arm pattern: from 1+2 3+4 7 to 1+2 3+4 9 (usually 1+2 3+4 5 or 1+2 4+5 7 in Bathycrinidae), (3) great number of consecutive muscular articulations in the distal arm, (4) almost flat and smooth synostoses beyond IIBr2 (rudimentary transverse synarthry in Bathycrinus), (5) few discoidal columnals in Septocrinus and largest Zeuctocrinus (but about 10 in the smallest paratype of Z. gisleni and more than 30 discoid columnals in Rouxicrinus) (usually 10 columnals or more in Bathycrinidae), (6) irregular syzygial stereom on the fulcral ridge of the distal columnals (regular secondary crenularium in the Bathycrinidae except for Bathycrinus volubilis Mironov, Discolocrinus thieli Mironov), (7) dense arrangement of Ps: when viewed from the side, every second or third Br bearing a P (P on every fourth Br in Bathycrinidae).

Septocrinidae differ from the ten-armed Caledonicrinus Avocat and Roux and Naumachocrinus A.H. Clark in having developed BB and radix, more distal position of P1, and in lacking cover and side plates (pinnule structure not known for Naumachocrinus).

Among the crinoids with five unbranched arms (Democrinus Perrier, Conocrinus Orbigny, Porphyrocrinus Gislén) and with arms branched beyond IBr2 (Phrynocrinus A.H. Clark, Porphyrocrinus thalassae Roux), Phrynocrinus shares several features with Septocrinus and Zeuctocrinus. All three genera have rod-like spicules in the ambulacral grooves, an oblique fulcral ridge on the muscular brachial articulations, almost smooth non-muscular brachial articulations (synostoses), and proxistele absent or with a few discoidal columnals. However Phrynocrinus differs significantly from septocrinids also in possessing a lobed stalk attachment disk, strongly inflated tegmen, and very long proximal pinnules attached by thick bases to the tegmen (A.H Clark, 1907; A.M. Clark, 1973; Imaoka et al., 1991, Donovan & Pawson, 1994; Kogo, 1998; Roux et al., 2002).