Onychocella Jullien, 1882

Taylor, Paul D., Martha, Silviu O. & Gordon, Dennis P., 2018, Synopsis of ‘ onychocellid’ cheilostome bryozoan genera, Journal of Natural History (J. Nat. Hist.) 52 (25 - 26), pp. 1657-1721 : 1662-1665

Genus Onychocella Jullien, 1882

( Figure 1 )

Type species

Cellepora angulosa Reuss, 1848 , by subsequent designation ( Canu 1900, p. 388). Miocene of Austria.

Diagnosis

Colony encrusting. Autozooids hexagonal; zooidal boundaries raised, with a medial groove. Cryptocyst extensive, granular. Gymnocyst lacking. Opesia subterminal, occupying about one-third of frontal surface, bell shaped ( Figure 1 (a,d)), lacking opesiular indentations and strong lateral constrictions, proximal edge concave; operculum semicircular. Ovicell immersed, the frontal wall of the distal zooid overhanging the opesiae of the brooding zooid which has a slightly enlarged opesia ( Figure 1 (c)). Avicularia vicarious, asymmetrical ( Figure 1 (b,e)), half the size or slightly less than an autozooid; opesiae inverted pear shaped or elliptical; rostrum long, curved or straight, acutely triangular, floor pustulose; condyles present on opposite sides of avicularian opesia; mandible curved, the convex side with a broad wing. No pore chambers.

Remarks

The Recent species Onychocella marioni Jullien, 1882 is often cited as the type species of Onychocella , usually followed by a remark that it is a likely or definite junior synonym of the fossil species Cellepora angulosa Reuss, 1848 . Unfortunately, and in contrast to most of the new genera he introduced, Jullien (1882) failed to designate a type species for Onychocella in his original description of the genus. The first author to give a type species appears to have been Canu (1900, p. 388), who chose the fossil species Cellepora angulosa . In the same paper Canu considered Onychocella marioni Jullien, 1882 to be a junior synonym of Onychocella angulosa ( Canu 1900, p. 386) . Although Canu maintained Cellepora angulosa as the type species of Onychocella in several subsequent publications (e.g. Canu 1911), he later claimed ( Canu and Bassler 1930, p. 23), mistakenly, that Jullien (1882) had designated Membranipora antiqua Busk, 1858 as the type species of Onychocella . Harmer (1926), who followed Canu (1900) in regarding Cellepora angulosa as the type species of Onychocella , synonymised Cellepora angulosa , Membranipora antiqua and Onychocella marioni . To correct what they believed to be an error, Canu and Bassler (1930) provided redescriptions of all three species and how to distinguish them. It was Bassler (1935, p. 159, 1953, p. G168), who erroneously indicated Onychocella marioni as the type species of Onychocella . Although this mistake was pointed out by Brown (1958, p. 40), some subsequent authors (e.g. Prenant and Bobin 1966; Gordon and Taylor 1999) have maintained Onychocella marioni as the type species of Onychocella , and in recent times it has become ubiquitous to indicate Onychocella marioni as the type species of this genus. The issue is complicated by the fact that Jullien (1882, p. 280) gave a complete description of O. marioni but only listed the type species chosen by Canu (1900) as ‘ Membranipora angulosa Reuss. Manzoni’, implying that he was referring to the species identified by Manzoni as this species and not necessarily the species in the sense of Reuss (1848), a publication to which he may not have had access.

Aside from the putative synonymy of Onychocella angulosa with O. marioni , it has also been suggested that O. antiqua is another synonym ( Waters 1891). Although Canu and Bassler (1930) regarded these three as separate species, other interpretations were proposed by later authors (e.g. Gautier 1962; Hayward 1974), with few regarding all three species as distinct. Most modern researchers tend to accept at least the synonymy of Onychocella marioni with Onychocella angulosa . The similarities evident in Figure 1 between a Recent colony identified as O. marioni ( Figure 1 (a–c)) and a Miocene colony of O. angulosa ( Figure 1 (d–f)) are clear, with the major differences being attributable to diagenesis of the fossil causing growth of epitaxial calcite into openings. However, a revision of the type material of all three species, which is beyond the scope of the current paper, would be needed to clarify the likely synonymies.

Whereas all three species described by Jullien (1882) when introducing Onychocella are extant and have encrusting colonies, he listed a further 26 fossil species with more disparate colony forms, including several erect species with bifoliate or vincularian (cylindrical) colonies. Some of these species are not closely related to Onychocella , notably Elea hexagona d’ Orbigny, 1852a which is a cyclostome rather than a cheilostome bryozoan ( Pergens 1889; Taylor 1994). It is clear that Jullien (1882) conceived the genus broadly. Indeed, a large number of fossil species with non-encrusting colonies have since been referred to Onychocella . Colony form is known to be labile among cheilostome bryozoans – several genera (e.g. Calyptotheca Harmer, 1957 ; Microporella Hincks, 1877b ; Thalamoporella, 1887 ) contain both encrusting and erect species – and therefore encrusting colony form may not be a useful character in diagnosing Onychocella .

Opesial shape varies among the species assigned to Onychocella by Jullien (1882) and subsequent authors. In addition to the bell-shaped opesiae of the type species (e.g. Cook 1964; fig. 11(a)), other species have semielliptical (e.g. O. meijeri Voigt, 1987 ; O. alveolata Taylor and McKinney, 2006 ) or ovoidal [e.g. O. cyclostoma ( Goldfuss, 1826) , see Voigt 1979] opesiae. Species with indentations at the proximolateral corners of the opesia have also been classified in Onychocella [e.g. O. sexangularis ( Goldfuss, 1826) , see Voigt 1979].

Spines are generally lacking in Onychocella , as in most other onychocellids except for Cheethamia , Hoplitaechmella and Tornipora (see below), but two Cretaceous species – O. propinqua ( von Hagenow, 1851) and O. spinifera Voigt, 1981a – have oral spines, a character that Voigt (1981b) interpreted as primitive. The occurrence of oral spines in early astogenetic stages at the minimum of Wilbertopora (see Cheetham et al. 2006), the oldest and arguably most primitive neocheilostome, lends support to Voigt’ s interpretation.

Immersed ovicells are perhaps ubiquitous in species that can be assigned unequivocally to Onychocella . These are very inconspicuous in the type species, evident only through the modification of the distal zooidal boundary which in ovicellate zooids is overlapped by the cryptocystal frontal wall of the distal zooid but in non-ovicellate zooids has a ridge and furrow like the rest of the zooidal boundary ( Figure 1 (c)). Sometimes the ovicell is expressed by a slight convexity in the proximal cryptocyst of the distal zooid. Acccording to Ostrovsky (2013), Onychocella and related genera have ‘escharelliform’ ooecia in which the ooecium is fused with the proximal cryptocyst of the distal zooid. The ooecium has the same surface texture as the cryptocyst of the distal zooid and is therefore described in the current paper as ‘cryptocyst-like’ although it is not strictly a cryptocyst.

A distinctive feature of some Cretaceous species of Onychocella is the occurrence of voids in the proximal cryptocystal frontal walls (e.g. O. alveolata Taylor and McKinney, 2006 ). Similar voids occur in the onychocellid genera Escharifora and Kamilocella gen. nov. (see below), and also in some genera belonging to another microporoidean family, Coscinopleuridae (see Voigt 1956; Koromyslova et al. 2018). It is unclear whether the voids (1) are primary structures of the cryptocyst, (2) result from subsequent resorption of parts of the cryptocyst by the living colony, or (3) are diagenetic features remaining after dissolution of patches of cryptocyst that were more soluble, perhaps composed of aragonite or high-Mg calcite.

Avicularia in most species that have been assigned to Onychocella are asymmetrical, with the rostrum angled or curved to the left or right so that it lies between the distal and a distolateral autozooid. However, some fossil species have been placed in Onychocella that have symmetrical avicularia with straight rostra, even in recent publications, e.g. O. pyriformis ( Goldfuss, 1826) (see Voigt 1979, p. 4, fig. 2); O. novaki ( Brydone, 1910) (see Zágoršek and Vodrážka 2006, p. 3, fig. 1); and O. schopforum Taylor, 2008 . Variability in the symmetry of avicularia may even occur within a species, as in O. alveolata Taylor and McKinney, 2006 (compare their pl. 79, fig. 2(a) with pl. 80, fig. 1(d)). There is also some variability in the shape of the avicularian opesia among species placed in Onychocella , with a proximal slit developed in species such as Onychocella acmon (d’ Orbigny, 1851) .

Range

Cretaceous (Cenomanian) to Recent.

Figures

Figure 1. Onychocella angulosa (Reuss). (a–c) Recent, Madeira, NHMUK 1911.10.1.640; (a) group of autozooids and avicularia; (b) avicularium; (c) two ovicellate autozooids (arrowed), two non-ovicellate autozooids and an avicularium. (d)–(f) Middle Miocene (Badenian), St Margarethen, Burgenland, Austria, NHMUK BZ 8212 (Pitt Collection); (d) group of autozooids and avicularia; (e) avicularium; (f) early astogeny with ancestrula at centre. Scale bars: a, d, f = 500 µm; b, c, e = 200 µm.