Hippomenella Canu 1917

Genus Hippomenella Canu & Bassler, 1917

Hippomenella Canu & Bassler, 1917: 41.

Hippomenella – Brown 1949: 517. — Harmer 1957: 1095. — Gordon 1984: 77. — Zabala & Maluquer 1988: 117.

Lepralia mucronelliformis Waters, 1899, by original designation.

Colonies encrusting, budding intrazooidal. Frontal shield partly umbonuloid, with two or more rows of lateral areolar pores encircling an imperforate suboral area; basal pore chambers with multiporous septula. Orifice with condyles, oral spines present. Ovicell hyperstomial, ectooecium uncalcified, endooecial surface finely pitted in parts but devoid of any other structures, not closed by the operculum. Adventitious avicularia present, occasionally dimorphic.

Since its introduction, the genus Hippomenella has been assigned to a number of different families. The confusion stems from the fact that the original generic diagnosis by Canu & Bassler (1917) included characters of several superficially similar, yet structurally distinct, Recent and fossil species, resulting in a poorly defined genus. Although Brown (1949) noticed this problem and described the lectotype of Hippomenella mucronelliformis in great detail, the type species still remained relatively poorly known, as SEM images have never been published to date.

The genus has previously been placed in the schizoporelloid families Hippoporinidae Osburn, 1952 (Brown 1958: 62), Hippopodinidae Levinsen, 1909 (Gordon 1984: 77), Schizoporellidae Jullien, 1883 (Gordon 1989: 43; Gordon et al. 1994; Gordon & d’Hondt 1997: 25), and very recently in the Escharinidae Tilbrook, 2006 (D.P. Gordon, pers. comm. 2011). Tilbrook (2006) and Hayward & Winston (2011) regarded Hippomenella as incertae sedis. After SEM analysis of the lectotype of H. mucronelliformis the generic diagnosis is here revised, and the genus transferred to the lepralielloid family Romancheinidae Jullien, 1888 for reasons specified below. Thus, Hippomenella is now being united with the genus Waters (1899) initially considered closely related: the species epithet mucronelliformis refers to its similarity with Mucronella coccinea Abildgaard, 1806, which today is placed in the romancheinid genus Escharoides Milne Edwards, 1836.

The presence of a ring scar (Fig. 3E), which defines the slightly reduced central umbonuloid part of the frontal shield in the type species, is one reason for removing the genus from the lepraliomorph Escharinidae to the umbonulomorph Romancheinidae. The previous assignment of Hippomenella to lepraliomorph families was partly due to its original placing in the Hippoporininae by Canu & Bassler (1917) [see the discussion on this subfamily and the family Hippoporinidae in Gordon (1984); both taxa are now considered as synonyms of Phidoloporidae Gabb & Horn, 1862]. Moreover, Brown (1949, 1952) and Gordon (1984) considered the New Zealand species Lepralia vellicata Hutton, 1873 to belong to Hippomenella. Despite the apparently similar structure of the frontal shield at the zooidal surface, Gordon (1984: 77) observed that the imperforate umbonuloid part of the shield of L. vellicata is extremely reduced, which was considered by him to support a placing of the genus in the Lepraliomorpha. The numerous pores along the marginal frontal shield of the above-mentioned species are all considered here to be areolar pores, as more than one may contribute to avicularium formation. The presence of over 20 avicularia that are scattered on the surface of a single zooid of L. vellicata (cf. Brown 1949: 519) supports this interpretation.

The other major source of confusion was a misconception of the ooecium structure of Hippomenella. The ooecium of H. mucronelliformis was not known when Canu & Bassler (1917) introduced the genus. Instead they described the ooecia of species that may be placed in the morphologically similar romancheinid genus Hippopleurifera Canu & Bassler, 1925 (see discussion in Hastings 1966), the genotype of which (Eschara biauriculata Reuss, 1847) is characterised by a pair of lateral fenestrae in the ooecium. Moreover, Brown (1949) considered the endooecium to be entirely perforated by pseudopores, and, based on observations of L. vellicata, Gordon (pers. comm. 2012) later regarded the ooecium type of Hippomenella as schizoporelloid. Thus, a number of species with a range of ooecial structures and morphologies were lumped together in Hippomenella. In contrast, in H. mucronelliformis the calcified endooecium is not perforated by pseudopores (as in the schizoporelloid type) but merely superficially pitted in the distolateral part (Fig. 3C, F), and it is lacking any other ooecial structures.

This ovicell type is, in turn, rather similar to the primary ooecium in the genus Escharoides, and therefore yet another argument for placing Hippomenella in the Romancheinidae. Prior to being covered by secondary calcification of the distal zooid, the endooecium in Escharoides shows a similarly pitted surface as in H. mucronelliformis. This can best be seen in fossil specimens of Escharoides in which the (presumably aragonitic) secondary calcification has vanished (see Berning 2006: figs 71, 73-79).

Comparative analyses of the frontal shield (e.g., the extent of the umbonuloid part) and the ooecium will be vital in order to unravel the relationship between Hippomenella and Hippopleurifera. For instance, in the genotype of Hippopleurifera almost the entire frontal shield is perforated by areolar pores, suggesting that the umbonuloid part is extremely reduced. The ooecial fenestrae in Hippopleurifera supposedly result from a thick but incomplete cover of secondary calcification produced by the distal zooid, whereas the exposed endooecium is densely perforated by tiny pseudopores or superficial pits. In contrast, in several other (fossil) species that have been assigned to Hippopleurifera the extent of the umbonuloid frontal shield seems to be similarly extensive as in H. mucronelliformis, and there is no secondary calcification from the distal zooid covering the imperforate endooecium. The occasionally occurring prominent ribs in some of these species (e.g., in the Pliocene Eschara sedgwicki Milne Edwards, 1836) are probably entirely of endooecial origin. Additionally, a third group comprises species with apparently schizoporelloid-like ovicells, i.e., with pseudopores that penetrate the endooecium (e.g., L. vellicata, Hippomenella amaralae Vieira et al., 2010, Hippopleurifera belizae Winston, 1984, Hippomenella ramula Hayward & Winston, 2011). A thorough revision of these taxa is, therefore, urgently needed but beyond the scope of the present paper. For this reason, the generic diagnosis given above is rather conservative and may need to be expanded again if species with other ooecial structures are included in Hippomenella.

Information on the ancestrula has also been omitted from the generic diagnosis because I doubt that the zooid identified and described as such by Brown (1949), which looks similar to a mature autozooid but with more proximally positioned avicularia (Fig. 3A), is indeed the ancestrula. In the type specimen (and also observed in another colony) the region immediately proximal to and around this zooid is occupied by other autozooids, which probably cover and disguise the true ancestrula. In fact, remnants of spines of the true ancestrula can still be seen that are situated close to the proximal margin of the first generation autozooid and which are not entirely covered by the overgrowing zooid. As all other romancheinid taxa have a tatiform ancestrula, I presume the same applies to Hippomenella.