Kjaerina (Kjaerina) Bancroft, 1929

Colmenar, Jorge, 2016, Ordovician rafinesquinine brachiopods from peri-Gondwana, Acta Palaeontologica Polonica 61 (2), pp. 293-326 : 295-296

Subgenus Kjaerina (Kjaerina) Bancroft, 1929

Type species: Kjaerina typa Bancroft, 1929 ; Cheney Longville Formation , Cheneyan (lower Katian), Cheney Longville, Shropshire ( UK) .

Species included: Kjaerina typa typa Bancroft, 1929 , Cheney Longville Formation, Cheneyan Stage (lower Katian), Shropshire ( UK); Kjaerina typa geniculata Bancroft, 1929 , Cheney Longville Formation, Cheney- an Stage (lower Katian), Shropshire ( UK); Strophomena bipartita Salter in Murchison, 1854, Alternata Limestone and Cheney Longville formations, Cheneyan Stage (lower Katian), Shropshire ( UK); Leptaena complanata Sowerby, 1839 , Aurelucian Stage (lower Sandbian), Shropshire ( UK); Kjaerina jonesi Bancroft, 1929 , Horderley Sandstone Formation, Burrellian Stage (upper Sandbian), Shropshire ( UK); Kjaerina hedstroemi Bancroft, 1929 , Horderley Sandstone Formation, Burrellian Stage (upper Sandbian), Shropshire ( UK); Rafinesquina poljensis Alichova, 1951 , Hirmuse Formation, Oandu Stage (lower Katian), East Baltic and NW Russia; Kjaerina hartae Jin, Caldwell, and Norford, 1995 , Red River Formation, Maysvillian Stage (lower Katian), southern Manitoba ( Canada); Kjaerina (Kjaerina) gondwanensis sp. nov. Portixeddu Formation, lower middle Katian, Sardinia ( Italy).

Emended diagnosis. —Rafinesquinine with abrupt dorsally directed geniculation (if present). Radial ornament parvicostellate. Concentric ornament consisting of irregular rugae sometimes covering the entire disc, sometimes restricted posterolaterally or even absent. Prominent median rib in the ventral valve often present. Dental plates always present, usually straight but may be gently curved and extending anteriorly into muscle-bounding ridges. Cardinal process bifid with plate-like lobes during all ontogeny. Socket ridges of variable strength bounding triangular dental sockets. Notothyrial platform anchor-shaped, continuous anteriorly with median ridge. Mesocardinal ridge sometimes present.

Remarks. —Since Bancroft (1929) erected Kjaerina and Hedstroemina for a group of very close species of Rafinesquininae with dorsally geniculate shells, their taxonomic validity have been questioned. Salmon (1942) considered both genera synonyms of Rafinesquina Hall and Clarke, 1892 , considering as specific characters the differences in the amount of curvature in the dorsal valve and in the presence of a conspicuous median rib on the ventral valve of Kjaerina . Spjeldnaes (1957) considered them as two different subgenera within Rafinesquina differing in the divergence of the muscle bounding ridges and in their valve convexity. Similarly, Williams (1963) and Muir-Wood and Williams (1965) considered Kjaerina and Hedstroemina as two independent subgenera within Kjaerina .

Hurst (1979) observed enough differences between these two taxa to consider them as independent genera. The latter author considered that Hedstroemina has a less strongly developed median rib, more expressed rugae, more convex and more strongly dorsally geniculate ventral valves, wider and more impressed ventral musculature, more heavily calcified bilobed cardinal process, and thinner socket ridges than Kjaerina .

In spite of that, after the main revisions of the family since Hurst’s (1979) work, Kjaerina and Hedstroemina have been considered as independent genera (Harper in Owen and Harper 1982; Cocks and Rong 2000; Cocks 2010), only one species remains today within Hedstroemina in addition to H. fragilis , its type species: Oepikina? inaequiclina Alichova, 1951 ( Cocks and Rong 2000). Hedstroemina almadenensis Villas, 1995 was ascribed to the same genus but it is transferred to Kjaerina (Villasina) subgen. nov. herein.

Several Late Ordovician rafinesquinines share common features of Kjaerina and Hedstroemina , it is necessary therefore to discuss their taxonomic status, to more detail herein.

Analysis of the morphologic variability of the new species K. (Kjaerina) gondwanensis described (see below) allows to rule out as genus diagnostic two of the previously mentioned features: the median rib, frequent in some species of Kjaerina , and the rugation, considered more typical of Hedstroemina . The development of median rib is highly variable within the studied population of K. (Kjaerina) gondwanensis , displaying a continuous series of forms among the specimens with a prominent median rib (Fig. 2C, G, I) and specimens with the rib poorly developed (Fig. 2D, E, H). Similar pattern is present in the degree of rugation in K. (Kjaerina) gondwanensis ; there is a continuity from specimens of without rugae (Fig. 2B, F), with rugae restricted posterolaterally (Fig. 2A, C–E, G, I) and rugae covering most of the shell surface (Fig. 2H). The variability of the rugation as an intraspecific feature was also stated for Kjaerina by Cocks (2010).

The strength of the geniculation also seems to be a variable intraspecific feature. For example, the type species of Kjaerina displays both geniculate ( K. typa geniculata ) and non-geniculate ( K. typa typa ) morphotypes. However, the geniculation in Kjaerina seems to be more abrupt than those displayed by Hedstroemina valves that appear to be more evenly convex ( Cocks 2010).

With respect to the divergence of the dental plates, they are supposed to be narrowly divergent in Kjaerina , whereas they would be narrowly to widely divergent in Hedstroemina . Comparing the divergence angle of the dental plates in all the species of both genera ( Table 1), it can be observed that the range of this feature in Hedstroemina fragilis is so large that it includes most of the means and ranges of this parameter in most of the species of Kjaerina . Thus, this character should not be used to differentiate between the discussed genera.

Ventral muscle field impression is a character too much variable among species and depends in many cases on the preservation of specimens.

The remaining diagnostic feature, i.e., strength of socket ridges has been observed in all the Kjaerina and Hedstroemina species. Typically, the socket ridges are thinner in most of Kjaerina species but specimens of K. bipartita display a similar expression of this character to the one observed in H. fragilis .

Finally, the supposedly more heavily calcified cardinal process lobes and thinner socket ridges of Hedstroemina need some comment. After revision of the topotypes of both genera housed in the Natural History Museum in London, UK, it turns out that the cardinal process lobes and the socket ridges are strongly variable features within the species of Kjaerina , including species with similar development to the one present in some of Hedstroemina . Moreover, the cardinal process lobes thicken in most brachiopod species during ontogeny, and its degree of calcification can not be used to distinguish between closely related genera.

In summary, due to the intraspecific variability in the majority of diagnostic characters of Kjaerina and Hedstroemina , without clear morphological boundaries between particular species, it does not seem to be justified to keep them in separate genera. Nevertheless, considering that there is a certain difference in the development of geniculation of their type species, it is proposed to maintain them as subgenera within Kjaerina , according to the criteria followed by Williams (1963) and Muir-Wood and Williams (1965). Kjaerina stands as the senior name for the genus, since Hedstroemina (= Rakverina Rõõmusoks, 1993 = Virunites Rõõmusoks, 2004 ) was introduced in the same paper 13 pages after Kjaerina ( Bancroft 1929) .