Enteropleura Kittl, 1912

Chen, Jin-Hua & Stiller, Frank, 2007, The halobiid bivalve genus Enteropleura and a new species from the Middle Anisian of Guangxi, southern China, Acta Palaeontologica Polonica 52 (1), pp. 53-61 : 56-58

Genus Enteropleura Kittl, 1912

Type species: Daonella guembeli Mojsisovics, 1874 (subsequent designation by Diener 1923: 52); Middle Anisian (Middle Triassic), western Hungary.

Species included: Enteropleura bittneri Kittl, 1912 ; Enteropleura lamellosa ( Kittl, 1912) ; Enteropleura jenksi Hopkin and McRoberts, 2005 ; Enteropleura walleri sp. nov.

Diagnosis (after Waller in Waller and Stanley 2005: 21).— Halobiidae with an alivincular ligament, hinge shorter than shell length, and ornamentation that is intermediate between that of Bositra and Daonella .

Discussion.— Kittl (1912) regarded the internal shell ridges as an important character of Enteropleura Kittl, 1912 , distinguishing it from Daonella Mojsisovics, 1874 . However, Krumbeck (1924) showed that the internal ridges described by Kittl (1912) are also present in both Daonella and Halobia Bronn, 1830 , and therefore regarded the genus name Enteropleura as invalid. Campbell (1994: 44), following Krumbeck (1924), regarded the internal ridges as growth tracks of the adductor muscle scar. Ichikawa (1958: 190) included Enteropleura as a subgenus in Daonella . McRoberts (2000: 600) considered Enteropleura as a separate genus but “poorly known”, which “may, after further study, be placed in the Halobiidae ”. Waller and Stanley (2005) and Hopkin and McRoberts (2005) recognised Enteropleura as being a valid genus within the Halobiidae . Waller (in Waller and Stanley 2005) further pointed out that Enteropleura has an alivincular ligament, which was first reported by Arthaber (1896: 194, fig. 12) in “ Posidonomya nov. spec. ” (= Enteropleura bittneri ).

The commarginal sculpture in Enteropleura is usually fine and regular (except for Enteropleura guembeli ), and the radial sculpture, if present, is fine and weak. This shell sculpture differs from that in most species of Daonella , in which the radial ribs are strong and the commarginal lirae are weak in late ontogeny. However, the radial sculpture is weak in some early species of Daonella , e.g., Daonella boeckhi Mojsisovics, 1874 , possibly indicating an intermediate position between Enteropleura and Daonella . The alivincular ligament and the sculpture of Enteropleura are very similar to those of Bositra De Gregorio, 1886 . But Bositra is relatively rounded or oval in shape, its hinge line is generally much shorter than the shell length, the auricles are very small or lacking, and the commarginal lirae are generally broader than those of Enteropleura . Enteropleura is closely related to both Bositra and Daonella and probably has phylogenetic relationships with these two genera. We agree with Waller (in Waller and Stanley 2005: 17, fig. 5) that Enteropleura probably is derived from a species of Bositra and evolved into early Daonella in the Middle Triassic.

valve parts with radial grooves (impressions of the internal radial ridges). K. Internal mould, posterodorsal part of left valve, paratype, NIGP 140180 ; showing the commarginal undulations on the posterior sector and their branching into lirae ventrally. L. Internal mould, dorsal part of right valve, paratype, NIGP 140181 ; L 2, posterodorsal part with narrow posterior auricle (arrow). M. Internal moulds, crowded valves, paratype, NIGP 140175 .

Based on morphological characters, the five Enteropleura species may be subdivided into the following two species−groups. The character of the morphological differences, however, does not justify the formal erection of new subgenera.

The Enteropleura guembeli group comprises only E. guembeli ( Mojsisovics, 1874) , originally described from the Balatonites Beds (= Balatonites balatonicus Zone ) of western Hungary. Characteristic features are a small size (usually 10–15 mm or less in length), a subcircular to suboval shell shape, and a sculpture of irregular commarginal rugae and very weak radial striae, lacking the fine, regular commarginal lirae that are characteristic for the species of the Enteropleura bittneri group.

The species of the Enteropleura bittneri group differ in the larger shell size (maximum length about 30 mm), the D−shape or Daonella shape, and the fairly regular sculpture of fine commarginal lirae and in most species additionally developed weak radial ribs. The shell surface usually can be subdivided into three contiguous radial sectors (triangular fields), which are differently sculptured. A small and very narrow posterior auricle is present. The group comprises the four species Enteropleura bittneri Kittl, 1912 from the Rahnbauerkogel fauna ( Balatonites balatonicus Zone ) of Grossreifling, Austria, Enteropleura lamellosa ( Kittl, 1912) from the Anisian of the Dinarides in Dalmatia, Croatia, Enteropleura jenksi Hopkin and McRoberts, 2005 from the Balatonites shoshonensis Zone, Favret Formation , Nevada, and Enteropleura walleri sp. nov. from Unit D (late Middle Anisian) of the Lower Member of the Banna Formation, Fengshan District, northwestern Guangxi.

Enteropleura bittneri and E. jenksi share the fine cancellate sculpture consisting of commarginal lirae and radial costellae, but the posterodorsal sector of E. jenksi is wider than that of E. bittneri , and the two species further differ in details of the beak and the shell outline. Enteropleura lamellosa is very similar to E. bittneri and E. jenksi in size, outline, and shell sculpture. However, in comparison to the latter two species, the commarginal lirae of E. lamellosa are slightly broader, the interspaces between the lirae are narrower, and the radial costellae appear weaker developed (fide the figures of Kittl 1912: pl. 10: 23, 24). With regard to general shell outline, Enteropleura lamellosa is similar to both Daonella and Enteropleura . Kittl (1912) assigned it to Daonella rather than to Enteropleura , because he could not observe the internal ridges. However, the presence or absence of the internal ridges is not significant for the distinction of Enteropleura from Daonella (Waller in Waller and Stanley 2005). The shell shape, hinge margin, and especially the shell sculpture of E. lamellosa , which consists of fine, regularly and closely spaced radials and commarginals, are much more similar to Enteropleura than to Daonella .