DURBAN MUS. NOVIT. 27 PALAEOLOPHID AND LIOSTREINE OYSTERS FROM THE CRETACEOUS OF SOUTHEAST AFRICA 29 Maastrichtian <strong>of</strong> Mt St-Pierre, Maestricht (Holland). In the writer=s opinion, this conspecificity has not been established and the oldest available name for the Dutch material is likely to be R. diluvianum (Linné), even though the latter was based on a rather different morphotype. Malchus et al. (1994: 120) noted that the A... many morphotypes under which Rastellum taxa occur has resulted in a plethora <strong>of</strong> names for <strong>Cretaceous</strong> forms@ and they were led to regard A... most Rastellum taxa from European <strong>Cretaceous</strong> strata ... [as belonging]... to R. diluvianum@. The writer finds it hard to support such a Aphilosophical taxonomy@ in which sweeping synonymies are suggested (but not substantiated by population analysis), in which all vaguely similar morphotypes within an 80 million year period <strong>of</strong> earth history are lumped together, and in which oyster evolution stagnates for vast periods <strong>of</strong> time. Certainly, it is not supported by the writer=s study <strong>of</strong> the Zululand fossil <strong>oysters</strong> where Rastellum occurs at three stratigraphic levels and, at each, is morphologically distinct. This indicates, at the very least, that there are changes in the structure and morphotype frequency <strong>of</strong> Rastellum populations, both in time and space, and that the evolution <strong>of</strong> the group did not stagnate. It must be reiterated, a species is a contemporaneous populations <strong>of</strong> individuals (not a hand-picked cluster <strong>of</strong> similar morphotypes from different stratigraphic levels) and, for fossil oyster taxonomy to progress, intraspecific variation must be documented and morphotype frequencies determined on a level-by-level basis. For reasons <strong>of</strong> nomenclatural priority this is particularly important with respect to the classic European species. Only then can populations from other stratigraphic levels and geographic regions be fruitfully compared, different species recognized, and justifiable synonymies upheld. Rastellum allobrogensis (Pictet and Roux, 1853) Figs 1-2 Ostrea allobrogensis Pictet and Roux 1853, pl. 49, fig. 1; Coquand 1869: 156, pl. 59, figs 8-10. Lopha diluviana (Linné) Rennie 1936: 316. Material 20 specimens, DNSM-P6903-6922, many fragmentary. Locality Some 70 m <strong>of</strong>f the lefthand side <strong>of</strong> the Pinda-Sodwana dirt road, about 200 m after the road crosses the Munywane stream; strata at this locality belong to the Douvilleiceras mammillatum Superzone (uppermost Lower Albian - lowermost Middle Albian). Palaeoecology The present material occurs close to the base <strong>of</strong> the Mzinene Formation, within a metre or two <strong>of</strong> the underlying unconformity with Bumbeni rhyolite, in ill-sorted yellow gritty to sandy limestones, with occasional small pebbles, associated with Douvilleiceras, brancoceratids, various heterodonts, the oyster Gyrostrea, the gastropod Pseudomelania and rare belemnites. The exposure is too small for adequate analysis, but evidence suggests a very nearshore environment, but without evidence for continual strong current or pounding wave action, i.e. in a sheltered bay (the poorly-sorted character <strong>of</strong> the sediments could be due to storm activity and/or bioturbation). The abraded nature <strong>of</strong> many <strong>of</strong> the robust Rastellum shells tends to suggest they may be an allochthonous component to this environment. Description Much <strong>of</strong> the material is worn and fine detail has been removed by abrasion. The shell is moderately large (max. height = 94.3 mm), thick, robust, more-or-less equivalve, subequilateral, higher than long (H/L = 1.12-1.40), slightly prosocline to orthocline, vertically ovate to weakly reniform and sometimes posteriorly curved. LV mostly oval, sometimes slightly curved to the posterior, moderately to strongly concave (W/H = 0.31-0.52), with prominent usually worn well-rounded umbo; beak generally abraded or masked by attachment scar or matrix. The attachment area is medium sized and situated on the posterior flank to the beak. The external surface is more-or-less uniformly convex, without an umbonal ridge. There are 17-20 prominent sharp-crested simple to bifurcating ribs which are crenulated by strong commarginal growth striae and produce deep plications to the commissure. The LA is gyrostreid, straightening in maturity, with a narrow resilifer separating the wide flat subequal bourrelets. There is a shallow umbonal cavity and narrow commissural shelf with prominent curbs dorsally and along the anteroventral margin; patches <strong>of</strong> chomata just below the LA are lacking, but DNSM-P6907 shows faint but dense relict lath chomata along its posterior margin. There is a large (height 35-38% <strong>of</strong> valve height, length 26-31% <strong>of</strong> valve length) reniform to subtrigonal adductor pad (not comma-shaped), truncate and slightly concave anterodorsally, at about midheight. The RV is oval to lenticular, shallowly convex to slightly concave (W/H = 0.22-0.24), with an inconspicuous umbo and opisthogyrous beak. The LA is not well preserved in the available material, most <strong>of</strong> it removed by abrasion. It seems to have been moderately large, with a narrow resilifer separating broad flattened bourrelets <strong>of</strong> equal width. There is no umbonal cavity and the commissural shelf is most prominent anteroventrally, due to a pronounced sunken gill cavity; in the better-preserved material dense fine lath chomata occur around almost the entire periphery, and are particularly conspicuous in the crests <strong>of</strong> plications, but there are no patches <strong>of</strong> chomata just below the LA. The large oval to elliptical adductor pad is truncate anterodorsally and situated slightly above midheight, close to the posterodorsal margin. In DNSM-P6918 and 6920 there is an umbonal ridge along the steep, relatively high anteroventral margin, with the remainder <strong>of</strong> the valve sloping gently to the posterior; both DNSM-P6908 and 6916 are shallowly concave. A single RV, DNSM-P6913, has a short posteriorly-curved Atail@, giving the valve outline a comma shape (Fig. 2G). The external ornament is not well preserved but in DNSM-P6920 strong narrow-crested ribs can be seen to diverge from a line marking the crest to the anteroventral face, producing deep plications to the commissure; the ribs are crenulated by prominent growth striae and cross the steep anteroventral face vertically, to meet the commissure at right angles. Intraspecific variation The available material is fairly consistent in its characters, particularly with respect to the shape <strong>of</strong> the LV, its convexity, shape <strong>of</strong> the attachment scar and rib density; RVs are a bit more variable, with one showing a short Arctostrea-like Atail@. Although there are no chomatal patches, relict lath chomata are faintly visible in the plications <strong>of</strong> on a number <strong>of</strong> specimens, particularly RVs, but it is not possible to decide whether their absence in some is a primary feature or due to abrasion or exfoliation <strong>of</strong> test.
30 PALAEOLOPHID AND LIOSTREINE OYSTERS FROM THE CRETACEOUS OF SOUTHEAST AFRICA DURBAN MUS. NOVIT. 27 A D B Fig. 1. Rastellum allobrogensis (Pictet & Roux). A-C. Internal, lateral and external views <strong>of</strong> an LV, DNSM-P6906. D. Internal view <strong>of</strong> an LV, DNSM-P6905. E. Internal view <strong>of</strong> an LV, DNSM-P6903. All x1. E F C
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