Tumulduria

Skovsted, Christian B., Kouchinsky, Artem, Bengtson, Stefan & Holmer, Lars E., 2014, The problematic early Cambrian fossil Tumulduria incomperta represents the detached ventral interarea of a paterinid brachiopod, Acta Palaeontologica Polonica 59 (2), pp. 359-365 : 360-362

Tumulduria as a brachiopod

The lateral fields of the broadly triangular plates of Tumulduria incomperta are highly variable in shape and size relative to specimen length, and lateral margins are usually uneven ( Fig. 1 ). The median ridge is very similar in morphology to the ridge-like pseudodeltidium of the ventral interarea of Cryptotreta and the lateral flanks are reminiscent of the flat, orthocline ventral interarea itself ( Pelman 1977; Laurie 2000). In width, the convex pseudodeltidium of T. incomperta typically varies from 200 to 500 μm, which is comparable to the pseudodeltidium found on complete ventral valves of Cryptotreta neguertchenensis (see Laurie 2000). The variable transverse lineation of the plates often shows a distinct deflection towards the oldest part of the plate (as indicated by the overlapping shell layers) across the ridge, which is also the case in the pseudodeltidium of C. neguertchenensis and presumably reflects the small embayment of this part of the ventral interarea ( Laurie 2000: fig. 85A). Comparison could also be made with the ventral interarea of Salanygolina obliqua Ushatinskaya, 1987 from the lower Cambrian of Mongolia, where a narrow and convex pseudodeltidium of similar morphology divides the flat, triangular interarea ( Holmer et al. 2009: text-fig. 4A).

Complete specimens of ventral valves are lacking in our collections, mirroring the absence of complete ventral valves in most collections of Cryptotreta and Salanygolina . In the case of Cryptotreta and Salanygolina , the lack of complete ventral valves is presumably a consequence of fragmentation due to structural weakness caused by the large umbonal foramen (see also Pelman 1977 and Holmer et al. 2009) and a similar umbonal foramen was probably present also in Tumulduria . However, one specimen exhibits the central ridge and one complete lateral half of the triangular interarea ( Fig. 2A ), effectively bridging the morphological gap between typical specimens of T. incomperta and complete ventral valves of other cryptotretid brachiopods. Although incomplete, this specimen shows that the interarea originally was a flat, obtusely triangular plate (the specimen is broken parallel to the pseudodeltidium and thus represents only the left lateral half of the interarea; Fig. 2A 1 ,A 2 ) with a well marked pseudodeltidium ( Fig. 2A 3 ). The plate exhibits uneven transverse growth-

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lines and an oblique, centrally placed flexure-line. The flexure line presumably represents a zone of weakness, and its inclination (about 30˚) corresponds roughly to the slope of the lateral margins in typical Tumulduria plates (around 25–40˚ in larger specimens in our collections; Fig. 1A–D ). A similar specimen was illustrated together with other specimens of Tumulduria as “Skeletal Problematicum 1” by Meshkova (1974: pl. 26: 3). The specimen illustrated by Meshkova (1974) appears to be more complete and preserves part of both lateral fields of the interarea, complete with flexure lines, pseudodeltidium and the posterior margin with a central embayment. Other fragmentary specimens in our collections preserve an acute linear fold which probably represents the lateral junction of the interarea and general shell surface and support the interpretation of the interarea as a flat, triangular plate separated from the rest of the shell by a sharp fold ( Fig. 2B ).

In most samples investigated by us, we find an association of the problematic fossil Tumulduria incomperta with indeterminable brachiopod shell fragments and flat to gently convex and weakly sulcate brachiopod valves ( Fig. 3 ). The associated shells are strongly reminiscent of the dorsal valves of cryptotretid brachiopods, including the genera Cryptotreta and Aldanotreta which are known to occur in the Tommotian of the Siberian Platform ( Pelman 1977, 1979). Although isolated dorsal valves of cryptotretids may be difficult to separate morphologically, the valves occurring with Tumulduria in the Pestrotsvet Formation differ from the dorsal valves of all described cryptotretids in having an extremely small interarea divided by an open triangular notothyrium.

In addition to the co-occurrence and morphological similarities of Tumulduria plates and associated cryptotretid dorsal valves, the apparent botryoidal shell structure is very similar between typical specimens of T. incomperta and co-occurring flat brachiopod valves (compare Figs. 1H 2 and 3D 2 ). The botryoidal structure is observable on the internal surface of all available specimens, but is not apparent on external surfaces. This similarity in preservation of the shell structure was also noted by Bengtson et al. (1987: 368) although the illustrated brachiopod valves were identified as belonging to Aldanotreta sunnaginensis ( Bengtson et al. 1987: fig. 7). Although the botryoidal structure is most likely of diagenetic origin, the close similarity in preservation provide a strong indication that the original composition and shell structure of typical Tumulduria plates and co-occurring cryptotretid brachiopod valves were identical or, at least, very similar.

Based on the observed occurrence patterns as well as morphological and ultrastructural similarities we conclude that Tumulduria incomperta is likely to represent the detached central portion of the ventral interarea of a cryptotretid brachiopod and that the associated brachiopod valves represent the dorsal valve of the same taxon.

Figures

Fig. 1. Paterinid brachiopod Tumulduria incomperta Missarzhevsky in Rozanov et al, 1969. Typical Tumulduria plates with pseudodeltidium and partially preserved lateral flanks representing ventral interarea. Aldan and Uchur River area, Nochoroicyathus sunnaginicus Zone, Siberia. A. NRM Br141128, sample 1825; A 1, view from above; A 2, oblique posterior view. B. NRM Br141129, sample 1825; B 1, view from above; B 2, oblique posterior view. C.NRM Br141130, sample 1825; C 1, view from above; C 2, oblique posterior view. D. NRM Br141131, sample 1825; D 1, view from above; D 2, oblique lateral view. E. NRM Br141132, sample JSP 1990 27/1; E 1, view from above; E 2, oblique posterior view. F. NRM Br141133, sample JSP 1990 27/1; F 1, view from above; F 2, oblique posterior view. G. NRM Br141134, sample JSP 1990 27/1; G 1, view from above; G 2, oblique posterior view. H. NRM Br141135, sample 1825; H , internal view; H , detail of internal surface with botryoidal structures.

Fig. 2. Paterinid brachiopod Tumulduria incomperta Missarzhevsky in Rozanov et al, 1969. Fragmentary valves. Aldan and Uchur River area, Nochoroicyathus sunnaginicus Zone, Siberia. A. NRM Br141136, sample 449, left lateral half of ventral interarea; A1, view from above; A2, oblique posterior view; A3, detail of pseudodeltidium in oblique view. B. NRM Br141137, sample 1832, fragment preserving junction of ventral interarea and ventral shell surface; B1, view from posterior; B2, oblique posterolateral view. C. NRM Br141138, sample 525, fragment of shell surface with fila and nick points.

Fig. 3. Paterinid brachiopod Tumulduria incomperta Missarzhevsky in Rozanov et al, 1969. Dorsal valves. Aldan and Uchur River area, Nochoroicyathus sunnaginicus Zone, Siberia. A. NRM Br141139, sample 1793; A 1, dorsal view; A 2, oblique anterior view; A 3, oblique posterior view; A 4, detail of cardinal area in oblique posterior view; A 5, oblique anterolateral view of internal surface. B. NRM Br141140, sample 1825; B 1, dorsal view; B 2, oblique posterior view. C. NRM Br141141, sample 1832; C 1, dorsal view; C 2, oblique posterior view; C 3, oblique anterior view of internal surface. D. NRM Br141142, sample 1832; D , internal view; D , detail of internal surface with botryoidal structures.