Nodobairdia mammillata, KOLLMANN, 1963

Forel, Marie-Béatrice, Poulet-Crovisier, Nathalie, Korat, Lidija & Maddocks, Rosalie F., 2024, Ornate Bairdiidae (Ostracoda) in 3 dimensions: exploring carapace morphology and pore canals of Triebelina van den Bold, 1946, Nodobairdia Kollmann, 1963 and Mirabairdia Kollmann, 1963, Comptes Rendus Palevol 23 (11), pp. 137-159 : 146-150

NODOBAIRDIA MAMMILLATA KOLLMANN, 1963

External structures

The type material of Nodobairdia mammillata (type species of Nodobairdia Kollmann, 1963 ) was incrusted with sediments that limited the observations, but allowing the recognition of its diagnostic features ( Kollmann 1963). The 3D investigation offers the unique opportunity to virtually remove all sediment masking inner and outer morphology and characters of the valves. Based on the size diagram in Forel & Moix (2020), the dimensions of the isolated LV studied here (Appendix 1) may correspond to a submature A-1 stage ( Fig. 6), while the carapace corresponds to a younger juvenile, possibly of A-3 stage (Fig. 7A-F).

The diagnostic features of Nodobairdia mammillata include three aligned subventral nodes, two dorsal nodes at LV and four subdorsal nodes at both valves, the anterior and posterior ones being oblong ( Kollmann 1963). In this species, the antero-dorsal and postero-dorsal nodes at LV are reduced in submature stages and adults, and the subventral nodes coalesce in adults ( Kollmann 1963). These features are observed in our largest LV ( Fig. 6A), confirming its submature stage, while the strong antero-dorsal node and unfused subventral nodes of the carapace (Fig. 7A, B) confirm that it represents a younger stage. The coalescence of the ventral row of nodes in the largest LV confirms the N. mammillata attribution and differentiates it from N. verrucosa Kollmann, 1963 .

The 3D investigation of the LV shows that the wall is actually thickened between the two dorsal nodes ( Fig. 6 A-C). The convexity in the lateral outline is formed by a dorsal ridge that seems to develop quite late during the ontogeny, as it is less prominent in the carapace (Fig. 7A-C). The macro-ornamentation features are covered with verrucae ( Figs 6A; 7A, B) that are also visible for instance in Kristan-Tollmann (1978, 1986) and Dépêche & Crasquin-Soleau (1992). The median surface, which is delimited by the subventral and subdorsal structures, displays a reticulum made of round, large and deep fossae; they are more developed in the isolated LV and mainly seen on individual scans ( Fig. 6H, I). The three subventral nodes delimit a flat and smooth ventral surface ( Figs 6G; 7D). The anterior extension, termed “nose”, was taken as a subgeneric marker of Triebelina (Nodobairdia) by Bolz (1971b), who considered it as antero-ventral marginal denticles interconnected by a calcitic layer, i.e., a frill. This structure appears to be hollow in the isolated LV (Fig. 8A, B), but it is impossible to confirm whether a radial pore canal is present. A tiny opening seems to be present at the extremity of the nose in the LV of the carapace, but the definition of the scan does not allow further precision. In both LV, the postero-ventral margin is thickened and bears three parallel rows of full spines that are more strongly developed in the largest LV; they also develop anteriorly and are interrupted by the oral concavity ( Fig. 6G). The thickened portion of the carapace is separated from the actual margin by a narrow flat area (Fig. 7D).

The lateral surface of Nodobairdia mammillata also displays at least 14 nodules that are distributed in a recognizable pattern at each valve (Fig. 8). These nodules have never been described, but they are visible in specimens from the Norian-Rhaetian of Australia ( Dépêche & Crasquin-Soleau 1992), Ladinian, Carnian and Rhaetian of Hungary ( Monostori & Tóth 2014) as well as Carnian of Sicily ( Crasquin et al. 2018). They are symmetrical between the valves and are here labelled from 1 to 14 in the LV and from 1’ to 14’ in the RV. All of them are recognized in the juvenile carapace (Fig. 7A-F) and submature LV(Fig. 7G), indicating that they may be fully acquired quite early in the ontogenetic development of the species. They are further described and discussed below.

Internal structures

The calcified inner lamella of the largest LV extends from the antero-dorsal margin to slightly higher than the posterior border; in lateral view it is very reduced along the ventral margin and shows maximum of width along the antero-ventral margin ( Fig. 6B). The vestibule is very narrow. The inner surfaces are carved by:

– four subdorsal cavities corresponding to the subdorsal nodes, in both valves. In the isolated LV, the cavities are shallower below the anterior and posterior nodes, corresponding to a thickening of the nodes themselves ( Fig. 6B);

– two dorsal cavities corresponding to the dorsal nodes, in the left valves. They are very reduced in the submature LV, corresponding to particularly thick areas of the valve;

– three deep ventral cavities in the carapace and an elongate lineation ventrally, with the three main cavities corresponding to the three aligned ventral nodes, nearly coalescing in the isolated LV ( Fig. 6D). Overall, the submature LV displays thicker structures compared to those of the younger carapace;

– the circular AMS spot is located in a shallow cavity around mid-length and slightly below mid-height in the isolated LV ( Fig. 6B). It is composed of at least nine subcircular to subrectangular individual scars organized into three rows: a ventral arcuate row of four scars, a median row (apparently incomplete) of three scars, and an upper row of two scars ( Fig. 6J). Other scars (frontal, mandibular) are not observed. The AMS pattern of Nodobairdia was never reported so far. Kristan-Tollmann et al. (1980) labelled the individual scars of Triassic ornate forms from a to g, surrounding a central h scar that can be doubled or tripled (see Forel & Chitnarin 2023 for summary). The h-scars are typically arranged horizontally next to each other, the inner one being larger than the outer one: this pattern is observed here in the N. mammillata LV with a possibly triple h-scar ( Fig. 6J). This observation is of major importance, as changes in the number of scars h are associated with the typical Triassic evolutionary stage of the AMS (Kristan-Tollmann et al. 1980). The d-scars are also generally divided into three to five small scars, but only one is observed here. The scars recognized here, from a to triple h, display the same position and relative size compared to other ornate Bairdiidae that have been so far studied ( Forel & Chitnarin 2023: fig. 5); they are considered homologous.

Kollmann (1963) mentioned a hinge composed of a simple bar in the RV, protruding at the anterior and posterior ends, and a corresponding furrow in the left valve, which is the usual condition in Bairdiidae . Kristan-Tollmann (1971) described the hinge as a long, straight and smooth bar with triangular anterior and posterior lists and corresponding furrow. The preservation of the hinge in both specimens does not allow any precise observation, but the scans of the carapace reveal the details of the dorsal contact of the valves: the RV is nested in the LV at the posterior end (Fig. 8D, E), the overlap of LV on RV develops until the mid-part of postero-dorsal node, is interrupted along dorsal border (Fig. 8D, F) and resumes from the anterior part of the antero-dorsal node to the anterior margin (Fig. 8D, G).

The carapace also allows observation of the ventral contact of the valves, which we here describe following the terminology used in Adamczak (1976) and Olempska (1999). As mentioned above, the RV is posteriorly nested into LV. The contact develops into a sort of contact groove (Fig. 8H, I) that progressively reduces and flattens until the anterior end of the posterior subventral node, where the ventral contact consists of RV being overlapped by LV (Fig. 8H, J). An anterior contact groove progressively redevelops around mid-part of the median subventral node until the anterior end of the anterior subventral node (Fig. 8H, K).

Auxiliary (bairdoppilate?) teeth and sockets are poorly developed on the carapace, possibly being less expressed in juveniles, but they are visible anteriorly on the isolated LV, more clearly on individual scans than on reconstruction (Fig. 8A, C). Kristan-Tollmann (1971) mentioned six to eight small teeth in the RV; five sockets are clearly seen in the studied LV, three additional being more questionable because of preservation.