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Dapcodinium tabulodiniopsis
Dapcodinium tabulodiniopsis sp. nov., Mantle et al. 2020, p. 38-39
Holotype: Mantle et al. 2020, Plate X, 4a, b
Type locality: Northern Carnarvon Basin
Local stratigraphical range: Late Triassic (Carnian–Norian)
Original description: Mantle et al. 2020:
Description:
Small, proximate, acavate to cavate, flattened subspheroidal dinoflagellate cysts with an obtusely angular or rarely conate apex and a rounded or flat-based antapex.
Wall: epicysts and hypocysts are of similar length and both may exhibit weakly angular profiles at the boundary of the apical and precingular plates, and the transition between the postcingular and antapical plates. Although most specimens are acavate, poorly preserved cavate forms with very thin-walled endocysts are common. The separation of the endophragm and periphragm in these is greatest at the antapex, but rare specimens are strongly camocavate. The surface ornament of the endocyst is psilate with poorly or undefined tabulation. The periphragm (or autophragm) is psilate to scabrate, with coarser baculae, clavae, and grana defining the sutures or surmounting low sutural ridges.
Tabulation: these sutures define a typical dapcodinioid tabulation of ?4′, 4a, 7′′, 7–8c, 7′′′, 3′′′′, ns. The broad, moderately laevorotatory cingulum (4–7 μm) is divided into 8 plates, and is offset by one-half to two-thirds the cingular width; it has a flat or weakly indented profile.
Archaeopyle: formed by the loss of apical and anterior intercalary plates. This appears to involve only the dorsal plates, probably 3′, 4′, 2a and 3a (Fig. 5d), but rare specimens appear to have also lost the 2′ and 1a plates.
Dimensions:
maximum length 31 μm (38 μm) 46 μm; maximum width 21 μm (28 μm) 36 μm
20 measured specimens
Remarks:
The broad cingulum, moderately narrow precingular plates, tabulation, and compound AI archaeopyle all strongly support a generic assignment to Dapcodinium. As noted earlier, the labelling of the posterior plates herein (Fig. 5d, e) follows that of Below (1987), thereby recognizing three antapical plates (1–3′′′′) rather than a single antapical plate (1′′′′) and two posterior intercalary plates (1–2p).
Affinities/Comparisons:
The strongly tabulate appearance and squat subspherical ambitus of Dapcodinium tabulodiniopsis sp. nov. are most reminiscent of Dapcodinium polyedricum and Dapcodinium priscum. However, the sutural ornament of coarse grana, baculae, or clavae impart a strongly denticulate edge to the sutures, and this readily distinguishes Dapcodinium tabulodiniopsis sp. nov. from these and all other
species of Dapcodinium. Note that some of the paratypes of Dapcodinium priscum illustrated by Evitt (1961, pl. 119, figs. 7, 9, 10, 13) appear to exhibit short baculae or grana along sutural boundaries, but these are never as coarse or as persistent as in Dapcodinium tabulodiniopsis sp. nov.
Holotype: Mantle et al. 2020, Plate X, 4a, b
Type locality: Northern Carnarvon Basin
Local stratigraphical range: Late Triassic (Carnian–Norian)
Original description: Mantle et al. 2020:
Description:
Small, proximate, acavate to cavate, flattened subspheroidal dinoflagellate cysts with an obtusely angular or rarely conate apex and a rounded or flat-based antapex.
Wall: epicysts and hypocysts are of similar length and both may exhibit weakly angular profiles at the boundary of the apical and precingular plates, and the transition between the postcingular and antapical plates. Although most specimens are acavate, poorly preserved cavate forms with very thin-walled endocysts are common. The separation of the endophragm and periphragm in these is greatest at the antapex, but rare specimens are strongly camocavate. The surface ornament of the endocyst is psilate with poorly or undefined tabulation. The periphragm (or autophragm) is psilate to scabrate, with coarser baculae, clavae, and grana defining the sutures or surmounting low sutural ridges.
Tabulation: these sutures define a typical dapcodinioid tabulation of ?4′, 4a, 7′′, 7–8c, 7′′′, 3′′′′, ns. The broad, moderately laevorotatory cingulum (4–7 μm) is divided into 8 plates, and is offset by one-half to two-thirds the cingular width; it has a flat or weakly indented profile.
Archaeopyle: formed by the loss of apical and anterior intercalary plates. This appears to involve only the dorsal plates, probably 3′, 4′, 2a and 3a (Fig. 5d), but rare specimens appear to have also lost the 2′ and 1a plates.
Dimensions:
maximum length 31 μm (38 μm) 46 μm; maximum width 21 μm (28 μm) 36 μm
20 measured specimens
Remarks:
The broad cingulum, moderately narrow precingular plates, tabulation, and compound AI archaeopyle all strongly support a generic assignment to Dapcodinium. As noted earlier, the labelling of the posterior plates herein (Fig. 5d, e) follows that of Below (1987), thereby recognizing three antapical plates (1–3′′′′) rather than a single antapical plate (1′′′′) and two posterior intercalary plates (1–2p).
Affinities/Comparisons:
The strongly tabulate appearance and squat subspherical ambitus of Dapcodinium tabulodiniopsis sp. nov. are most reminiscent of Dapcodinium polyedricum and Dapcodinium priscum. However, the sutural ornament of coarse grana, baculae, or clavae impart a strongly denticulate edge to the sutures, and this readily distinguishes Dapcodinium tabulodiniopsis sp. nov. from these and all other
species of Dapcodinium. Note that some of the paratypes of Dapcodinium priscum illustrated by Evitt (1961, pl. 119, figs. 7, 9, 10, 13) appear to exhibit short baculae or grana along sutural boundaries, but these are never as coarse or as persistent as in Dapcodinium tabulodiniopsis sp. nov.