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Alterbidinium fusiforme
Etymology. From the Latin fusus, meaning extended, in reference to the elongate, spindle-shaped ambitus of the cyst.
Holotype. Plate 1A, B, Sample 17 (GSC C-711377), slide E6-14 A, EF J30/0.
Paratypes. Plate 1C, F, Sample 17, slide E6-14 A, EF X38/4; Plate 1D, E, Sample 14, slide E6-13 A, EF P28/3; Plate 1G–I, Sample 18, slide E6- 20 I, EF F24/3; Plate 2A, Sample 17, slide 6E-14 A, EF L29/1; Plate 2B, C, Sample 18, slide E6-20 H, EF L32/2; Plate 2D, G, Sample 18, slide 6E- 20 H, EF D32/0; Plate 2E, F, H, I, Sample 18, stub 24D02, specimen 17 (CEDiT2024P185).
Type locality. Likely northwestern Denman Island (c. ?49.58-124.84), British Columbia, Canada (Fig. 1B, locality 3), based on consistency with assemblages from this section.
Type stratum. Upper Cedar District Formation, lower upper Campanian.
Repository. Geological Survey of Canada.
Diagnosis. A fusiform, dorso-ventrally compressed species of Alterbidinium? that is marginally to broadly circumcavate, rarely bicavate. Pericyst thin, smooth, transparent. Endocyst subcircular, ellipsoidal, or ovoidal. Cingulum weakly expressed by a shallow furrow. Sulcus may be marked by a longitudinal periphragm fold. Periarcheopyle intercalary type I2a, steno- to isodeltaform hexa.
Description. Cyst fusiform, dorso-ventrally compressed, marginally to broadly circumcavate, and rarely bicavate. Periphragm thin, distin guished by a tapering, blunt apical horn and left antapical horn as seen from dorsal view, which may be narrowly acuminate. Horns may present with folds along their lateral margins reflecting periphragm concavity toward the polar extremities. Endocyst subcircular to elongate elliptical or ovoidal, typically longer than wide with a length-to-width ratio ranging from 0.93 to 2.6. Periphragm surface essentially smooth —psilate to shagreenate—occasionally with a longitudinal fold of varying relief transecting the length of the ventral surface (Plate 1B). The apical horn expresses a rounded apical pore complex (Plate 2H). A vestigial right antapical horn may present as a periphragm bulge (Plate 1C), but most commonly, the second antapical horn is reduced to such an extent as to be virtually indiscernible on most specimens. The cingulum is distinguished only by ventrolateral folds or a shallow transverse furrow with no displacement. The periarcheopyle is inferred intercalary type I(2a).
Dimensions. Holotype: pericyst length =88.3 μm; pericyst width =41.3 μm; endocyst length =39.2 μm; endocyst width =34.1 μm; endocyst length-to-width ratio =1.2. Range: pericyst length =71.6 (85.7) 104.8 μm; pericyst width =25.7 (31.8) 41.3 μm; endocyst length =30.6 (40.2) 53.4 μm; endocyst width =20.0 (26.3) 34.2 μm; endocyst length-to-width ratio =0.93 (1.59) 2.57. Specimens measured: 30. Values presented in Appendix B, Table B.1.
Comparison. Alterbidinium? fusiforme sp. nov. is most similar to Alterbidinium? novozealandicum described by Schiøler et al.(2001) from the middle to upper Campanian Herring Formation of South Island, New Zealand, in possessing an elongate, fusiform periphragm with blunt apical horn, diminished right antapical horn, and acuminate left antapical horn, but differs in having a clearly defined endocyst as well as a faintly expressed, yet discernable cingular furrow. Alterbidinium? fusiforme sp. nov. is questionably assigned to the genus due to the lack of consistency in the expression of a reduced right antapical horn, unlike the uncertainty surrounding A.? novozealandicum on the basis of endocyst ambiguity. Alterbidinium? novozealandicum is also considerably larger, with its smallest dimensions exceeding those of A.? fusiforme sp. nov. by twofold. The close affinity between A.? fusiforme sp. nov. and A.? novozealandicum is most apparent when considering the potential for endocyst development. Schiøler et al.(2001, p. 148) draw attention to the possibility of an endocyst ambitus in the latter species noting that for some specimens “minute amorphous particles (diameter 0.5–1.5 μm), or larger aggregates of those particles, were observed occupying a sub- circular to oval area located centrally . . . outlining parts of a sub- circular to oval perimeter” (Schiøler et al., 2001, figs. 4D, E; 5D). This orientation of particulate debris in A.? novozealandicum suggests an artifactual reflection of an extremely delicate endocyst that would otherwise be characterized by broad circumcavation. Schiøler et al.(2001) also noted the similarity of A.? novozealandicum to the Berriasian Fusiformacysta salasii described by Morgan(1975) from southern Australia, although Fusiformacysta, a Middle Jurassic to Early Cretaceous genus, is definitively acavate and possesses a 3P archeopyle (Riding and Helby, 2001). Likewise, Isabelidinium marshallii described from the same middle to upper Campanian strata of New Zealand as A.? novozealandicum (Roncaglia, 2000), differs in having a rhomboidal ambitus, centrally located endocyst with width typically exceeding length, and larger overall size.
Stratigraphic range. Lower Campanian–lower upper Campanian.
Occurrence. Alterbidinium? fusiforme sp. nov. has been recovered from outcrops of the upper Trent River Formation along the Puntledge River (Sample 9) and Trent River (samples 11–14) and from coastal exposures of the upper Cedar District Formation along western Denman Island (samples 15–19) within the Comox OA, British Columbia, Canada.
Holotype. Plate 1A, B, Sample 17 (GSC C-711377), slide E6-14 A, EF J30/0.
Paratypes. Plate 1C, F, Sample 17, slide E6-14 A, EF X38/4; Plate 1D, E, Sample 14, slide E6-13 A, EF P28/3; Plate 1G–I, Sample 18, slide E6- 20 I, EF F24/3; Plate 2A, Sample 17, slide 6E-14 A, EF L29/1; Plate 2B, C, Sample 18, slide E6-20 H, EF L32/2; Plate 2D, G, Sample 18, slide 6E- 20 H, EF D32/0; Plate 2E, F, H, I, Sample 18, stub 24D02, specimen 17 (CEDiT2024P185).
Type locality. Likely northwestern Denman Island (c. ?49.58-124.84), British Columbia, Canada (Fig. 1B, locality 3), based on consistency with assemblages from this section.
Type stratum. Upper Cedar District Formation, lower upper Campanian.
Repository. Geological Survey of Canada.
Diagnosis. A fusiform, dorso-ventrally compressed species of Alterbidinium? that is marginally to broadly circumcavate, rarely bicavate. Pericyst thin, smooth, transparent. Endocyst subcircular, ellipsoidal, or ovoidal. Cingulum weakly expressed by a shallow furrow. Sulcus may be marked by a longitudinal periphragm fold. Periarcheopyle intercalary type I2a, steno- to isodeltaform hexa.
Description. Cyst fusiform, dorso-ventrally compressed, marginally to broadly circumcavate, and rarely bicavate. Periphragm thin, distin guished by a tapering, blunt apical horn and left antapical horn as seen from dorsal view, which may be narrowly acuminate. Horns may present with folds along their lateral margins reflecting periphragm concavity toward the polar extremities. Endocyst subcircular to elongate elliptical or ovoidal, typically longer than wide with a length-to-width ratio ranging from 0.93 to 2.6. Periphragm surface essentially smooth —psilate to shagreenate—occasionally with a longitudinal fold of varying relief transecting the length of the ventral surface (Plate 1B). The apical horn expresses a rounded apical pore complex (Plate 2H). A vestigial right antapical horn may present as a periphragm bulge (Plate 1C), but most commonly, the second antapical horn is reduced to such an extent as to be virtually indiscernible on most specimens. The cingulum is distinguished only by ventrolateral folds or a shallow transverse furrow with no displacement. The periarcheopyle is inferred intercalary type I(2a).
Dimensions. Holotype: pericyst length =88.3 μm; pericyst width =41.3 μm; endocyst length =39.2 μm; endocyst width =34.1 μm; endocyst length-to-width ratio =1.2. Range: pericyst length =71.6 (85.7) 104.8 μm; pericyst width =25.7 (31.8) 41.3 μm; endocyst length =30.6 (40.2) 53.4 μm; endocyst width =20.0 (26.3) 34.2 μm; endocyst length-to-width ratio =0.93 (1.59) 2.57. Specimens measured: 30. Values presented in Appendix B, Table B.1.
Comparison. Alterbidinium? fusiforme sp. nov. is most similar to Alterbidinium? novozealandicum described by Schiøler et al.(2001) from the middle to upper Campanian Herring Formation of South Island, New Zealand, in possessing an elongate, fusiform periphragm with blunt apical horn, diminished right antapical horn, and acuminate left antapical horn, but differs in having a clearly defined endocyst as well as a faintly expressed, yet discernable cingular furrow. Alterbidinium? fusiforme sp. nov. is questionably assigned to the genus due to the lack of consistency in the expression of a reduced right antapical horn, unlike the uncertainty surrounding A.? novozealandicum on the basis of endocyst ambiguity. Alterbidinium? novozealandicum is also considerably larger, with its smallest dimensions exceeding those of A.? fusiforme sp. nov. by twofold. The close affinity between A.? fusiforme sp. nov. and A.? novozealandicum is most apparent when considering the potential for endocyst development. Schiøler et al.(2001, p. 148) draw attention to the possibility of an endocyst ambitus in the latter species noting that for some specimens “minute amorphous particles (diameter 0.5–1.5 μm), or larger aggregates of those particles, were observed occupying a sub- circular to oval area located centrally . . . outlining parts of a sub- circular to oval perimeter” (Schiøler et al., 2001, figs. 4D, E; 5D). This orientation of particulate debris in A.? novozealandicum suggests an artifactual reflection of an extremely delicate endocyst that would otherwise be characterized by broad circumcavation. Schiøler et al.(2001) also noted the similarity of A.? novozealandicum to the Berriasian Fusiformacysta salasii described by Morgan(1975) from southern Australia, although Fusiformacysta, a Middle Jurassic to Early Cretaceous genus, is definitively acavate and possesses a 3P archeopyle (Riding and Helby, 2001). Likewise, Isabelidinium marshallii described from the same middle to upper Campanian strata of New Zealand as A.? novozealandicum (Roncaglia, 2000), differs in having a rhomboidal ambitus, centrally located endocyst with width typically exceeding length, and larger overall size.
Stratigraphic range. Lower Campanian–lower upper Campanian.
Occurrence. Alterbidinium? fusiforme sp. nov. has been recovered from outcrops of the upper Trent River Formation along the Puntledge River (Sample 9) and Trent River (samples 11–14) and from coastal exposures of the upper Cedar District Formation along western Denman Island (samples 15–19) within the Comox OA, British Columbia, Canada.