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Gonyaulacysta desmos
Original description (Poulsen, 1991):
Gonyaulacysta jurassica (Deflandre 1938) desmos subsp. nov.
Plate 1, figs. 3, 6
Gonyaulacysta jurassica (Deflandre 1938) emend. Norris & Sarjeant 1965 emend. Sarjeant, 1982; Sarjeant, 1982, text-fig. 2a, pl. 4, fig. 8, pl. 5, fig. 1.
Derivation of Name. Greek Seguos (desmos), link, (transitional form).
Diagnosis. A subspecies of Gonyaulacysta jurassica which exhibits only partial development of the hypopericoel, i.e., there is a marginal separation of the hypoperiphragm and hypoendophragm, whereas the two wall layers are in contact at the mid-antapical region.
Holotype. Oxford Clay (1 m below the Lower Calcareous Grit Formation) Cayton Bay, Yorkshire, DGU Catalogue No.: 1989-NEP-1.
Paratype. Oxford Clay (27 m below the Lower Calcareous Grit Formation) Cayton Bay, Yorkshire, DGU Catalogue No.: 1989-NEP-2.
Occurrence. Praecordatum, Bukowskii and Costicardia ammonite subzones of the early Oxfordian in England.
Discussion. Gonyaulacysta jurassica desmos subsp. nov. appears to be a transitional form between the subspecies G.jurassica adecta, which has an acavate hypocyst, and the subspecies G. jurassica jurassica which has a well developed hypopericoel. The new subspecies of G. jurassica is proposed because itis morphologically distinct and it has potential stratigraphic significance. All three subspecies occur together in the early Oxfordian.
STRATIGRAPHIC DISTRIBUTION
In Europe, Gonyaulacysta jurassica first appears in the early Bathonian (Woollam and Riding, 1983; Riding et al., 1985; Davies, 1985; Herngreen and de Boer, 1978). In East Greenland, Svalbard, and Franz Josef Land, the first appearance of Gonyaulacysta jurassica is in the Callovian
(Bjaerke, 1980; Piasecki 1980; Smelror, 1987; 1988 a, b). In the Sverdrup Basin the first appearance of G. jurassica was also reported as Callovian (Johnson and Hills, 1973) or late Bathonian to early Callovian (Davies, 1983). Sarjeant (1982) determined that the earliest occurrences of G. juras-
sica are restricted to the subspecies adecta. G. jurassica jurassica first appears in the carly Oxfordian (Sarjeant, 1982: Riding, 1984).
The first appearance of Gonyaulacysta jurassica desmos (Text-Figure 3) was reported by Sarjeant (1982, p. 14), as the Praecordatum ammonite Subzone of the Mariae ammonite Zone (early Oxfordian) in his discussion of the development of hypopericoel in G. jurassica. The first
appearance of G. jurassica desmos as reported herein also occurs in the Praecordatum ammonite Subzone. In East Greenland the range base occurs just above the first ap- pearance of Wanaea fimbriata, the inception of which marks the base of the Mariae ammonite Zone (early Oxfor-
dian) (see Woollam and Riding, 1983; Riding, 1984).
The extinction of Gonyaulacysta jurassica desmos in England is within the Bukowskii ammonite Subzone of the Cordatum ammonite Zone (early Oxfordian). However, one poorly preserved specimen has been recorded in the overlying Costicardia ammonite Subzone (Upper Oxford Clay, Ham Cliff). The last appearance of G. jurassica adecta is in the late Oxfordian, Regulare ammonite Zone (Thomas and Cox, 1988).
The illustration of Gonyaulacysta jurassica jurassica from the Regulare ammonite Zone (late Oxfordian) provided by Riding (1984, fig. 4.6: 1987, fig. 8.8) shows similarities to G. jurassica desmos. The posterior part of the endophragm. however, is not in sharp focus and is not in contact with the periphragm (Riding, written commun.,1988).
The range top of Gonyaulacysta jurassica jurassica seems to vary geographically. In Poland the last Occurrence is in the Autissiodonensis Zone (latest Kimmeridgian) (unpubl. data of the present author). In Britain, the extinction is reported (Woollam and Riding, 1983; Riding, 1984) to be within the Pectinatus ammonite Zone (latest early Volgian).
Gonyaulacysta jurassica (Deflandre 1938) desmos subsp. nov.
Plate 1, figs. 3, 6
Gonyaulacysta jurassica (Deflandre 1938) emend. Norris & Sarjeant 1965 emend. Sarjeant, 1982; Sarjeant, 1982, text-fig. 2a, pl. 4, fig. 8, pl. 5, fig. 1.
Derivation of Name. Greek Seguos (desmos), link, (transitional form).
Diagnosis. A subspecies of Gonyaulacysta jurassica which exhibits only partial development of the hypopericoel, i.e., there is a marginal separation of the hypoperiphragm and hypoendophragm, whereas the two wall layers are in contact at the mid-antapical region.
Holotype. Oxford Clay (1 m below the Lower Calcareous Grit Formation) Cayton Bay, Yorkshire, DGU Catalogue No.: 1989-NEP-1.
Paratype. Oxford Clay (27 m below the Lower Calcareous Grit Formation) Cayton Bay, Yorkshire, DGU Catalogue No.: 1989-NEP-2.
Occurrence. Praecordatum, Bukowskii and Costicardia ammonite subzones of the early Oxfordian in England.
Discussion. Gonyaulacysta jurassica desmos subsp. nov. appears to be a transitional form between the subspecies G.jurassica adecta, which has an acavate hypocyst, and the subspecies G. jurassica jurassica which has a well developed hypopericoel. The new subspecies of G. jurassica is proposed because itis morphologically distinct and it has potential stratigraphic significance. All three subspecies occur together in the early Oxfordian.
STRATIGRAPHIC DISTRIBUTION
In Europe, Gonyaulacysta jurassica first appears in the early Bathonian (Woollam and Riding, 1983; Riding et al., 1985; Davies, 1985; Herngreen and de Boer, 1978). In East Greenland, Svalbard, and Franz Josef Land, the first appearance of Gonyaulacysta jurassica is in the Callovian
(Bjaerke, 1980; Piasecki 1980; Smelror, 1987; 1988 a, b). In the Sverdrup Basin the first appearance of G. jurassica was also reported as Callovian (Johnson and Hills, 1973) or late Bathonian to early Callovian (Davies, 1983). Sarjeant (1982) determined that the earliest occurrences of G. juras-
sica are restricted to the subspecies adecta. G. jurassica jurassica first appears in the carly Oxfordian (Sarjeant, 1982: Riding, 1984).
The first appearance of Gonyaulacysta jurassica desmos (Text-Figure 3) was reported by Sarjeant (1982, p. 14), as the Praecordatum ammonite Subzone of the Mariae ammonite Zone (early Oxfordian) in his discussion of the development of hypopericoel in G. jurassica. The first
appearance of G. jurassica desmos as reported herein also occurs in the Praecordatum ammonite Subzone. In East Greenland the range base occurs just above the first ap- pearance of Wanaea fimbriata, the inception of which marks the base of the Mariae ammonite Zone (early Oxfor-
dian) (see Woollam and Riding, 1983; Riding, 1984).
The extinction of Gonyaulacysta jurassica desmos in England is within the Bukowskii ammonite Subzone of the Cordatum ammonite Zone (early Oxfordian). However, one poorly preserved specimen has been recorded in the overlying Costicardia ammonite Subzone (Upper Oxford Clay, Ham Cliff). The last appearance of G. jurassica adecta is in the late Oxfordian, Regulare ammonite Zone (Thomas and Cox, 1988).
The illustration of Gonyaulacysta jurassica jurassica from the Regulare ammonite Zone (late Oxfordian) provided by Riding (1984, fig. 4.6: 1987, fig. 8.8) shows similarities to G. jurassica desmos. The posterior part of the endophragm. however, is not in sharp focus and is not in contact with the periphragm (Riding, written commun.,1988).
The range top of Gonyaulacysta jurassica jurassica seems to vary geographically. In Poland the last Occurrence is in the Autissiodonensis Zone (latest Kimmeridgian) (unpubl. data of the present author). In Britain, the extinction is reported (Woollam and Riding, 1983; Riding, 1984) to be within the Pectinatus ammonite Zone (latest early Volgian).