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Microdinium avocetianum
Microdinium avocetianum Riding, 2002, p.4-6,8, pl.1, figs.1-16; text-fig.2.
Holotype: Riding, 2002, pl.1, figs.5-6.
Age: late Tithonian.
Microdinium avocetianum sp. nov. (Plate 1, figs 1-16; Text-Figure 2).
Derivation of name. From the Avocet-1A well.
Description. A small, ellipsoidal to subquadrangular species of Microdinium which may be slightly elongate or equant and is dorsoventrally flattened. The autophragm is dark, smooth to scabrate, occasionally irregularly microreticulate, and 0.5 urn thick. The paratabulation is indicated by low, smooth, undulose to irregularly denticulate parasutural crests. These parasutural crests vary from 0.5 urn to 2 urn in height, including the denticles. The denticles may be up to 1 urn high and are solid, thorn-like and may be sharp or blunt/rounded distally. The parasutural crests and denticles increase in height antapically; and the denticles are more common toward the antapex. The paratabulation formula is ?pr, 4', 7", 6c, 6'", lp, 1"", 5s. Paraplate 1"' (Iu) is significantly smaller than 6'" (VI) and lp(X) is not displaced antapically. The precingular paraplate series is relatively short and paraplate ps (Z) is long and prominent. Archeopyle apical, type (4a), with a simple operculum which is free. The principal archeopyle suture is angular and coincides with the parasutures between the apical and precingular paraplate series.
Holotype and type locality. Plate 1, figs 5,6, specimen CPC 36310, housed in the Commonwealth Palaeontological Collection (CPC) of AGSO, Canberra, ACT 2601, Australia. Avocet-1A well, Timor Sea, offshore northwestern Australia, ditch cuttings sample at 1770.0 -1775.0 m. This interval is within the Flamingo Formation and is of late Tithonian age (Pseudoceratium iehiense Range Zone, Subzones 4ciia to 4cic). The geographical coordinates of the Avocet-1A well are: latitude 11° 22' 18.05"S and longitude 125° 45' 22.29"E (see Text-Figure 1).
Dimensions. These are all in micrometers (µm); 40 specimens were measured. Min.(Mean)Max.
Overall cyst length (excl. operculum): 22 (31) 42
Overall cyst width: 22 (29) 44
The holotype is 33 µm long and 31 µm wide. The measured specimens are from a sidewall core sample at 1771.50 m and ditch cuttings from 1775.0-1780.0 m and 1770.0-1775.0 m in the Avocet-IA well (Text-Figure 1).
Remarks. Microdinium avocetianum is distinguished by its relatively small size, darkened autophragm, low, irregularly denticulate crests and the paratabulation pattern. The species is typically slightly elongate, however some loisthocysts (Sarjeant et al., 1987, p. 26, 27) may rarely be equant or broader than long (Plate I).Microdinium avocetianum meets the three non-paratabulation-related criteria for partiform cysts in that it is small, has relatively robust autophragm and is not markedly elongate (Evitt, 1985, p. 112). The parasutural crests indicate a partiform gonyaulacalean paratabulation pattern of ?pr, 4', 7", 6c, 6'", lp, 1"", 5s (Plate 1, Text-Figure 2). The number of apical paraplates was deduced from the morphology of the principal archeopyle suture. The paratabulation style is like that of Microdinium opacum and Glyphanodinium facetum Drugg 1964 (see Evitt, 1985, text-figs 5.18A, D, G). Crucially, the intra-parasulcal ls/ps (Im/Z) parasuture clearly abuts 1'" (Iu) as opposed to lp (X) (Text-Figure 2). This is principally due to the large, elongate nature of the posterior parasulcal paraplate (Z). Furthermore, paraplate 1 '" (Iu) is relatively large and is not part of the parasulcus (Text-Figure 2). In sexiform gonyaulacacean forms, this paraplate is typically incorporated into the parasulcus and is relatively small (for example Riding, 1987, text-fig. 13). Despite the relatively large size of paraplate 1'" (Iu), it is significantly smaller than 6'" (VI). This means that paraplate lp (X) is not displaced antapically, thereby also displacing the l""/lp (Y/X) parasuture in the direction of the antapex, which is typical of many partiform species (Evitt, 1985, text-fig. 5.18E). The greater size of paraplate 6'" (VI) than 1'" (Iu) also means that 3'" (III) is lateral, as opposed to partially middorsal. The paracingulum of M. avocetianum is relatively prominent, slightly laevorotatory, and comprises six paraplates. By contrast, the parasulcus typically has much reduced or entirely suppressed parasutures. Paraplates rs (Ii) and Is (Im) and the flagellar scar are typically vaguely indicated (Plate 1). Despite this, five parasulcal paraplates including the flagellar scar can frequently be discerned (Text-Figure 2). The precingular paraplate series is significantly shorter than the postcingular paraplates (Plate 1 ); this disparity in size between the epicyst and hypocyst is typical of the partiform group (Evitt, 1985, p. 116). The middorsal precingular paraplate (4) is subdivided once into 4i and 4u of Taylor Evitt (Text-Figure 2). Because of this, and the fact that Taylor-Evitt paraplate 1 i is not reduced in size or incorporated into the parasulcal area, there are seven precingular paraplates. Loisthocysts were largely observed and few isolated opercula were encountered. The principal archeopyle suture appears to be somewhat expanded in many specimens; this gaping effect is probably a result of dorsoventral crushing. No accessory archeopyle sutures were observed. The apical paraplate series (opercula) are relatively flat in longitudinal view. However, the few observed opercula and the principal archeopyle suture indicate that four apical and no anterior intercalary paraplates are present. The presence of a preapical paraplate could not be unequivocally determined. This apparent lack of anterior intercalary paraplates is unsurprising given the disparity in size of paraplates 1'" (Iu) and 6'" (VI). Partiform taxa which superficially resemble the sexiform pattern, such as Microdinium opacum, typically lack anterior intercalaries (see Evitt, 1985, text-figs 5.18A, D, G; p. 116). Microdinium avocetianum is only the second species of Microdinium to be described from the Jurassic; the majority of representatives of this genus are Cretaceous and/or Paleogene (Williams et al., 1998, p. 399-404). The only other Jurassic species is M. jurassicum Riding & Helby 2001 from the Callovian-Oxfordian of Australasia (Riding and Helby, 2001a). This species represents the formalization of Microdinium sp. A (M.P. 619) of Helby et al. (1988, text-figs 10G, H) from Kawhia, New Zealand. It is therefore possible that M. jurassicum and M. avocetianum may be related and part of the same phylogenetic lineage. Microdinium avocetianum has been recorded from the late Tithonian of the Timor Sea region in many open file reports of R. Helby as 'Meiourogonyaulax avocetiana . Furthermore, it was reported from the Upper Jurassic of the Carnarvon Basin, Western Australia as Meiourogonyaulax sp. A by Parker (1986, p. 78, text-fig. 5.29, pi. 12, fig. 7). Microdinium avocetianum is morphologically similar to Jansoniajurassica Pocock 1972 and J. manifesta. Riding & Walton in Riding et al. 1991 (see below). Riding et al. (1991) interpreted J. jurassica and J. manifesta as forms which preferred fresh to brackish water paleoenvironments (see also Pocock, 1972). Specifically, these authors viewed J. manifesta as an opportunistic taxon, which thrived in brackish water paleoenvironments established during the early parts of a transgressive phase. Tykoezinski et al. (2000, p. 90) stated that some species of Jansonia Pocock 1972 emend. Riding & Walton in Riding et al. 1991 maybe polyhaline, thriving at times of lowered salinity, but tolerant of normal marine salinities. It is possible therefore, that M. avocetianum may be similarly polyhaline; however, it has not been observed in flood proportions in lagoonal faciès. In the Avocet 1A well at 1771.50 m it occurs in a diverse marine microplankton association of 75 species, which is interpreted as being open marine. Microdinium avocetianum is associated with a diverse and generally abundantmarine microplankton assemblages. These are described in more detail by Bint and Marshall (1994) and Riding and Helby (2001b).
Comparison. Microdinium avocetianum differs from other species of this genus in being relatively small and having a smooth, darkened autophragm, irregularly denticulate parasutural crests and al p (X) paraplate which is ventral in position. The majority of the 45 validly described species of Microdinium are larger than M. avocetianum. The Australasian Callovian-Oxfordian species Microdinium jurassicum is similar in overall morphology and size to M. avocetianum. However M. jurassicum is consistently microreticulate, has distally smooth/undulate parasutural crests and may be partially tuberculate (Riding and Helby, 2001a). Microdinium avocetianum is similar toM. glabrum Cookson & Eisenack 1974, M. opacum and the generotype, M. ornatum. The autophragm of these species is smooth and M. ornatum is similar in size. However, the lp/1"" (X/Y) parasuture is antapical in M. glabrum and M. ornatum and the parasutural crests are distally smooth in all three forms. Microdinium ornatum also has perforate parasutural crests and may also exhibit intratabular tubercles, which are absent in M. avocetianum. Microdinium angulare (Below 1987) Lentin & Williams 1989 has irregularly denticulate parasutural crests, but has an antapical lp/1"" (X/Y) parasuture and has dense nontabular ornamentation. The majority of the other representatives of Microdinium are characterized by low relief, nontabular ornamentation (granules, short spines, verrucae etc.), in contrast to being psilate. Examples of these forms areM. bait ens (Below 1987) Lentin & Williams 1989, M. carpentieriae Slimani 1994, M. consaeptum (Below 1987) Lentin & Williams 1989, M. deconinckii Slimani 1994, M. densigranulatum (Below 1987) Lentin & Williams 1989, M. dentatum Vozzhennikova 1967, M? fibratum Batten & Lister 1988 and M. granocarinatum (Below 1987) Lentin & Williams 1989. Species oí Microdinium with reticulate autophragm include M. cassiculum Wilson 1984, M. jurassicum and M. reticulatum Vozzhennikova 1967. Several species have spinose parasutural crests; these include M. carpentierae Slimani 1994, M? crinitum Davey 1969, M. dentatum Vozzhennikova 1967, M. distinctum Davey 1969 and M. setosum Sarjeant 1966. Microdinium bensonii Slimani 1994, M. marheineckii Slimani 1994 and M. mariae Slimani 1994 all have penitabular ornamentation. Microdinium? alatum Conrad 1941 ex Sarjeant 1967 has high, striate parasutural crests and is densely areolate. The paraplates of M carinatum (Below 1987) Lentin & Williams 1989 are characterized by large perforations, mostly arranged in penitabular positions, which may indicate reflected trichocyst pores (Below, 1987). Microdinium? horridum (Below 1987) Lentin & Williams 1989 and M? reteinvolvatum (Below 1987) Lentin & Williams 1989 both have a perforate autophragm. The former also has extremely prominent gonal spines formed by extensions of the parasutural crests (Below, 1987, pi. 17, figs 1-6). Microdinium avocetianum also superficially resembles the Late Jurassic species Meiourogonyaitlax mombasaensis Mungai in Jiang et al. 1992 in morphology, having a smooth, robust autophragm and a full gonyaulacalean paratabulation indicated by low parasutural crests. Meiourogonyaulax mombasaensis, however, has a sexiform gonyaulacalean paratulation pattern, is significantly larger than Microdinium avocetianum and lacks denticulate parasutural crests (Jiang et al., 1992). This new species also closely resembles all four species of Jansonia due to its relatively small size, partiform paratabulation and robust, darkened autophragm. Jansonia is extremely similar to Microdinium in morphology, but the genera were separated by Riding and Walton in Riding et al (1991) on the basis of paleoecological preference and the nature of the autophragm. The most similar species is /. manifesta, however this species is slightly more elongate, has a more rounded ambitus, has distally smooth parasutural ridges, has a more prominent (i.e. higher) paracingulum and autophragm covered by dense, branching ridges (Riding et al., 1991). Furthermore, the paratabulation of/, manifesta differs fromM. avocetianum in several ways. The parasulcal paraplates in/, manifesta are not reflected and the 7" paraplate in M. avocetianum is not elongate (Riding et al., 1991, textfig. 5). Jansonia jurassica lacks prominent parasutural features (Pocock, 1972; Jansonius, 1986) and /. scarffei Tykoezinski et al. 2000 has a deeply incised paracingulum. Jansoniapsilata Martinez et al. 1999 is significantly smaller thanM. avocetianum, is equant, has smooth autophragm and has low, distally smooth parasutural ridges.
Stratigraphical and Geographical Distribution. Microdinium avocetianum has been observed throughout Australia and adjacent regions. The species occurs in the Flamingo Formation and coeval strata (late Tithonian) in the Timor Sea, offshore northwestern Australia (Whittam et al., 1996, text-fig. 5). The material described herein is from the late Tithonian Flamingo Formation of the Avocet- 1A well. This species is also present in the late Tithonian of the Carnarvon Basin, offshore Western Australia. It is most prominent throughout the late Tithonian middle to upper Pseudoceratium iehiense Range Zone (Subzones 4ciia to 4cic) in this region (Foster, 2001, text-fig. 2). Reworked forms may also be intermittently present throughout the Berriasian to the Valanginian-Hauterivian of the Carnarvon Basin. The Pseudoceratium iehiense Range Zone spans the Jurassic-Cretaceous boundary and is late Tithonian to early Berriasian in age. Microdinium avocetianum is confined to the late Tithonian part of this Zone, the age of which was discussed in Helby et al. (1987, text-fig. 19).
Holotype: Riding, 2002, pl.1, figs.5-6.
Age: late Tithonian.
Microdinium avocetianum sp. nov. (Plate 1, figs 1-16; Text-Figure 2).
Derivation of name. From the Avocet-1A well.
Description. A small, ellipsoidal to subquadrangular species of Microdinium which may be slightly elongate or equant and is dorsoventrally flattened. The autophragm is dark, smooth to scabrate, occasionally irregularly microreticulate, and 0.5 urn thick. The paratabulation is indicated by low, smooth, undulose to irregularly denticulate parasutural crests. These parasutural crests vary from 0.5 urn to 2 urn in height, including the denticles. The denticles may be up to 1 urn high and are solid, thorn-like and may be sharp or blunt/rounded distally. The parasutural crests and denticles increase in height antapically; and the denticles are more common toward the antapex. The paratabulation formula is ?pr, 4', 7", 6c, 6'", lp, 1"", 5s. Paraplate 1"' (Iu) is significantly smaller than 6'" (VI) and lp(X) is not displaced antapically. The precingular paraplate series is relatively short and paraplate ps (Z) is long and prominent. Archeopyle apical, type (4a), with a simple operculum which is free. The principal archeopyle suture is angular and coincides with the parasutures between the apical and precingular paraplate series.
Holotype and type locality. Plate 1, figs 5,6, specimen CPC 36310, housed in the Commonwealth Palaeontological Collection (CPC) of AGSO, Canberra, ACT 2601, Australia. Avocet-1A well, Timor Sea, offshore northwestern Australia, ditch cuttings sample at 1770.0 -1775.0 m. This interval is within the Flamingo Formation and is of late Tithonian age (Pseudoceratium iehiense Range Zone, Subzones 4ciia to 4cic). The geographical coordinates of the Avocet-1A well are: latitude 11° 22' 18.05"S and longitude 125° 45' 22.29"E (see Text-Figure 1).
Dimensions. These are all in micrometers (µm); 40 specimens were measured. Min.(Mean)Max.
Overall cyst length (excl. operculum): 22 (31) 42
Overall cyst width: 22 (29) 44
The holotype is 33 µm long and 31 µm wide. The measured specimens are from a sidewall core sample at 1771.50 m and ditch cuttings from 1775.0-1780.0 m and 1770.0-1775.0 m in the Avocet-IA well (Text-Figure 1).
Remarks. Microdinium avocetianum is distinguished by its relatively small size, darkened autophragm, low, irregularly denticulate crests and the paratabulation pattern. The species is typically slightly elongate, however some loisthocysts (Sarjeant et al., 1987, p. 26, 27) may rarely be equant or broader than long (Plate I).Microdinium avocetianum meets the three non-paratabulation-related criteria for partiform cysts in that it is small, has relatively robust autophragm and is not markedly elongate (Evitt, 1985, p. 112). The parasutural crests indicate a partiform gonyaulacalean paratabulation pattern of ?pr, 4', 7", 6c, 6'", lp, 1"", 5s (Plate 1, Text-Figure 2). The number of apical paraplates was deduced from the morphology of the principal archeopyle suture. The paratabulation style is like that of Microdinium opacum and Glyphanodinium facetum Drugg 1964 (see Evitt, 1985, text-figs 5.18A, D, G). Crucially, the intra-parasulcal ls/ps (Im/Z) parasuture clearly abuts 1'" (Iu) as opposed to lp (X) (Text-Figure 2). This is principally due to the large, elongate nature of the posterior parasulcal paraplate (Z). Furthermore, paraplate 1 '" (Iu) is relatively large and is not part of the parasulcus (Text-Figure 2). In sexiform gonyaulacacean forms, this paraplate is typically incorporated into the parasulcus and is relatively small (for example Riding, 1987, text-fig. 13). Despite the relatively large size of paraplate 1'" (Iu), it is significantly smaller than 6'" (VI). This means that paraplate lp (X) is not displaced antapically, thereby also displacing the l""/lp (Y/X) parasuture in the direction of the antapex, which is typical of many partiform species (Evitt, 1985, text-fig. 5.18E). The greater size of paraplate 6'" (VI) than 1'" (Iu) also means that 3'" (III) is lateral, as opposed to partially middorsal. The paracingulum of M. avocetianum is relatively prominent, slightly laevorotatory, and comprises six paraplates. By contrast, the parasulcus typically has much reduced or entirely suppressed parasutures. Paraplates rs (Ii) and Is (Im) and the flagellar scar are typically vaguely indicated (Plate 1). Despite this, five parasulcal paraplates including the flagellar scar can frequently be discerned (Text-Figure 2). The precingular paraplate series is significantly shorter than the postcingular paraplates (Plate 1 ); this disparity in size between the epicyst and hypocyst is typical of the partiform group (Evitt, 1985, p. 116). The middorsal precingular paraplate (4) is subdivided once into 4i and 4u of Taylor Evitt (Text-Figure 2). Because of this, and the fact that Taylor-Evitt paraplate 1 i is not reduced in size or incorporated into the parasulcal area, there are seven precingular paraplates. Loisthocysts were largely observed and few isolated opercula were encountered. The principal archeopyle suture appears to be somewhat expanded in many specimens; this gaping effect is probably a result of dorsoventral crushing. No accessory archeopyle sutures were observed. The apical paraplate series (opercula) are relatively flat in longitudinal view. However, the few observed opercula and the principal archeopyle suture indicate that four apical and no anterior intercalary paraplates are present. The presence of a preapical paraplate could not be unequivocally determined. This apparent lack of anterior intercalary paraplates is unsurprising given the disparity in size of paraplates 1'" (Iu) and 6'" (VI). Partiform taxa which superficially resemble the sexiform pattern, such as Microdinium opacum, typically lack anterior intercalaries (see Evitt, 1985, text-figs 5.18A, D, G; p. 116). Microdinium avocetianum is only the second species of Microdinium to be described from the Jurassic; the majority of representatives of this genus are Cretaceous and/or Paleogene (Williams et al., 1998, p. 399-404). The only other Jurassic species is M. jurassicum Riding & Helby 2001 from the Callovian-Oxfordian of Australasia (Riding and Helby, 2001a). This species represents the formalization of Microdinium sp. A (M.P. 619) of Helby et al. (1988, text-figs 10G, H) from Kawhia, New Zealand. It is therefore possible that M. jurassicum and M. avocetianum may be related and part of the same phylogenetic lineage. Microdinium avocetianum has been recorded from the late Tithonian of the Timor Sea region in many open file reports of R. Helby as 'Meiourogonyaulax avocetiana . Furthermore, it was reported from the Upper Jurassic of the Carnarvon Basin, Western Australia as Meiourogonyaulax sp. A by Parker (1986, p. 78, text-fig. 5.29, pi. 12, fig. 7). Microdinium avocetianum is morphologically similar to Jansoniajurassica Pocock 1972 and J. manifesta. Riding & Walton in Riding et al. 1991 (see below). Riding et al. (1991) interpreted J. jurassica and J. manifesta as forms which preferred fresh to brackish water paleoenvironments (see also Pocock, 1972). Specifically, these authors viewed J. manifesta as an opportunistic taxon, which thrived in brackish water paleoenvironments established during the early parts of a transgressive phase. Tykoezinski et al. (2000, p. 90) stated that some species of Jansonia Pocock 1972 emend. Riding & Walton in Riding et al. 1991 maybe polyhaline, thriving at times of lowered salinity, but tolerant of normal marine salinities. It is possible therefore, that M. avocetianum may be similarly polyhaline; however, it has not been observed in flood proportions in lagoonal faciès. In the Avocet 1A well at 1771.50 m it occurs in a diverse marine microplankton association of 75 species, which is interpreted as being open marine. Microdinium avocetianum is associated with a diverse and generally abundantmarine microplankton assemblages. These are described in more detail by Bint and Marshall (1994) and Riding and Helby (2001b).
Comparison. Microdinium avocetianum differs from other species of this genus in being relatively small and having a smooth, darkened autophragm, irregularly denticulate parasutural crests and al p (X) paraplate which is ventral in position. The majority of the 45 validly described species of Microdinium are larger than M. avocetianum. The Australasian Callovian-Oxfordian species Microdinium jurassicum is similar in overall morphology and size to M. avocetianum. However M. jurassicum is consistently microreticulate, has distally smooth/undulate parasutural crests and may be partially tuberculate (Riding and Helby, 2001a). Microdinium avocetianum is similar toM. glabrum Cookson & Eisenack 1974, M. opacum and the generotype, M. ornatum. The autophragm of these species is smooth and M. ornatum is similar in size. However, the lp/1"" (X/Y) parasuture is antapical in M. glabrum and M. ornatum and the parasutural crests are distally smooth in all three forms. Microdinium ornatum also has perforate parasutural crests and may also exhibit intratabular tubercles, which are absent in M. avocetianum. Microdinium angulare (Below 1987) Lentin & Williams 1989 has irregularly denticulate parasutural crests, but has an antapical lp/1"" (X/Y) parasuture and has dense nontabular ornamentation. The majority of the other representatives of Microdinium are characterized by low relief, nontabular ornamentation (granules, short spines, verrucae etc.), in contrast to being psilate. Examples of these forms areM. bait ens (Below 1987) Lentin & Williams 1989, M. carpentieriae Slimani 1994, M. consaeptum (Below 1987) Lentin & Williams 1989, M. deconinckii Slimani 1994, M. densigranulatum (Below 1987) Lentin & Williams 1989, M. dentatum Vozzhennikova 1967, M? fibratum Batten & Lister 1988 and M. granocarinatum (Below 1987) Lentin & Williams 1989. Species oí Microdinium with reticulate autophragm include M. cassiculum Wilson 1984, M. jurassicum and M. reticulatum Vozzhennikova 1967. Several species have spinose parasutural crests; these include M. carpentierae Slimani 1994, M? crinitum Davey 1969, M. dentatum Vozzhennikova 1967, M. distinctum Davey 1969 and M. setosum Sarjeant 1966. Microdinium bensonii Slimani 1994, M. marheineckii Slimani 1994 and M. mariae Slimani 1994 all have penitabular ornamentation. Microdinium? alatum Conrad 1941 ex Sarjeant 1967 has high, striate parasutural crests and is densely areolate. The paraplates of M carinatum (Below 1987) Lentin & Williams 1989 are characterized by large perforations, mostly arranged in penitabular positions, which may indicate reflected trichocyst pores (Below, 1987). Microdinium? horridum (Below 1987) Lentin & Williams 1989 and M? reteinvolvatum (Below 1987) Lentin & Williams 1989 both have a perforate autophragm. The former also has extremely prominent gonal spines formed by extensions of the parasutural crests (Below, 1987, pi. 17, figs 1-6). Microdinium avocetianum also superficially resembles the Late Jurassic species Meiourogonyaitlax mombasaensis Mungai in Jiang et al. 1992 in morphology, having a smooth, robust autophragm and a full gonyaulacalean paratabulation indicated by low parasutural crests. Meiourogonyaulax mombasaensis, however, has a sexiform gonyaulacalean paratulation pattern, is significantly larger than Microdinium avocetianum and lacks denticulate parasutural crests (Jiang et al., 1992). This new species also closely resembles all four species of Jansonia due to its relatively small size, partiform paratabulation and robust, darkened autophragm. Jansonia is extremely similar to Microdinium in morphology, but the genera were separated by Riding and Walton in Riding et al (1991) on the basis of paleoecological preference and the nature of the autophragm. The most similar species is /. manifesta, however this species is slightly more elongate, has a more rounded ambitus, has distally smooth parasutural ridges, has a more prominent (i.e. higher) paracingulum and autophragm covered by dense, branching ridges (Riding et al., 1991). Furthermore, the paratabulation of/, manifesta differs fromM. avocetianum in several ways. The parasulcal paraplates in/, manifesta are not reflected and the 7" paraplate in M. avocetianum is not elongate (Riding et al., 1991, textfig. 5). Jansonia jurassica lacks prominent parasutural features (Pocock, 1972; Jansonius, 1986) and /. scarffei Tykoezinski et al. 2000 has a deeply incised paracingulum. Jansoniapsilata Martinez et al. 1999 is significantly smaller thanM. avocetianum, is equant, has smooth autophragm and has low, distally smooth parasutural ridges.
Stratigraphical and Geographical Distribution. Microdinium avocetianum has been observed throughout Australia and adjacent regions. The species occurs in the Flamingo Formation and coeval strata (late Tithonian) in the Timor Sea, offshore northwestern Australia (Whittam et al., 1996, text-fig. 5). The material described herein is from the late Tithonian Flamingo Formation of the Avocet- 1A well. This species is also present in the late Tithonian of the Carnarvon Basin, offshore Western Australia. It is most prominent throughout the late Tithonian middle to upper Pseudoceratium iehiense Range Zone (Subzones 4ciia to 4cic) in this region (Foster, 2001, text-fig. 2). Reworked forms may also be intermittently present throughout the Berriasian to the Valanginian-Hauterivian of the Carnarvon Basin. The Pseudoceratium iehiense Range Zone spans the Jurassic-Cretaceous boundary and is late Tithonian to early Berriasian in age. Microdinium avocetianum is confined to the late Tithonian part of this Zone, the age of which was discussed in Helby et al. (1987, text-fig. 19).