- © 2000 by AASP Foundation
Following a restudy of the type material of Muderongia simplex Alberti 1961, this species and the informally-named Volgian morphotype Muderongia sp. A of Davey (1979) are deemed to be unequivocally conspecific. They both have angular endocysts, the extremities of which extend into the pericystal horns, thereby engendering a cornucavate cyst organisation. The specific diagnosis of Muderongia simplex is emended to note this morphologic phenomenon. The distinctive Lower Cretaceous (Ryazanian to Hauterivian–Barremian) circumcavate representatives of Muderongia which have been extensively referred to as Muderongia simplex in the literature are assigned to the new species Muderongia endovata. The distinction between the genera Muderongia and Phoberocysta is maintained, contrary to a recent synonymisation.
During an investigation into the marine palynostratigraphy of the uppermost Jurassic to Lower Cretaceous sediments of northwest Europe, Davey (1979) illustrated and briefly informally described Muderongia sp. A from the Middle Volgian (= Upper Kimmeridgian sensu Anglico) of southern England. This distinctive and abundant morphotype has been found to be present throughout Europe in significant proportions and is confined to the Middle Volgian. Its stratigraphic extent has been determined to range between the Rotunda and Okusensis/Kerberus zones (Davey, 1979; 1982; Woollam and Riding, 1983; Riding, 1984; Riding and Thomas, 1988; 1992; Riley et al., 1989; Partington et al., 1993; Riding et al., 1993), thus it is a reliable biostratigraphic index fossil. Muderongia sp. A has never been formalised due to taxonomic problems which were detailed by Riding and Thomas (1988) and Barron (1989). These taxonomic difficulties relate to the relationship of Muderongia sp. A to the type material of Muderongia simplex Alberti 1961 and are set out in detail below. It is the purpose of this paper to clarify and stabilise the taxonomic positions of these two forms. In this paper, the standard ammonite zones are treated as being chronostratigraphic units (Wimbledon and Cope, 1978; Callomon, 1984). Therefore, ammonite zones are referred to by the specific name only, e.g., Albani Zone.
THE TAXONOMIC POSITION OF MUDERONGIA SP. A OF DAVEY (1979) AND MUDERONGIA SIMPLEX SENSUDAVEY (1979)
Muderongia is the oldest known ceratioid dinoflagellate cyst genus. It has an apical archeopyle, is cavate and is characterised by five or six horns formed by the periphragm (one apical, two paracingular/lateral and one or two antapical) (Monteil, 1991). It emerged during the latest Jurassic, however, the genus is most diverse in the Early Cretaceous, and many of the constituent species are key biostratigraphic markers (Text-Figures 1⇓, 2⇓). Davey (1979) described Muderongia sp. A as differing from Muderongia simplex Alberti 1961 due to its distinctive angular, as opposed to subspherical, endocyst. Davey (1979) further commented that the endocyst of Muderongia sp. A may extend into the proximal part of the antapical horns. In fact, the endocyst of Muderongia sp. A of Davey (1979) often markedly extends into the proximal portion of all the four or five horns (e.g., Riding and Thomas, 1988, pl. 1, fig. 8⇓). This partial extension of the endocyst into the horns formed by the pericyst in Muderongia sp. A engenders a characteristically cornucavate cyst organisation, which contrasts markedly with the typically circumcavate cyst organisation of the Lower Cretaceous forms attributed to Muderongia simplex (see, for example, Davey, 1979, pl. 2, figs 7, 8). No intermediate morphotypes between these two forms have been observed.
It was considered that Muderongia sp. A of Davey (1979) was a distinct taxon which awaited formalisation (Fisher and Riley, 1982). However, Riding and Thomas (1988) and Barron (1989) noted that, according to the original illustrations and the description of Muderongia simplex by Alberti (1961), this species may have both angular and subspherical endocysts. Alberti (1961) stated “the inner body (endocyst) lies close to the outer margin of the cyst (pericyst), often it stretches out somewhat into the horns” (present authors’ emphasis) (the German to English translation is by W. A. S. Sarjeant, unpubl.). Alberti (1961) figured both circumcavate and cornucavate forms (Alberti, 1961, pl. 2, figs 1, 2, 4–6; pl. 12, figs 1, 2), i.e. Muderongia simplex sensu Davey (1979) and Muderongia sp. A of Davey (1979) respectively. The endocyst of the holotype of Muderongia simplex (Alberti, 1961, pl. 2, fig. 4) appears to be markedly subangular. This observation was confirmed by a restudy of Alberti’s collection by one of us (NEP). Although the holotype is lost, other figured specimens of Alberti (1961, pl. 2, figs 1, 2; pl. 12, fig. 2) have angular endocysts which extend into the pericystal horns, in addition to forms with subspherical endocysts (pl. 2, figs. 5, 6; pl. 12, fig. 1). Riding and Thomas (1988) and Barron (1989) recognised the taxonomic confusion between Muderongia sp. A of Davey (1979) and Muderongia simplex, and the former authors suggested a restudy of the original material of Alberti (1961). Poulsen (1996) also noted this confused taxonomic situation and included both Muderongia sp. A of Davey (1979) and Muderongia simplex in his emended Muderongia simplex. Poulsen (1996) emended the specific diagnosis of Muderongia simplex so as to include forms with both angular and subspherical endocysts, effectively formalising the description of all the specimens figured by Alberti (1961). The treatment of Muderongia sp. A by Monteil (1991) was also reviewed and clarified by Poulsen (1996) (see below). Muderongia microperforata (Davey 1982) Monteil 1991 and Muderongia tomaszowensis Alberti 1961 were both considered to be subspecies of Muderongia simplex by Poulsen (1996). However, we herein maintain Muderongia microperforata and Muderongia tomaszowensis as separate species, and in addition separate Muderongia endovata sp. nov. from Muderongia simplex.
Davey (1979) and Helby (1987) suggested that Muderongia sp. A of Davey (1979) may be related to Senoniasphaera jurassica (Gitmez & Sarjeant 1972) Lentin & Williams 1976 and the genus Senoniasphaera Clarke & Verdier 1967 respectively. However, Muderongia is classified in the Family Ceratiaceae; prior to the publication of Fensome et al. (1993), it was termed an unequivocal ‘psuedoceratioid’ genus (Poulsen and Riding, 1992). By contrast, Senoniasphaera belongs to the Family Areoligeraceae and is a ‘compressed gonyaulacoid form’ (e.g., Bint, 1986). Monteil (1991) appeared to agree with Helby’s (1987) assertion that Muderongia sp. A of Davey (1979) belonged in Senoniasphaera, as he illustrated this morphotype as Senoniasphaera tabulata Backhouse & Helby, in Helby, 1987 (Monteil, 1991, pl.1, figs 3a–3c). This implicit synonymisation is rejected here as Senoniasphaera tabulata differs from Muderongia sp. A of Davey (1979) in that it has relatively small paracingular protrusions (as opposed to lateral horns), the cavation is minimal except at the paired antapical horns, and it has complete and well-expressed paratabulation by parasutural ridges (Helby, 1987, text-figs 21–23).
In summary, therefore, the holotype of Muderongia simplex Alberti 1961 (Alberti, 1961, pl. 2, fig 4) is clearly cornucavate; see Stover et al. (1996, pl. 5, fig. 13). Furthermore, one of us (NEP) has confirmed this by a restudy of the type material in 1994. Therefore, we contend that Muderongia simplex sensu stricto has an angular endocyst and is cornucavate and the circumcavate forms with subspherical endocysts are a new species, herein named as Muderongia endovata sp. nov. The latter forms are largely Lower Cretaceous and have been erroneously and extensively assigned to Muderongia simplex.
Division Dinoflagellata (Bütschli 1885)
Class Dinophyceae Pascher 1914
Subclass Peridiniphycidae Fensome et al. 1993
Order Gonyaulacales Taylor 1980
Suborder Ceratiineae Fensome et al. 1993
Family Ceratiaceae Willey & Hickson 1909
Muderongia mcwhaei Cookson & Eisenack 1958.
Monteil (1991) deemed Phoberocysta to be a junior synonym of Muderongia as part of a major revision of these Upper Jurassic–Lower Cretaceous genera. Both Muderongia and Phoberocysta are cavate, ceratioid genera with apical archeopyles. The genera differed, prior to Monteil’s (1991) revision, principally in that Muderongia is a proximate cyst with a smooth periphragm and Phoberocysta is proximochorate, its periphragm being surmounted by parasutural and intraparatabular processes. This distinction is a major morphologic feature and intergradation between the genera has not been demonstrated; furthermore, the two genera have different stratigraphic ranges. Muderongia emerged in the latest Jurassic (mid Volgian) and has been reported throughout the Early Cretaceous into the Albian (Costa and Davey, 1992; Riding and Thomas, 1992). The inception of Phoberocysta is in the late Ryazanian and the range top of the longest ranging species, Phoberocysta neocomica (Gocht 1957) Millioud 1969, is early Aptian (Heilmann-Clausen, 1987; Costa and Davey, 1992). Furthermore, the consistent presence or absence of processes is a well established criterion for generic separation. For example, the following pairs of genera are characterised by the respective absence and presence of processes or other major discrete ornamentation: Endoceratium–Pseudoceratium, Mendicodinium–Ctenidodinium, Odontochitina–Xenascus, Pareodinia–Gochteodinia, Subtilisphaera–Palaeohystrichophora and Rhombodinium–Wetzeliella. Endoceratium–Pseudoceratium and Odontochitina–Xenascus are, like Muderongia and Phoberocysta, ceratioid genera. In a recent index of dinoflagellate cyst genera and species, Lentin and Williams (1993) upheld all the taxonomic proposals of Monteil (1991). However, previous and subsequent major studies on dinoflagellate cysts generally and ceratioid cysts specifically have all maintained the separate generic identities of Muderongia and Phoberocysta (e.g., Wall and Evitt, 1975; Stover and Evitt, 1978; Evitt, 1985; Bint, 1986; Helby, 1987; Fensome et al., 1993; Stover et al., 1996; Williams et al., 1998). In this study, we recognise both genera and reject the synonymisation of Muderongia and Phoberocysta proposed by Monteil (1991). We prefer to retain the generic synopsis and modified description of Stover and Evitt (1978, pp. 66, 67), which was considered by Helby (1987, p. 298) to represent an emendation. Furthermore, we also reject the synonymisation by Monteil (1991) of Muderongia extensiva Duxbury 1977 with Muderongia mcwhaei Cookson & Eisenack 1958 as the former consistently has significantly longer horns than the latter. Additionally, these two species are confined to separate provinces; Muderongia extensiva is confined to northwest Europe, whereas Muderongia mcwhaei is restricted to Australasia and the South Atlantic region. We also reject the placing of Phoberocysta tabulata Raynaud 1978 into Muderongia by Monteil (1991) because of the lack of prominent parasutural processes, in this species.
Muderongia simplex, Alberti, 1961, p. 12, pl. 2, fig. 6, pl. 12, fig. 1 (non. pl. 2, figs. 1, 2, 4, 5, pl. 12, fig. 2); Millioud, 1967, pl. 2, fig. 16; Brideaux, 1976, pl. 44.2, fig. 1; Duxbury, 1977, p. 55, pl. 14, fig. 9; Harris, 1977, pl. 20, fig. 6; Duxbury, 1978, pl. 3, fig. 2; Raynaud, 1978, pl. 1, fig. 8; Thusu, 1978, pl. 2, fig. 4; Ashraf, 1979, p. 136, pl. 6, fig. 10; Davey, 1979, pl. 2, figs. 7–8; Piasecki, 1979, fig. 15; Dörhöfer and Davies, 1980, figs. 34D, 36C–G; Morgan, 1980, pl. 19, figs. 7–10; Williams and Bujak, 1980, pl. 5, figs. 2, 4; Below, 1981, pl. 1, fig. 6⇓; Woollam and Riding, 1983, pl. 8, figs. 7–8; Thusu and Van der Eem, 1985, pl. 53, fig. 4; Williams and Bujak, 1985, text-fig. 33.16; Aarhus et al., 1986, fig. 10o; Davey, 1987, pl. 5, fig. 14; Habib and Drugg, 1987, pl. 4, fig. 1; Heilmann-Clausen, 1987, pl. 1, figs. 9–10; Thusu et al., 1988, pl. 29, figs. 8, 9; ?Barron, 1989, pl. 2, figs. 8, 9; Colin et al., 1992, pl. 4, figs. 11, 12; Costa and Davey, 1992, pl. 3.2, fig. 4; Poulsen, 1996, pp. 57, 58, pl. 23, fig. 5.
Derivation of Name.
The specific epithet refers to the ovoidal shape of the endocyst.
A species of Muderongia, the thin periphragm of which forms one apical horn, two equatorial/lateral horns and one antapical horn giving a typical rhomboid to rounded subpentagonal ceratioid amb. The area of the periphragm in the right antapical area, close to the 5′″/6′″ parasuture, forms a prominent shoulder or protrusion which is a reduced or incipient right antapical horn. The left antapical horn is close to the 1p/1″″ parasuture and is the longest and most prominent. The horns are subconical, tapering distally and with rounded, closed terminations. The equatorial/lateral horns, however, may be subrhombic with a medial concavity at the distal extremities at the position of the paracingulum. The equatorial/lateral horns are frequently longer posterior to the position of the paracingulum and may point slightly in an antapical direction. The endocyst is ovoidal to longitudinally elongate subspherical; it may have a small antapical boss. The ovoidal/subspherical endocyst engenders a circumcavate cyst organisation. Periphragm continuous, thin, smooth to microscabrate; endophragm continuous, relatively thick, smooth, scabrate or microgranulate. Archeopyle apical, type (4A). Paratabulation corniform gonyaulacacean, indicated, sometimes partially, by low, smooth, discontinuous ridges on the periphragm. Paracingulum indicated by low ridges on the periphragm and frequently a medial concavity on the equatorial/lateral horns. Parasulcus indicated by low ridges on the periphragm and the parasulcal notch, offset to the left.
Specimen MPK 1275, Housed in the palynological collections of the British Geological Survey, Keyworth, Nottingham, UK.
Locality and Horizon.
Specimen figured as Muderongia simplex Alberti, 1961 (pl. 2, fig. 6). Housed in the palynological collections of the Bundesanstalt für Geowissenschaften und Rohstoffe Aussenstelle, Berlin, Germany.
Locality and Horizon.
The Valanginian of the Dabendorf Borehole, Germany (slide 2a/D1 of Alberti, 1961).
The holotype measures 104.5 μm in overall length and 75.5 μm in overall width; the endocyst is 64.5 μm long and 55.5 μm wide. The paratype is 118.0 μm in overall length and 92.5 μm in overall width; the endocyst measures 76.0 μm in length and 72.5 μm in width.
Assemblage of 35 specimens measured (μm):
|Overall cyst length: (including operculum)||98.5||107.5||118.5|
|Overall cyst width:||63.5||69.5||78.0|
|Endocyst length: (including operculum)||62.5||66.0||71.5|
The periphragm of Muderongia endovata sp. nov. is normally psilate and the pericoel is typically narrow. The horns are usually relatively short in comparison to other representatives of the genus, however the length of these may vary significantly within the species.
Stratigraphical and Geographical Distribution.
Muderongia endovata sp. nov. is a geographically extremely widespread species which has been reported from the Early Cretaceous of arctic Canada, Australia, Europe, Greenland, the Middle East, offshore North Africa, the North and South Atlantic, Norway, and Spitsbergen (see synonymy listing, above). It is present, in significant proportions, (attributed to Muderongia simplex) from the late Ryazanian (Stenomphalus zone) to the late Hauterivian (Marginatus/Variabilis zones) in northwest Europe (Text-Figure 1⇑; Duxbury, 1978; Davey, 1979; 1982; Woollam and Riding, 1983; Costa and Davey, 1992). Duxbury (1977) and Heilmann-Clausen (1987) stated that this form extends into the early–mid Barremian. It is possible that either the species is present sporadically and in low numbers in Lower Barremian strata or that these Barremian records represent reworking. Schiøler (1992) illustrated a specimen of this species (as Muderongia simplex subsp. simplex) from the Upper Cretaceous (Coniacian) of Bornholm, Denmark, which is intepreted as representing stratigraphical recycling from the late Ryazanian to late Hauterivian.
Barron (1989, pl. 2, figs. 8, 9) illustrated a single specimen which he attributed to Muderongia simplex (i.e. Muderongia endovata) from the Middle Volgian of northeast Scotland. This specimen differs from typical Lower Cretaceous specimens of Muderongia endovata in that it has relatively long equatorial/lateral and antapical horns. It is therefore questionably attributed to Muderongia endovata and is most likely to be an aberrant specimen. Poulsen (1996, pl. 23, fig. 5) also illustrated a rather squat specimen of Muderongia endovata (as Muderongia simplex) from ammonite-bearing strata from the Middle Volgian of Poland. This record is from the Scythicus Subzone of the Scythicus Zone, which is equivalent to the Albani Zone. Thus it appears that Muderongia endovata may be extremely rare in the mid Volgian of Europe, absent during the late Volgian–early-earliest late Ryazanian and reappearing in large numbers in the late Ryazanian (Stenomphalus Zone).
Muderongia endovata sp. nov. differs from all other species of this genus in having relatively short, straight and blunt apical, lateral and antapical horns and an ovoidal to subspherical endocyst, giving a circumcavate cyst organisation. The lateral horns are of the Axial type L I of Monteil (1991), meaning that they are aligned with the paracingulum, indented and show equal development of precingular and postcingular extensions (Monteil, 1991, table 3). Thus all species with ‘bent’ and ‘curved’ lateral horns (types L II and L IV of Monteil (1991) respectively) differ profoundly from Muderongia endovata in both lateral horn morphology and general shape. These taxa comprise Muderongia australis, M. asymmetrica, M. crucis, M. extensiva, M. macwhaei, M. staurota, M. testudinaria and M. tetracantha. Furthermore, although Muderongia endovata has a single antapical horn, it is not axial (i.e. type ATP I of Monteil, 1991) as the horn is distinctly offset to the left and there is a reduced or incipient horn in the right antapical area. Hence, Muderongia endovata is deemed to have ‘joined’ (type ATP II) antapical horns of Monteil (1991, table 4). This means that Muderongia aequicornus, M. pariata and M. tomaszowensis differ from M. endovata as they have ‘not joined’ (type ATP III) and axial antapical horn types (Monteil, 1991). Additionally, Muderongia longicorna has two long, subequal antapical horns, which may be pointed distally. Microperforate periphragm is a characteristic feature of Muderongia microperforata, M. perforate and M. siciliana. Furthermore, M. siciliana is a small species with unequally developed lateral horns (Torricelli, 1996). Muderongia simplex differs from M. endovata in having a rounded rectangular to subpentagonal endocyst, engendering a cornucavate cyst organisation.
A species of Muderongia, with a distinctly ceratioid outline and which is dorsoventrally compressed. The thin periphragm forms five prominent horns which comprise a single apical horn (axial, type AP I), two indented lateral horns (axial, type L I) and two antapical joined horns (type ATP I). The left antapical horn is well developed and normally is over double the size of the right antapical horn. The horns are typically conical, proximally wide and rounded distally; the distal extremities may be closed or perforate. The endocyst is oval, giving rise to a circumcavate cyst organisation. The periphragm is thin, psilate to scrabrate; the endophragm is continuous, relatively thick and psilate, scabrate or granulate. Archeopyle apical, type (4A), the principal suture of which indicates a distinctly left-offset parasulcal notch. Operculum free. Paratabulation is indicated by the archeopyle, the paracingulum which is frequently marked by low ridges and/or indentations of the lateral horns, and the parasulcus which may be indicated by the offset parasulcal notch.
Plate 2, figure 9 of Alberti (1961), housed in the palynological collections of the Bundesanstalt für Geowissenschaften und Rohstoffe Aussenstelle, Berlin, Germany. The holotype (pl. 2, fig. 8 of Alberti, 1961) is lost (Dr. J. Strahl, written comm.).
Locality and Horizon of the Neotype.
Turonian of the Pirna Borehole, Sachen, Germany, slide 2a of Alberti (1961). The latter author considered that the specimens of Muderongia perforata from the Turonian of the Pirna Borehole may have been reworked.
Holotype, 144 μm in length (width not measured); range 123 μm to 166 μm in length and 108 μm to 116 μm in width according to Alberti (1961). Neotype, 191 μm in length and 165 μm in width; endocyst 109 μm in length and 83 μm in width. Paratype, 148 μm in length and 109 μm in width; endocyst 83 μm in length and 67 μm in width.
The emendation of this species is largely related to the establishment of a neotype because the holotype cannot been located. We agree with Monteil (1991) that this species should be considered separate from Muderongia pariata Duxbury 1983 (see comparison), contrary to a synonymisation by Aarhus et al. (1990).
Stratigraphical and Geographical Distribution.
Valanginian of the Dabendorf Borehole, Germany (personal observation, NEP) and Turonian (possibly reworked, Alberti 1961) of the Pirna borehole, Germany.
Muderongia perforata differs from most other species of Muderongia in having distally perforate horns; Muderongia microperforata has a microperforate periphragm; M. pariata has only one antapical horn.
Muderongia simplex, Alberti, 1961, p. 12, pl. 2, figs. 1, 2, 4, pl. 12, fig. 2 (non. pl. 2, figs. 5, 6, pl. 12, fig 1); Gitmez and Sarjeant, 1972, pp. 241–242, pl. 15, figs. 1–2; ? Thusu, 1978, pl. 2, fig. 6; Duxbury, 1980, pl. 11, fig. 3; Fisher and Riley, 1980, p. 323, pl. 4, fig. 3; Poulsen, 1996, pp. 57, 58, pl. 23, figs. 2–3; Bailey et al., 1997, fig. 4I.
Muderongia sp. A, Davey, 1979, p. 64, pl. II, figs. 4–5; Davey and Riley, 1978, pl. 3, fig. 3; Davey, 1982, p. 30, pl. 9, figs. 1–3; Riding, 1984, pl. 5, fig. 1; Thusu and Vigran, 1985, pl. 52, fig. 5; Van Helden, 1986, p. 196, pl. 5, fig. 3; Riding and Thomas, 1988, p. 78, 80, pl. 1, fig. 8, pl. 2, fig. 8; Barron, 1989, p. 200, pl. 2, fig. 5; Riding and Thomas, 1992, pl. 2.18, fig. 6; Fensome et al., 1996, pl. 2, fig 1.
A species of Muderongia, the thin periphragm of which forms one apical, two equatorial/lateral and two antapical horns, giving a typically rhomboid to rounded subpentagonal ceratioid amb. The apical, equatorial/lateral and the left antapical horns are highly variable in length, however all may be relatively prominent. The right antapical horn is close to the 5′″/6′″ parasutural boundary and is consistently significantly reduced in size. The left antapical horn is close to the 1p/1″″ parasuture. The horns are normally conical, tapering slightly distally and with blunt, rounded, closed terminations. The equatorial/lateral horns, however, may be parallel-sided with a slight medial concavity at the distal extremities at the position of the paracingulum (i.e. indented or ‘notched’). The endocyst is rounded rectangular to subpentagonal; it approaches the shape of the pericyst in that the endophragm extends slightly into the horns. The relatively small size of the right antapical horn means that the endocyst frequently does not penetrate into this incipient horn or protuberance. The rounded/subangular endocyst engenders a cornucavate cyst organisation; the two cyst layers are normally in contact in the areas between the horns. Periphragm continuous, thin, smooth to microscabrate; endophragm continuous, relatively thick, smooth, scabrate, occasionally having low discontinuous anastomosing crests. Archeopyle apical, type (4A), operculum free. Paratabulation generally not indicated, presumably corniform gonyaulacacean; rarely it may be faintly indicated by low, discontinuous ridges on the periphragm. Paracingulum frequently indicated by low ridges on the periphragm and/or a medial concavity on the equatorial/lateral horns. Parasulcus may be indicated by the parasulcal notch, offset to the left.
The original holotype, Plate 2, figure 4 of Alberti (1961), is lost (J. Strahl, written comm.). We therefore must nomimate a neotype specimen and hereby nominate the best specimen of Muderongia sp. A of Davey (1979). This is considered to be figure 5 on Plate 2 of Davey (1979). This specimen was illustated by Monteil as Senoniasphaera tabulata (Monteil, 1991, pl. 1, figs. 3a–3c). The specimen is British Geological Survey (BGS) figured specimen MPK 1271 and is curated in the palynological collections of the Basin Analysis and Stratigraphy Group, BGS, Nottingham, UK.
Locality and horizon.
The original holotype, which is now lost, was from the Valanginian of the Dabendorf Borehole, near Berlin, Germany (Alberti, 1961, p. 12). The neotype is from the BGS Warlingham Borehole, Surrey (NGR TQ 3476 5719; Worssam and Ivimey-Cook, 1971). The core sample at 696.65m is from the Upper Kimmeridge Clay (Rotunda zone).
Holotype, 151 μm in length and 121 μm in width according to Alberti 1961, p. 12). The specimen of Alberti (1961, pl. 2, fig. 1) measures 198 μm in length and 148 μm in width; its endocyst is 99 μm in length and 82 μm in width. The material of Alberti (1961) thus appears to be significantly larger than the material below, which is largely from England.
Assemblage of 30 specimens measured (μm):
|Overall cyst length: (including operculum)||87.5||112.0||131.0|
|Overall cyst width:||69.5||79.5||96.5|
|Endocyst length: (including operculum)||70.5||77.5||84.5|
The main expression of intraspecific variation within Muderongia simplex is the length of the horns. Some morphotypes, particularly those from the uppermost Kimmeridgian, frequently have relatively short apical, paracingular and antapical horns (e.g., Davey, 1979, pl. 2, fig. 4; Riding and Thomas, 1988, pl. 1, fig. 8). However, these features, aside of the right antapical horn, may be prominent (e.g., Fisher and Riley, 1980, pl. 4, fig. 3; Riding and Thomas, 1988, pl. 2, fig. 8). The majority of Volgian representatives have relatively long horns(e.g.,Bailey etal., 1997,text-fig. 4i⇑). This morphostratigraphical aspect requires further detailed study. The periphragm is readily folded, reflecting its thinness. Note that the neotype was illustated by Monteil as Senoniasphaera tabulata (Monteil, 1991, pl. 1, figs. 3a–3c).
Stratigraphical and Geographical Distribution.
Muderongia simplex has been recorded from the Upper Jurassic (Middle Volgian) and Lower Cretaceous (Valanginian–Hauterivian) of Europe (e.g., Davey, 1979), North Africa (Thusu and Vigran, 1985) and the western Atlantic (Habib, 1978; Van Helden, 1986). It is present consistently in the Middle Volgian from the Rotunda to the Okusensis/ Kerberus zones (Text-Figure 1⇑; Riding, 1984; Riley et al., 1989; Riding and Thomas, 1992). In the North Sea Basin and adjacent regions, there is an acme of Muderongia simplex between the Rotunda and the Albani zones (Text-Figure 1⇑; Riley et al., 1989; Riding et al., 1993). In Denmark, Muderongia simplex has been recorded from the Albani Zone from the Aars-1 borehole, side track 3. Records from the Lower Cretaceous are extremely scarce and include those of the holotype from the Valanginian of the Berlin area, Germany (Alberti, 1961), material from the Berriasian to lowermost Valanginian from offshore eastern Florida (Habib, 1978) and questionable specimens from the Valanginian–Hauterivian of Spitsbergen (Thusu, 1978, pl.2, fig. 6) and the Barremian of eastern England (Duxbury, 1980, pl. 11, fig. 3). The range top of consistent Muderongia simplex as defined herein in the Okusensis/Kerberus zones is considered to be a reliable biostratigraphic marker. This means that it is possible that the holotype, the material of Habib (1978) and the possible specimens of Thusu (1978) and Duxbury (1980) represent reworked material.
Muderongia simplex differs from all other species of this genus in having a cornucavate cyst organisation with significant incursion of the endocyst into the pericystal horns. Muderongia tomaszowensis Alberti 1961 is typically mildly cornucavate and may be delphicavate or circumcavate (Monteil, 1991). Furthermore, this species has relatively short horns and a characteristically rounded, four-sided endocyst. The endocysts of Muderongia crucis Neale & Sarjeant 1962, Muderongia extensiva Duxbury 1977, Muderongia longicorna Monteil 1991, Muderongia macwhaei Cookson & Eisenack 1958 and Muderongia tetracantha (Gocht 1957) Alberti 1961 may be subangular and extend slightly into the proximal parts of the four horns of these species. However, these forms are readily distinguished from Muderongia simplex by their characteristic long and slender horns. Furthermore, the paracingular horns of these species are consistently indented and bent, the distal portion pointing in an antapical direction, i.e. are of type LII of Monteil (1991).Muderongia aequicornus Århus in Århus et al. 1990 has axial (straight) lateral horns like Muderongia simplex. There may, however, be antapical protrusions in the epicyst of Muderongia aequicornus, but these do not extend into the antapical pericystal horns (Monteil, 1991, pl. 2, fig. 9). Muderongia asymmetrica Brideaux 1977 and Muderongia testudinaria Burger 1980 have two long, tapering, distally pointed antapical horns and Muderongia microperforata (Davey 1982) Monteil 1991 is characterised by microperforate periphragm. Muderongia australis Helby1987, Muderongia endovata sp. nov., Muderongia pariata Duxbury 1983, Muderongia perforata Alberti 1961 and Muderongia staurota Sarjeant 1966 all have ovoidal or subspherical endocysts.
Muderongia simplex, Alberti, 1961, pl. 2, fig. 5.
A species of Muderongia, the thin periphragm of which forms four prominent horns and one minor protuberance. There are a single apical horn, two lateral-equatorials and one left antapical horn; the minor protuberance is in a right antapical position close to the 5′″/ 6′″ parasutural boundary. The shape is ceratioid, compressed dorsoventrally, giving an angular, cruciform appearance. The horns are normally conical with closed, rounded, distal terminations. Thus, M. tomaszowensis has axial lateral horns (type L I) and an axial antapical horn (type ATP I) (Monteil, 1991). The endocyst is subcircular to rhombic in dorsoventral outline; it extends moderately into the lateral horns. The endocyst is rounded antapically. Or, it may extend towards the prominent antapical horn, without penetrating into the horn, thereby giving rise to an apically and antapically circumcavate or hypocavate cyst organisation. The species is cornucavate; the two cyst layers are normally in contact in the areas between the horns. The periphragm is thin, psilate to scabrate and the endophragm is continuous, relatively thick, psilate, scabrate or granulate. Archeopyle apical, type (4A), with the margin indicating a distinct left offset of the parasulcal notch, operculum free. Paratabulation indicated by archeopyle, paracingulum (marked by low ridges and indentation of the lateral horns) and the parasulcus which may be delimited by the parasulcal notch and a narrow, sinuous depression.
Pl. 2, fig. 13 of Alberti (1961), housed in the palynological collections of the Bundesanstalt für Geowissenschaften und Rohstoffe Aussenstelle, Berlin, Germany. The holotype (pl. 2, fig. 12 of Alberti, 1961) is lost (Dr. J. Strahl, written comm.).
Specimen assigned to Muderongia simplex by Alberti (1961, pl. 2, fig. 5) and housed in the palynological collections of the Bundesanstalt für Geowissenschaften und Rohstoffe Aussenstelle, Berlin, Germany.
Locality and Horizon.
The neotype is from the Valanginian of the Tomaszow Borehole, Poland, slide 8/Val. of Alberti (1961). The paratype is from the Lower Hauterivian of the Werle-3 Borehole, Mecklenburg, Germany, slide 2a/ 14 of Alberti (1961).
Holotype, 140 μm in overall length and 112 μm in overall width; range 135–175 μm in length and 104–140 μm in width (Alberti, 1961). The neotype is 191 μm in length and 165 μm in width (endocyst 109 μm in length and 83 μm in width). The paratype is 148 μm in length and 109 μm in width (endocyst 83 μm in length and 67 μm in width). The length of this species varies significantly and ranges from large specimens c.190 μm long to smaller specimens c.135 μm in length.
Monteil (1991, p. 477) emended Muderongia tomaszowensis to include specimens with processes, i.e. species belonging to Phoberocysta. This is not followed in this study and our emendation herein excludes proximochorate forms (see also the generic remarks above and Poulsen, 1996). This species is here confined to proximate specimens and is characterised by four prominenthorns; the right antapical horn is significantly reduced to a protuberance.
Stratigraphic and Geographical Distribution.
Muderongia tomaszowensis is confined to the Valanginian to Lower Hauterivian (Lower Cretaceous) of France, Germany and Poland (Alberti, 1961; Monteil, 1991). The holotype and neotype of Muderongia tomaszowensis are from the Valanginian of the Dabendorf borehole, Berlin, Germany and the Tomaszow borehole, Poland, respectively. The paratype is from the Lower Hauterivian of the Werle-3 Borehole, Mecklenburg, Germany.
Muderongia tomaszowensis differs from the majority of the other species of the genus in having only four prominent horns, i.e. one apical horn, two lateral horns (axial type L I) and a single antapical horn (axial type ATP I). Muderongia crucis and Muderongia staurota both have bent lateral horns, and Muderongia tetracantha has curved, antapically tapering lateral horns (Monteil, 1991). Furthermore, Muderongia pariata has finely perforated periphragm in the distal ends of the horns. The principal difference between M. tomaszowensis and Muderongia simplex is that the latter has five horns and a rounded rectangular to subpentagonal endocyst which is strongly extended into the horns. Joined antapical horns typify Muderongia endovata; of these the left horn is small and the right horn is a reduced or incipient structure. Muderongia australis is distinguished by a slightly larger right antapical horn (joined antapical horns, type ATP II) and bent lateral horns (type L II) with short extension posts and Mudeongia macwhaei has bent lateral horns with long posterior distal extensions.
In this study, Muderongia and Phoberocysta are deemed to be separate genera. This contribution has also comprehensively reviewed and emended three species of Muderongia originally described by Alberti (1961). These forms are M. perforata, M. simplex and M. tomaszowensis. The new species M. endovata is described. This study grew out of the need to solve the long standing taxonomic confusion surrounding Muderongia simplex and Muderongia sp. A of Davey (1979). This confusion has been resolved via a restudy of the type material of Muderongia simplex. Due chiefly to the rather broad original specific definition of Muderongia simplex by Alberti (1961), most workers have incorrectly attributed the Lower Cretaceous forms with subcircular endocysts to Muderongia simplex. The older, largely Jurassic morphotypes with subangular endocysts were termed Muderongia sp. A (Davey, 1979). We have shown that Muderongia simplex and Muderongia sp. A of Davey (1979) are unequivocally conspecific. Therefore, we have erected the new species Muderongia endovata for the distinctive Lower Cretaceous circumcavate forms with subcircular endocysts.
We are greatly indebted to Dr. J. Strahl (Bundesanstalt für Geowissenschaften und Rohstoffe Aussenstelle Berlin, Germany) for the opportunity to examine the Alberti palynological collection. The authors are also grateful to Drs Hans Gocht and Wolfgang Wille (University of Tübingen, Germany) and Dr. Eckart Schrank (University of Berlin, Germany) for their help in locating the Alberti collection. N. E. Poulsen and J. B. Riding publish with the permission of the Directors, of the Geological Survey of Denmark and Greenland and the British Geological Survey (NERC) respectively.