Lower Jurassic ( Pliensbachian ) ammonites from Bornholm , Baltic Sea , Denmark

Ammonites seem to have been first reported from Bornholm by Malling & Grönwall in 1909. They described a fauna including an ammonite which they correctly recognised as Aegoceras centaurus (d’Orbigny), now placed in Beaniceras, from the locality of Stampen, south-east of Rønne (Fig. 1). This established the presence of rocks dated to the Pliensbachian Stage of the Lower Jurassic marking a different zonal horizon from other ammonites reported from Bornholm, and the locality has not been redescribed. Malling (1914, p. 269) mentioned three species of ammonites but named only one of them, as Aegoceras pettos, i.e. Coeloceras pettos (Quenstedt). This may be the form recorded below as Coeloceras grenouillouxi (d’Orbigny). Malling (1920) listed six species of ammonites without description or illustration. The chief contribution to ammonite palaeontology was made by Höhne (1933) who studied the clay pits at Rønne where strata belonging to the Hasle Formation were exposed and collected ammonites in place and from the excavated material. He described and figured four species from which he concluded that the Jamesoni, Ibex and Spinatum Zones were present. This dating is revised below. However, Höhne was principally concerned with the structure and petrography of the coal-bearing sediments and his palaeontological work seems to have been a sideline. Lower Jurassic (Pliensbachian) ammonites from Bornholm, Baltic Sea, Denmark

Ammonites seem to have been first reported from Bornholm by Malling & Grönwall in 1909.They described a fauna including an ammonite which they correctly recognised as Aegoceras centaurus (d 'Orbigny), now placed in Beaniceras, from the locality of Stampen, south-east of Rønne (Fig. 1).This established the presence of rocks dated to the Pliensbachian Stage of the Lower Jurassic marking a different zonal horizon from other ammonites reported from Bornholm, and the locality has not been redescribed.Malling (1914, p. 269) mentioned three species of ammonites but named only one of them, as Aegoceras pettos, i.e.Coeloceras pettos (Quenstedt).This may be the form recorded below as Coeloceras grenouillouxi (d 'Orbigny).Malling (1920) listed six species of ammonites without description or illustration.
The chief contribution to ammonite palaeontology was made by Höhne (1933) who studied the clay pits at Rønne where strata belonging to the Hasle Formation were exposed and collected ammonites in place and from the excavated material.He described and figured four species from which he concluded that the Jamesoni, Ibex and Spinatum Zones were present.This dating is revised below.However, Höhne was principally concerned with the structure and petrography of the coal-bearing sediments and his palaeontological work seems to have been a sideline.
Lower Jurassic (Pliensbachian) ammonites from Bornholm, Baltic Sea, Denmark Desmond T. Donovan and Finn Surlyk The Jurassic succession of the island of Bornholm in the Baltic Sea includes the Hettangianlowermost Pliensbachian Rønne Formation, the Lower Pliensbachian Hasle Formation, the Upper Pliensbachian -(?)Lower Aalenian Sorthat Formation and the (?)Upper Aalenian -Bathonian Bagå Formation.Ammonites are only known from the Hasle Formation, and all available ammonite specimens from this formation are described and figured.Material reported by previous authors has been re-examined, together with previously undescribed specimens.Nine genera and eleven species are recognised.The ammonites show that the rocks from which they were collected fall into the Lower Pliensbachian (Carixian) Substage.The lowermost subzone proved in the Hasle Formation is the basal Taylori Subzone of the Jamesoni Zone of the Northwest European Subboreal standard zonation, which marks a marine transgression over underlying marginal marine beds without ammonites belonging to the Rønne Formation.All subzones of the Jamesoni Zone are proved together with the Valdani Subzone of the Ibex Zone.The Upper Pliensbachian (Domerian) Substage, previously reported, is now thought to be unproved by ammonite evidence.The underlying and overlying Rønne and Sorthat Formations are dated by dinoflagellate cysts and terrestrial palynomorphs, and the ammonite occurrences provide important control points for palynostratigraphic dating of the succession.
No reports on ammonites have appeared in the literature since 1933.The collection examined here belongs to the Geological Museum, University of Copenhagen, and includes a larger number of species than had been previously recognised, eleven species being described below.None of these are new, and although Malling & Grönwall (1909) and Höhne (1933) both described new varieties, these are now thought to lie within the range of variation of well-known species from elsewhere.

Stratigraphy
The Jurassic of the island of Bornholm in the Baltic Sea consists of a thickly developed Hettangian-Bathonian succession, whereas Upper Jurassic deposits are absent.Four formations and a number of members are recognized (Gravesen et al. 1982;Koppelhus & Nielsen 1994;Michelsen et al. 2003, this volume).The Hettangianlowermost Pliensbachian Rønne Formation rests on Triassic redbeds of the Kågeröd Formation and onlaps Lower Palaeozoic sediments or deeply eroded crystalline basement.The Rønne Formation is subdivided into the lacustrine Munkerup Member of Early Hettangian age, the Upper Hettangian -lowermost Pliensbachian paralic Sose Bugt Member and the partly correlative Upper Sinemurian tidally-influenced Galgeløkke Member.The formation is overlain by shoreface sandstones and subordinate clay beds of the Lower Pliensbachian Hasle Formation.
The age of the uppermost part of the Rønne Formation is somewhat uncertain.A few dinoflagellate cysts belonging to Nannoceratopsis senex and Mendicodinium reticulatum with known ranges from the Lower Pliensbachian to the Middle Bajocian were reported from the top of the formation by Surlyk et al. (1995).The oldest ammonites from the overlying Hasle Formation belong to the basal Pliensbachian Jamesoni Zone (Taylori Subzone), and it was tentatively suggested that the dinocysts may have an earlier appearance than previously documented (Surlyk et al. 1995).Two specimens of the ammonite Platypleuroceras brevispina described here from the Sose Odde locality were probably collected in a clay ironstone bed at the top of the Sose Bugt Member (23.1-23.4 m in Surlyk et al. 1995, fig 4).They indicate the Brevispina Subzone of the Jamesoni Zone.The base of the Hasle Formation may thus be diachronous younging from the Taylori Subzone on   Gry 1969 andGravesen et al. 1982).
the Rønne-Hasle Block to the Brevispina Subzone on the Arnager-Sose Block.
The Jurassic succession is capped by the paralic, lacustrine and fluvial Upper Pliensbachian -Toarcian (?Lower Aalenian) Sorthat Formation which is overlain by the Bagå Formation of (?Late Aalenian) Bajocian-Bathonian age.The ammonites described in this paper are all from the heterolithic lower parts of the Hasle Formation.
The Hasle Formation is 80-140 m thick and is exposed along the west coast of Bornholm between the towns of Rønne and Hasle.A small outcrop occurs west of Sose Odde on the south coast (Fig. 1).The formation was deposited during an overall rise in relative sea level following deposition of the paralic Rønne Formation (Gravesen et al. 1982;Surlyk & Noe-Nygaard 1986;Koppelhus & Nielsen 1994;Surlyk et al. 1995;Michelsen et al. 2003, this volume).The formation consists mainly of brown weathering hummocky cross-stratified finegrained sandstones.Chamositic-oolitic levels and layers of pebbles and pebble-rich zones occur throughout.Intercalated clay beds are common in the Rønne area, where a succession belonging to the formation was formerly exposed at the locality of Rønne Lervarefabrik (Fig. 2).In the literature and in collections, the locality is known as 'Rönne-Lervarefabrik' (Höhne 1933), 'Rønne Lervarefabriks ny Grav' (Malling 1914), 'Rønne Lervarefabrik' (Gry 1969), 'Grube Rønne-Lervarevaerk', and 'Ny Østre Lergrav, Rønne'.All ammonites found in the clay pits at Rønne appear to come from this locality which is here called Rønne Lervarefabrik.Höhne (1933) also described a locality which he called 'Rönne Teglverk, Neue Grube', but this is a different locality which exposed slightly younger coal-bearing deposits of the Sorthat Formation.In the section of this paper on systematic palaeontology the ammonite-bearing locality is given as Rønne Lervarefabrik, in each case followed by the locality names written on the original dockets in brackets.
The first thorough description of a marine fauna from the Jurassic of Bornholm was provided by Lundgren (1879) who referred it to the Middle Lias.His material mainly came from loose blocks and the fauna was revised by Moberg (1888) in a study of a contemporaneous fauna found in situ in south-east Skåne, southern Sweden.
A rich marine invertebrate fauna was found in 1906 at the mouth of the Stampeå stream on the south coast of Bornholm and was described by Malling & Grönwall (1909).The fauna includes 56 species and shows great resemblance to the Pliensbachian of north-west Germany.It was referred to the Lower Pliensbachian Centaurus Zone on the basis of the presence of the ammonite Aegoceras centaurus d'Orbigny var.bornholmiensis Malling & Grönwall. Malling (1911) listed a marine fauna with 38 species of bivalves, scaphopods, gastropods, belemnites, ammonites, Hybodus and plesiosaur teeth and rib bones from the low cliffs south of Hasle harbour described by Gravesen et al. (1982), Surlyk & Noe-Nygaard (1986) and Larsen & Friis (1991).The ammonite was identified as Arietites falcaries (Quenstedt) Malling (1920).

Affinities of the ammonite fauna
All the genera and species now recorded from Bornholm are also found in Germany and England, and most of them have been recorded from north Germany (Hoffmann 1982).One or two species which were not listed by Hoffmann (1982;e.g. Coeloceras grenouillouxi) are closely related to ones that are recorded from north Germany (e.g. C. pettos).The fauna is therefore typical of the Boreal Province.The only point worthy of remark is the absence of the genus Polymorphites which is usually common in Boreal Jamesoni Zone faunas, but in view of the small number of ammonites examined its absence may not be significant.The ammonites show that the rocks from which they were  (1933).
collected fall into the Lower Pliensbachian (Carixian) Substage.The lowermost subzone proved in the Hasle Formation is the basal Taylori Subzone of the Jamesoni Zone of the NW European Subboreal standard zonation (Page 2003, this volume), which marks a marine transgression over underlying marginal marine beds without ammonites belonging to the Rønne Formation.
All subzones of the Jamesoni Zone are proved, together with the Valdani Subzone of the Ibex Zone (Fig. 3).

Material
A total of about 35 specimens representing nine genera and eleven species are described from the Hasle Formation.

Systematic palaeontology
Full synonymies for some species would be very long; references are only given here to citations relevant to the discussion and to occurrences on Bornholm.Type specimen.The original of Quenstedt (1845, plate 6, fig. 5a, b) was designated lectotype by Buckman (1912, p. viii).
Discussion.Full synonymy and description can be found in Howarth & Donovan (1964, p. 295).
Stratigraphical horizon.Howarth & Donovan (1964) noted that in well-dated north-west European localities, the species is restricted to the lower part of the Jamesoni Zone.
Material.A specimen now numbered 1987/94 was described and figured by Höhne (1933)  Höhne (1933) described and figured from Rønne Lervarefabrik (Grube Rønne-Lervarevaerk) a fragment of a smooth ammonite showing parts of typical Tragophylloceras suture lines.It does not appear to be among the specimens examined by us.This is apparently the Phylloceras listed from bed 11 of his section (Höhne 1933, p.11).It could be T. numismale as Höhne thought, but is not now considered to be specifically identifiable.
The Bornholm examples agree well with Oppel's (1854) somewhat stylised drawing of the lost holotype and with Schlatter's (1980) photographic figures.The species is strongly compressed, the whorl thickness being about 25% of the diameter.The venter is fastigate rather than sharp as in some oxycones.On the body chamber of large individuals, the fastigate venter is lost, the whorl becomes thicker and the venter broadly rounded as shown by Quenstedt (1885, plate 37, fig.3), a specimen with whorl height 110 mm and thickness 47 mm.The ribbing persists to varying diameters on different individuals.The more nearly complete example from Bornholm (Plate 1, figs 7-9) is about 107 mm in diameter, retaining some of the body chamber, but much larger specimens are known from elsewhere.
Stratigraphical horizon.The genus Radstockiceras probably ranges from the upper Raricostatum Zone to the mid-Davoei Zone (Donovan 1994).In the type Pliensbachian in southern Germany, it is almost restricted to the Polymorphus Subzone (Schlatter 1980, p. 52;table 2) but in northern Germany it seems to range through much of the Jamesoni Zone and possibly into the Ibex Zone (Hoffmann 1982, p. 152).It is not, therefore, a good indicator of stratigraphical horizon.

Family Eoderoceratidae Spath 1929
Genus Paramicroderoceras Dommergues et al. 1994 It was previously noted by Donovan (1990, p. 258-259) that a number of evolute, bituberculate species have been commonly, but wrongly, referred to Microderoceras by continental authors.Donovan (1990) then placed them in Tetraspidoceras Spath 1926, as did Spath himself (e.g.Spath 1938, p. 5, fig. 1a).However, Tetraspidoceras as defined by the type species, Am. quadrarmatus Dumortier, has inner whorls with the umbilicus only about 35% of the diameter, ornamented with numerous fine ribs and pairs of small tubercles at intervals.The evolute, strongly tuberculate form is only developed on the body chamber.In contrast, the forms now under discussion show little, if any, change of shell form or ornament with growth.Dommergues et al. (1994, p. 36) have also pointed out that Tetraspidoceras is inappropriate for these species and have proposed the new genus Paramicroderoceras for them.Nomenclature.The status of Quenstedt's 'trinomina' is doubtful.Hölder (1958, p. 22) has stated that this name had not been previously used.As it has been employed as a species name by authors it is retained here.
Discussion.The single specimen from Bornholm agrees well with the lectotype.Schlatter (1980), studying the German material from the type area of C. pettos, treated C. grenouillouxi as separate from C. pettos.However, Dommergues & Mouterde (1978, p. 346), reviewing abundant material from the type locality of C. grenouillouxi at Cottards (Cher), France, concluded that the assemblage showed considerable variability and they regarded C. grenouillouxi as a synonym of C. pettos.
Nomenclature.Simpson's (1843) unillustrated specimen was first figured by Blake (1876), and later, photographically, by Buckman (1913).Ammonites marshallani was not figured until Howarth (1962) who regarded it as a synonym of aculeatum.Ammonites decussatum Simpson (1843, p. 25) is probably also a synonym (Howarth 1962, p. 109).The species is somewhat vari-able and several other specific names of authors are probably synonyms.
Type.The holotype of Apoderoceras aculeatum is lost.
A paratype (Whitby Museum: no.177) was figured by Buckman (1913,  Nomenclature.There are two common forms of small Phricodoceras, one of which has strong tubercles at the ventral ends of the ribs; tubercles half-way along the ribs, if present, die out at a small size (c.20 mm).This is P. taylori.Ammonites cornutus Simpson 1843 is a synonym, as Simpson himself recognised in 1884 (p. 105).The other form has the inner tubercles persisting to a larger size, and this is Ammonites quadricornutus Simpson 1855 (holotype figured by Buckman 1911, plate 32).Unfortunately, Schlegelmilch in his picturebook of Lias ammonites has confused the two forms and illustrates P. quadricornutum under the name of P. taylori (Schlegelmilch 1976, plate 27, fig. 3) and P. taylori as P. cornutum (Schlegelmilch 1976, plate 28, fig. 1).
Discussion.Phricodoceras includes both small species (taylori, quadricornutum) and large ones (lamellosum d'Orbigny, non J. de C. Sowerby) and is probably dimor-phic, as noted by Dommergues (1978) who interpreted Ammonites lamellosus d'Orbigny as the macroconch of P. taylori.Only the small forms (microconchs) have been reported from Bornholm.They are typical examples of P. taylori.Stratigraphical horizon.In Tethyan areas, Phricodoceras has a long stratigraphical range, from the Upper Sinemurian to the Upper Pliensbachian.In Northwest Europe, however, it characterises the lowermost part of the Jamesoni Zone (Taylori Subzone), and it is likely to mark this horizon in Bornholm.In Burgundy (Dommergues 1987) and in southern Germany (Schlatter 1980), Phricodoceras occurs mainly above Apoderoceras (Taylori and Nodogigas zonules of Page 2003, this volume), and it is possible that both these levels are represented in Bornholm. . 1976. /809-812, replicas of Höhne's (1933) ) four specimens referred to above, from bed 53 in the section at Rønne Lervarefabrik recorded by Höhne (1933, p. 11, 12) (1980).

Material
After studying abundant material, Dommergues (1987, p. 141) concluded that the other names included in the above synonymy had been applied to forms which fall within the range of variation of P. brevispina.
The specimens from Bornholm are typical of the species.Dommergues (1987, p. 141, 142) believed the species to be dimorphic.The Bornholm examples are all macroconchs according to Dommergues' interpretation, complete with body chambers at diameters of about 100 mm.

Stratigraphical horizon.
The species characterises the Brevispina Subzone of the Jamesoni Zone, and ranges throughout the subzone according to Schlatter (1980, table 2).In Burgundy, Dommergues (1987, p. 29) regarded the species as characteristic of the lower part of the subzone, replaced by species of the group of P. submuticum in the upper part.Type.The specimen figured by Quenstedt ( 1856) is lost.

Material
A neotype was designated and figured by Schlegelmilch (1976).
Discussion.The species is characterised by compressed whorls, close, regular ribbing, and strong ventral ornament.There are small but equally prominent inner and outer spines.The Bornholm examples are typical of the species.Plate 13, fig. 3 in Höhne (1933) is the side view only of a specimen which we have not seen.It may belong to P. caprarium.Bornholm individuals with body chamber are 55-60 mm in diameter when complete; specimen no.1987/86 is about 61 mm in diameter with three-quarters of a whorl of body chamber, and an unnumbered specimen (Plate 5, figs 7-9) has a body chamber of about three-quarters of a whorl at 58 mm diameter and is complete with the aperture of the shell.
The species is smaller than later species of Platypleuroceras.Dommergues (1987, p. 152) regarded P. caprarium as a macroconch and illustrated examples ranging from about 45 mm to 84 mm in diameter.He illustrated as P. (m) sp. 3 (Dommergues 1987, plate 6, figs 23-30) specimens ranging from 29 to 44 mm diameter which he thought might be the microconchs of the species.The Bornholm examples would all be macroconchs according to Dommergues' (1987) definition.
Stratigraphical horizon.Detailed collecting of the type Pliensbachian by Schlatter (1980) showed the species to be the earliest species of Platypleuroceras, though occurring wholly above the range of Phricodoceras.Hoffmann (1982, p. 260) recorded it below, as well as in association with, Phricodoceras in north Germany.
Nomenclature.Quenstedt (1845, p. 88;1849, plate 4, fig. 1) described and figured Ammonites jamesoni var.latus.His illustration is of a fragment of an outer whorl which cannot be satisfactorily interpreted.Dommergues (1987) has adopted the name in the interpretation of Schlatter (1980).
The species is distinguished from U. jamesoni (J.de C. Sowerby), as interpreted by the neotype designated by Donovan & Forsey (1973, p. 12, plate 4, fig. 3a, b), by having fewer ribs throughout, and stronger ornament on the body chamber.Dommergues (1987, p. 115, fig. 28) regarded the species as later than U. jamesoni in his evolutionary lineage.
The species is large.Schlatter (1980, plate 14, fig. 1) figured (as U. aff.confusa) a wholly septate example 117 mm in diameter, and Dommergues (1987, plate 10, figs 1, 2) figured one still septate at 132 mm diameter.The Bornholm specimen is smaller, with about threeeighths of a whorl of body chamber at a diameter of about 120 mm, and inner whorls mostly preserved as an impression.
Stratigraphical horizon.In the type Pliensbachian the species occurs throughout the Jamesoni Subzone and just ranges into the succeeding Ibex Zone (Schlatter 1980).Material. No. 1987/85, recorded  An incompletely preserved ammonite, no.1987/84, differs from the species of Platypleuroceras and Uptonia recorded above in having nearly smooth inner whorls, followed by coarse, blunt ribs.It bears some resemblance to forms figured, for example by Schlatter (1980, plate 16, fig. 3), as Acanthopleuroceras maugenesti (d'Orbigny).If this identification is correct it indicates the Ibex Zone, Valdani Subzone.The locality is recorded only as Bornholm Island.
Types.There are twenty specimens of Am. centaurus in the d'Orbigny collection (J.-L.Dommergues and R. Mouterde in: Fischer 1994, p. 69).No lectotype has been designated.The holotype of Beaniceras senile (original of Buckman 1918, plate 126) is in the British Geological Survey collection no.47092.Aegoceras centaurus d'Orbigny var.bornholmiensis Malling & Grönwall was evidently based on the single figured specimen which is therefore the holotype.We have not seen this specimen.
Discussion.Phelps (1985) regarded the species as belonging to a new (but unnamed) subgenus (of Androgynoceras).Pending the naming of such a subgenus, the species is here retained in Beaniceras.
D'Orbigny (1844) illustrated only the cadicone inner whorls.Spath (1938, p. 109) separated B. senile Buckman from B. centaurus on the basis of its contracted bodychamber with arched venter in contrast to the flatter venter of the preceding whorl.However, material from the type area figured by Dommergues & Mouterde (1978) shows that in the adult, the body chamber, about half a whorl long and 20 mm diameter, ceases to expand with growth and corresponds with the form named B. senile by Buckman (1918).
Phelps (1985), who revised the stratigraphy and taxonomy of Beaniceras, figured (plate 1, fig.14) as centaurus an ammonite which is indistinguishable from the holotype of B. senile, which he presumably regarded as a synonym.
Malling & Grönwall (1909) described and figured as Aegoceras centaurus d'Orbigny var.bornholmiensis an ammonite from their locality at Stampen, Bornholm which is very similar to Beaniceras senile, and is therefore placed in synonymy.It should be noted, however, that if it is desired to uphold Buckman's (1924) species B. senile, then the name bornholmiensis has priority.
Stratigraphical horizon.Phelps (1985) recognised a Centaurus Zonule near the top of the Valdani Subzone, and this has been adopted by Page (2003, this volume).
Material.Malling and Grönwall's (1909) figured specimen has not been seen by us, and its present location is not known.Discussion.The whorl fragments were identified by Höhne (1933) as close to Amaltheus spinatus, i.e.Pleuroceras in modern nomenclature.If correct this would indicate the Domerian Stage, Spinatum Zone.However, the fragments do not look like Pleuroceras as far as can be seen from the poor illustration, reproduced here (Fig. 4).They appear to show bifurcating ribs, which do not occur in Pleuroceras, and do not show the interrupted keel characteristic of that genus.The V-shaped ribs arise from an umbilical tubercle, with thickened, forwardly curved outer ends.It is to be noted that while the specimen illustrated as fig. 4 in plate 14 (Höhne 1933) is shown with its anterior end upwards, the corresponding ventral view (fig.5) is wrongly oriented, with the anterior end downwards.There are several Jurassic and Cretaceous ammonite genera which have the characters described above, and it is not considered safe to identify these fragments from the figures.

Ammonite
Discussion.This whorl fragment was also wrongly identified by Höhne (1933) as Amaltheus.It has straight ribs each of which appears to bear three tubercles or spines.It is not considered to be identifiable from the figures.

Plate 1
All figures are at natural size.All figures are at natural size.

Fig. 1 .
Fig.1.Geological sketch map of the island of Bornholm in the Baltic Sea showing the position of localities mentioned in the text (modified fromGry 1969 andGravesen et al. 1982).

Fig. 3 .
Fig.3.Pliensbachian zonal and subzonal subdivision (modified fromPage 2003, this volume).On the basis of the ammonite evidence, all four of the subzones of the Jamesoni Zone and the Valdani Subzone of the Ibex Zone (shaded subzones) are represented on Bornholm.

(Quenstedt 1884)
plate 25, fig.7) was refigured by Schlegelmilch(1976, plate 24, fig.4) and stated to be lectotype.It is in the geological collections at the University of Tübingen, Germany.Discussion.The single example from Bornholm agrees well with the lectotype.It is a phragmocone 100 mm in diameter.The last preserved part of the shell has been damaged by injury during life.The figured examples attributed to this species are all phragmocones, the later whorls and body chamber being unknown.Am.