new jellyfish taxa from the upper jurassic ... - Wiley Online Library

[Palaeontology, Vol. 49, Part 6, 2006, pp. 1287–1302]

NEW JELLYFISH TAXA FROM THE UPPER JURASSIC LITHOGRAPHIC LIMESTONES OF CERIN (FRANCE): TAPHONOMY AND ECOLOGY by CHRISTIAN GAILLARD*, JACQUELINE GOY, PAUL BERNIER*, JEAN PAUL BOURSEAU, JEAN CLAUDE GALL§, GEORGES BARALE*, ERIC BUFFETAUT– and SYLVIE WENZ** *UMR CNRS 5125 – Paleóenvironnements et Paleóbiosphe`re, Universite´ Claude Bernard – Lyon 1, Geóde – 2, rue Raphae¨l Dubois, F-69622 Villeurbanne Cedex, France; e-mails: [email protected]; [email protected]; [email protected] Institut Oceánographique, 195 rue Saint Jacques, 75005 Paris, France; e-mail: [email protected] Universite´ Claude Bernard – Lyon 1, Geóde – 2, rue Raphae¨l Dubois, F-69622 Villeurbanne Cedex, France; e-mail: [email protected] §Universite´ Louis Pasteur, Ecole et Observatoire des Sciences de la Terre, 1 rue Blessig, F-67084 Strasbourg Cedex, France; e-mail: [email protected] –UMR CNRS 5125 – Paleóenvironnements et Paleóbiosphe`re, 16 cour du Lie´gat, F-75013 Paris, France; e-mail: [email protected] **Museúm National d’Histoire Naturelle, De´partement Histoire de la Terre, Paleóbiodiversite´ et Paleóenvironnements, CP 58, 8 rue de Buffon, F-75005 Paris, France Typescript received 8 September 2005; accepted in revised form 12 January 2006

Abundant well-preserved jellyfish impressions are described from the Cerin Lagersta¨tte (Ain, eastern France). The enclosing sediments are lithographic limestones deposited in a Late Kimmeridgian lagoon lying on an emergent reef complex. Two new taxa of Scyphozoa are proposed: Paraurelia cerinensis gen. et sp. nov. (abundant) and Paraurelia sp. A (rare), and two new taxa of Cubozoa: Bipedalia cerinensis gen. et sp. nov. (rare) and Paracarybdea lithographica gen. et sp. nov. (very rare). Rapid covering by a microbial mat helped the preservation of the animals. Many specimens of Paraurelia cerinensis are deformed by slippage down the palaeoslope, which characterizes the margin of the lagoon. Their resultant morphology and their orientation clearly indicate the downslope direction. Tentacles of Bipedalia cerinensis and Paracarybdea lithographica are also orientated according to the palaeoslope. The jellyfish were probably dead individuals occasionally introduced into the Cerin lagoon. However, another hypo-

Abstract:

Fossil jellyfish are rarely preserved in rocks and as a result are little studied (Wade 1994). Some exceptional cases of organic preservation occur, e.g. Progonionemus (Limnomedusae) in shale layers of the Triassic Gre`s a` Voltzia Formation in eastern France (Grauvogel and Gall 1962). Impressions are more frequent but still rare. Owing to their non-skeletal body, jellyfish need special taphonomic conditions to be preserved, including fine-grained sedimentation. In Mesozoic deposits they are best known from Upper Jurassic lithographic limestones such as the famous Lagersta¨tte of Solnhofen (Bavaria, Germany: Kieslinger 1939; Kuhn

ª The Palaeontological Association

thesis may be considered with reference to the model of the present-day jellyfish lakes in Palau (Caroline Islands, Western Pacific). Jellyfish could have lived in the more oxygenated upper layer of water of the Cerin lagoon that allowed pelagic life. This situation could have corresponded to short periods of easier communication between the open sea and the lagoon. Jellyfish are only found in the lower beds of the lithographic limestones and their distribution illustrates the supposed evolution of the Cerin lagoon. Initially, it was deep, mainly flooded, with possibly autochthonous jellyfish and allochthonous animals indicating clear marine influence. Later, the lagoon shallowed and its sediments often emerged with marginal marine burrows and plants indicating increasing terrestrial influence. Key words: jellyfish, Lagersta¨tte, Cerin, Upper Jurassic,

France, taphonomy, ecology.

1961; Barthel et al. 1990). Recently, jellyfish from the rather similar lithographic limestones of Cerin (southern Jura Mountains, Ain, France; Text-fig. 1) have been discovered during the course of scientific excavation from 1975 to 1995 (Barale et al. 1985; Bernier et al. 1991a). An important, well-preserved assemblage has yielded some interesting new information. The aim of this paper is to describe new Jurassic jellyfish taxa, consider some taphonomic aspects, discuss the possible ecology of the jellyfish fauna and revisit the interpretation of the Cerin lagoon ecosystem in the light of these discoveries.

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TEXT-FIG. 1.

Location of the scientific excavation of Cerin

(Ain, France).

MATERIAL AND METHODS Cerin is situated in the southern Jura Mountains, about 80 km east of Lyon (Text-fig. 1). The scientific excavation has taken place in a disused quarry where the lithographic limestone had been worked during the nineteenth century (Bourseau et al. 1984). A bed-by-bed study including the systematic investigation of upper bedding surfaces measuring 75–150 m2 was carried out. Many recent palaeontological data have been published on trace fossils (Bernier et al. 1982, 1984; Gaillard et al. 1994b, 2003), algae (Bernier et al. 1991b), ammonites (Enay et al. 1994), asteroids (Breton et al. 1994), ophiuroids (Bourseau et al. 1991), echinoids (Bourseau et al. 1994), fish (Wenz et al. 1994) and pterosaurs (Buffetaut et al. 1990) from the site.

The Cerin Lithographic Limestone Formation accumulated during the Late Kimmeridgian ⁄ Early Tithonian (Enay et al. 1994). It consists of lower laminites, including rare flints overlain by the lithographic limestone s.s. where laminites alternate with thicker beds. Schematically, the thickness of micritic beds gradually increases from a few millimetres at the base to 5–25 cm at the top of the sequence. The rock is a typical lithographic limestone composed of a very pure, fine micrite (Bernier 1994). The grain size is generally 2–4 lm. Like the Solnhofen lithographic limestone, the Cerin lithographic limestone may be regarded as an obrutionary stagnation deposit (Seilacher et al. 1985). Sedimentary structures and trace fossils indicate frequent emergence (Bernier et al. 1982, 1984, 1991a; Gall et al. 1985; Gaillard et al. 1994b). The precise palaeoenvironment corresponds to the margin of a shallow tropical lagoon located on an extinct coral reef complex (Bernier 1984; Bernier et al. 1994). This dead coral reef complex belonged to a wide, shallow, carbonate platform bordering the deep marine Tethys Ocean (Text-fig. 2). Jellyfish are found mainly in the lower lithographic limestones. They are present only in a few beds (Textfig. 3). They are abundant in beds 28 and 84 (941 specimens observed on a surface of 75 m2 on bed 84), and scarce (1–6 specimens) in beds 38, 202F, 217 and 221A, representing less than 1 per cent of the excavated beds. Jellyfish are well preserved on the surface of the very spectacular bed 84 (Text-fig. 4). On this bed, the sizes (mean diameter) of the best preserved jellyfish were measured (744 specimens). The orientation of the deformed specimens was systematically recorded (178 specimens).

T E X T - F I G . 2 . Palaeogeography of Western Europe with location of the Cerin and Solnhofen fossil deposits, modified from the Early Tithonian map of Thierry and Barrier (2000). C, Cerin; S, Solnhofen.

GAILLARD ET AL.: NEW JURASSIC JELLYFISH TAXA

The best specimens were photographed in situ and, if possible, extracted. Unfortunately, because of the thinness of the enclosing bed and the abundance of joints, only a few delicate jellyfish with long tentacles were collected. Although exceptional, the preservation of these jellyfish cannot lead to a very accurate description and their systematic attribution remains questionable. We have used

T E X T - F I G . 3 . Stratigraphical and environmental setting of the jellyfish in the lithographic limestones of Cerin, modified from Gaillard et al. (1994b).

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the more recent zoological classification proposed by Franc (1994a, b) instead of the palaeontological classification proposed by Harrington and Moore (1956a, b). Repository. All the specimens collected are housed in the Centre Commun des Collections de Geólogie (C3G) de l’Universite´ Claude-Bernard, Lyon 1.

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A

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C D

T E X T - F I G . 4 . A–D, jellyfish at the top of bed 84, field photographs. Abundant specimens are Paraurelia cerinensis. Long arrows (A, D) indicate the initial downslope direction. Short arrows (A, D) point to specimens of Bipedalia cerinensis; the downslope direction is clearly indicated in D by the orientation of the tentacles of B. cerinensis.


SYSTEMATIC PALAEONTOLOGY (C. Gaillard and J. Goy)

Class SCYPHOZOA Goette, 1887 Order SEMAEOSTOMATIDA Agassiz, 1862 Family AURELIIDAE Agassiz, 1862 Genus PARAURELIA gen. nov.

Type species. Paraurelia cerinensis sp. nov.; monotypic. Derivation of name. After the genus Aurelia.

Diagnosis. Fossil jellyfish showing a morphological resemblance to the recent genus Aurelia.

Paraurelia cerinensis sp. nov. Text-figure 5A–C

Derivation of name. After the locality of Cerin. Types. Holotype FSL 501835 (Text-fig. 5A); paratype FSL 501829 (Text-fig. 5B). Type locality and horizon. Scientific excavation of Cerin (Ain, France), Late Kimmeridgian, Lithographic Limestone Formation, bed 84.

Diagnosis. Discoidal umbrella bordered by a marked thickening. Umbrella diameter mainly < 105 mm. Margin entire. Four marked central organs in form of a horseshoe or almost complete circle. Tentacles not observed.

A

B

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Description. In its undeformed state (see below for taphonomy), the umbrella corresponds to a slight circular depression in the sediment. Diameter is 15–105 mm with an average of 60 mm. Holotype diameter is 58 mm. The imprint of the thickening that characterizes the border of the umbrella corresponds to a deeper, regular, external furrow. The four central symmetrical smooth imprints probably correspond to gonads. Other impressions may represent the rhopalia and mouth arms but they are very poorly preserved. Rhopalia, which are sensory structures located on the border of the umbrella, are visible between the marginal lappets (Textfig. 5A, C). Exceptionally, very slight mouth arms may be discerned (Text-fig. 5A). Tentacles, which are very small and numerous (more than 1000) in modern Aurelia, have not been observed in this material.

Discussion. The description is based on abundant material; 941 specimens were numbered and 744 measured at the top of bed 84. This species is significantly different from all other species previously known from Upper Jurassic deposits. The closest species is Hydrocraspedata mayri Kolb, 1951 described from the Upper Jurassic Plattenkalk of Bavaria (Eichsta¨tt, Solnhofen Basin). Both species have an entire circular margin. There is some resemblance to juvenile specimens of H. mayri that exhibit only four organ imprints, interpreted by Kolb (1951) as mouth arms. The three main characteristics of H. mayri showing significant differences are: (1) larger size (diameter, 114 mm for juveniles, 200 mm for adults); (2) occurrence of two kinds of imprints in the umbrella, four central mouth tentacles (striated imprints) and four more peripheric gonads (the last only for adult specimens); (3) occurrence of numerous fine radial striae in the velum (?impressions of statocysts). H. mayri was referred to either Leptomedusae or Trachymedusae by Kolb (1951).

C

Paraurelia cerinensis gen. et sp. nov., field photographs, top of bed 84. A, holotype, FSL 501835. B, paratype, FSL 501829. C, uncollected specimen. Note the four central symmetrical imprints probably corresponding to gonads. m, possible mouth arms; r, possible rhopalia.

TEXT-FIG. 5.

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It is referred to trachylinid medusae (Hydromedusae) in the Treatise on Invertebrate Paleontology (Harrington and Moore 1956b) but such attribution is doubtful (Fu¨rsich and Kennedy 1975).

Occurrence. Beds 28 and 217 (bed 38?).

Class CUBOZOA Werner, 1973 Order CARYBDEIDA Gegenbaur, 1856

Occurrence. Beds 28 and 84 (beds 38, 202F, 217 & 221A?); more than 1000 specimens observed.

Family CARYBDEIDAE Gegenbaur, 1856 Genus BIPEDALIA gen. nov.

Paraurelia sp. A Text-figure 6

Material. Very few specimens, only one being well preserved: FSL 501883. Horizon. Late Kimmeridgian, Lithographic Limestone Formation of Cerin, bed 217. Description. Discoidal umbrella bordered by two marked thickenings. Diameter of the umbrella, 120 mm. Margin entire. Irregularly spaced organs in the central part of the umbrella are probably gonads. Four short, thin, mouth arms.

Discussion. This species is referred to Paraurelia because there are clear similarities to P. cerinensis (shape of the umbrella, gonads, absence of tentacle). It is characterized by the presence of stronger prominent mouth arms.

Derivation of name. From the disposition of pedalia in groups of two. Type species. Bipedalia cerinensis sp. nov.; monotypic.

Diagnosis. Fossil jellyfish resembling the recent genus Tripedalia but with four groups of two pedalia, each having one tentacle. Discussion. This genus is referred to Cubomedusae, which are characterized by marginal tentacles connected to the umbrella by a basal expansion termed the pedalia. Bipedalia appears to be an intermediate genus between two recent genera: Carybdea with four pedalia, each bearing one tentacle and Tripedalia with four groups of three pedalia each bearing one tentacle.

Bipedalia cerinensis sp. nov. Text-figure 7A–C

Derivation of name. After the locality of Cerin. Holotype. FSL 501808 (Text-fig. 7A). Type locality and horizon. Scientific excavation of Cerin (Ain, France), Late Kimmeridgian, Lithographic Limestone Formation, bed 84.

Diagnosis. Dome-shaped umbrella with a thickening at its border. Umbrella margin with four groups of two tentacles, each tentacle on a large spatula-shaped structure or pedalia. Four marginal sense organs or rhopalia between the groups of pedalia.

T E X T - F I G . 6 . Paraurelia sp. A, FSL 501883, top of bed 217. Note the mouth arms in the right lower part of the photograph.

Description. Specimens are relatively rare and generally deformed, like the umbrella, which is usually crushed. In the best-preserved examples, eight long, thin tentacles are clearly visible (Text-fig. 7A, C). Pedalia are obvious triangular parts at the proximal end of each tentacle (Text-fig. 7A, C). Organs under the umbrella are not clearly visible. Rhopalia are probably located between the groups of pedalia (Text-fig. 7C). The location of gonads could be along a more or less visible groove (Text-fig. 7A, C). Umbrella 45–70 mm in diameter. Total length 200–350 mm. Tentacles 100–260 mm long and 2–3 mm in diameter. Holotype measurements: umbrella diameter, 70 mm;


A

B

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C

T E X T - F I G . 7 . Bipedalia cerinensis gen. et sp. nov., field photographs, top of bed 84. A, holotype, FSL 501808. B–C, uncollected specimens. g, possible location of gonads; p, pedalia; r, rhopalia; t, tentacle.

total length, 340 mm; tentacle length, 270 mm; tentacle diameter, 3 mm.

Genus PARACARYBDEA gen. nov.

Derivation of name. After the genus Carybdea.

Discussion. This species is significantly different from all other Upper Jurassic species. Quadrimedusina quadrata (Haeckel 1869) is an Upper Jurassic carybdeid jellyfish that is very different from Bipedalia cerinensis owing to its clearly square shape and the absence of visible tentacles. Leptobrachites trigonobrachius Haeckel, 1869 is relatively common in Upper Jurassic (Tithonian) lithographic limestones at Solnhofen. This species shows some similarities in shape and size but is a rhizostomatid. Appendages of well-preserved specimens, which are fixed in the central part of the umbrella, are short and with rather wide mouth arms instead of long, thin tentacles. Occurrence. Bed 84, 14 specimens observed.

Type species. Paracarybdea lithographica sp. nov.; monotypic.

Diagnosis. Fossil jellyfish showing a morphological resemblance to the Recent genus Carybdea.

Paracarybdea lithographica sp. nov. Text-figure 8A–B

Derivation of name. After the lithographic limestone enclosing the fossil jellyfish. Types. Holotype FSL 501825-1 (Text-fig. 8A); paratype FSL 506560-1 (Text-fig. 8B).

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A

B

T E X T - F I G . 8 . A, Paracarybdea lithographica gen. et sp. nov., holotype exhibiting four well-preserved pedalia and a long thin tentacle crossing a Paraurelia cerinensis specimen; FSL 501825-1. B, Paracarybdea lithographica gen. et sp. nov., paratype (bent specimen) superimposed on a Bipedalia cerinensis specimen; FSL 506560-1. p, pedalia; t, tentacle; P.l., Paracarybdea lithographica, P.c., Paraurelia cerinensis, B.c., Bipedalia cerinensis. Arrows indicate the downslope direction; top of bed 84.

Type locality and horizon. Scientific excavation of Cerin (Ain, France), Late Kimmeridgian, Lithographic Limestone Formation, bed 84.

Diagnosis. Dome-shaped umbrella with a thickening along its border. Umbrella margin with four groups of one tentacle, each tentacle on a strong pedalia. Description. Specimens are rare and poorly preserved. Four pedalia are clearly visible. Tentacles are long and thin. Organs under the umbrella are not visible. Measurements of the holotype: umbrella diameter, 50 mm; total length, 220 mm; length of tentacle, 150 mm, diameter, 2Æ5 mm.

Discussion. This species shows very strong similarities to Bipedalia cerinensis. Both have thin tentacles at the end

of strong pedalia. The only important difference is the number of pedalia and tentacles, but this may be difficult to observe because of poor preservation and ⁄ or possible superimposition of specimens; e.g. additional possible pedalia visible in Text-figure 8A probably belong to another superimposed specimen. However, specimens referred to Paracarybdea lithographica could possibly be poorly preserved Bipedalia cerinensis. Text-figure 8B shows a characteristic Paracarybdea lithographica with only four pedalia and tentacles superimposed on a Bipedalia cerinensis with six visible tentacles. Occurrence. Bed 84, five specimens observed.


PALAEOBIOGEOGRAPHICAL CONSIDERATIONS Comparison between Cerin and Solnhofen jellyfish associations The famous Solnhofen fossil deposit (Bavaria, Germany) has yielded the only well-known Upper Jurassic jellyfish fauna. The enclosing rocks are also lithographic limestones and jellyfish occur only as a result of exceptional taphonomic conditions. The Solnhofen fauna is more diverse than that of Cerin. It was mainly studied by Haeckel (1865, 1866, 1869, 1874), Ammon (1886, 1908), Maas (1902, 1906), Kuhn (1938), Kieslinger (1939) and Kolb (1951) A revised list of all the described jellyfish was given by Kuhn (1961) as follows: Acalepha deperdita Beyrich, 1849 ‘Medusites’ bicinctus, ‘M.’ staurophorus, ‘M.’ circularis, ‘M.’ porpitina Haeckel, 1869 ?Quadrimedusina quadrata (Haeckel, 1869) Epiphyllina (Paraphyllites) distincta (Maas, 1906) Eulithota fasciculata Haeckel, 1869 ¼ Solnhofenistomites stechowi Kuhn, 1938 Hydrocraspedata mayri Kolb, 1951 ?Leptobrachites (¼ Pelagiopsis Brandt) trigonobrachius Haeckel, 1869 ‘Acraspedites’ ‘Palaegina’ Cannostomites multicirratus Maas, 1902 Semaeostomites zitteli Haeckel, 1874 Rhizostomites admirantus Haeckel, 1866 ¼ R. lithographicus Haeckel, 1866, ¼ Hexarhizites insignis Haeckel, 1870, ¼ Myogramma speciosum Maas, 1902, ¼ M. speciossimum Ammon, 1908, ¼ Ephyropsites jurassicus Ammon, 1908.

Hydrocraspedata and Eulithota are probably Hydromedusae; the others forms are Scyphomedusae. The most common genus is Rhizostomites in different modes of preservation (upper- or underside of the umbrella) (Kieslinger 1939). Rhizostomites, which commonly reaches 50 cm in diameter, is larger than the largest Cerin jellyfish, which rarely reach 10 cm in diameter. The Cerin and Solnhofen assemblages clearly differ in both composition and the richness of the associations (diverse in Solnhofen, poor in Cerin). In spite of a rather similar palaeogeographical situation (Text-fig. 2), these differences are related to different depositional conditions, the Cerin environment being a small, very shallow, restricted lagoon (Bernier et al. 1991a; Gaillard et al. 1994b), whereas a large, relatively deep lagoon with less restricted communication with the open sea (Barthel et al. 1990) was present at Solnhofen.

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Comparison with Recent Mediterranean jellyfish fauna The Cerin jellyfish fauna is composed of Scyphomedusae, which are common in modern seas. Paraurelia cerinensis and P. sp. A are very probably related to extant Aurelia aurita, which is a common cosmopolitan jellyfish. It is an opportunistic eurythermal and euryhaline genus occurring in a wide range of marine environments. It is known from almost all seas (Lindquist 1959; Maaden 1959; Franc 1994a). Bipedalia cerinensis and Paracarybdea lithographica are Cubomedusae, recent taxa of which mainly occur along coastal and shallow waters of archipelagos in tropical and subtropical regions (Franc 1994b). Their occurrence is consistent with the geographical and climatic contexts hypothesized for the Cerin lagoon. The tropical Cerin lagoon was linked to the Mesozoic Tethys (Text-fig. 2), which corresponds to an ancient configuration of the present-day Mediterranean Sea. Recent Aurelia aurita and Carybdea marsupialis, which coexist in the Mediterranean Sea, are possibly related to the fossil jellyfish taxa described from Cerin.

TAPHONOMY Jellyfish fossilization Few studies have been conducted on jellyfish fossilization (Scha¨fer 1941; Hertweck 1966; Mu¨ller 1984, 1985; Barthel et al. 1990; Bruton 1991). Information gained from specimens encountered at Cerin lead to some interesting conclusions concerning taphonomic aspects. All of the jellyfish in the lithographic limestones are visible on top surfaces of beds. Most are dorsal side up. All are known only from impressions on the sediment. The imprint of the lower part of the jellyfish body is attested to by the good preservation of gonads and umbrella border. This is clear for Paraurelia cerinensis, which is very similar to experimental moulds of living Aurelia aurita (Bruton 1991). The shrivelled aspect of some Solnhofen specimens, which argues for hypersalinity (Barthel et al. 1990), has not been observed at Cerin. The preservation potential of jellyfish is very slight. Two favourable conditions occurred in the Cerin lagoon: a substrate composed of very fine grained sediment and a restricted environment. Moreover, Cerin jellyfish were also probably well preserved because they were rapidly covered by a thin microbial mat, an occurrence that is often evident when the tops of beds have been deformed by sliding or tearing (Gall et al. 1985; Bernier et al. 1991) (Text-fig. 9). Two beds containing jellyfish (28, 38) exhibit an upper microbial mat with sliding and tearing structures. It is assumed that microbial mats are present at the

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A

B

T E X T - F I G . 9 . A, microbial mat folded by sliding on the slope of the lagoon border; top of bed 178. B, microbial mat torn by sliding on the slope of the lagoon border; top of bed 280.

top of each bed and that they played an important role in preservation (Gall et al. 1985; Seilacher et al. 1985) of very fragile organisms such as jellyfish.

Taphonomy of bed 84 Jellyfish imprints occur at the top of bed 84. They are covered by a very thin lamina (1 mm thick), which probably corresponds to an undisturbed microbial mat. When cleaved, this mat frequently sticks to overlying bed 85 (Text-fig. 10). Only 77 per cent of the Paraurelia cerinensis specimens occurring at the top of bed 84 exhibit a normal circular shape; the remainder are deformed specimens. This deformation corresponds to a preferential unsticking and sliding of the side of umbrellas orientated up-slope. Their initial circular shape was progressively transformed into a D-shape (Text-fig. 11). The symmetry axis of these deformed jellyfish shows an evident preferential orientation. They are mainly parallel to the main line of slope, which is permanent and demonstrated by various features:

sliding and tearing of microbial mats (Text-fig. 9), rills with soft cobbles, and the organization of some terrestrial vertebrate tracks (Bernier et al. 1982, 1991a; Gall et al. 1985). The convexity of the deformed jellyfish clearly indicates the downslope direction (Text-figs 4A, 11–12). Specimens referred to Bipedalia cerinensis and Paracarybdea lithographica also have a preferential orientation down the slope. Their tentacles are parallel to the line of the main slope and also clearly indicate the downslope direction (Text-figs 4D, 8). However, most of the jellyfish are rather regularly arranged on the sediment surface. Apparently they generally adhered to the bottom, probably because of rather rapid microbial mat development. Only a few specimens were transported a short distance, subsequently deformed (Textfig. 11D) and partly overlapped each other on the slope of the lagoon margin before being covered.

ECOLOGY Compared to the Solnhofen lithographic limestones, where jellyfish are more common and more diverse (Kuhn 1961; Barthel et al. 1990; Frickhinger 1994), the Cerin lithographic limestones contain only four species of jellyfish, occurring only in a few beds. This indicates that the Solnhofen Basin was more favourable for jellyfish, probably because of easier communication with the open sea. It was a wide, deep lagoon with a top layer of oxygenated water probably inhabited by diverse pelagic organisms (Barthel et al. 1990). The Cerin lagoon was smaller and shallower, and subaerial exposure of its margins occurred frequently (Gaillard et al. 1994b). There are three possible hypotheses to explain the presence of jellyfish in the lagoon.

Allochthonous pelagic jellyfish? Modern jellyfish are pelagic organisms that occasionally become stranded on beaches. This stranding is

T E X T - F I G . 1 0 . Schematic drawing showing the occurrence of a microbial mat at the top of bed 84, which is likely to have aided the preservation of jellyfish.The white arrow indicates the main jointing.


A

B

C

D

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Deformed specimens of Paraurelia cerinensis caused by sliding. A, slightly deformed specimen. B, association of deformed and undeformed specimens indicating moderate sliding. C, strongly deformed specimen with a typical D-shape. The straight border is the uphill side and represents a horizontal line (H). D, deformed and overlapped specimens indicating a general sliding direction. All specimens from bed 84; arrows indicate the downslope direction. TEXT-FIG. 11.

T E X T - F I G . 1 2 . Downslope orientation of deformed jellyfish (Paraurelia cerinensis); 178 measured specimens, bed 84.

associated with blooms and frequently observed (UNEP 1991) (Text-fig. 13). Late Jurassic jellyfish also lived in the open sea and were occasionally introduced into the Cerin lagoon. The abundant fauna of beds 28

and 84 may be related to jellyfish blooms in the sea associated with exceptional conditions (e.g. storms), but why are they not mixed with abundant other animals? Many open marine organisms (e.g. fish and ammonites) were frequently introduced into the lagoon in this way and fossilized in other beds. Efficient sorting is a possibility because jellyfish are very buoyant and the Cerin lagoon was poorly connected to the open sea. This poor connection may also explain the infrequent occurrence. The gaussian size-frequency distribution of 744 similar specimens from bed 84 (Text-fig. 14A) may reflect transport (Boucot 1953). Hence, the hypothesis of allochthonous jellyfish is possible but not certain. It is consistent with the littoral situation of the beds examined (Bernier et al. 1982, 1991a; Gall et al. 1985; Gaillard et al. 1994b, 2003). On the other hand, some features, mainly the extreme domination of jellyfish in some beds, are consistent with the idea of an autochthonous jellyfish fauna. Some present-day restricted lagoons have dense jellyfish populations. Thus, the occurrence of an occasional rich and undiversified assemblage in the Cerin

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Autochthonous benthic jellyfish?

TEXT-FIG. 13.

Swarming of present-day Aurelia aurita in the

Gulf of Aquaba.

lagoon possibly indicates an opportunistic jellyfish fauna (r strategy) adapted to special environmental conditions.

Some scyphomedusae can live upside-down on the bottom of shallow pools in tropical seas. Dense populations occur and survive because of an efficient symbiosis with zooxanthellae. A good example is known from Cassiopea populations in small restricted pools on Aldabra Island (Seychelles, Western Indian Ocean) (Drew 1972). Jellyfish populations occur in these shallow pools, about 200 m inland from the lagoon of the atoll. Water exchange with the sea occurs through karstic circulation. Cassiopea is dominant and covers about 15 per cent of the available bottom (Drew 1972, fig. 1). In such an environment, where food supply is low, the level of dependence of the jellyfish on symbiotic algae is very important. The carbon fixation (about 75 lg ⁄ cm2 ⁄ h) must be almost entirely attributed to the activity of the algae (Drew 1972). These are situated within the endoderm cells of the club-shaped vesicles on the oral arms of the jellyfish, and the inverted position ensures their exposure to sunlight. The Aldabra lagoon (Stoddart 1967) was compared with the Cerin lagoon and many environmental similarities were noted (Gaillard et al. 1994a). Cassiopea only lives in very small pools situated within the coral island of Aldabra. Specimens are never found in the widespread sedimentation areas of lime mud that characterize the border of the lagoon and which could be invoked for the Cerin area. Another significant difference is that Cassiopea mouth lobes, where photosynthetic algae are concentrated, are strongly developed. As shown by their imprints, this kind of morphology is never observed in the Cerin species, which were probably not adapted to

T E X T - F I G . 1 4 . A, size-frequency distribution of Paraurelia cerinensis; 744 measured specimens, bed 84. B, size-frequency distribution of extant Aurelia aurita population in Eil Malk Jellyfish Lake, Palau; 283 measured specimens (after Hamner et al. 1982).


this unusual benthic life, and do not support this hypothesis.

Autochthonous pelagic jellyfish? Another interesting model is provided by the tropical marine lakes of Palau (Caroline Islands, West Pacific) (Text-fig. 15). Like the Aldabra pools, they are situated within coral islands, but they are stagnant. The 5-ha-wide and 30-m-deep Eil Malk Jellyfish Lake has been well studied (Hamner and Hauri 1981; Hamner et al. 1982; Landing et al. 1991). It is a meromictic saline lake with stratified water and anoxic hypolimnion. Tidal cycles occur and sea-water enters the lake at the surface but does not mix vertically with the deep lake water. The upper water layer, 15 m thick, is oxygenated and inhabited by a large planktonic population of jellyfish, copepods and fish. High turbidity leads to important primary productivity. At the chemocline a dense floating bacterial plate degrades and solubilizes the particulate material. The lower water layer is anoxic, cloudy but without turbidity, and enriched in H2S, NH3 and PO43–. The lake is stable, aseasonal and shows no cycles in its planktonic population, which is dominated by jellyfish (Mastigias and Aurelia). However, the jellyfish population is not regularly distributed in the lake. During the day 70 per cent of the Mastigias population are between 0 and 2Æ5 m deep and migrate horizontally searching for the greatest amount of sunlight. During the night they migrate vertically between the surface and the chemocline. This unusual behaviour seems to be related to light and the nutrient requirements of their symbiotic zooxanthellae. This symbiotic association seems to be essential. Associated Aurelia aurita, which is a jellyfish similar to Paraurelia cerinensis, shows the same pattern of vertical distribution but feeds heavily on copepods. It is possible that if killed, e.g. by the disturbance or disappearance of the oxygenated top water layer, the

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jellyfish population would be deposited on the anoxic bottom where taphonomic conditions are favourable for preservation. The size-frequency of such a population is characteristic, with numbers decreasing slowly and regularly with size and age (Text-fig. 14B). This results from the continuous recruitment of ephyrae and the long life of adults in the absence of pelagic predators (Hamner et al. 1982). The size-frequency distribution of Cerin jellyfish forms a bell-shaped curve (Text-fig. 14A), which is rather different from a ‘life assemblage’ according to Boucot (1953). It possibly results from limited transport within the lagoon, perhaps from its centre to its border. The numerous deformed specimens suggest stranding on a sloping marginal marine mud-flat. In conclusion, although the Cerin ecosystem was probably rather different, this last hypothesis deserves consideration.

PALAEOENVIRONMENTAL IMPLICATIONS According to the last hypothesis above, some palaeoenvironmental implications may be deduced. The Cerin lagoon is usually regarded as having been filled with poisonous anoxic water. It is assumed that pelagic and benthic animals, which came from various environments, were occasionally introduced and gathered in the lagoon where they died after a relatively short time (Barale et al. 1985; Bernier and Gaillard 1990). The possible occurrence of an autochthonous population of jellyfish could attest to the unusual persistence of a top layer of well-oxygenated water allowing a specific pelagic fauna to thrive. This situation could correspond to short periods of more open access between the sea and the lagoon. The lagoon was also rather deep, allowing the occurrence of a lower layer of anoxic water, facilitating the unusual preservation of these fragile organisms. Beds containing abundant jellyfish are only situated at the base of the lithographic limestones (beds 28, 84),

T E X T - F I G . 1 5 . The Jellyfish Palau lakes model: poor communication with the open sea, stagnant waters and autochthonous jellyfish population in the oxygenated upper layer of water.

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where they are associated only with rare other organisms. They then become very rare (beds 202F, 217, 221 A) and disappear. When occurring rarely, they are associated with more abundant and diverse organisms (molluscs, brachiopods, echinoderms, crustaceans, fish and plants). All jellyfish occur beneath the transition level after which terrestrial influences become dominant (Text-fig. 3). The history of the Cerin lagoon may be summarized as two periods separated by the deposition of this transition level. The lagoon was first relatively deep, mainly flooded, with the presence of jellyfish, fish and ammonites indicating clear marine influence (Text-fig. 3), most of these marine organisms consisting of open marine animals exceptionally introduced into the lagoon. However, some specialized organisms, including jellyfish, may have been able to live in the restricted lagoon. The lagoon then progressively filled with mud, became less deep and often partly emergent with a dominance of autochthonous marginal marine burrows (Tubularina lithographica) and allochthonous land plants (Zamites) indicating increasing terrestrial influence. In conclusion, the jellyfish distribution in the lithographic limestones strongly supports the previously inferred evolution of the Cerin lagoon (Text-fig. 3). Acknowledgements. This work was supported by the CNRS (UMR 5125 Paleóenvironnements et Paleóbiosphe`re: Scientific team of the Cerin scientific excavation and Scientific team of Paleóećosyste`mes aquatiques: structure et fonctionnement). Financial support and equipment were also provided by the Conseil Geńe´ral de l’Ain, the Museum of Natural History of Lyon. We are very grateful to G. Viohl (Jura Museum, Eichsta¨tt) for his help with documentation, and to anonymous reviewers for constructive suggestions for improving the text. We are also very grateful to J. C. Reniaud and G. Sirven for logistic assistance, A. Armand for drawing assistance, N. Podevigne for photographic printing, A. Prieur for collection managment, and D. Batten fro improving our English.

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