Gastropod egg capsules from the Lower Cretaceous of Russia preserved by calcitization

Palaeogeography, Palaeoclimatology, Palaeoecology 466 (2017) 326–333

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Gastropod egg capsules from the Lower Cretaceous of Russia preserved by calcitization Michał Zatoń a,⁎, Alexandr A. Mironenko b, Kamila Banasik a a b

University of Silesia, Faculty of Earth Sciences, Będzińska 60, PL-41-200 Sosnowiec, Poland Geological Institute of Russian Academy of Sciences, Pyzhevski Lane 7, 119017 Moscow, Russia

a r t i c l e

i n f o

Article history: Received 9 June 2016 Received in revised form 18 November 2016 Accepted 29 November 2016 Available online 1 December 2016 Keywords: Gastropoda Egg capsules Calcitization Taphonomy Ammonites Cretaceous

a b s t r a c t Small-sized (0.8–1.6 mm in diameter), circular to oval, three-dimensionally preserved calcitized structures have been found embedded within ammonite body chamber moulds from the Lower Aptian (Lower Cretaceous) of Russia. The characteristic morphology, consisting of a flat attachment base and convex upper hemisphere possessing an apical, tiny, circular opening indicate that these structures represent gastropod (possibly Caenogastropoda) egg capsules. Originally, the egg capsules were attached to the empty shells of the ammonites Deshayesites and Sinzovia which later were embedded within carbonate concretions. The preservation of the egg capsules resulted from both their deposition within a suitable, cryptic habitat provided by the empty ammonite shells, and the quick cementation of the infilling sediment which not only sufficiently protected the capsules from external environment, but also created a suitable, closed microenvironment for fossilization. The calcitization of the egg capsules may have occurred under low pH conditions in an environment characterized by a very low concentration of phosphorous ions essential for phosphatization. So far, such structures are known from a few examples derived from different stratigraphic horizons and geographic locations. Those which are known have been reported in the form of pyritized, phosphatized, carbonaceous and even bioimmured fossils. The calcitized gastropod egg capsules presented here indicate, that such structures may in fact be preserved by a wide array of fossilization modes in different paleoenvironments/microenvironments. Thus, such fossils seem to be much more common in the fossil record than previously considered. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Aquatic gastropods produce a variety of egg-containing gelatinous masses, strings, ribbons and tough capsules (e.g., Soliman, 1987; Rawlings, 1999; Przeslawski, 2004). However, only the latter ones have a chance to be fossilized and enter the fossil record (e.g., Zatoń et al., 2009; Zatoń and Mironenko, 2015a). The structures interpreted as fossilized gastropod egg capsules are relatively rare in the fossil record, and have a patchy distribution in both time and space, being mainly known from the Jurassic and Cretaceous of Europe. Unlike egg capsules from the Jurassic, which have been repeatedly reported, those from the Cretaceous are known only from two occurrences. Although such structures have been known for a long time (e.g., see Lundgren, 1878 for the examples from the Lower Jurassic of Sweden), they have not been paid sufficient attention until the work of Kaiser and Voigt (1977) who described small, circular fossils preserved on a bivalve shell from the Lower Jurassic (Pliensbachian) of Germany, interpreting them as remnants of gastropod egg capsules. Later, Kaiser and Voigt (1983) found more of these structures, differing from each other in morphology, ⁎ Corresponding author. E-mail address: [email protected] (M. Zatoń).

http://dx.doi.org/10.1016/j.palaeo.2016.11.048 0031-0182/© 2016 Elsevier B.V. All rights reserved.

preserved on the ammonite moulds from the Pliensbachian of Germany. They compared them to the egg capsules produced by extant neritimorph and columbellid (Neogastropoda) gastropods. Pyritized hemispherical structures preserved on fossil wood and similar to gastropod egg capsules were also reported from the Pliensbachian of Germany by Riegraf and Schubert (1991) and Schubert et al. (2008). Zatoń et al. (2009) described tiny, circular structures preserved in the form of carbonaceous impressions on cardiniid bivalves from the Hettangian deltaic deposits of Poland. Later, Zatoń et al. (2013) reported on the first gastropod (neritimorph) egg capsules preserved by bioimmuration. The latter find, detected on the mould of a large volutid gastropod from the Maastrichtian of the Netherlands, was also the first report of such structures from the Cretaceous. Recently, Zatoń and Mironenko (2015a), for the first time described egg capsules of caenogastropod affinity preserved by phosphatization from the latest Jurassic (Volgian) of Russia. These three-dimensionally preserved capsules are definitely the best preserved fossils of this kind, enabling the observation of both their exterior and interior parts. Zatoń and Mironenko (2015b) also reported neritimorph-like egg capsule remnants preserved on a Lower Cretaceous (early Aptian) ammonite mould from Daghestan in Russia. Possible gastropod egg capsules preserved as carbonaceous compressions on non-marine bivalve moulds

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are also known from the Miocene of Czech Republic (Mikuláš and Dvořák, 2001). Here we report on an occurrence of structures most probably representing gastropod egg capsules from the Lower Cretaceous of Russia. However, unlike all such fossils mentioned above, these were preserved by calcitization. 2. Material and methods 2.1. Material and its provenance The fossils interpreted as egg capsules were found in body chambers of five, out of a total of 225, Lower Aptian ammonites belonging to the species Deshayesites volgensis Sasonova (two specimens) and Sinzovia sazonovae Wright (three specimens). The ammonite specimens were found within carbonate concretions in the Lower Aptian shales outcropping at the Volga river bank, not far from the town of Khvalynsk, Saratov region, south-western Russia (Fig. 1A–B, see also Gavrilov et al., 2002). The carbonate concretions (20–50 cm in length) occur in black, bioturbated, bituminous shales of the Deshayesites volgensis–Volgoceratoides schilkovkensis ammonite Zone, 2.5–3 m above the noticeable horizon of the ‘Aptian plate’ (Fig. 1C, see also Gavrilov et al., 2002). The concretions contain only three ammonoid species: Sinzovia sazonovae Wright, Deshayesites volgensis Sasonova and small heteromorphic Volgoceratoides schilovkensis Mikhailova & Baraboshkin together with fish bones and scales, rare and small bivalve (pectinids, arcticids and inoceramids) and gastropod (actenoids, turbinids, cerithioids) shells, as well as ammonite aptychi. The Lower Aptian shale layers crop out along the Volga river from Ulyanovsk almost to Saratov (Baraboshkin et al., 2003: Fig. 4), and occupy a large area from the Oka–Tsna swell in the North toward the northern Caspian Lowland (Gavrilov et al., 2002). The black shales formed mainly within the Deshayesites volgensis = Deshayesites forbesi ammonite Zone, and thus simultaneously with one of the episodes of the global OAE-1

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event known as OASE-1a subevent (Gavrilov et al., 2002). The thickness of the shales varies greatly in different localities. The black shales, the thickness of which varies greatly in particular localities, consist of non-uniform layers differing in mineralogical, as well as paleontological characteristics (Gavrilov et al., 2002). It is clear that in that part of the basin, the anoxic conditions prevailed during the formation of the deposits below the ‘Aptian plate’, as bioturbation traces and benthic fauna are lacking (Gavrilov et al., 2002). Later on, during the sedimentation of the concretions-bearing deposits, the anoxia was infrequent as evidenced by the presence of bioturbations and periodically occurring benthic fauna.

2.2. Methods The egg capsules were investigated using a binocular microscope and TESCAN VEGA SEM with a BSE detector at the Paleontological Institute of the Russian Academy of Sciences in Moscow, and the environmental scanning electron microscope (ESEM) Philips XL30 at the Faculty of Earth Sciences of the University of Silesia in Sosnowiec, Poland. In both cases, the specimens were inspected in uncoated state in low-vacuum conditions. Images were generated using secondary (SE) and back-scattered electrons (BSE). In order to determine the exact composition of the egg capsules, three capsules were isolated from the ammonite Deshayesites volgensis (GIUS 9-3670/2) and analysed using WITec alpha 300R Confocal Raman Microscope equipped with an air-cooled solid laser (488 nm) and a CCD camera operating at −61 °C. The laser radiation was coupled to a microscope through a single-mode optical fibre with a diameter of 3.5 μm. An air Zeiss LD EC Epiplan-Neofluan DIC (100/0.75NA) objective was used. Raman scattered light was focused a broad band single mode fibre with effective Pinhole size about 30 μm and monochromator with a 600 mm−1 grating. The power of the laser at the sample position was 42 mW. Integration times of 3s with accumulation of 10 scans and a

Fig. 1. Locality and lithostratigraphy. A–B. Sketch-maps showing the sampled locality (indicated by an asterisk) at the Volga River bank near the town of Khvalynsk in the Saratov region, south-western Russia. C. Schematic lithostratigraphical section of the ammonite-bearing Lower Aptian deposits. The egg-capsule-bearing ammonites come from the carbonate concretions (marked) occurring within the black shales (black) which are underlain and overlain by clays (grey) (simplified after Gavrilov et al., 2002).

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resolution 3 cm−1 were chosen. The monochromator was calibrated using the Raman scattering line of a silicon plate (520.7 cm−1). Additionally, the same specimens were also subjected for mineralogical analysis using PANalytical X'Pert Pro MPD-PW 3040/60 X-ray diffractometer (XRD). For XRD, the capsules were powdered and situated on zero-background sample holder (ZBH). XRD data were collected using the diffractometer equipped with a Theta-Theta geometry, using a lamp with Cu anode. Generator settings were 40 kV and 40 mA. Data were collected in the range of 10–90°2Θ with a 0.02°2Θ step size and 600 s counting time. The interpretation and quantitative analysis of the collected data were carried out by means of the HighScore + Software. Both equipments are housed at the Faculty of Earth Sciences, University of Silesia, Sosnowiec. The specimens are housed at the Moscow State University Museum, Russia, abbreviated MSU 120 and at the Faculty of Earth Sciences of the University of Silesia in Sosnowiec, Poland, abbreviated GIUS 9-3670/1-2.

3. Results 3.1. Characteristics of the ammonites The shells of the Lower Aptian ammonites Deshayesites volgensis (Deshayesitidae) and Sinzovia sazonovae (Aconeceratidae) are brevidomic with their body chamber length about 180°. Deshayesites shells bear coarse ribs (Fig. 2A–B), whereas those of Sinzovia are relatively smooth (Fig. 2C–D). Sinzovia shells are oxyconic, with narrow umbilicus and crenulated keel, whereas the shells of Deshayesites are more evolute, with a ribbed ventral side. Both genera were migrants from the Tethyan realm. They preferred warm-water, marine basins: Deshayesites were spread on the territory of South Europe: France, Caucasus, Turkmenistan (Bogdanova and Mikhailova, 2004; Bersac and Bert, 2015); Sinzovia had more global distribution up to South America (Riccardi et al., 1987). All Aconeceratidae were relatively small-sized ammonites and their macroconchs had diameter of 5–6 cm (Riccardi et al., 1987). Shells of Deshayesitidae during certain times were considerably larger, but in the

layer with carbonate concretions both these genera were identical in size and relatively small. Both genera have an aptychus-type of lower jaws (Doguzhaeva and Mutvei, 1990; Doguzhaeva et al., 1995; Rogov and Mironenko, 2016). The shape of their aptychi corresponds to the shape of ammonite apertures. Sinzovia aptychi are more elongated and smooth, whereas aptychi of ornamented Deshayesites are more ribbed, with a thicker calcitic layer (Rogov and Mironenko, 2016). Sinzovia is one of the most studied ammonoid genera in terms of paleobiology (it is well known in literature as “Aconeceras trautscholdi”). The structure of its embryonic shell (Tanabe et al., 2008), as well as its radula are well-known (Doguzhaeva and Mutvei, 1990). Sinzovia is also the first ammonite genus for which a direct evidence of ovoviviparity is known (Mironenko and Rogov, 2015). 3.2. Characteristics of the egg capsules The fossils interpreted as egg capsules are well-visible on the surface of internal moulds of Deshayesites (Fig. 2A–B) and Sinzovia (Fig. 2C–D) body chambers. The number of capsules in particular ammonite specimen varies widely from 37 up to even 178 in one ammonite. Their occurrence is also variable with respect to particular body chamber parts (Table 1). The capsules may occur as isolated specimens (Fig. 2A–B) or in groups (Fig. 2C–D), often forming characteristic chains consisting of several specimens (Fig. 2D). In a ribbed Deshayesites most capsules are located inside shallow ribs (Fig. 2A–B), whereas in smooth Sinzovia shells they are randomly distributed (Fig. 2C–D). The capsules are light in colour and translucent. The capsules are three-dimensionally preserved and thus their visible part is a circular or oval in outline base (depending on the location on a flat surface or inside the concavity of a rib), while the rest of the capsules (apical parts and walls) are embedded in the body chamber mould. When extracted from the mould, each of the capsules has similar characteristics, having a flat attachment base capped by a hemisphere (Fig. 3A, D–H). The capsule base is surrounded by an irregular rim which extends beyond the base (Fig. 3A, D–E). The upper side of the hemisphere has a tiny, circular opening (Fig. 3A, D–E, H), the diameter

Fig. 2. Ammonites with preserved gastropod egg capsules. A. Deshayesites volgensis (a bite-mark is indicated by white arrow) with preserved egg capsules (exemplified by arrows), MSU 120/4. B. Deshayesites volgensis and some examples of the egg capsules (arrowed), GIUS 9-3670/2. C. Sinzovia sazonovae with a body chamber hosting abundant egg capsules, MSU 120/5. D. Sinzovia sazonovae with clustering egg capsules indicated by an arrow, MSU 120/6. Scale bars at the ammonites 5 mm.

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Table 1 Data on ammonite shells size, and the number and size of the egg capsules preserved. Specimen

Ammonite species

Shell measurements [mm]

No. of egg capsules on right side

No. of egg capsules on left side

No. of egg capsules on ventral side

Total no. of egg capsules

Size of egg capsules [mm]

MSU 120/4

Deshayesites volgensis

6

29

11

46

1.0–1.6 (mean = 1.39, n = 10)

MSU 120/5

Sinzovia sazonovae

71

97

10

178

0.8–1.3 (mean = 0.99, n = 10)

MSU 120/6

Sinzovia sazonovae

–

104

–

104

0.9–1.1 (mean = 1.0, n = 10)

GIUS 9-3670/1

Sinzovia sazonovae

30

32

20

82

0.8–1.2 (mean = 1.21, n = 10)

GIUS 9-3670/2

Deshayesites volgensis

D = 38 Wb = 12 Wh = 16.5 D = 30 Wb = 8 Wh = 16 D = 49 Wb = 11 Wh = 24.5 D = 33 Wb = 9 Wh = 14 D = 25.5 Wb = 6.5 Wh = 10

6

18

13

37

0.9–1.2 (mean = 1.08, n = 10)

of which is similar in both Deshayesites and Sinzovia capsules, ranging from 154 μm to 187 μm. On the concave surface left by extracted capsule, characteristic, globular structures having smooth surface and margins occur (Fig. 3B–C). These are associated with tiny framboid pyrites in some places (bright spots in Fig. 3C). Although the capsules are

translucent, no additional structures could be observed within the infilling. With respect to morphology and general characteristics, the capsules are similar in each ammonite studied here; however, their outline may be circular (Fig. 3D–F, H) or more oval (Fig. 3A). Their size also differs, ranging from 0.8 to 1.6 mm in the longest axis (Table 1). The

Fig. 3. ESEM photomicrographs of selected egg capsules. A–C. Remnants after the egg capsule originally deposited within the body chamber of Deshayesites volgensis (MSU 120/4). A. Oval concavity within the body chamber mould left by extracted egg capsule. The apical opening and peripheral rim are well-visible. B–C. Globular microstructures preserved on the concavity walls. White spots in C are framboid pyrite crystals. D. Circular in outline egg capsule extracted from the body chamber mould of Deshayesites volgensis (GIUS 9-3670/2). The apical opening (arrowed) and irregular peripheral rim are well-visible. E–H. Egg capsules preserved on the body chamber mould of Sinzovia sazonovae (MSU 120/5). E. Concavities left by two capsules. Apical opening well-preserved in the specimen on the right (arrowed). F. Three capsules embedded within the body chamber mould as only their bases are visible as circular structures, and one trace of the capsule preserved as a concavity (arrowed). G. Concavity within the body chamber mould after the egg capsule showing its original shape. H. Concavity within the body chamber mould. The deformation in its upper part indicates that originally the egg capsule wall was soft and flexible. I. Similar, but phosphatized egg capsule preserved within the body chamber of the Upper Jurassic ammonite Craspedites nekrassovi from Russia.

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biggest (more elongated) capsules occur in Deshayesites specimens, while in the Sinzovia specimens, they are more circular in outline and thus slightly smaller. Originally the capsules must have been soft and flexible. This is supported by deformations preserved in the upper hemisphere (Fig. 3E, H), as well as in attachment base (Fig. 3G). The latter are especially well-visible in those capsules which originally were deposited on uneven surfaces, such as the concavities of the ribs. In such cases, the resulting shape of the base is characteristically convex. 3.3. Composition of the egg capsules The Raman analysis showed that the egg capsules investigated are composed of calcite (Fig. 4). In the Raman spectrum of calcite the following bands are distinguished: 156 cm−1, 283 cm− 1, 714 cm− 1, 1087 cm−1, 1438 cm−1, 1750 cm−1. Characteristic Raman bands related to CO3 group are as follow: ν1 (symmetric stretching) and ν4 (symmetric bending) modes are 1087 cm− 1, 714 cm− 1, respectively (Buzgar and Apopei, 2009). Asymmetric stretching vibration (ν3) is attributed to a line at 1438 cm−1. The asymmetric bending (ν2) vibration mode of CO3 group does not appear in the Raman spectrum. A weak band observed at 1750 cm− 1 may be regarded as the combination bands of ν1 + ν4 modes (Gunasekaran et al., 2006). The lower wave numbers of calcite (156 cm−1, 283 cm−1) arise from the external vibrations of the CO3 groups that involve translatory and rotatory oscillations of these groups (relative translations between the cation and anionic groups, see Buzgar and Apopei, 2009; Gunasekaran et al., 2006). The XRD also showed that the capsules are built by a Mg-calcite [unit cell parameters are a = 4.9707(1), c = 16.9718(6)Å], having about 4 mol.% MgCO3, without any additions of aragonite. 4. Discussion 4.1. Identification of the fossils as gastropod egg capsules In order to support the identity of the fossils as gastropod egg capsules, important features pointing to such an affiliation must be provided. In the present case, the critical features linking the fossils with aquatic gastropod egg capsules are their mode of occurrence, characteristic morphology and size. The fossil egg capsules occur only in the ammonite body chamber and are absent in the phragmocone, which indicates that these structures are not randomly formed during shell cementation and

Fig. 4. Raman spectrum of calcite forming the egg capsule.

diagenesis. In one Deshayesites specimen (MSU 120/4) bearing a ventral bite mark (see Klompmaker et al., 2009), they occur between an aperture and the shell damage. This location indicates that they were attached to the ammonite shell after it has been injured. The hemispheres are embedded within the carbonate material forming the internal moulds of the ammonite body chamber. This indicates that they were attached to the inner side of the empty shell wall. Such a mode of occurrence of these structures is known in other ammonite body chambers from the Lower Jurassic (Kaiser and Voigt, 1983), Upper Jurassic (Zatoń and Mironenko, 2015a) and Lower Cretaceous deposits (Zatoń and Mironenko, 2015b). One may suggest that being preserved in the ammonites, the observed hemispherical objects may represent the fossilized ammonite eggs. However, the objects studied herein and those reported in the literature (see also Zatoń and Mironenko, 2015a), occur as attached to the body chambers over their whole length. Thus, they probably do not represent fossilized ammonite eggs (e.g., Etches et al., 2009). The observed hemispherical structures do not represent parasite-induced structures either. Such “pearls”, forming by an occlusion of parasites by a mantle-induced shell secretion, would be preserved as distinct pits on the ammonite moulds (see De Baets et al., 2011, 2015). Instead, they are clearly embedded within the mould and are not covered with ammonoid shell layers. The characteristic morphology of the hemispherical objects, here preserved three-dimensionally which is a rare phenomenon concerning such structures (Zatoń and Mironenko, 2015a), points to gastropods as producers. As in Recent egg capsules of aquatic gastropods, the specimens discussed herein also possess a flat base for attachment to a variety of firm and hard substrates. However, unlike the neritid gastropod egg capsules (Adegoke et al., 1969; Kano and Fukumori, 2010), those presented here possess a characteristic circular opening at the top of the upper hemisphere. This morphology, characterized by a flat, circular attachment base and a hemisphere with an apical opening is similar to some of the Recent gastropods of the clade Caenogastropoda, namely rissoids (Rissoidae, see Lebour, 1934; Graham, 1988), muricid (Muricidae) (see D'Asaro, 1991; Pastorino et al., 2007; Pastorino and Penchaszadeh, 2009), and volutid (Volutidae) neogastropods (Matthews-Cascon et al., 2010). Recently, very similar, but phosphatized, gastropod egg capsules having an apical opening were found to be attached within the body chamber of Late Jurassic ammonites from Russia by Zatoń and Mironenko (2015a, see also Fig. 3I). As was indicated by the latter authors, despite the striking morphological similarity of the Jurassic egg capsules to those produced by the Recent rissoids and muricids, there is no any evidence and certainty that those were also produced by either of these groups of gastropods. Exactly the same may be stated with respect to the egg capsules discussed here. It is because: 1) the apical openings in the rissoid, muricid and volutid egg capsules are much larger (e.g., Graham, 1988; Pastorino et al., 2007; Matthews-Cascon et al., 2010), 2) egg capsules of muricid and volutid neogastropods are much larger (20 mm and more) than those reported here, and 3) neogastropods have become common and diverse much later, since the Late Cretaceous, although some muricoids are known from the Valanginian (e.g., Kaim, 2004). The additional problem of their specific affiliation is the preservation of the egg capsules in isolation from the parent gastropod(s). Anyway, taking the shared similarities with the rissoid and muricid gastropod egg capsules into account, it may be stated that the fossil capsules described here were produced by some representatives of Caenogastropoda, as has also been recently suggested for the Upper Jurassic phosphatized capsules by Zatoń and Mironenko (2015a). Although morphologically similar egg capsules differ in size in particular ammonites, these may have been produced by the same gastropods. It is wellknown that egg capsules of the Recent gastropod species may vary widely in size. For example, a volutid Voluta ebraea may produce egg capsules in a wide range of sizes ranging from 15 to 29.5 mm in diameter (Matthews-Cascon et al., 2010). Quite similar size range of the apical openings of the capsules preserved in both ammonite genera may somewhat also point to their origin from the same gastropod.

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4.2. Paleoenvironment Gastropod egg capsules described herein are nearly identical to the capsules, which were found in Late Jurassic ammonites (Zatoń and Mironenko, 2015a); however, the deposits containing the ammonites with preserved capsules originated in different paleoenvironments. Late Jurassic capsules studied by Zatoń and Mironenko (2015a) were found in sandy phosphatic concretions, which were formed in relatively shallow water not far from sea shore (Kiselev, 2012). The ammonites in these concretions are chaotically arranged and are partially fragmented, which indicates formation of these clusters above the storm wave-base. The ammonites with capsules described herein, on the contrary, were found within the black shale-hosted carbonate concretions. These shales are bioturbated, but does not show any traces of wave turbulence (Gavrilov et al., 2002). Ammonites in the shales, as well as in the concretions are generally distributed within a single layer, without any traces of displacement, sometimes with aptychi preserved in their body chambers. In comparisons to very abundant ammonites, the benthic faunas preserved in the concretions are rare and small-sized (up to a few mm in length). These include gastropods (actenoids, turbinids, cerithioids) and bivalves (pectinids, arcticids and inoceramids). All these features point to relatively greater water depth in the area of formation of these concretions. Therefore, gastropods which produced these egg capsules had an ecological flexibility and could have lived not only in shallow-water environment, but also in relatively deep-water, poorly oxygenated conditions. It is important to note, that in such a relatively deep-water environment with more or less soft muddy bottom, only ammonite shells were suitable shelters for spawning. There were no large bivalve or gastropod shells which were available for gastropods in the coastal Late Jurassic environment. Therefore, we can assume that the distribution of these gastropods depended on ammonites and on the presence of their empty shells on the sea floor. 4.3. Preservation of the egg capsules Although relatively rarely reported from the fossil record, the structures interpreted as gastropod egg capsules are preserved in a wide array of preservational modes, being dolomitized (probably secondarily, Kaiser and Voigt, 1983), pyritized (Riegraf and Schubert, 1991; Schubert et al., 2008; Zatoń and Mironenko, 2015b), phosphatized (Zatoń and Mironenko, 2015a), preserved by bioimmuration (Zatoń et al., 2013) or as carbonaceous compressions (Mikuláš and Dvořák, 2001; Zatoń et al., 2009), and even as calcitized capsules as in the present case. This suggests that such structures, although still rarely reported, may in fact be fossilized in various diagenetic conditions, and thus should be more common in the fossil record. However, the gastropod egg capsules reported so far have exclusively been found on bivalve, gastropod and ammonite moulds. This unequivocally suggests that before being fossilized, the egg capsules must have originally been deposited within the empty shells of these molluscs. Thus, the empty shells not only played the role of spawning grounds for gastropods, but also served as natural shelters for the capsules enhancing their chances for fossilization (Zatoń and Mironenko, 2015a, 2015b). Especially this concerns empty body chambers of ammonite shells which not only provided a cryptic hard substrate for the egg capsule deposition, but also a suitable, closed microenvironment for their fossilization. Although originally the egg capsules described may have been tough, they certainly were soft and flexible what is evidenced by the partial collapse of the upper hemisphere in some of the specimens (Fig. 3E, H). Similar phenomena were observed in Late Jurassic egg capsules described by Zatoń and Mironenko (2015a). Most probably these both kinds of egg capsules had similar composition as the other caenogastropod egg capsules, consisting of organic compounds such as proteins, carbohydrates and mucopolysaccharides (e.g., Tamarin and Carriker, 1967; Soliman, 1987; Hawkins and Hutchinson, 1988; Rawlings, 1999; Matthews-Cascon et al., 2010). Interesting is the fact

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that morphologically similar egg capsules as those described here and also deposited in ammonite body chambers, are known to be preserved by phosphatization (Zatoń and Mironenko, 2015a), what has already been stated above. It is well-known that phosphatization is a means of exceptional preservation of soft tissues, whereas calcite rarely preserves soft tissues with a similar level of biological fidelity (e.g., Briggs and Wilby, 1996; Broce et al., 2014). Indeed, even in such structures as the gastropod egg capsules discussed, the differences between those being phosphatized and calcitized are evident. Those which were phosphatized not only retained their three-dimensionality, but also show the distinct capsule walls and empty cavity inside (see Zatoń and Mironenko, 2015a) which originally was presumably filled by a protein-gell fluid (Hawkins and Hutchinson, 1988; Rawlings, 1999). In the calcitized specimens, on the other hand, the whole volume of a capsule is uniformly replaced by calcite which also filled the interior between the base and upper hemisphere. Low taphonomic fidelity has also been observed in some Early Cambrian calcitized animal embryos (Broce et al., 2014). Therefore, it is clear that the egg capsules may have either been phosphatized or calcitized depending on local diagenetic conditions surrounding a decaying structure. It is currently wellknown, that such a “switch” between calcium carbonate and calcium phosphate precipitation can be turn-on or turn-off in a microenvironment surrounding the decaying tissues depending on a microbiallydriven changes in pH, where phosphatization occurs at lowered (~6.3) pH conditions, and calcitization sets up when pH rises (Briggs and Wilby, 1996; Briggs and Kear, 1994; Wilby et al., 1996; Sagemann et al., 1999). However, it is experimentally proved (Martin et al., 2003) that the lower pH alone is insufficient to promote phosphatization if the concentration of phosphorous is too low, and under such conditions the invertebrate eggs may be calcitized (Martin et al., 2003). Indeed, unlike the Late Jurassic ammonite shells preserving phosphatized egg capsules (Zatoń and Mironenko, 2015a), the present ones were buried in completely different paleoenvironment, devoid of any phosphatic deposits, indicating that originally the availability of phosphorous was very low (see Gavrilov et al., 2002: Table 1). Also the size of the capsules may have been too small to provide sufficient amount of phosphorous for phosphatization during their decay. Thus, it is quite possible that calcitization of the egg capsules occurred under low, characteristic for phosphatization, pH conditions in an anoxic, closed microenvironment (presence of tiny pyrite framboids) characterized by a very low amount of important phosphorous ions (see Martin et al., 2003). The gastropod egg capsules are preserved within the body chambers of ammonites which, in turn, were embedded within carbonate concretions undergoing calcitization either. Due to supersaturation of dissolved carbonate, the concretions must have cemented rapidly during early diagenesis (e.g., McCoy et al., 2015), as is evidenced from excellent preservation of the ammonites which bear no signs of compaction (for other examples see e.g., Zatoń and Marynowski, 2004; Landman and Klofak, 2012). In the body chambers, the egg capsules are attached to the inner shell wall and embedded within the carbonate (micritic) infilling. Thus, the three-dimensional preservation of the egg capsules resulted from both 1) the enclosing of host ammonite shells within the concretions, preventing the shells from compaction which would have also destroyed the capsules, and 2) early cementation of the body chambers infilling which, due to its rapid hardening, may have sufficiently protected the capsules from external environment. Similar early cementation of the sediment filling the bivalve shells hosting gastropod egg capsules from the Lower Jurassic of Poland, seems to be a prerequisite not only for the capsules preservation but also for the survival of chitin contained within them (Wysokowski et al., 2014). The globular structures observed on the concave surface left by extracted capsule somewhat resemble fossilized calcifying bacterial colonies noted by Robin et al. (2015): Fig. 3D). If true, these may indicate that calcitization of originally organic egg capsules (or at least their walls) may have been bacterially-mediated. The presence of framboid pyrites may further attest for bacterial sulphate reduction of the egg

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capsule-containing organic matter. Similar to the experimental mineralization of the invertebrate eggs undertaken by Martin et al. (2003), their calcitization may have been a quick process taking a few weeks only. Although some putative, embryonic shells have been noted in some of the capsules by Kaiser and Voigt (1983): Fig. 3B), none of such remains have been noted within the calcitized egg capsules studied here, simply because the potential embryos either developed and already hatched, or degraded completely before calcitization of the capsule interior. 5. Conclusions The structures described in the present paper were identified as gastropod egg capsules on the basis of their characteristic mode of occurrence, morphology and size. The capsules bear distinct features which may link them to the Caenogastropoda clade; however, their specific affiliation to the certain gastropod group is not possible. Nevertheless, the ecological flexibility of these gastropods can be noted as their capsules were found in both shallow-water sediments, as well as in relatively deep-water black shales. Although their presence in ammonite body chambers is not new and similar fossils interpreted as gastropod egg capsules have previously been detected in Jurassic and Lower Cretaceous deposits, their preservation in the form of calcite has never been detected up till now. This kind of mineralization widens the possible modes of preservation of gastropod egg capsules which so far have been reported as pyritic, carbonaceous, phosphatic and even bioimmured fossils. So diverse preservational pathways indicate that such delicate structures may be preserved under various diagenetic conditions, which, in turn, increase their chances to be found in different paleoenvironments and geographical locations. In fact, the present case clearly shows that gastropod egg capsules are more abundant in the fossil record than previously considered. Moreover, such calcitized and translucent egg capsules are suitable for searching of potentially preserved pre-veliger embryos and embryonic shells, even though such have not been detected during the current study. Acknowledgments The research was supported for A.A.M. by RFBR Grant No. 05-1506183. Tomasz Krzykawski (Sosnowiec) is cordially thanked for performing the XRD analysis. Zuzanna Wawrzyniak (Sosnowiec) is acknowledged for linguistic correction of the manuscript. We are very grateful to Andrey Devyatkin (Saratov, Russia) for the donation of specimens for this study. Andrzej Kaim (Warsaw) and an anonymous reviewer are thanked for their remarks, suggestions and corrections. Andrzej Kaim and Krzysztof Hryniewicz (Warsaw) are acknowledged for their help in determination of gastropods and bivalves. References Adegoke, O.S., Dessauvagie, T.F.J., Yoloye, V.L.A., 1969. Hemisphaerammina-like egg capsules of Neritina (Gastropoda) from Nigeria. Micropaleontology 15, 102–106. Baraboshkin, E., Alekseev, A.S., Kopaevich, L.F., 2003. Cretaceous paleogeography of the North-Eastern Peri-Tethys. Palaeogeogr. Palaeoclimatol. Palaeoecol. 196, 177–208. Bersac, S., Bert, D., 2015. Two ammonite species under the same name: revision of Deshayesites deshayesi (d'Orbigny, 1841) based on topotype material (Lower Aptian, Lower Cretaceous, Northeast of France). Ann. Paleontol. 101, 265–294. Bogdanova, T.N., Mikhailova, I.A., 2004. Origin, evolution and stratigraphic significance of the superfamily Deshayesitaceae Stoyanow, 1949. Bulletin de I′Institut royal des Sciences naturelles de Belgique, Sciences de la Terre 74, 189–243. Briggs, D.E.G., Kear, A.J., 1994. Decay and mineralization of shrimps. PALAIOS 9, 431–456. Briggs, D.E.G., Wilby, P.R., 1996. The role of the calcium carbonate-calcium phosphate switch in the mineralization of soft-bodied fossils. J. Geol. Soc. Lond. 153, 665–668. Broce, J., Schiffbauer, J.D., Sharma, K.S., Wang, G., Xiao, S., 2014. Possible animal embryos from the Lower Cambrian (stage 3) Shuijingtuo Formation, Hubei Province, South China. J. Paleontol. 88, 385–394. Buzgar, N., Apopei, A.I., 2009. The raman study of certain carbonates. Analele Ştiinţifice Ale Universităţii “Al. I. Cusa” Iaşi. Geologie 55, 97–112. D'Asaro, C.N., 1991. Gunnar Thorson's world-wide collection of prosobranch egg capsules: Murricidae. Ophelia 35, 1–101. De Baets, K., Klug, C., Korn, D., 2011. Devonian pearls and ammonoid-endoparasite coevolution. Acta Palaeontol. Pol. 56, 159–180.

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