BGBiogeosciencesBGBiogeosciences1726-4189Copernicus GmbHGöttingen, Germany10.5194/bg-12-6945-2015Stable isotope study of a new chondrichthyan fauna (Kimmeridgian,
Porrentruy, Swiss Jura): an unusual freshwater-influenced isotopic
composition for the hybodont shark AsteracanthusLeuzingerL.leuzinger.lea@gmail.comKocsisL.https://orcid.org/0000-0003-4613-1850Billon-BruyatJ.-P.SpezzaferriS.VennemannT.Département des Géosciences, Université de
Fribourg, Chemin du Musée 6, 1700 Fribourg, SwitzerlandSection d'archéologie et paléontologie, Office de
la culture, République et Canton du Jura, Hôtel des Halles, 2900
Porrentruy, SwitzerlandInstitut des Dynamiques de la Surface Terrestre,
Université de Lausanne, Quartier UNIL-Mouline, Bâtiment Géopolis, 1015 Lausanne,
SwitzerlandUniversiti Brunei Darussalam, Faculty of Science, Geology
Group, Jalan Tungku Link, BE 1410, Brunei Darussalamnow at: CRILAR, 5301 Anillaco, La Rioja, ArgentinaL. Leuzinger (leuzinger.lea@gmail.com)7December201512236945695412June201512August201530October201525November2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://bg.copernicus.org/articles/12/6945/2015/bg-12-6945-2015.htmlThe full text article is available as a PDF file from https://bg.copernicus.org/articles/12/6945/2015/bg-12-6945-2015.pdf
Chondrichthyan teeth (sharks, rays, and chimaeras) are mineralized in isotopic
equilibrium with the surrounding water, and parameters such as water
temperature and salinity can be inferred from the oxygen isotopic composition
(δ18Op) of their bioapatite. We analysed a new chondrichthyan
assemblage, as well as teeth from bony fish (Pycnodontiformes). All specimens
are from Kimmeridgian coastal marine deposits of the Swiss Jura (vicinity of
Porrentruy, Ajoie district, NW Switzerland). While the overall faunal
composition and the isotopic composition of bony fish are generally
consistent with marine conditions, unusually low δ18Op values
were measured for the hybodont shark Asteracanthus. These values are
also lower compared to previously published data from older European Jurassic
localities. Additional analyses on material from Solothurn (Kimmeridgian, NW
Switzerland) also have comparable, low-18O isotopic compositions for
Asteracanthus. The data are hence interpreted to represent a so far
unique, freshwater-influenced isotopic composition for this shark that is
classically considered a marine genus. While reproduction in freshwater or
brackish realms is established for other hybodonts, a similar behaviour for
Asteracanthus is proposed here. Regular excursions into lower
salinity waters can be linked to the age of the deposits and correspond to an
ecological adaptation, most likely driven by the Kimmeridgian transgression
and by the competition of the hybodont shark Asteracanthus with the
rapidly diversifying neoselachians (modern sharks).
Introduction
Chondrichthyan remains are common in the Mesozoic fossil record of Western
Europe, and in many different palaeoenvironmental settings (e.g. lagoonal,
open marine, reduced salinity) (Duffin and Thies, 1997; Müller, 2011;
Underwood, 2002). Their teeth are predominantly composed of fluor-apatite,
the most resistant variety of apatite (Vennemann et al., 2001) and are
continuously shed and replaced, except in chimaeras (Cappetta, 2012; Stahl,
1999). In addition to their abundance, their mechanical and chemical
resistance make them an ideal material for stable isotope analyses. They
mineralize in isotopic equilibrium with the surrounding water, hence their
primary oxygen isotopic composition (δ18Op) reflects
that of the ambient water at a given temperature when they formed (Kolodny et
al., 1983; Longinelli and Nuti, 1973). This makes them a valuable
palaeoenvironmental proxy, used in numerous studies (e.g. Kocsis et al., 2007;
Lécuyer et al., 2003; Vennemann et al., 2001).
This research is based on fossil material – mainly chondrichthyans – found
between 2000 and 2011 during controlled palaeontological excavations
conducted by the Paléontologie A16 team (PAL A16, canton of Jura, NW
Switzerland). All fossiliferous sites are located in the vicinity of
Porrentruy (Ajoie district) and are related to the building of the Trans-Jura
highway (A16). The Ajoie region is part of the Tabular Jura (Marty et al.,
2007), mainly consisting of subhorizontal Mesozoic (Oxfordian and
Kimmeridgian) strata.
During the Kimmeridgian, the Ajoie region was a shallow-marine carbonate
platform at a palaeolatitude of about 30∘ N (Marty, 2008) and
surrounded by the Central and London Brabant massifs, the Tethys, and the Paris
Basin (Fig. 1). The palaeoclimate was semi-arid with high seasonality
(Philippe et al., 2010; Waite et al., 2013). The platform had a very complex
morphology due to the basement structure and sea-level changes occurred
during its depositional history. These processes induced several episodes of
emersion suggested by numerous dinosaur footprints (Marty, 2008; Marty et
al., 2007) and hardgrounds, followed by erosion and reworking. Lateral
changes in water depth potentially occurred at a very local scale (Jank et
al., 2006; Waite et al., 2013). The record of ammonites typical of the boreal
and Tethyan domains show that the study area was influenced by water masses
from both the Tethys and Paris Basin (Colombié and Rameil, 2007; Comment
et al., 2011).
Palaeogeographical map of the study site and surroundings (Late
Kimmeridgian, modified from Comment et al., 2011). CH = Switzerland,
palaeolatitude of Porrentruy =∼ 30∘ N. Emerged land
is outlined, darker grey corresponds to deeper water. Upper left
corner: present-day geographical position of Porrentruy (⋆) and other
European sites (•) of previously published studies and providing
geochemical data compared in Fig. 5. The shaded square delimits the area
detailed in the palaeogeographical map.
Based on phosphate oxygen isotope analyses obtained from this Late Jurassic
chondrichthyan fauna, this study proposes answers to the following questions:
(1) is there any unexpected isotopic composition for the associated marine
fauna recorded in Porrentruy? (2) Are the Porrentruy isotopic data unique so
far, or comparable to other European localities? (3) What do we learn about
the palaeoecology of the hybodont shark Asteracanthus based on the
isotopic composition?
Material and methods
The chondrichthyan dental material of the PAL A16 collection is rich and
diverse, comprising more than 2000 fossils. Sharks and rays (subclass
Elasmobranchii) are represented by the hybodont sharks – the extinct sister
group of modern sharks (Maisey et al., 2004) (order Hybodontiformes:
“Hybodus”, Planohybodus, Asteracanthus) – the
modern sharks (subcohort Neoselachii, order Carcharhiniformes:
Palaeoscyllium, Corysodon; order Heterodontiformes:
Heterodontus, Paracestracion; order Protospinaciformes:
Protospinax; order Squatiniformes: Pseudorhina) and rays
(superorder Batomorphii, order Rajiformes: Belemnobatis,
Spathobatis). Chimaeras (superorder Holocephali (sensu Stahl, 1999), order Chimaeriformes: Ischyodus) are also present.
The investigated material comes from the Kimmeridgian Reuchenette Formation
and more precisely from the latest Early Kimmeridgian (Cymodoce
ammonite zone, Banné Marls) and up to the Late Kimmeridgian
(Mutabilis ammonite zone, Corbis Limestones
and Eudoxus ammonite zone, lower Virgula Marls) (Fig. 2).
Except for Asteracanthus and Ischyodus remains that are of
a considerable size and were collected directly on the field, the material
consists predominantly of microfossils resulting from sediment sieving.
The oxygen isotopic composition of phosphate from biogenic apatite was
measured on rays, the chimaeroid Ischyodus and the hybodonts
Asteracanthus and Hybodus. Bioapatite of bony fish
Pycnodontiformes was also analysed for comparison. Stratigraphically, samples
were selected from different beds in order to cover all units of the studied
section (Fig. 2). Additionally, Kimmeridgian material from the neighbouring
Natural History Museum of Solothurn was analysed for comparison.
Simplified stratigraphic profile of the Porrentruy area with
third order orbital cycles and section yielding the studied chondrichthyan
material. Numbers indicate geological age in millions of years. SB =
sequence boundary, ts = transgressive surface, mfs = maximum flooding
surface.
Fossil material from the study site of Porrentruy. (a): teeth of
Asteracanthus. Left: adult specimen (SCR010-1125).
Right: juvenile specimen (SCR004-221) to scale
and magnified. Occlusal (top) and lateral (bottom) views. (b): left
prearticular bone of Pycnodontiformes with teeth (specimen SCR010-1204).
Photographs by PAL A16.
The best mineralized part of the teeth, the enameloid, was isolated in
Pycnodontiformes and Asteracanthus (Fig. 3). From eleven of the
Asteracanthus teeth, dentine was also analysed in parallel to
examine any isotopic differences between the tissues. In the case of
chimaeroid dental plates the densest parts were selected. For the very small
material (1–5 mm) – as in rays and Hybodus – several isolated
teeth were analysed together as bulk samples of enameloid and dentine. Due to
the small size, only the outer aspect of this material was sampled as it was
visibly the best preserved, i.e. not worn-out teeth and/or with ornamentation
well defined and light-grey in colour. After manual removal of the largest
part possible of the root, the most dentine-free teeth were used for
analysis.
From the Porrentruy material, 38 samples of Asteracanthus teeth (27
enameloid and 11 dentine), 7 of Ischyodus dental plates and 13 of
Pycnodontiformes teeth were analysed; in addition, 4 bulk samples for
Hybodus and 3 for rays were investigated. From the Solothurn
material, enameloid of 9 Asteracanthus and 3 Pycnodontiformes teeth
were added for comparison. Altogether, a total of 77 analyses were made.
The sample powders (at least 2 mg per sample) were pre-treated following the
procedure of Koch et al. (1997), and the PO43- ion of the apatite was
separated and precipitated as silverphosphate (e.g. Kocsis, 2011; O'Neil et
al., 1994). NBS–120c phosphorite reference material was processed in
parallel with the samples. Generally, triplicates of each sample were
analysed together with two in-house phosphate standards (LK–2L:
12.1 ‰ and LK–3L: 17.9 ‰) to correct the results. The samples
were analysed in a high-temperature conversion elemental analyser (TC/EA)
coupled to a Finningan MAT Delta Plus XL mass spectrometer at the University
of Lausanne after the method described in Vennemann et al. (2002). The data
are expressed in permil and reported as δ18Op on the
VSMOW scale (Vienna Standard Mean Ocean Water). The overall analytical error
is taken as 0.3 ‰, however individual samples often reproduced
better. For the NBS-120c an average value of 21.3 ± 0.3 ‰
(n= 6) was obtained. This is somewhat lower than the mean reported value of
21.7 ‰ (e.g. Halas et al., 2011), but no correction was applied to the
values measured as the small offset is thought to be due to heterogeneity in
the sedimentary phosphorite and its different response to pretreatments
compared to the enameloid of the teeth sampled.
The oxygen isotope composition of unaltered fish teeth
(δ18Op) is a function of both water temperature and
isotopic composition of ambient water (δ18Ow) during
tooth growth (Kolodny et al., 1983; Lécuyer et al., 2013; Longinelli and
Nuti, 1973). Here below is the phosphate fractionation equation of
Lécuyer et al. (2013) used for calculating the temperature of sea water:
T(∘C)=117.4(± 9.5)-4.50(± 0.43)⋅(δ18Op-δ18Ow).
For marine fauna, the global, average seawater isotopic composition
(δ18Ow) can be used as an approximation that is assumed
to be equal to -1 ‰ for the ice-free Late Jurassic seawater (e.g.
Shackleton and Kennett, 1975).
Results
For the Porrentruy samples, the bioapatite oxygen isotope compositions have a
range between 17.0 and 21.9 ‰, with an overall average value of
18.8 ± 0.9 ‰ (n= 65). These values can be grouped into three
ranges: (1) values of bulk samples (Hybodus and rays) and
Ischyodus that are between 18.5 and 19.8 ‰ (average
19.2 ± 0.4 ‰, n= 14); (2) enameloid values of
Asteracanthus, averaging 18.1 ± 0.6 ‰
(17.0–19.7 ‰, n=27) and (3) those of Pycnodontiformes with an
average of 19.8 ± 1.0 ‰ (18.2–21.9 ‰, n= 13). The
average value of 18.9 ± 0.8 ‰ (17.7–20.0 ‰, n=11) in
the Asteracanthus' dentine is significantly different from the
equivalent enameloid sampled from the same teeth demonstrated by Student's
t test: t(20) = 2.98, p < 0.01.
For the Solothurn comparison material, an average of
18.7 ± 0.9 ‰ (n=9) and 19.4 ± 0.7 ‰ (n=3) was
obtained for Asteracanthus and Pycnodontiformes teeth respectively.
All of the data are available and detailed in the Supplement.
Associated fauna and palaeoecology
The associated fauna of the Porrentruy material is characteristic of a
coastal marine environment, with notably a rich marine bivalve assemblage,
sea urchins and over 600 ammonites (Comment et al., 2011; Marty and
Billon-Bruyat, 2009). Among vertebrates, coastal marine turtles
(Plesiochelyidae) (Anquetin et al., 2014; Püntener et al., 2014, 2015)
and crocodilians (Thalattosuchia) (Schaefer, 2012) are common.
During the Late Jurassic, modern sharks were expanding and diversifying,
while hybodonts were declining and restricted more to environments of reduced
salinity, or even freshwater, where modern sharks were less represented
(Kriwet and Klug, 2008; Rees and Underwood, 2008; Underwood, 2002). In our
assemblage however, hybodonts and rays clearly dominate (86 % of the dental
material). This suggests that conditions are still favourable to hybodonts in
Porrentruy, unlike in neighbouring localities from southern Germany
(Nusplingen, Solnhofen) or France (Cerin), where hybodonts are scarce or
absent. Our chondrichthyan assemblage (see Sect. 2) is rather similar to
that in northern Germany (e.g. in Oker) (Duffin and Thies, 1997; Thies,
1995), also dominated by hybodonts and rays. There, the fauna is associated
with conditions of reduced salinity (Underwood and Rees, 2002; Underwood and
Ward, 2004; Underwood, 2002, 2004). The chimaeroid Ischyodus must
also be regarded as one of the most abundant chondrichthyans, even if
representing only 3 % of the remains. Indeed, its nonrenewable and less
resistant dentition and the relatively low amount of dental elements per
individual (six dental plates against hundreds to thousands of teeth for
sharks and rays) (Stahl, 1999) easily lead to an underestimate of its
abundance. Interestingly, most of the few modern sharks (Neoselachii) of our
assemblage (i.e. Heterodontus, Palaeoscyllium,
Protospinax, Pseudorhina) are thought to have had a benthic
lifestyle (Underwood, 2002; Underwood and Ward, 2004), supporting a
well-oxygenated bottom water, which is also indicated by the invertebrate
fauna.
Discussionδ18Op values from the Porrentruy material:
palaeoecological indications
Values of bulk samples (Hybodus and rays) and Ischyodus
have a similar range and could reflect either a similar habitat for these
groups, or a similar diagenetic alteration. Since they correspond to
dentine-bearing samples – i.e. tissues that are more easily altered than
enameloid – and given that the dentine samples of Asteracanthus
tend to similar values, the least resistant tissue of all these specimens
could have been affected by alteration during diagenesis. Diagenetically
altered isotopic values for dentine or bone are expected in fossil samples
(see Lécuyer et al., 2003; Sharp et al., 2000; Pucéat et al., 2003).
Therefore, in order to discuss ancient ecological parameters, we focus on
enameloid samples in the rest of the text.
The isotopic compositions of Pycnodontiformes and
Asteracanthus enameloid samples are considered not to have been
altered, partly because of their original histological structure when
examined with a microscope, their black-blueish colour when subjected to
cathodoluminescence, and the generally good preservation potential for
enameloid when not recrystallized (e.g. Kohn and Cerling, 2002). The distinct
range in values between Asteracanthus and Pycnodontiformes
enameloid, both when compared to one another and to dentine-bearing samples
within the same group, further supports preservation of original values.
Also, the fact that an Asteracanthus enameloid value measured on a
tooth is lower than its dentine counterpart from the same tooth shows that
the enameloid did not experience intense alteration, unlike the dentine that
clearly recrystallized. Entirely altered specimens would give a similar
value, whatever the tissue analysed. The same can be inferred from the
isotopic difference between Asteracanthus and Pycnodontiformes
enameloid values, which would be expected to result in similar values if they
would have experienced the same diagenetic alteration (see Fischer et al.,
2012). Because of these reasons, the significant differences in
δ18Op values of Asteracanthus and
Pycnodontiformes enameloid from Porrentruy (Student's t test, t(38)=6.36, p < 0.01) are
interpreted as reflecting actual differences in the living conditions rather
than in the alteration process (Fig. 4).
Graphic representation of the δ18Op values
(average, standard deviation, end members) measured for Porrentruy in this
study and their corresponding water temperature using Eq. (1). Comparable
water temperatures for all taxa require different δ18Ow
values, which relate to salinity. Bulk and dentine values might have suffered
diagenesis. Note the strong difference between δ18Ow of
Pycnodontiformes and Asteracanthus enameloid values (i.e. distinct
palaeoenvironments) when similar ecological T is assumed. The wide value
range of Pycnodontiformes indicates a tolerance to salinity fluctuations
occurring within the platform, and possibly a living area broader than the
shallow-marine platform. No attempt to define the final
δ18Ow values or water temperatures is made here.
Water temperatures calculated with Eq. (1) from enameloid
δ18Op of Pycnodontiformes and Asteracanthus
differ by 7.4 ∘C (1.6 ‰). The two taxa are found in the same
deposits and such a temperature difference is not plausible neither
laterally, nor vertically, given that the water depth did not exceed a few
tens of metres in the study area (Waite et al., 2013). Most of our
Pycnodontiformes δ18Op values (18.2 to 21.9 ‰)
indicate marine conditions, since they are comparable with the isotopic
composition measured on several marine vertebrate taxa from the Late Jurassic
of western Europe (18.5 to 22.8 ‰) (see Billon-Bruyat et al., 2005;
Dromart et al., 2003; Lécuyer et al., 2003). Those values are consistent
with the marine conditions indicated by the associated fauna of Porrentruy.
When used in Eq. (1), the Pycnodontiformes δ18Op
values give a mean temperature range that is also consistent with the
palaeogeographical settings of the study site (23.9 ± 4.4 ∘C,
n= 13). However, the range in values is quite wide (see Fig. 4) and
can be interpreted as a tolerance to salinity fluctuations for this taxon,
since some of those bony fish are known to be euryhaline and are probably
poor environmental indicators (Kocsis et al., 2009; Poyato-Ariza, 2005).
Semi-confined lagoons induced by local depth differences on the platform and
subjected to higher evaporation rates during the dry season would have been
characterized by a higher salinity and thus higher isotopic composition,
potentially recorded by Pycnodontiformes. For the lowest value
(18.2 ‰), an influence of a slightly reduced salinity cannot be
excluded. On the other hand, the highest values can also be interpreted as
reflecting a deeper, cooler environment around the platform. The good state
of preservation of Pycnodontiformes remains and the presence of several
mandibles and tooth palates suggest that the material was not transported
over long distances.
The preservation of the fine ornamentation of Asteracanthus teeth
also suggests that they lived in the vicinity, even if the isotopic
composition of Asteracanthus is significantly different from that of
Pycnodontiformes. Also, the associated record of several large
Asteracanthus fin spines in marly deposits of the Lower
Virgula Marls (lagoonal deposits indicating a low-energy context)
(see Waite et al., 2013) argues against long distances of sediment transport
for those relatively large fossils (up to 26.5 cm long), supporting an
autochthonous character of this genus. Moreover, the preservation of the root
in several Asteracanthus teeth – an indication of post-mortem
embedding rather than tooth loss in hybodonts (Underwood and Cumbaa, 2010) –
also precludes transport. Yet, temperatures obtained with
Asteracanthus enameloid samples using the Eq. (1) are higher
(average 31.3 ± 2.9 ∘C, n= 27). This could imply a
habitat closer to the sea surface but would then also suggest a possible
influence of more evaporative conditions on the oxygen isotope composition of
the water with δ18Ow values higher than the global
average used above (i.e. 1 ‰). For example, 0 ‰ as proposed by
Lécuyer et al. (2003) for low-latitude marginal seas with high
evaporation rates. However, higher δ18O values of water would also
result in higher temperatures calculated with an average of 35.8 ∘C
and a maximum reaching 41.0 ∘C, which are considered unrealistic. A
more consistent explanation is to consider Asteracanthus as living
in a freshwater-influenced environment, i.e. an environment with a lower
δ18Ow value (Fig. 4).
Shark nurseries in reduced salinity environments for
Asteracanthus?
Assessing the tooth replacement rate of an extinct shark is difficult, and
studies of such rates are scarce (e.g. Botella et al., 2009). However,
Asteracanthus possesses a crushing dentition composed of a rather
small amount of large teeth (see figure in Rees and Underwood, 2008, p. 136)
organized in a relatively low number of files and rows (sensu
Cappetta, 2012); hence, a relatively slow replacement rate is likely,
compared to other sharks with numerous slender, cuspidated teeth adapted to
clutch and tear their prey. This implies that the δ18Op
values of Asteracanthus potentially reflect an average of the
surrounding water parameters over a relatively longer growing period. The
lower δ18Op values of Asteracanthus, compared
to typical Late Jurassic marine compositions (see data from marine
vertebrates of other studies in Sect. 5.1), corresponds either to a constant
brackish living environment or to a marine environment with regular
excursions into fresh- or brackish waters (or vice-versa). As
Asteracanthus remains were not re-sedimented nor transported over
long distances, it can be proposed that they partly inhabited the marine
realm, as indicated by the associated fauna, but not continuously. Lateral
salinity changes are readily caused by rainy winters coupled with an
irregular morphology of the platform, creating marked depth differences and
lagoons (Waite et al., 2013) where the proportion of meteoric water could
have been important. However, excursions into more distant
brackish and/or freshwater realms can also be considered. Extant elasmobranchs that
occupy different environmental niches during relatively long period of their
lives (not necessarily with salinity variations) can do so for different
reasons: seasonal environmental changes, reproduction, and development in a
distinct environment during the first ontogenetic stages (White and
Sommerville, 2010).
More than 130 Asteracanthus teeth were found in the Porrentruy
excavation sites. Only four of them appeared to be clearly undersized
(< 1 cm) (Fig. 3). As illustrated in Rees and Underwood (2008,
p. 136), the size difference between lingual-most and labial-most teeth of
any file is quite small in Asteracanthus medius. Even if a stronger
heterodonty cannot be excluded for other species of the genus, it seems more
likely that the clearly undersized dental material belonged to juvenile
individuals. The record of hundreds of submillimetric fish remains such as
dermal denticles resulting from sieving of hundreds of kilograms of sediments
exclude a taphonomic bias linked to the size of the teeth.
Asteracanthus juveniles could have spent the first period of their
life in estuaries, rivers or lagoons, sheltered from predators such as
crocodilians or the bony fish Caturus. Extant euryhaline bull shark
females (Carcharhinus leucas) and their juveniles are known to have
a similar behaviour (Jenson, 1976; Pillans et al., 2005), as is the case for
some small hybodont sharks (Fischer et al., 2011; Klug et al., 2010). The
location of this environment with reduced salinity remains open, especially
since some sharks are known to migrate across very long distances, e.g. the
blacktip shark (Castro, 1996). Regarding the fish faunal composition of
Porrentruy, salinity fluctuations within the study area cannot be excluded.
Two of the most abundant bony fish taxa recorded – Pycnodontiformes and
“Lepidotes” – are known to tolerate salinity fluctuations (Amiot
et al. 2010; Kocsis et al., 2009; Poyato-Ariza, 2005). Additionally,
several chondrichthyan taxa recorded are potential indicators of reduced
salinity: the chimaeroid genus Ischyodus was reported in Jurassic
freshwater deposits of Russia (Popov and Shapovalov, 2007) and can therefore
not be considered as strictly marine. The modern shark
Palaeoscyllium, relatively scarce but present in our fossil
assemblage, is the oldest modern shark known to tolerate freshwater, so far
only in the Cretaceous though (Sweetman and Underwood, 2006). Finally, and as
mentioned above, hybodonts and rays are in some cases also linked to reduced
salinity conditions (Duffin and Thies, 1997; Thies, 1995). Salinity
fluctuations (from pliohaline to brachyhaline) are supported by different
ostracods assemblages in the study site (Schudack et al., 2013), yet they
overwhelmingly indicate brachyhaline conditions in our sample sections.
Comparison of δ18Op values (average, standard
deviation, end members) of Pycnodontiformes and Asteracanthus
enameloid samples from Porrentruy, Solothurn and other European
localities through time. The stratigraphical position is approximate and
corresponds to Early, Middle, Late divisions of each stage. The approximate
geographical positions of previously studied localities (Dromart et al.,
2003; Lécuyer et al., 2003; Billon-Bruyat et al., 2005) are shown in
Fig. 1. Detailed localities and stratigraphic positions are available in the
Supplement.
In Fig. 5, the oxygen isotopic compositions of Pycnodontiformes and
Asteracanthus enameloid samples measured in this study are shown for
the Porrentruy and Solothurn localities and compared to previously published
data from others – mostly older – Swiss, French, and British Jurassic
localities (Billon-Bruyat et al., 2005; Dromart et al., 2003; Lécuyer et
al., 2003). Generally, the Porrentruy Asteracanthusδ18Op values – especially in the Late Kimmeridgian –
are lower than in other studies, while Pycnodontiformes values are
comparable. The material from Solothurn (Kimmeridgian) – a locality with
similar palaeoenvironment but under Tethyan influence only – shows some
affinities with the Porrentruy material, for instance with unusually low
oxygen isotope values for several Asteracanthus. The Porrentruy
Asteracanthusδ18Op values tend to get lower
in the Upper Kimmeridgian but this trend must be considered with caution due
to the relatively small amount of Lower Kimmeridgian samples.
This global comparison suggests that the low δ18Op
values measured for Asteracanthus here are likely linked to the age
of the deposits. Interestingly, a tolerance of Asteracanthus to
salinity variations has briefly been mentioned by Kriwet (2000), based on its
presence in the younger deposits of the Purbeck and Wealden group in southern
England (Woodward, 1895). Asteracanthus remains from freshwater
deposits are also recorded in the Upper Cretaceous of Sudan (Buffetaut et
al., 1990). The present data indicate an adaptation to a wider salinity range
through time and in the Kimmeridgian already, maybe in response to the
spectacular diversification of modern sharks in the marine realms of Western
Europe at the end of the Jurassic (Cuny and Benton, 1999). Also, the
shallow-water platform of NW Switzerland may have somehow represented a
shelter for the hybodonts, still dominating the shark fauna around
Porrentruy. The high sea-level in the Kimmeridgian (Hardenbol et al., 1998)
could have opened new niches in shallow-water environments that was
influenced by freshwater run-offs. These new living places could have
provided shelter and nursery ground for Asteracanthus.
This is the first isotopic evidence of a euryhaline ecology for the large,
durophagous shark Asteracanthus, classically considered as marine
(Agassiz, 1843; Rees and Underwood, 2006, 2008).
Concluding remarks
Most of the δ18Op values of enameloid measured in
the hybodont shark Asteracanthus are too low to reflect fully marine
conditions.
Comparisons with geochemical data of older European Jurassic localities
confirm the unusual character of the Asteracanthus isotopic
compositions measured in the material from this study. This new
freshwater-influenced isotopic composition of Asteracanthus is
likely linked to a change in its ecology through geologic time, as suggested
by similar results obtained with Kimmeridgian material from Solothurn. The
Kimmeridgian transgression (i.e. opening of new shallow-water niches) (see
Fig. 2) and probably competing stress from quickly diversifying neoselachians
could have played an important role in the adaptation to brackish and
freshwater realms.
A predominantly marine ecology is proposed for
Asteracanthus, combined with regular excursions into
freshwater and/or brackish environments, possibly for reproduction purposes
considering the rarity of juvenile material in the marine, depositional
environment.
The Supplement related to this article is available online at doi:10.5194/bg-12-6945-2015-supplement.
Acknowledgements
We thank the PAL A16 team for making available the fossil material, as well
as precious field details, geological data and figures, and also for
inspiring discussions. The photographs of the material were kindly taken by
Bernard Migy and Olivier Noaillon. We really appreciate the help of S. Thüring
from the Natural History Museum of Solothurn for providing
additional fossil material for comparative purposes and the assistance of
scientists and technicians at the University of Fribourg (B. Grobéty, A. Foubert,
P. Dietsche) for preparing tooth thin-sections and making available
microscopes. Thank you to G. Cuny for his help in the identification of the
fossil material. We also want to thank C. Meier for details on the
stratigraphy of Solothurn. Thank you to M. Hechenleitner for his help in
finalizing this paper. The authors are also grateful for the constructive
comments of two reviewers, J. Fischer and R. Amiot.
This study was funded by the Section d'archéologie et paléontologie
(Canton Jura) and the Federal Road Office from Switzerland. L. Kocsis received
support from the Swiss National Science Foundation (SNF
PZ00P2_126407) while this research was conducted.Edited by: E. J. Javaux
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