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).

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).

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.

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 Asteracanthus δ18Op 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 Asteracanthus δ18Op 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).