Provenance of MPCA 632
The presence of calcareous nannofossils in the MPCA 632 sample leaves no doubts about the unfeasibility that MPCA 632 was collected in the Early Triassic Vera Formation of the Los Menucos Group as indicated by Bogan et al. (2013). Coccolithophores and calcareous nannofossils first appear in the fossil record in Late Triassic marine sediments (Bown et al., 2004, Demangel et al., 2023). During the late Carnian (approximately 227 Ma) they had a consistent presence, increasing their richness and abundance during the Norian and Rethian times, followed by a significant extinction at the end of the Triassic. Rates of speciation were significantly high in the Late Triassic, Early Jurassic and Tithonian-Berriasian (Bown et al., 2004). Therefore, according to the most recent geochronological dating of the Vera Formation in the Lower Triassic (Falco et al. 2020), the provenance of MPCA 632 from this geological unit is untenable. Furthermore, the fossiliferous content and tectonic setting of the Vera Formation are clearly continental.
The inventory card corresponding to MPCA 632 was recently found at the Museo Carlos Ameghino. The locality indicated in this card is “Las Lajas, Neuquen” (Fig. 2b). Geological surveys summarized in Zapala geological map 3969-I (Leanza et al., 2005) and a Field Guide of the Jurassic of the Neuquén Basin (Gulisano and Gutierrez Pleimling, 1994) show that there are no Triassic outcrops in the vicinity of that locality (Fig. 1). The only outcrops in the area are Jurassic and Cretaceous in age.
Polycostella beckmannii Thierstein is considered a reliable Late Jurassic nannofossil biomarker for the Neuquén Basin (Leanza et al., 2020). Its First Occurrence (FO) defines the base of the NJ20-B calcareous nannofossil Zone in the early Tithonian (Bralower et al., 1989). This nannolith has been recognized in the subsurface of Neuquén Basin (Vennari et al., 2017; Concheyro in Aguirre-Urreta et al., 2019) and in two sections of the Los Catutos Member of the Vaca Muerta Formation. In these sections, Polycostella beckmannii correlates with the Windhauseniceras internispinosum ammonite Zone which has been restricted to the late Tithonian (Zeiss and Leanza, 2010; Riccardi et al., 2011). Recently, the first occurrence (FO) of P. beckmanni has been reported in the lower Tithonian of the Tethyan areas, in the NJT15 calcareous nannofossil biozone (Casellato and Erba, 2021), and is confirmed in the Neuquén basin (Concheyro and Lescano, 2021). Therefore, due to its stratigraphic value, the sole presence of P. beckmanni in BACF-NP 4220–4224 strongly supports the referral of MPCA 632 to the Tithonian.
The fossil record of Watznaueria fossacincta, W. barnesiae, and Cyclagelosphaera margerelii extend from the Bajocian to the Maastrichtian (Fig. 4). They are long ranging taxa, characteristic of marine environments from the Middle Jurassic until the end of the Cretaceous, and are almost always present in Cretaceous nannofloras. They are also cosmopolitan taxa. Watznaueria biporta, is frequent in marine Cretaceous sediments, but it has also been recorded in the Late Jurassic (Tithonian; Bown et al., 1998; Nannotax 3; Fig. 4). In addition, the presence of scarce specimens of Crepidolithus sp. confirms the Jurassic age of the sample, since the genus biochron extends from the Hettangian to the top of Tithonian (Bown and Cooper, 1998; Nannotax 3).
The association of Watznaueria fossacincta, Watznaueria barnesiae, Watznaueria biporta, Cyclagelosphaera margerelii, Polycostella beckmannii and Crepidolithus most closely resembles the nannofossil association found in the Los Catutos Member, Vaca Muerta Formation, at Cantera Loma Negra (Scasso and Concheyro, 1999). These same taxa together with the strictly Jurassic Ethmorhabdus gallicus, Crepidolithus crassus, and Schizosphaerella punctulata characterize the nannoflora of the Los Catutos Member, supporting a middle Tithonian age for this unit (Scasso and Concheyro, 1999). It should be noticed that Ethmorhabdus gallicus, Crepidolithus crassus, and Schizosphaerella punctulate, recognized in los Catutos ( Scasso and Concheyro, 1999) are fragile taxa which would not resist the strong diagenetic process inferred for MPCA 632.
Jurassic outcrops in the area of Las Lajas, Neuquén, are composed by sediments belonging to Cuyo and Mendoza groups (Leanza et al., 2005). Among them, the micritic limestone in MPCA 632 is macroscopically very similar to the 40 samples from three profiles of the Los Catutos Member of the Vaca Muerta Formation analyzed by Scasso and Concheyro (1999).
Compared with the limestones and marls of the Los Catutos Member of the Vaca Muerta formation, the isotopic composition of the MPCA 632 samples is slightly different. Scasso et al. (2015) obtained δ18O values between − 3.5 and − 5.5‰, with average values of -4.87‰ for limestones and − 4.43‰ for marls, and δ13C values between − 1 and + 1.55‰ with average of 0.93‰ for limestones and 1.025‰ for marls.
The MPCA 632 samples gave values of δ18O ranged from − 2.20‰ to -2.18‰, and the average was − 2.19 showing a variation with respect to those (-3.5 to -5.5‰) obtained by Scasso et al. (2005). The oxygen isotopic composition of carbonates is quite sensitive to temperature. Therefore, the negative δ18O values of carbonates would be the result of isotopic fractionation as consequence of an increase in temperature during diagenesis. The isotopic carbon variation observed by Scasso et al. (2005) seems to be caused by local paleoenvironmental conditions during the initial marine transgression in the Neuquén Basin.
The δ13C values of Los Catutos limestones and marls samples representing the marine facies of Los Catutos Member (Scasso et al, 2005) are restricted in a very narrow range varying from − 1 to 1.5‰. Our data reveal that the δ13C values of the samples ranged from 1.97 to 1.99‰ with the average being 1.98‰. This carbon isotopic composition was similar to the global seawater of the Tithoniann (Veizer et al., 1999). These values are reflecting optimal conditions for the development of an abundant fauna of marine invertebrates as well as a continuous burial of organic carbon.
In addition, the study on carbonate isotopes carried out by Rodriguez Blanco et al. (2018) in samples from the Tithonian-Early Valanginian succession in the Neuquén Basin show few variations and coincide with the global isotope values (0 and + 3) reports for these stratigraphic interval (Katz et al., 2005). In such a way, the data mentioned in this contribution, although scarce, can be considered similar to those published by the last-mentioned authors.
Considering provenance indicated in the inventory card (Fig. 2b), the nannofossil association, very especially the presence of Polycostella beckmannii (Figs. 3–4), the lithology, the isotopic composition (Supplementary Data), and plotting the δ13C values obtained for the MPCA 632 samples together with the extensive data provided by Rodriguez Blanco et al. (2022), it is most likely that MPCA 632 was collected from outcrops of the Los Catutos Member of the Vaca Muerta Formation.
Gouiric-Cavalli (2016) described a new caturid taxon from the Jurassic of Neuquén (middle Tithonian Los Catutos Member, Vaca Muerta Formation) under the name Catutoichthys olsacheri. Unfortunately, the holotype and only known specimen of Catutoichthys olsacheri is an incomplete fish, including only a few poorly preserved skull bones and a comparison with MPCA 632 is not possible. However, it is likely that MPCA 632 represents a specimen of this taxon.
Distribution of Caturoidei
Confirming the suspicions of López-Arbarello and Ebert (2023), the micropaleontological, chemical and lithological analyses of MPCA 632 demonstrate that the caturoid specimen described by Bogan et al. (2013) does not come from the Lower Triassic of Los Menucos Group. Instead, the specimen must have been collected from outcrops of the Upper Jurassic–Lower Cretaceous Vaca Muerta Formation, most probably from its Tithonian Los Catutos Member.
The only other record of a caturoid in the Triassic is “Caturus” insignis (Kner, 1866), from the Norian of Seefeld, Austria (López-Arbarello and Ebert, 2023). After close examination of the holotype TLM F.117, the senior author has been able to confidently exclude “Caturus” insignis (Kner, 1866) from the family Catuidae or the superfamily Caturoidei (López-Arbarello pers. obs.). The superfamily Caturoidea is diagnosed by the presence of sharply carinate acrodin tooth caps on the larger jaw teeth; an extremely slender rod-like maxilla; a relatively high number of branchiostegal rays (22 or more on each side); haemal spines broadly spatulate in the transverse plane; preural haemal and neural spines near the caudal peduncle region strongly inclined to a nearly horizontal orientation (Grande and Bemis, 1998). None of these features is present in the holotype of “Caturus” insignis, which nevertheless share an overall resemblance with caturoids and might be an early member on the lineage leading to this superfamily (López-Arbarello pers. obs.).
With the exclusion of “Caturus” insignis from the group, the fossil record of Caturoidea is restricted to the Jurassic–Lowest Cretaceous (López-Arbarello and Ebert, 2023: Table 1). Except for the youngest records in the Lower Cretaceous of Spain (El Montsec and Las Hoyas, Martín-Abad and Poyato-Ariza, 2013), all other caturoid records are from marine environments. Caturoid Bauplan, streamlined body and narrow caudal peduncle, indicate a pelagic habitat (Friedman et al., 2020). The first confident record of a caturoid outside todays Europe is “Caturus” dartoni Eastman, 1899, from the Bathonian (lower Sundance Formation) of Hot Springs, South Dakota, in North America. The holotype of “C.” dartoni (NMNH 4792) is a very incomplete specimen and none of the diagnostic features of the Caturoidea is preserved. However, other specimens from the Sundance and Wanakah formations were studied by Schaeffer and Patterson (1984), who discussed the resemblance of this species with European species of Caturus, including several features: the morphology of the jaws, dentition, paired fins and scales, the presence of at least 24 branchiostegal rays and hemichordacentra. In particular they highlighted the fusion of hypurals 1–3, a feature shared with the Early Jurassic Caturus heterurus from Lyme Regis, UK, and C. smithwoodwardi, from Holzmaden, Germany.
The next younger record of a Caturoid west of the Tethys is Caturus deani Gregory, 1923, from the Upper Jurassic Jagua Formation (Oxfordian) of Cuba (Iturralde-Vinent and Ceballos Izquierdo, 2015). The holotype of this species, an imperfectly preserved skull AMNH FF 6371 (7930), shows a very slender maxilla distinct of caturoid fishes. Later on, in the Tithonian, caturoids are recorded in the Vaca Muerta Formation of Argentina (Gouiric-Cavalli, 2016 and this study). Therefore, according to their known fossil record, the dispersal of caturoids towards the west most probably occurred through the Hispanic Corridor (Fig. 5) and started in the Middle Jurassic as suggested by López-Arbarello and Ebert (2023).
The Hispanic Corridor started developing during the Sinemurian, probably intermittently at those early stages, and it was well-established as a shallow marine connection by the Pliensbachian–Toarcian (Damborenea et al., 2012). Studies on bivalves show that, during the Early Jurassic, the Hispanic Corridor functioned as a filter, allowing the dispersion of benthonic littoral species and simultaneously representing a barrier for neritic species (Aberham, 2001; Damborenea et al., 2012). The Hispanic Corridor became an effective dispersal route for neritic taxa only during the Middle Jurassic, during the rift-drift transition and the opening of the Caribbean Seaway (Gasparini et al., 2000; Damborenea et al., 2012; Pindell et al., 2020). Therefore, the first West appearance of a caturoid, “Caturus” dartoni in the Sundance Sea coincides with the establishment of this very successful spreading path which had driven the evolution of other vertebrate groups, in particular the marine reptiles (Bardet et al., 2014).