Diversity and paleoenvironmental implications of an elasmobranch assemblage from the Oligocene–Miocene boundary of Ecuador

Elasmobranch paleodiversity

The elasmobranch assemblage described herein from the Montañita-Olón site comprises 27 taxa (including three of indeterminate taxonomy) of squalomorphs, galeomorphs and batoids (Figs. 3–8; Table 1; Tables S1 and S2). A general descriptive taxonomy of the dental elements of key species (first and rare records in Ecuador and Tropical America) is presented in Data S1.

Squalomorphs, with seven species, are the second most diverse and abundant group of sharks in the Montañita-Olón assemblage (Fig. 9; Table 1), and their remains are represented exclusively by isolated teeth (Table S1). The identified taxa include the hexanchiforms Heptranchias cf. H. howellii (Reed, 1946) (Figs. 3A–3H; Data S1) and Hexanchus cf. H. griseus (Bonnaterre, 1788) (Figs. 3I–3O; Data S1), the squaliforms Centrophorus cf. C. granulosus (Bloch & Schneider, 1801) (Figs. 3P–3W; Data S1) and Dalatias sp. (Figs. 3X and 3Y; Data S1), the echinorhiniforms cf. Echinorhinus sp. (Figs. 4A and 4B; Data S1), and Paraechinorhinus cf. P. barnesi Welton in Pfeil, 1983 (Figs. 4C and 4D; Data S1), and the pristiophoriform Pristiophorus sp. (Figs. 4E–4H). With the exception of Pristiophorus sp., all the above-mentioned species are reported for the first time in the fossil record of Ecuador. Heptranchias cf. H. howellii, Centrophorus cf. C. granulosus, Dalatias sp., cf. Echinorhinus sp. and Paraechinorhinus cf. P. barnesi are reported for the first time in the fossil record of the ECP.

Galeomorphs (isolated teeth as well as vertebral remains) represent the most diverse and abundant group in the elasmobranch assemblage from the Montañita-Olón site (Fig. 9). This group of sharks is characterized by a total of 15 species within 14 genera and nine families of Orectolobiformes, Lamniformes and Carcharhiniformes (Table 1). Lamniforms represent the most diverse group in the overall assemblage of the Montañita-Olón site. With 10 taxa, it includes: Isurus cf. I. oxyrinchus Rafinesque, 1810 (Figs. 4L–4S; Data S1), an indeterminate lamnid species (represented by and isolated tooth, see Figs. 6A–6C; Data S1), Mitsukurina cf. M. lineata (Probst, 1879) (Figs. 4T–4Z; Data S1), Carcharias sp. (Figs. 5A and 5B; Data S1), Odontaspis sp. (Figs. 5C–5K; Data S1), Otodus (Carcharocles) cf. O. angustidens (Agassiz, 1833–1843) (Figs. 5L–5Q; Data S1), Parotodus benedenii (Le Hon, 1871) (Figs. 5R–5T; Data S1), Megalolamna paradoxodon (Shimada et al., 2017) (Figs. 5U–5X; Data S1), and Alopias cf. A. exigua (Probst, 1879) (Figs. 6D–6H). Carchariniforms are represented by Carcharhinus gibbesii (Woodward, 1889) (Figs. 6N–6S; Data S1), the most abundant taxon in the assemblage (Table S2), as well as Galeocerdo aduncus (Agassiz, 1833–1843) (Figs. 6T–6W), Physogaleus contortus (Gibbes, 1849) (Figs. 7A–7F), Hemipristis serra (Agassiz, 1833–1843) (Figs. 7G–7J), and Sphyrna sp. (Figs. 7K–7N). A group of 25 semi articulated vertebrae (Figs. 7O and 7P) was also collected in the Montañita-Olón site inside a concretion (Fig. 2C); however, due to the lack of diagnostic elements, a more accurate identification than “Carcharhiniformes indet.” is not possible. The presence of an incomplete tooth of the whale shark Rhincodon sp. (Figs. 4I–4K; Data S1) represents the only record of an orectolobiform shark in the Montañita-Olón site. With the exception of Carcharias sp. and H. serra, all the above-mentioned galeomorph species are reported for the first time in the fossil record of Ecuador. Rhincodon sp., Mitsukurina cf. M. lineata, Odontaspis sp., Otodus (Carcharocles) cf. O. angustidens, Alopias cf. A. exigua, Alopias latidens and Carcharhinus gibbesii are reported for the first time in the fossil record of the ECP.

The batoids, with only one genus and two species, are the least diverse group from the Montañita-Olón assemblage (Fig. 9; Table 1). Only a few isolated teeth of the mobulids Mobula fragilis (Cappetta, 1970) (Figs. 8A–8K; Data S1) and Mobula sp. (Figs. 8L–8R; Data S1) are herein reported. Mobula fragilis is here reported for the first time in the fossil record of Ecuador and the ECP. Although some isolated mobulid teeth have been reported before from Neogene deposits of the Central Eastern Pacific of Ecuador (Carrillo-Briceño, Aguilera & Rodríguez, 2014) and Panama (Carrillo-Briceño et al., 2018), the morphological pattern of the Mobula sp. specimens from the Montañita-Olón site has not been recognized before in the ECP. In addition, two eroded, broken and non-diagnostic caudal spine fragments (Figs. 8S and 8T) are referred herein to Myliobatiformes indet.

Paleobathymetric analysis

Two paleobathymetric methods were applied to the elasmobranch fauna from the Montañita-Olón site: (1) an adaptation of Nolf’s method which calculates the percentage of species that share an assigned depth range (Nolf & Brzobohatý, 1994) and (2) the weighted method (Perez et al., 2017) by resampling the mean differences and using the relative abundance as weight estimator of the mean. Nineteen of the 24 total taxa, which were represented by 373 specimens (88.39% of total studied material), were included in the analysis. We used only taxa that have comparable modern analogs that allow us to estimate their depth ranges (Carrillo-Briceño et al., 2015a). This allows us to obtain more accurate estimations. For taxa without modern representatives, their presence is assumed in the estimated depth range.

Both results were comparable. For the Nolf technique, 73.7% of the studied taxa are represented in a depth range of 100–200 m (Fig. 10B). For the weighted paleobathymetry, a range between 87.8 and 378.0 m with a mean of 192 m was estimated (Fig. 10C). Our results suggest that the fossils were most likely deposited in an outer neritic (open shelf) environment. Of these two applied techniques, the weighted method provides more precision, but the Nolf method allows a better visualization of the depth distribution for each taxa in the assemblage (Fig. 10). The results obtained using these two techniques, improves the data interpretation in our study.

Diversity composition and biostratigraphy significance

Elasmobranchs from the Oligocene and early Miocene have not been reported from Ecuador, and their fossil record has been restricted to younger strata (Longbottom, 1979; Carrillo-Briceño, Aguilera & Rodríguez, 2014). The elasmobranch assemblage described here from the Montañita-Olón site includes at least 27 taxa (Table 1). This represents the most diverse elasmobranch assemblage known from Ecuador and for the Oligocene–Miocene boundary of Tropical America. The assemblage includes 13 extinct taxa (e.g., Heptranchias cf. H. howellii, Paraechinorhinus cf. P. barnesi, Mitsukurina cf. M. lineata, Otodus (Carcharocles) cf. O. angustidens, P. benedenii, M. paradoxodon, Alopias cf. A. exigua, A. latidens, C. gibbesii, G. aduncus, P. contortus, H. serra and M. fragilis) with a worldwide paleodistribution (Cappetta, 2012; Cappetta, Gregorová & Adnet, 2016; Szabó & Kocsis, 2016a; Shimada et al., 2017). From the overall elasmobranch assemblage at Montañita-Olón site, 19 species are reported here for the first time in the fossil record of Ecuador (Table 1). Ten species, including the squalomorphs Heptranchias cf. H. howellii, Centrophorus cf. C. granulosus, Dalatias sp., cf. Echinorhinus sp., the galeomorphs Rhincodon sp., Mitsukurina cf. M. lineata, Odontaspis sp., Carcharhinus gibbesii and the batoids Mobula fragilis and Mobula sp., are reported for the first time in the fossil record of the ECP. Records of Paraechinorhinus cf. P. barnesi, Otodus (Carcharocles) cf. O. angustidens, Alopias cf. A. exigua, A. latidens, are reported for the first time in the fossil record of South America. The presence of Rhincodon sp. from Ecuador, together with the record from the late Oligocene (Chattian) of Eastern USA (Cicimurri & Knight, 2009), represent the oldest records for this taxon. The Heptranchias cf. H. howellii specimens from the Montañita-Olón site (see Data S1) clearly resemble teeth of H. howellii from the Oligocene of North America (Welton, 1974), early Miocene of Colombia (Carrillo-Briceño et al., 2016b) and other regions (Cappetta, Gregorová & Adnet, 2016). The presence of Paraechinorhinus cf. P. barnesi in the Oligocene–Miocene of Ecuador increases its paleobiogeographic distribution and represents the oldest record of the genus in the Americas, as P. barnesi was known only from the middle Miocene of California (USA) (Pfeil, 1983; Cappetta, 2012).

As mentioned above in the results section, mobulid teeth identified as Mobula sp. have been reported before in the ECP from the Middle Miocene-early Pliocene of Ecuador (Carrillo-Briceño, Aguilera & Rodríguez, 2014, fig. 5) and late Miocene-Pliocene of Panama (see Perez et al., 2017; Carrillo-Briceño et al., 2018, table S6 and references therein). However, the morphological pattern of the Mobula sp. specimens from the Montañita-Olón site (Figs. 8L–8R) differ from those teeth reported from Ecuador and Panama. The two Mobula sp. specimens referred to here (MPM-1384), as well as those specimens illustrated by Carrillo-Briceño et al. (2016a, fig. 11.19–21) from the early Miocene of Venezuela, resemble teeth of the living species Mobula munkiana Notarbartolo Di Sciara (1987) (Adnet et al., 2012, fig. 4). Nevertheless, due to the scarcity of fossil and comparative material, a more accurate specific identification is not possible at this time. The batoids are the less diverse group from the Montañita-Olón assemblage with only benthopelagic representatives (Table S2). The absence of other batoids, especially those associated benthonic habitats, could likely be a result of bias in sampling. However, the benthonic batoids could also be associated with the ecological and environmental conditions that prevailed during the deposition of the Dos Bocas Formation, (e.g., Cappetta, Gregorová & Adnet, 2016).

To date, the elasmobranch assemblage from the Montañita-Olón site is the only one known from Oligocene–Miocene boundary of the ECP. Other Oligocene elasmobranchs from the region are unknown, and only a few early Miocene assemblages are restricted to southern Peru (Landini et al., 2019). The elasmobranch assemblage reported by Landini et al. (2019) from the Chilcatay Formation which is characterized by 22 taxa, clearly contains faunal differences in comparison with those from the Montañita-Olón site, where only a few galeomorphs (e.g., Isurus oxyrinchus, Carcharias, P. benedenii, M. paradoxodon, G. aduncus, P. contortus and H. serra) were present in both geological units. A few early Miocene assemblages from the southernmost areas of the Eastern Pacific in Chile have also been described (Villafaña & Rivadeneira, 2018; Villafaña et al., 2019). As in the Montañita-Olón site, early Miocene assemblages from Chile include taxa such as Carcharias, Odontaspis, Isurus (Villafaña et al., 2019), including a new species of sawshark †Pristiophorus humboldti, Villafaña et al. (2019) from the Navidad Formation. The presence of Pristiophorus sp. in the Montañita-Olón site represents the oldest record for this taxon in the ECP, although the taxon has been reported from the Eocene of southernmost Magallanes Region (Otero et al., 2013). The isolated rostral spines reported here show the typical characters described for the genus Pristiophorus from the Eastern Pacific (Carrillo-Briceño et al., 2013; Staig et al., 2015; Villafaña et al., 2019). However, the use of rostral spines is not recommended for species identification due to their high variability and insufficiency as a diagnostic character (Underwood & Schlögl, 2013; Engelbrecht et al., 2017; Villafaña et al., 2019). By contrast, the teeth display enough characters to assign specimens to lower taxonomic levels. Therefore, we prefer to identify to the genus level until more material is available. In addition, two coprolites with abundant bony fish remains and some isolated rostral spines of Pristiophorus were found in the Montañita-Olón site (Fig. S2). Although it is difficult to identify the possible producer of these coprolites, there is no doubt that sawfishes were part of the predator’s diet.

Like Pristiophorus, other elasmobranch taxa such as Dalatias, Carcharias, and Isogomphodon, were at the end of the Neogene regionally extirpated from the Eastern Pacific, but with extant representatives still inhabiting the Western Atlantic (Carrillo-Briceño et al., 2018). The presence of Dalatias sp. in the Montañita-Olón site is the oldest record of this taxon in the Eastern Pacific. This record from Ecuador together with the one known from the late Miocene–Pliocene of the Atacama Region in Chile (Villafaña & Rivadeneira, 2018), suggest that Dalatias was present in the Eastern Pacific from the late Oligocene to the end of the Neogene, when it became regionally extinct. The presence of Mitsukurina cf. M. lineata in the Montañita-Olón site also suggests the extirpation of the genus from the Eastern Pacific (see Carrillo-Briceño et al., 2018, table S4).

A late Oligocene–early Miocene age has been suggested for the Dos Bocas Formation on the basis of radiometric dating (Witt et al., 2019). According to Witt et al. (2019), the U-Pb zircon dating for the Montañita-Olón outcrop in the Montañita area (Fig. 2B) yielded an age of 23.5 ± 0.4 Ma, with a younger cluster average of 22.9 ± 0.6 Ma. These results agree with a late Oligocene age proposed for the unit using faunal composition (Olsson, 1931; Bristow, 1975; Tanaka et al., 2017). In addition, the age of the Dos Bocas Formation (Montañita-Olón site) proposed here is also supported by the presence of the typical Oligocene megatooth species Otodus (Carcharocles) cf. O. angustidens from other localities around the world (Baut & Génault, 1999; Gottfried & Fordyce, 2001; Reinecke, Stapf & Raisch, 2001; Szabó & Kocsis, 2016b).

Paleoenvironmental inferences

A shallow protected environment has been suggested as the most plausible depositional environment for the Dos Bocas Formation (see Tanaka et al., 2017; Witt et al., 2019, and references therein). The base of the fossiliferous outcrop consists of massive, moderately sorted, fine to medium-grained sandstone with angular quartz-feldspathic clasts and probably glauconite rounded green grains (Fig. 2). The matrix is micritic and volcanogenic, possibly bentonitic. According to Tanaka et al. (2017), bedding is massive to indistinct, suggesting little influence by traction currents or storm waves, and in turn implying a quiet setting; estuarine or mid-shelf is possible. Witt et al. (2019, fig. 8A) reported for the outcrops of the Dos Bocas Formation in the Montañita-Olón area, local large arthropod burrows and some evidence of patch reefs of bivalves and worms, suggesting that these beds were deposited in a protected shallow-water environment, likely under a period of strong tectonic deformation. Nevertheless, arthropod burrows are not exclusively from shallow water environments, and our field observations suggest that the thick layer of large arthropod burrows and the probable evidence of patch reefs with fragmentary mollusks reported by Witt et al. (2019) for the Montañita-Olón area are represented towards the top of the section (Fig. 2A). In contrast, micropaleontological evidence suggests for the Dos Bocas Formations, by Ordoñez, Jiménez & Suárez (2006) and Witt et al. (2019, fig. 3C), an upper platform environment. In our bathymetric analysis of the Montañita-Olón site (Dos Bocas Formation), 73.7% of the studied elasmobranch taxa are represented in a depth range of 100–200 m (Fig. 10). For the weighted paleobathymetry, a range between 87.8 and 378.0 m with a mean of 192 m was estimated. Our results suggest that the fossils were most likely deposited in an outer neritic (open shelf) environment, which coincides with the upper platform environment suggested by Witt et al. (2019, fig. 3C). Additional support for this estimation is derived from the presence of the benthopelagic squalomorph sharks Heptranchias cf. H. howellii, Hexanchus cf. H. griseus, Centrophorus cf. C. granulosus, Dalatias sp. and cf. Echinorhinus sp., whose extant representatives usually prefer deep-water environments near the continental slope (see Table S2). The extinct Heptranchias cf. H. howellii has been reported from other localities of the Americas and Europe as a species with deep environment preferences (Cappetta, Gregorová & Adnet, 2016; Carrillo-Briceño et al., 2016b and references there in). Paraechinorhinus cf. P. barnesi is another extinct taxon associated with deep-water paleoenvironments (Welton, 1979; Pfeil, 1983). In addition, the presence of “Goblin shark” Mitsukurina cf. M. lineata in the Montañita-Olón assemblage could support our bathymetric estimation. The extant Mitsukurina bottom-dwelling species is usually living in deep waters, on the outer continental shelves and upper slopes (Compagno, 1984a; Cappetta, Gregorová & Adnet, 2016); although it should not be ruled out that the species could also have occasional visits to the neritic areas (Ebert & Stehmann, 2013). As an example, an isolated tooth assigned to Mitsukurina was referred from an infralittoral environment during the early Miocene of Costa Rica (Laurito et al., 2014).

Some of the benthopelagic and pelagic galeomorph taxa of the Montañita-Olón assemblage (Table S2) have been associated with both coastal and open-water environments in the fossil record, as well as for their living representatives (Laurito, 1999; Aguilera, 2010; Cappetta, 2012; Compagno, 1984b, Purdy et al., 2001; Compagno, Dando & Fowler, 2005; Ebert & Stehmann, 2013; Carrillo-Briceño et al., 2013, 2015b, 2016a, 2018, 2019; Perez et al., 2017; Landini et al., 2019, and references there in). Taxa such as Isurus cf. I. oxyrinchus, Carcharias sp., Odontaspis sp., Otodus (Carcharocles) cf. O. angustidens, Parotodus benedenii, M. paradoxodon, Alopias cf. A. exigua, G. aduncus, P. contortus and H. serra have been reported with a wide global distribution during the Cenozoic (Gottfried & Fordyce, 2001; Cappetta, 2012; Reinecke et al., 2014; Carrillo-Briceño, Aguilera & Rodríguez, 2014; Carrillo-Briceño et al., 2015a), suggesting significant distances over oceanic basins and wide environmental ranges. The extinct Carcharhinus gibbesii, with 146 isolated teeth in the total sample, is the most abundant taxon in the elasmobranch assemblage from the Montañita-Olón site. Like the above-mentioned sharks, C. gibbesii was an oceanodromous species with a wide distribution in North America, Europe and Tropical America during the Oligocene–early Miocene (Cicimurri & Knight, 2009; Reinecke et al., 2014; Carrillo-Briceño et al., 2016b, 2019). In the early Miocene of Colombia, C. gibbesii was reported in both shallow and deep-water environments (Carrillo-Briceño et al., 2016b, 2019).

Other vertebrates reported for the Montañita-Olón outcrops include the dolphin Urkudelphis chawpipacha Tanaka et al., 2017, and a Pan-Cheloniidae sea turtle (Cadena, Abella & Gregori, 2018). However, no environmental conditions were inferred on the basis of these taxa. Bony fishes, including the tail of a Scombridae (Fig. 8U), and dental battery and isolated tooth of Labridae (Figs. 8V–8X), were collected in the Montañita-Olón outcrops (Fig. 2C). The caudal peduncle of Scombridae in the Montañita-Olón is assigned to †Eocoelopoma Woodward, 1901 by having preural vertebrae 2–4 abruptly shortened and a hypural plate formed by hypurals 1–4 (-5) (Monsch & Bannikov, 2011). Eocoelopoma is a primitive scombrid closely related to the Sardini + Thunnini clade and cannot be confused with Thunnini because the reduction of the preural vertebrae in this tribe is extremely pronounced (Monsch & Bannikov, 2011). The genus Eocoelopoma is known from the Paleocene of Turkmenistan, early Eocene of England, and from Equatorial Africa without specific age. This report for the Montañita-Olón represents the first record of the genus for Tropical America (Bannikov, 1985; Monsch, 2004; Monsch & Bannikov, 2011). The presence of Eocoelopoma sp. in the Montañita-Olón locality suggests open to oceanic marine environments, due to the paleoenvironmental inferences suggested in other localities where this taxon was reported (Friedman et al., 2016). The caudal skeleton of Eocoelopoma presents an typical interlocking of vertebrae, which are well adapted to a pelagic mode of life (Fierstine & Walters, 1968). A second specimen from the Montañita-Olón locality represented by upper pharyngeal bones with phyllodont condition, where the teeth are developed in distinct stacks, is diagnostic of Hypsigenyini (Bellwood et al., 2019), and the multiple oblique tooth rows (Figs. 8V and 8W) are diagnostic characters for †Trigonodon (Bellwood et al., 2019). Hypsigenyini are restricted to the Miocene-Pliocene of Europe and Africa (Schultz & Bellwood, 2004; Bellwood et al., 2009), and Trigonodon in only known from the Miocene of Europe by the type species †Trigonodon oweni Sismonda in Michelotti (1847). Although Trigonodon has been referred from the early Miocene of Costa Rica (Laurito et al., 2014), a future detailed taxonomic revision for our specimens from Ecuador and those from Costa Rica could help give new insights on the taxonomy of these fossil Hypsigenyini fishes from Tropical America. According to Schultz & Bellwood (2004), Trigonodon was a shallow-water fish possibly living primarily on coral reefs and associated hard-grounds. The presence of shallow-water components in the Montañita-Olón site could be explained by elements being washed into deeper water by turbidity currents or slumping (Vialle, Adnet & Cappetta, 2011).