The occurrence and diversity of elasmobranchs from the Oligocene–Miocene boundary from Tropical America is poorly known in comparison with the paleodiversity from younger Neogene intervals of the region. Here we describe a new elasmobranch assemblage from the rich fossil site of Montañita-Olón (Dos Bocas Formation, Santa Elena, Ecuador), where other vertebrates have already been described: for example, sea turtles and cetaceans. We report a total of 27 elasmobranch taxa, 19 of which are new fossil records for Ecuador, 10 new records for the Central Eastern Pacific and four new records for South America. Additionally, in order to reconstruct the environment where these marine remains were deposited, we performed abundance, paleobathymetric and habitat preference analyses, concluding that they were likely deposited in an outer neritic (open shelf) environment. The study of Oligocene and early Miocene marine elasmobranchs faunas in Tropical America is key to addressing the issues in the evolutionary history of this group.
The Oligocene–Miocene transition (OMT) was an important period for the evolutionary history of the marine biota in the northern margins of South America, especially due to the significant changes that affected marine biota in the Pacific and proto-Caribbean region (
The outcrops of the Dos Bocas Formation studied herein correspond to a cliff of approximately 800 m in length along the coastline between the towns of Montañita and Olón (
(A and B) Outcrops in the Montañita area. (C) Section showing stratigraphic provenance of fossil specimens. U-Pb zircon dating corresponds with the sample CP705 (age of 23.5 ± 0.4 Ma, with a younger cluster average of 22.9 ± 0.6 Ma) referred by
The fossil elasmobranch fauna from the Montañita-Olón site consists of 424 cranial (teeth and rostral spines) and postcranial (vertebrae and caudal spines) elements (
(A–H)
(A and B) cf.
(A and B)
(A–C) Lamnidae indet. (Lower anterior tooth (MPM-1377)). (D–H)
(A–F)
(A–K)
Superorder | Order | Family | Genus | Taxon |
---|---|---|---|---|
Squalomorphii | Hexanchiformes | Heptranchidae | ||
Hexanchidae | ||||
Squaliformes | Centrophoridae | |||
Dalatiidae | ||||
Echinorhiniformes | Echinorhinidae | cf. | ||
† | † | |||
Pristiophoriformes | Pristiophoridae | |||
Galeomorphii | Orectolobiformes | Rhincodontidae | ||
Lamniformes | Lamnidae | |||
Indet. | Indet. | |||
Mitsukurinidae | ||||
Odontaspididae | ||||
†Otodontidae | † | † | ||
† | † | |||
† | † | |||
Alopiidae | ||||
† | ||||
Carcharhiniformes | Carcharhinidae | † | ||
† | ||||
† | † | |||
Hemigaleidae | † | |||
Sphyrnidae | ||||
Indet. | Indet. | Indet. | ||
Batomorphii | Myliobatiformes | Mobulidae | † | |
Indet. | Indet. | Indet. |
First fossil record in Ecuador.
An analysis of abundance for the Montañita-Olón site assemblage was carried out using percentages of specimens by family, genera and species. In addition, we performed a paleobathymetric analysis following the methodology of
The elasmobranch assemblage described herein from the Montañita-Olón site comprises 27 taxa (including three of indeterminate taxonomy) of squalomorphs, galeomorphs and batoids (
Squalomorphs, with seven species, are the second most diverse and abundant group of sharks in the Montañita-Olón assemblage (
Galeomorphs (isolated teeth as well as vertebral remains) represent the most diverse and abundant group in the elasmobranch assemblage from the Montañita-Olón site (
The batoids, with only one genus and two species, are the least diverse group from the Montañita-Olón assemblage (
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 (
Both results were comparable. For the Nolf technique, 73.7% of the studied taxa are represented in a depth range of 100–200 m (
(A) Blue lines indicate the common paleobathymetric range for each taxa, arrows indicate that the bathymetric range is greater, a dashed red line indicates that there is not information for the taxa and that was removed from the analysis. The gray shadow indicates the 95% confidence intervals for the more probable depth range for this assemblage. (B) For the Nolf technique, 73.7% of the studied taxa are represented in a depth range of 100–200 m. (C) The weighted paleobathymetric estimation indicate a mean of 192 m after resampling with 10,000 simulations. The
Elasmobranchs from the Oligocene and early Miocene have not been reported from Ecuador, and their fossil record has been restricted to younger strata (
As mentioned above in the results section, mobulid teeth identified as
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 (
Like
A late Oligocene–early Miocene age has been suggested for the Dos Bocas Formation on the basis of radiometric dating (
A shallow protected environment has been suggested as the most plausible depositional environment for the Dos Bocas Formation (see
Some of the benthopelagic and pelagic galeomorph taxa of the Montañita-Olón assemblage (
Other vertebrates reported for the Montañita-Olón outcrops include the dolphin
We report 27 elasmobranch taxa, of which 19 are new fossil records for Ecuador, 10 new records for the Central Eastern Pacific and four new records for South America. This elasmobranch fossil assemblage represents the most diverse known from Ecuador and for the Oligocene–Miocene boundary of Tropical America. The elasmobranch habitat preferences and paleobathymetric analyses support the hypothesis that the Montañita-Olón site was likely deposited in an outer neritic (open shelf) environment. Future work and new findings could help improve and refine our interpretations. The assemblage from the Montañita-Olón site increases the fossil record of the region and represents a critical window into marine tropical vertebrate faunas in the ECP during the OMT, a critical moment in the evolutionary history of the marine biota of Tropical America.
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Abbreviations: indeterminate (Indet.).
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Abbreviations: minimun (Mn), maximun (Mx), meters (m) and indeterminate (Indet.). Lifestyle: Benthic (B), Benthopelagic (Bp); Pelagic (P). Preferred Habitat Neritic (N), Oceanic (O), Sublitoral (S), Bathyal (Bt), Abysal (Ab), Hadal (Ha), Epipelagic (E), Mesopelagic (M), Bathypelagic (Bp).
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A. MPM-1389 and B. MPM-1390) with isolated rostral teeth of
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We thank students and University (UPSE) staff involved in the fieldwork, especially Xavier Guncay. Thanks to the curators of the paleontological collections referred to in the methods section for their valuable assistance and permission to review fossil collections. Thanks to Marcelo Sánchez-Villagra and the Evolutionary Morphology and Palaeobiology group at the Palaeontological Institute and Museum at the University of Zurich for their support. Special thanks to the Center for Microscopy and Image Analysis of the University of Zurich for their assistance and support performing the scanning electron microscopy analysis. To Ana Balcarcel for the revision of the preliminary version of the manuscript. We greatly appreciate comments and suggestions from Editor Andrew A. Farke and the two reviewers David Cicimurri and Victor Perez.
The authors declare that they have no competing interests.
The following information was supplied relating to field study approvals (i.e., approving body and any reference numbers):
Ecuadorian Instituto Nacional de Patrimonio Cultural (INPC) approved this field study (Excavation permit, Code: No 0039-DR5.INPC.2015).
The following information was supplied regarding data availability:
Raw data and script are available in
All the fossil specimens described here are available at the “Museo Paleontológico Megaterio” (MPM-) at the Universidad Estatal Peninsula de Santa Elena, Ecuador. Every specimen or group of specimens is assigned with a catalog number using the acronyms “MPM-”. Data on fossil specimens with their respective catalog numbers are available in