542
Journal of Paleontology 91(3):512–547
Isistius brasiliensis; however, they also noted that other living species of Isistius prefer shallow water and even make daily vertical migrations through the water column. If the preferred depth of I. brasiliensis were used as a proxy, its inclusion would have increased the mean average to ~470 m, but the estimate would still remain in the bathyal zone. The estimate for the Piña Sandstone facies is deeper than what was determined by Carrillo-Briceño et al. (2015a) based on chondrichthyan evidence alone. However, Collins et al. (1996) reported that the Chagres Formation was deposited in 200–500mdepth based on foraminiferal assemblages. Therefore, the depth inferred herein for the Piña Sandstone facies of the Chagres Formation would still fall within the range predicted by Collins et al. (1996) and is permissible given the presence of other deep-water taxa such as whales and billfishes (Uhen et al., 2010; Carrillo- Briceño et al., 2015a; Velez-Juarbe et al., 2015). Despite the fact that both the Chucunaque Formation and the
Gatun Formation are interpreted as having been deposited in the neritic zone, the paleobathymetric prediction for theChuncunaque Formation is nearly twice as deep as the Gatun Formation. This can be explained by the presence of pelagic taxa such as Isurus and Alopias in the Chucunaque Formation, which are absent in the Gatun Formation (Pimiento et al., 2013a). Furthermore, there are at least six taxa from the Chucunaque Formation (Isurus oxyrinchus, Alopias superciliosus, Alopias cf. A. vulpinus, Carcharhinus falciformis, Carcharhinus obscurus,and Sphyrna lewini), comprising 15% of the chondrichthyan fauna, that have depth ranges beyond the neritic zone (Table 3). The presence of these taxa may indicate the influence of deeper oceanic waters in the Bayano Basin (e.g., via upwelling systems) or may be explained by the migratory habits of these sharks. Many oceanic species migrate to shallow, coastal waters to give birth (Compagno, 1984, 1998; Bass et al., 1986;Mundy, 2005), which aids in reducing competition between juveniles and adults, and may help to prevent cannibalism (Compagno, 1990). Likewise, Carrillo-Briceño et al. (2014) recognized a chondrichthyan fauna, fromthemiddle Miocene to early Pliocene of Ecuador, indicating a shallow- to deep-water assemblage and suggested that it may reflect a short platform environment bordered by deep water. The late Miocene Curré Formation (Laurito and Valerio, 2008) from Costa Rica bears a sparse chondrichthyan assemblage (N = 89), relative to the Chucunaque Formation; however the two faunas are similar given that both are dominated by the order Carcharhiniformes, but also contain the genus Isurus. Laurito and Valerio (2008) inferred a neritic depth based on the chondrichthyan fauna and suggested that the depth is likely less than 35m based on the occurrence of Pholididae traces. The presence of deep-water taxa along the Pacific Shelf of Costa Rica, Panama, and Ecuador during the late Miocene supports the idea of a proximal open ocean environment influencing the neritic Chucunaque Formation. Our estimates presented here are a reflection of the
taxonomic composition of the three localities studied and are in agreement with previous studies. However, there are obvious limitations to this method; namely, ignoring a large number of specimens either due to an inability to identify to the species level or because the species is extinct and lacks a modern analog to constrain its depth prefence. Further, there is undoubtedly some degree of preservational and/or collection bias, either due
to uneven sampling efforts between screenwashed and surface collected material, or as an unavoidable result of thanatocoe- nosis (i.e., accumulation and deposition after death), as was suggested by Cappetta and Nolf (1991) in their paleobathy- metric analysis. Even so, using the subsample of taxa that do have modern analogs and taking into account their relative abundance offers a quantitative, probabilistic approach that in theory will provide a more accurate paleodepth estimate. Likewise, Carrillo-Briceño et al. (2015c) employed a similar probabilistic approach using maximum likelihood estimation to predict the paleobathymetry of the Miocene Uitpa Formation in Colombia based on chondrichthyan occurrences.
Paleobiogeography.—The chondrichthyan fauna fromtheGatun Formation has mixed biogeographic affinities, with evidence of taxa currently restricted to the Atlantic including Carcharhinus perezi, Rhynchobatus luebberti, Mobula hypostoma,and Taeniurops aff. T. grabata and Pacific-restricted taxa including Myliobatis cf. M. californica, Rhinoptera cf. R. steindachneri,and Mobula munkiana (Pimiento et al., 2013a). Of the 15 extant taxa observed from the Chucunaque Formation, all occur in both the Atlantic and the Pacific oceans today, with the possible exception of C. aff. macloti, given that extant C. macloti is restricted to the Pacific. Also, it is possible that the Type II Mobula teeth observed in the Chucunaque Formation actually belong to Mobula hypostoma, which would mean that there is an Atlantic-restricted species on the Pacific side of the evolving Panamanian isthmus. If even one of these species were accurately identified as representing a species that is currently restricted to the opposing side of the isthmus, it would support the hypothesis that there was a continued marine connection between 9Mya and 10Mya. The Chucunaque Formation of Lago Bayano is roughly
contemporaneous with the upper part of the Gatun Formation (Hendy, 2013; Pimiento et al., 2013a). The Gatun Formation of the Panama Canal Basin is considered to have had a Caribbean affinity based on benthic foraminiferal assemblages (Collins et al., 1996), whereas the Chucunaque Formation of the Bayano Basin has been interpreted as having a Pacificaffinity (Coates et al., 2004). The two distinct foram assemblages have been utilized to interpret restriction of deep-water circulation by ~8 million years ago; however, this deep-water connection is thought to have returned ~6 million years ago based on Pacific foraminifera in the bathyal Chagres Formation (Collins et al., 1996). Despite the fact that there was, likely, ongoing shoaling of the Isthmus of Panama and restriction of deep-water circulation (Montes et al., 2012a, 2012b; Sepulchre et al., 2014), there must have still remained a marine connection between the Caribbean Sea and Pacific Ocean ~10 million years ago to allow for exchange of marine vertebrates (Pimiento et al., 2013a). Sepulchre et al. (2014) incorporated neodymium isotopic data into a model in order to resolve the history of water masses connecting the Pacific to the Caribbean, and found that a seaway 50–200m deep would likely have been present ~10 million years ago, which is consistent with the depth estimates for the Gatun and Chucunaque Formations, derived from the weighted paleobathymetric analysis (Table 4; Fig. 14). The Lago Bayano fauna is therefore highly significant
because it represents the first described Miocene chondrichthyan fauna from the Pacific shelf of Panama. The only other Miocene
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