Perez et al.—Miocene sharks and rays from Lago Bayano, Panama
chondrichthyan faunas that have been described from the Pacific shelf of the evolving Central American isthmus are from the middle Miocene Punta Judas Formation (Laurito, 2004) and the late Miocene Curré Formation (Laurito and Valerio, 2008) from Costa Rica. The Curré Formation was tentatively assigned as being late Miocene because of the occurrence of chondrichthyan taxa typical of the Miocene and terrestrial mammal remains (Laurito andValerio, 2005, 2008;Valerio and Laurito, 2008). The co-occurrence of terrestrial taxa as well as marine fauna in the Curré Formation is very similar to that of the late Miocene Alajuela Formation (MacFadden et al., 2015, 2017), providing evidence that at least some emergent land was present within the Central American isthmus. Coates and Obando (1996) recon- structed the CAS during the late Miocene as an archipelagic strait on the basis of tectonic evidence and nannofossil assemblages. Alternatively, it has been proposed that a continuous peninsula connected Panama to North America as early as 19 million years ago, based on comparable body sizes between contemporaneous land mammals found in North America and Panama (Kirby and MacFadden, 2005; Kirby et al., 2008). However, neither reconstruction excludes the notion of a marine connection between the Pacific Ocean to the Caribbean Sea throughout the Miocene. After uplift of the Isthmus of Panama and complete closure
of the CAS many taxa had populations in both the Pacific and Atlantic, with a few species becoming restricted to either side. With the information available we cannot yet ascertain if the reason these taxa became restricted is due to the closure of the CAS, but it is highly probable that the uplift of the Isthmus of Panama was a contributing factor. Given that chondrichthyan taxa are highly mobile, and that many are capable of trans- oceanic migrations, it is possible that they could continue to exchange between the Pacific and Atlantic via a meridional route after complete closure of the CAS. However, temperature restrictions would make this unlikely. A more reasonable explanation for the co-occurrence of Atlantic-restricted and Pacific-restricted taxa between 9 and 10 million years ago is that there was in fact a marine connection between the Caribbean Sea and the Pacific Ocean.
Conclusion
This overview of the chondrichthyan fauna from the late Miocene Chucunaque Formation reveals the most prolific and diverse assemblage from Panama, with at least 31 taxa, eight of which are new to the fossil record of Panama. Furthermore, the Lago Bayano fauna is the first described Miocene chondrichthyan assemblage from the Pacific shelf of the Panamanian isthmus, and 87Sr/86Sr isotopic ratios reported provide the first geochronology for the region. At least 15 of the taxa identified have affinities with extant species, which offers the opportunity to utilize the habitat preferences of the corresponding living species to make inferences regarding functional diversity, paleoenvironment, and paleobio- geography. The Bayano ecosystems are dominated by generalist sharks (with cutting-grasping type dentitions) and filter-feeding
rays.Aweighted paleobathymetric analysis provides evidence for a paleoenvironment in the neritic zone, with amean average estimate of 110m. Finally, the geographic ranges of the fauna indicate mixed geographic affinity, which offers insight into a marine connection
543
between the Caribbean Sea and the Pacific Ocean during a time of active shoaling in Central America.
Acknowledgments
This work was funded
byU.S.National Science Foundation (NSF) grant 0966884 (OISE, EAR, DRL), 1358919 (EAR, OISE), and the Smithsonian Tropical Research Institution (STRI). We appreciate additional funding, in-kind support, and field assistance by C. Jaramillo, C. Montes,D.Ramirez,
C.Suarez, L. Londoño, and V. Zapata (STRI), and R. Portell (FLMNH). We also acknowledge the generosity and guidance of the local Embrerá at Puente Bayano, in particular N. Ortega and S. Quiroz. In addition, R. Perez is acknowledged for donating the Toyota vehicles used for fieldwork, and the Dirección de Recursos Minerales are thanked for providing collecting permits. We thank R. Leder and
S.Moran for help with macro photographs.We also appreciate the assistance from A. Heatherington with SEM images, D. Jones, G. Kamenov, and A. Waite for their help with the Sr-isotope analyses, the insightful comments of J. Carrillo-Briceño, as well as guidance on statistical methods (especially developing the R code for the bathymetric analyses) provided by M. Kowaleski. AForschungskredit postdoctoral fellowship from the University of Zürich (FK-15-105) supported C.P. This is a contribution of the NSF Partnerships for International Research and Education Panama Canal Project and the University of Florida Contribution to Paleobiology number 762.
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