Perez et al.—Miocene sharks and rays from Lago Bayano, Panama It hasbeenarguedthatwhile most worksonancient
chondrichthyan faunas provide valuable information about taxonomic diversity, they are somehow limited in their ecological interpretations because they ignore species functions in ecosystems (Moore, 2001; Hooper et al., 2002). To address this issue, functional diversity is often measuredasameanstoquantifythe impact of species on ecosystems (Petchey and Gaston, 2006). The correlation that exists between toothmorphology and diet can be used to apply a functional diviersity approach to the study fossil chondrichthyan faunas (e.g., Bertolini, 1933; Moss, 1977;
Cappetta, 1986, 1987, 2012; Frazzetta, 1988; Kent, 1994). Speci- fically, a classification scheme has been outlined by Kent (1994) in
which chondrichthyan dentitions are subdivided into nine types: five homodont forms (cutting, grasping, clutching, crushing, and vestigial) and four heterodont forms (cutting-grasping, grasping- cutting, grasping-crushing, and clutching-crushing). A more com- plex scheme was developed by Compagno (1990) that grouped sharks, rays, and chimaeroids into ecomorphotypes based on varying life histories, which incorporates the morphology, habitat, and behavior of each taxon. These ecomorphotypes have been utilized in numerous studies of extant sharks, especially those pertaining to conservation (Martin, 2005; Zhou and Griffiths, 2008; Lucifora et al., 2011; Grogan et al., 2012; Ritter, 2014), but have not been employed inmany paleontological studies. Here,we integrate dentition types and ecomorphotypes as a measure of functional diversity. This approach provides novel information regarding the function of chondrichthyan species in the ecosystems adjacent to the CAS during the late Miocene. This paper describes the taxonomy and systematics of the
chondrichthyan fauna from Lago Bayano and interprets the diversity, paleoenvironment, and paleobiogeographic significance of this new and prolific marine vertebrate fauna. These interpreta- tions will be based in functional diversity analyses and a unique approach toward analyzing paleobathymetry. We will show that this newdiscovery sheds a light on the dynamics of ancientmarine faunas in the New World tropics during the lateMiocene.
Geologic setting
The thick sequence of Cretaceous through Neogene sediments that occupy the central lowlands paralleling the San Blas and Darien highlands ofDarien Province in eastern Panama forms the Bayano, Chucunaque-Tuira, and Atrato basins (Stewart, 1966; Duque-Caro, 1990; Coates et al., 2004). The Chucunaque-Tuira and Atrato basins were subject to a detailed review by Coates et al. (2004) who established the lithostratigraphy used herein, provided biostratigraphic and paleobathymetric context, and interpreted their geological history. The Bayano Basin was not investigated, however, and our present understanding of the geology is built around the geological reconnaissance of Stewart (1966), extrapolation of Coates et al. (2004), and more recent mapping as a part of a broader study into the tectonic history of the Panama Isthmus (Montes et al., 2012a, 2012b). Attempts to map this region, locate contacts, and measure stratal thickness have been challenged by lack of exposure, discontinuous outcrops (often submerged or forested), and obscure folding and faults. In a general sense, the succession in the Lago Bayano area consists of Cretaceous volcanic intrusives (Darien Formation), Oligocene– early Miocene agglomerates (Porcona Formation), early–middle
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Miocene limestone (Clarita Formation), turbiditic sandstone and claystone (Membrillo Formation), overlain by late Miocene fossiliferous and conglomeratic sandstone and siltstone (Chucu- naque Formation) (Terry, 1956; Stewart, 1966; Coates et al., 2004). These units can be observed along the shoreline of southern Lago Bayano, Rio Maje, Rio Tigre, Carratera Panamericana, and minor roads (Fig. 1; Table 1). Stewart’s (1966) original description of the geology in this
region was done prior to the damming of the Rio Bayano and the formation of the artificial lake that is now Lago Bayano. In this initial description he noted the presence of chondrichthyan remains in Miocene-aged sandstones of the Chucunaque Formation in the deeply incised Rio Bayano and its tributaries. Other marine fossils (e.g., molluscs and foraminifera) were recognized in other Miocene-aged units, but chondrichthyan remains were restricted to the sandstone facies of the Chucu-
naque Formation. The construction of the Bayano Dam in 1976 flooded 350 km2 of rainforest to form Lago Bayano, making it difficult to reconstruct original sediment composition and sedi- mentary structures of the Chucunaque Formation at many of the chondrichthyan-bearing localities. The chondrichthyan remains utilized in this study are left as a residue among reworked gravel, sand, and mud grains along island shorelines. In the southeastern-most portion of the lake, exposed uneroded blocks comprise strongly weathered and sparsely fossiliferous orange mudstone. The islands in the northern portion of southern Lago Bayano contain a more varied range of facies, including fossi- liferous, gritty orange sandstone and a fine-grained tuffaceous white sandstone. In much of the Darien Province, the Chucunaque Formation
of Shelton (1952) is Messinian in age (7.1–5.6 Ma), although Coates et al. (2004) suggested that it could be older than 9.4Ma in the western part of their study area on the basis of calcareous nannofossil biostratigraphy. This age is consistent with 87Sr/86Sr dates of 10–9.5 Ma derived from calcerous Lindapectin shells that were deposited in association with the chondrichthyan remains on the shoreline of STRI 290138 (N9.1552,W78.7824) and STRI 300032 (N 9.1411, W 78.7545; Tables 1, 2). In July 2015, a research group returned to Lago Bayano and observed unusually low lake levels, which resulted in some minor exposures of the in situ fossiliferous layer along STRI 290116 and STRI 300032. Calcareous microfossils picked from this in situ layer were dated using 87Sr/86Sr isotopic ratios from STRI 300032, and found to be consistent with the ages of the Lindapectin (personal communication, A. Waite, 2016). This fauna is therefore Tortonian in age and correlative with the other important chondrichthyan-bearing units in Central and South America, including the Gatun Formation and Alajuela Formation of central Panama (Gillette, 1984; Pimiento et al., 2013a; MacFadden et al., 2017), Angostura Formation of Ecuador (Carrillo-Briceño et al., 2014), and upperUrumacro Formation of Venezuela (Carrillo-Briceño et al., 2015b).
Materials and methods
In 2010 we began to collect fossils from Miocene sediments exposed along the islands of Lago Bayano (Fig. 1). The sharks and ray teeth are most easily collected when lake levels are low and fossiliferous sedimentary zones are exposed.
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