Perez et al.—Miocene sharks and rays from Lago Bayano, Panama


four chondrichthyan-bearing Miocene deposits in Panama, only one tooth identified as Carcharias hasbeenreportedfromthe Chagres Formation (Carillo-Briceño et al., 2015a). Furthermore, there were 12 ecomorphotypes represented by


the chondricthyan fauna of Lago Bayano, most of which represent littoral taxa (Table 3; Fig. 13.2). The littoral ecomorphotype refers to relatively non-specialized taxa that inhabit continental


shelves and feed on moderately sized prey comprised of bony fishes, crustaceans, and cephalopods, among other organisms (Compagno, 1990). Within the littoral habitus there are more specialized ecomorphotypes, including the cancritrophic (demersal sharks primarily feeding on bottom dwelling crustaceans), teuthitrophic (diet primarily consisting of cephalopods), sphyrnid (littoral to semilittoral with a specialized cephalofoil bowplane), eurytrophic (omnivores trending toward apex predator), and archipelagic (apex predators), that were also observed (Table 3; Fig. 13.2). Compagno (1990) grouped the eagle rays and cownose rays (Myliobatidae) with the devilrays (Mobulidae) into the aquilopelagic habitus because of their pectoral-driven locomotion, despite their very different feeding adaptations (Adnet et al., 2012). The aquilopelagic habitus represents ~15% of the chondrichthyan fauna from the Chucunaque Formation. The oceanic habitus (~1.5%) is represented by three ecomorphotypes: macroceanic, tachypelagic, and microceanic. The remaining taxa are batoids placed in the rajobenthic (~1.5%) and rhynchobathic (<0.1%) ecomorphotypes. Based on our functional diversity analyses, the ecosystems of Lago Bayano were dominated by generalist taxa that lived in nutrient-rich waters with abundant zooplankton.


Paleobathymetry.—The weighted paleobathymetry method was applied to the chondrichthyan fauna of the Chucunaque Formation, as well as the Gatun Formation and the Piña Sandstone facies of the Chagres Formation. For the Chucunaque Formation, 15 species of the 31 total taxa were included, which were represented by 525 specimens (37% of the total number of material collected). For the Gatun Formation, 17 of the 26 taxa identified by Pimiento et al. (2013a) were included, which represented ~32% (N = 257) of the specimens identified. For the Piña Sandstone facies of the Chagres Formation, 12 of the 28 taxa identified by Carrillo-Briceño et al. (2015a) were


541


included, which represented ~14% (N = 70) of the specimens identified. This paleobathymetry method was not tested on the Rio Indio facies of the Chagres Formation because the chondrichthyan fauna therein is comprised of only four taxa identified to the species level that have modern analogs (Carrillo-Briceño et al., 2015a). Our results (Table 4; Fig. 14) show that the Chucunaque


Formation was most likely deposited in a neritic environment (mean depth of 110 m). Previous studies (see Coates et al., 2004) proposed that this formation was deposited in the inner-neritic to upper-bathyal zone at different localities, based on benthic foraminiferal asssemblages. Variability in paleobathymetry throughout the Neogene succession and within individual sections was thought to correspond to spatial and temporal differences in sediment deposition and basin tectonics. Never- theless, the river outcrops surveyed by Coates et al. (2004) were deposited in adjacent sedimentary basins (Chucunaque-Tuira and Sambu), whereas Lago Bayano resides on the western margin of the Bayano Basin, which likely experienced reduced subsidence and less accommodation space. Therefore there is little issue with the chondrichthyan fauna indicating a neritic environment for the Chucunaque Formation of Lago Bayano. Similar to the Chucunaque Formation, the Gatun was


deposited in a neritic environment (mean depth of 55 m), whereas the Piña Sandstone facies was deposited within the upper bathyal zone (mean depth of 370 m). The Chucunaque Formation and the Gatun Formation are both dominated by the order Carcharhiniformes (~78% and ~83%, respectively), which is the most diverse and abundant group alive today in nearshore environments (Compagno, 1990; Kent, 1994). In contrast, the deeper water Piña Sandstone facies of the Chagres Formation is dominated by the order Squaliformes (~70%). The most abundant taxon identified from the Chagres Formation, Isistius sp. (N = 272), was not included in the paleobathymetric analysis because it was only identified to the generic level, and hence the depth range can only be inferred from the different living species within this genus or via other fossil occurrences. Carrillo-Briceño et al. (2015a) reported that Isistius (~70% of the Piña Sandstone facies) occurs at depths of 0–3700 m, usually between 0 and 1000 m, based on the extant species


Figure 14. Paleobathymetric estimates for the Chucunaque Formation, Gatun Formation, and the Piña Sandstone facies of the Chagres Formation, showing the mean average depth. Depth ranges used in this analysis can be found in the Systematic Paleontology section as well as in Table 3.


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