540
Journal of Paleontology 91(3):512–547
(Fig. 2) indicate that it is unlikely that more collecting (either surface prospecting or screewhasing) will result in many additional taxa. Hence, the sample utilized in this study comprises an accurate representation of the chondrichthyan biodiversity present in the Chucunaque Formation. There are three additional marine Miocene formations from Panama that have documented chondrichthyan remains: the early Miocene Culebra Formation with 12 taxa represented by 45 dental remains and 10 vertebral centra (Pimiento et al., 2013b); the middle–late Miocene Gatun Formation with 26 taxa represented by 800 dental remains (Pimiento et al., 2013a); and the late Miocene Chagres Formation with 30 taxa represented by 513 dental remains (Carrillo-Briceño et al., 2015a). The 1422 teeth collected and 31 taxa described herein from Lago Bayano comprise the most prolific and diverse chondrichthyan fauna known from Panama. Despite significant collection effort, all of these samples are subject to some degree of collection bias, and likely taphonomic bias. As such, proportions are reported to aid in the juxtaposition of these faunas, but should still be interpreted with caution. Carcharhinus is by far the most abundant genus present in
the Chucunaque Formation, representing over 50% of specimens identified (Fig.12).Species of thegenus Carcharhinus are the most abundant and diverse sharks in modern nearshore environ- ments, with over 30 different species attributed to this genus (Compagno, 1984; Kent, 1994; Naylor and Marcus, 1994). Carcharhinus is also the most abundant genus represented in the Gatun Formation (~40%; Pimiento et al., 2013) and tied with Hemipristis as themost abundant genus in the Rio IndioMember of the Chagres Formation (~32%; Carrillo-Briceño et al., 2015a). In contrast, the deeperwater Piña Sandstone facies of the Chagres Formation is dominated by specieswithin the order Squaliformes, with only ~11%of the chondrichthyan fauna being represented by the genus Carcharhinus (Carrillo-Briceño et al., 2015a). The second most abundant genus in the Chucunaque
Formation is Mobula (~12%; Fig. 12). Mobulids (Mobula and Manta rays) are the largest stingrays alive today, and typically have a pelagic, coastal habit in tropical and sub-tropical waters (Notarbartolo Di Sciara, 1987; Adnet et al., 2012). Mobula has been recognized from the Gatun Formation of Panama, but is much less abundant (~1%; Pimiento et al., 2013a). The paucity of Mobula in the paleontology literature can likely be attributed to preservation and collection bias. Mobula teeth are extremely small and have a very thin enameloid, and yet they are very common in the Chucunaque Formation. Increased screenwashing efforts in other tropical and sub-tropicalNeogene localitiesmay revealmore Mobula occurrences.
Functional diversity.—Six of the nine dentition types described by Kent (1994) were observed: cutting, crushing, clutching, vestigial, cutting-grasping, and grasping-cutting (Table 3; Fig. 13.1). The generalist, cutting-grasping type dentition was the most prevalent, being represented by ~69% of the total
sample.This dentition type is the most common today, and it allows for a wide variety of prey options, but it is most commonly found among sarcophagous (i.e., those eating fleshy prey such as bony fishes and marine mammals) and ichthyophagous (those eating fish) sharks (Kent, 1994). The second most commondentition type is the vestigial form (~12%), which is essentially a specialized tooth form for filter
Figure 13. Two proxies for functional diversity. (1) Relative abundance of different dentition types as described by Kent (1994); (2) Relative abundance of different ecomorphotypes as described by Compagno (1990).
feeding, reflecting a planktivorous diet (Kent, 1994). This dentition type is represented by species within the genus Mobula. However, teeth of Mobula are not solely for filter feeding, but also possess some characteristics indicative of clutching and/or grinding. It has even been suggested that Mobula teeth may play a role during reproduction (Michael, 1993). Third is the clutching type (~7%), which is more typical of small to moderately sized sharks that primarily feed on small fishes and hard-bodied, benthic taxa (i.e., crustaceans, molluscs, and echinoids) by grabbing and gripping their prey with powerful shortened jaws (Kent, 1994). The fourth and fifth most common dentition types are the cutting type and the crushing type, both comprising 5.75% of the specimens identified. The cutting type is adapted for sarcophagous taxa (Kent, 1994). The crushing type is a specialized form for feeding on armored benthic invertebrates (i.e., durophagous), such as molluscs and crustaceans, and is most typically found in bottom-dwelling taxa (Kent, 1994). Lastly, a very small portion, 0.5%, had a grasping-cutting type dentition, which is represented by taxa in the genus Isurus. This dentition type relies more on capturing rather than dismembering the prey, whichmay be amore effective feeding strategy when operating in open water and feeding on more active prey items. Kent (1994) attributed this dentition type to ichthyophagous and teuthyopha- gous (i.e., those feeding on squid) sharks. It is alsoworth noting the absence of any taxa that have solely a grasping type dentition, such as Carcharias, which is a relatively common genus in Miocene deposits and inmodern nearshore environments. In fact, among the
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