BackgroundAs we know from modern species, nursery areas are essential shark habitats for vulnerable young. Nurseries are typically highly productive, shallow-water habitats that are characterized by the presence of juveniles and neonates. It has been suggested that in these areas, sharks can find ample food resources and protection from predators. Based on the fossil record, we know that the extinct Carcharocles megalodon was the biggest shark that ever lived. Previous proposed paleo-nursery areas for this species were based on the anecdotal presence of juvenile fossil teeth accompanied by fossil marine mammals. We now present the first definitive evidence of ancient nurseries for C. megalodon from the late Miocene of Panama, about 10 million years ago.Methodology/Principal FindingsWe collected and measured fossil shark teeth of C. megalodon, within the highly productive, shallow marine Gatun Formation from the Miocene of Panama. Surprisingly, and in contrast to other fossil accumulations, the majority of the teeth from Gatun are very small. Here we compare the tooth sizes from the Gatun with specimens from different, but analogous localities. In addition we calculate the total length of the individuals found in Gatun. These comparisons and estimates suggest that the small size of Gatun's C. megalodon is neither related to a small population of this species nor the tooth position within the jaw. Thus, the individuals from Gatun were mostly juveniles and neonates, with estimated body lengths between 2 and 10.5 meters.Conclusions/SignificanceWe propose that the Miocene Gatun Formation represents the first documented paleo-nursery area for C. megalodon from the Neotropics, and one of the few recorded in the fossil record for an extinct selachian. We therefore show that sharks have used nursery areas at least for 10 millions of years as an adaptive strategy during their life histories.
Abstract.-This newly described chondrichthyan fauna from the late Miocene Chucunaque Formation of Lago Bayano reveals a prolific and highly diverse assemblage from Panama, and one of the richest shark faunas from the Neotropics. Strontium geochronology indicates an age of 10-9.5 Ma for the chonrichthyan-bearing strata. Field efforts resulted in 1429 identifiable specimens comprising at least 31 taxa, of which at least eight are new to the documented fossil record of Panama. With this information an analysis of functional diversity was conducted, indicating ecosystems dominated by generalist species feeding upon a wide range of organisms, from plankton to marine mammals. A probabilistic approach of paleobathymetric estimation suggests a neritic environment. Previous studies based on foraminifera have suggested that the Chucunaque Formation had a greater Pacific Ocean affinity, making this the first Miocene chondrichthyan fauna described from the Pacific shelf of Panama. However, our geographic comparisons show that this fauna has mixed Caribbean and Pacific biogeographic affinities, which likely supports the previously purported connection between chondrichthyan faunas during the late Miocene.
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