Date Published: January 20, 2017
Publisher: Public Library of Science
Author(s): Bruce J. MacFadden, Douglas S. Jones, Nathan A. Jud, Jorge W. Moreno-Bernal, Gary S. Morgan, Roger W. Portell, Victor J. Perez, Sean M. Moran, Aaron R. Wood, William Oki Wong.
The late Miocene was an important time to understand the geological, climatic, and biotic evolution of the ancient New World tropics and the context for the Great American Biotic Interchange (GABI). Despite this importance, upper Miocene deposits containing diverse faunas and floras and their associated geological context are rare in Central America. We present an integrated study of the geological and paleontological context and age of a new locality from Lago Alajuela in northern Panama (Caribbean side) containing late Miocene marine and terrestrial fossils (plants, invertebrates, and vertebrates) from the Alajuela Formation. These taxa indicate predominantly estuarine and shallow marine paleoenvironments, along with terrestrial influences based on the occurrence of land mammals. Sr-isotope ratio analyses of in situ scallop shells indicate an age for the Alajuela Formation of 9.77 ± 0.22 Ma, which also equates to a latest Clarendonian (Cl3) North American Land Mammal Age. Along with the roughly contemporaneous late Miocene Gatun and Lago Bayano faunas in Panama, we now have the opportunity to reconstruct the dynamics of the Central America seaway that existed before final closure coincident with formation of the Isthmus of Panama.
The late Miocene was an important time to understand the biodiversity dynamics in the Neotropics. No place within this region was more important than Panama because it functioned as both a gateway and barrier to, respectively, interoceanic and intercontinental dispersal of Neotropical biotas. Despite this importance, until recently our knowledge of late Miocene biotas has been restricted to marine faunas such as the hyperdiverse, Gatun Formation, which preserves an excellent record of both invertebrates  and chondrichthyans . In contrast to the marine record, up to now no terrestrial macrofossils have been reported in Panama from the time interval between the early Miocene of the Panama Canal localities  and the late Pleistocene of the Azuero Peninsula . The apparent lack of terrestrial sedimentation, as evidenced by the paucity of outcrops during the late Miocene portion of this interval, likely resulted from the rapid uplift and consequent erosion during the formation of the isthmus . This hiatus of almost 19 million years in the non-marine fossil record of Panama is nevertheless critical to understanding the terrestrial faunal dynamics before and after the final closure of the Central American Seaway and rise of the Isthmus during the Pliocene.
This study integrates three domains of investigation: i.e., lithostratigraphy, Sr-isotope dating, and paleontology. The lithostratigraphic work was done in the field by measuring and describing stratigraphic sections and locating these via GPS. With regard to Sr-ratio dating, fossils were collected from our composite measured section as documented below. Powdered low-magnesium calcite samples were drilled from the interior of each specimen using a hand-held Dremel tool with a carbide dental burr. Approximately 0.01 to 0.03 g of powder was recovered from each fossil sample and these were analyzed according to standard techniques . The powdered samples were dissolved in 100μl of 3.5 N HNO3 and then loaded onto cation exchange columns packed with strontium-selective crown ether resin (Eichrom Technologies, Inc.) to separate Sr from other ions. Sr isotope analyses were performed on a Micromass Sector 54 Thermal Ionization Mass Spectrometer equipped with seven Faraday collectors and one Daly detector in the Department of Geological Sciences at the University of Florida. Sr was loaded onto oxidized tungsten single filaments and run in triple collector dynamic mode. Data were acquired at a beam intensity of about 1.5 V for 88Sr, with corrections for instrumental discrimination made assuming 86Sr/88Sr = 0.1194. Errors in measured 87Sr/86Sr are better than ±0.00002 (2σ), based on long-term reproducibility of NIST 987 (87Sr/86Sr– 0.71024). Age estimates were determined using the Miocene portion of Look-Up Table Version 4:08/03 associated with the Sr isotopic age model .
The Alajuela Formation includes a >25 m-thick basal package of interbedded, clast-supported conglomerates and litharenite sandstones that grades into an ~85 m-thick package of calcareous sandstones and calcarenites, representing a transition from tide-dominated, potentially estuarine, coastal environments to wave-dominated, shallow-water carbonate environments [10,18]. The 82 m-thick composite section (Fig 3), measured in proximity to the fossil localities on the southern extent of Lago Alajuela (Table 1), is subdivided into three distinctive lithological intervals to summarize major facies transitions during this transgression.
So far as lithostratigraphic studies in the field demonstrate, no suitable volcanic units crop out within our measured sections of the Alajuela Formation. The occurrence of original shell material from marine fossils therefore provided an opportunity to determine the age of the unit using 87Sr/86Sr geochronology. Mollusk shells (scallops and oysters) were collected from a ~2 m thick interval about 10 to 12 m above the base of the composite section and below a prominent fossiliferous conglomerate (Fig 3). As will become important below in the discussion with respect to the age of diagnostic fossils, this zone also approximates the levels where the terrestrial vertebrates were collected.
Silicified woods occur as rounded to subangular clasts with low sphericity that often occur as float. These clasts tend to be concentrated in the lower part of the section and occasionally found as part of the coarse-fraction in Interval 1 (Fig 3). Based on 60 samples that we collected, the mean wood clast length is 7.3 cm (s = 2.4 cm) with an observed range of 3 cm to 29 cm. (Rounded chert clasts up to about 4 cm long also occur within the formation.)
Two types of preservation commonly occur in the Alajuela Formation invertebrate fossils from Lago Alajuela: 1) body fossils whose hard parts originally consisted of calcite (e.g., bryozoans, scallops, oysters, decapods, and echinoderms) and 2) more commonly, internal and external molds of organisms whose hard parts originally consisted of aragonite (corals and the majority of mollusks). Representative photographs of mollusks, arthropods, and echinoderms are presented in Figs 7 and 8.
The discovery of a new fossil assemblage of plant, invertebrate, and vertebrate fossils from the Alajuela Formation at Lago Alajuela has the potential to advance our understanding of the evolution of tropical marginal marine and terrestrial ecosystems in Central America, as well as further address the current debate about the formation of the isthmus. The calibration of this assemblage using Sr-isotope ratio dating to 9.77 Ma ± 0.22 Ma provides precise temporal control otherwise unknown for the late Miocene in Central America. This indicates contemporaneity with the Gatun and Chucunaque formations in Panama as well as a late Clarendonian (Cl3) North American Land Mammal age as this biochron is otherwise known in higher latitude North America. We understand that we have not yet sampled the full range of ancient diversity at Lago Alajuela; we hope that further discoveries will be forthcoming with additional field work. Likewise, the taxonomic assignments that we present here will likely become more refined as individual taxonomic groups are compared in more detail in the future.