Aldo F. Rincon

Systematics and biogeography of crocodylians from the Miocene of Panama

ABSTRACT Despite the fact that fossil crocodylians have been recovered from the Panama Canal Zone starting with initial excavations in 1912, detailed studies have been lacking. Recent excavations of the canal have resulted in new discoveries of many vertebrate fossils, including the first known Miocene crocodylian skulls from Central America. These fossil skulls from the early-middle Miocene represent two new taxa with distinct morphology that is shared with extinct and extant caimans (Caimaninae). A cladistic analysis of 32 alligatorid and three outgroup taxa, scored for 75 characters, resulted in 1210 equally most parsimonious cladograms, all of which suggest that Culebrasuchus mesoamericanus, gen. et sp. nov., is the sister taxon to all previously known Caimaninae. Additionally, the analysis suggests that Centenariosuchus gilmorei, gen. et sp. nov., is the sister taxon to a caimanine clade that includes Purussaurus from the Miocene of South America. In fact, teeth very similar to those of Purussaurus have also been recovered from the Panama Canal. Given these South American affinities, we suggest that these early caimanines dispersed across saltwater. This is a potentially surprising result, because all extant alligatorids lack the salt glands that would have been necessary for the marine dispersal required to reach Central America during the Miocene. Unlike Miocene mammals that all have North American affinities, the Miocene crocodylians of Panama represent a ‘melting pot’ with taxa of disparate origins living together at the southern extreme of Central America.

First North American fossil monkey and early Miocene tropical biotic interchange

New World monkeys (platyrrhines) are a diverse part of modern tropical ecosystems in North and South America, yet their early evolutionary history in the tropics is largely unknown. Molecular divergence estimates suggest that primates arrived in tropical Central America, the southern-most extent of the North American landmass, with several dispersals from South America starting with the emergence of the Isthmus of Panama 3–4 million years ago (Ma). The complete absence of primate fossils from Central America has, however, limited our understanding of their history in the New World. Here we present the first description of a fossil monkey recovered from the North American landmass, the oldest known crown platyrrhine, from a precisely dated 20.9-Ma layer in the Las Cascadas Formation in the Panama Canal Basin, Panama. This discovery suggests that family-level diversification of extant New World monkeys occurred in the tropics, with new divergence estimates for Cebidae between 22 and 25 Ma, and provides the oldest fossil evidence for mammalian interchange between South and North America. The timing is consistent with recent tectonic reconstructions of a relatively narrow Central American Seaway in the early Miocene epoch, coincident with over-water dispersals inferred for many other groups of animals and plants. Discovery of an early Miocene primate in Panama provides evidence for a circum-Caribbean tropical distribution of New World monkeys by this time, with ocean barriers not wholly restricting their northward movements, requiring a complex set of ecological factors to explain their absence in well-sampled similarly aged localities at higher latitudes of North America.

Extinct Peccary “Cynorca” Occidentale (Tayassuidae, Tayassuinae) from the Miocene of Panama and Correlations to North America

Abstract Recently collected specimens of the extinct tayassuine peccary “Cynorca” occidentale (and another indeterminant tayassuid) are described from new excavations along the southern reaches of the Panama Canal. Fossil peccaries were previously unknown from Panama, and these new tayassuid specimens therefore add to the extinct mammalian biodiversity in this region. “Cynorca” occidentale occurs in situ in the Centenario Fauna (new name) from both the upper part of the Culebra Formation and overlying Cucaracha Formation, thus encompassing a stratigraphic interval that includes both of these formations and the previously described and more restricted Gaillard Cut Local Fauna. “Cynorca” occidentale is a primitive member of the clade that gives rise to modern tayassuines in the New World. Diagnostic characters for “C.” occidentale include a retained primitive M1, reduced M3, and shallow mandible, and this species is small relative to most other extinct and modern tayassuine peccaries. Based on the closest biostratigraphic comparisions (Maryland, Florida, Texas, and California), the presence of “C.” occidentale indicates an interval of uncertain duration within the early Hemingfordian (He1) to early Barstovian (Ba 1) land mammal ages (early to middle Miocene) for the Centenario Fauna, between about 19 and 14.8 million years ago. Based on what is known of the modern ecology of tayassuines and previous paleoecological interpretations for Panama, “C.” occidentale likely occupied a variety of environments, ranging from forested to open country habitat mosaics and fed on the diverse array of available plants.

New Turtles (Chelonia) from the Late Eocene Through Late Miocene of the Panama Canal Basin

Abstract Four distinct fossil turtle assemblages (Chelonia) are recognized from the Panama Canal Basin. The oldest, from the late Eocene–early Oligocene Gatuncillo Formation, is dominated by podocnemidid pleurodires. The early Miocene Culebra Formation includes both podocnemidids and trionychids. The early to middle Miocene Cucaracha Formation includes taxa classified in Geoemydidae (including Rhinoclemmys panamaensis n. sp.), Kinosternidae (represented by Staurotypus moschus n. sp.), large testudinids, trionychids, and podocnemidids, and finally, the late Miocene Gatun Formation records cheloniid sea turtles. These fossils include the oldest known representatives of Rhinoclemmys, the oldest record of kinosternids in Central America with a more extensive southern paleodistribution for Staurotypus and staurotypines in general, early occurrences of giant tortoises in the Neotropics, the oldest occurrence of soft-shell turtles in the tropics, the oldest late Eocene–early Oligocene Neotropical occurrences of podocnemidids. The Panamanian fossil turtles represent clades that are primarily endemic to North America, showing their very early arrival into the Neotropics prior to the complete emergence of the Isthmus of Panama, as well as their first contact with Caribbean-South American pleurodires by the early Miocene.

First Central American Record of Anthracotheriidae (Mammalia, Bothriodontinae) from the Early Miocene of Panama

ABSTRACT A new species of bothriodontine anthracothere, Arretotherium meridionale, is described from the early Miocene (Arikareean North America Land Mammal Age) Las Cascadas fossil assemblage in Panama, Central America. Fossils of A. meridionale are the first record of an anthracothere from the New World Tropics. Among anthracotheres, A. meridionale is most similar to A. acridens from the middle Arikareean from Texas in having a relatively deep and robust jaw, high and sharp cusps on the lower molars, short c—p1 diastema, and absence of a mesiolingual metacristid. A. meridionale differs from other species in being generally larger, two lower incisors (rather than three), prehypocristid never reaching the postprotocristid, more apical junction between postprotocristid and postmetacristid, mesiolingual entocristid transversely notching preentocrisitid, and transverse valley tapered lingually by prehypocristid. Although cladistic analysis of 28 anthracotheriids coded for 51 characters supports a relationship between A. meridionale and A. acridens, some presumably convergent dental characteristics are also similar to certain Oligocene-Miocene Eurasian bothriodontines. The presence of Arretotherium in the Las Cascadas Formation in Panama, and absence in the later Centenario Fauna, shows that primitive bothriodontines entered into Central America by the early Miocene before disappearing from the New World during the late early-middle Miocene.

Fossil evidence for earliest Neogene American faunal interchange: Boa (Serpentes, Boinae) from the early Miocene of Panama

ABSTRACT Isolated precloacal vertebrae from the early to middle Miocene Gaillard Cut of Panama represent the first Central American fossil record of the extant boine snake Boa and constrain dispersal of the genus into Central America from South America as no younger than approximately 19.3 Ma. Boa from the Las Cascadas fossil assemblage and the Centenario Fauna represent the oldest record of terrestrial southern vertebrate immigration into Central America, and demonstrate American interchange by the earliest Neogene. Interchange of snakes precedes contiguous terrestrial connection between Central and South America by approximately 17 Ma, necessitating dispersal across an approximately 100 km wide marine strait. The biogeographic history of snakes across the Neotropics is distinct from the mammalian record, and indicates a more complicated assembly of New World vertebrate faunas than previously recognized.

New floridatragulines (Mammalia, Camelidae) from the early Miocene Las Cascadas Formation, Panama

ABSTRACT Early Miocene floridatraguline camels are characterized by having an elongate snout, shallow and narrow symphysial area, and relatively primitive dentitions. Their fossil record is restricted to subtropical assemblages including the Hemingfordian Thomas Farm Local Fauna (L. F.) in Florida, the Zoyotal L. F. in Mexico, and the Arikareean Castolon L. F. in Texas. Here we describe the first floridatraguline camels from the early Miocene Las Cascadas Formation, Panama Canal area, Central America. We describe two new species that are similar to Aguascalientia wilsoni from the Zoyotal L. F. in having (1) a primitive lower dental formula, (2) brachydont teeth, (3) an unusually elongate jaw with caniniform c1 and p1 that are well separated by a diastema, (4) a long and narrow mandibular symphysis, (5) lower molars with small intercolumnar pillars, (6) an m3 hypoconulid divided by lingual and labial selenes, and (7) no diastema between p2 and p3. Aguascalientia panamaensis, sp. nov., and Aguascalientia minuta, sp. nov., are represented by partial lower dentitions and isolated upper molars. Both new species are distinct from A. wilsoni in having (1) p1 and c1 similar in size, (2) less inflected paraconids on lower premolars, and (3) less reduced premolars. Interpreted primitive characters are similar to a small, unnamed camel from the earliest Miocene Buda L. F. of Florida (middle late Arikareean NALMA). The Las Cascadas fossil assemblage probably constitutes a distinctive Arikareean (Ar3–Ar4) faunal province characterized by the arrival of northern immigrants into a small continental basin connected with North American continental terrains.

Temporal Calibration and Biochronology of the Centenario Fauna, Early Miocene of Panama

New excavations along the Panama Canal have yielded a growing Early Miocene assemblage of mammals referred to as the Centenario Fauna. Despite the area’s proximity to South America, the mammals of the Centenario Fauna have entirely North American affinities. The Centenario Fauna is distributed throughout a ∼115-m stratigraphic interval encompassing the uppermost Culebra and Cucaracha Formations within the Panama Canal basin. Previously published ages constrain the age of the lower limit of the Centenario Fauna to no younger than ∼19 Ma, but the upper limit has remained problematical. A fresh exposure of the Cucaracha tuff, a prominent marker horizon within our measured sections, has yielded two new radioisotopic determinations: (1) an 40Ar/39Ar age of 18.96 ± 0.90 Ma and (2) a U-Pb zircon age of 18.81 ± 0.30 Ma. In addition, magnetostratigraphic data indicate that the Centenario Fauna occurs within chron C5Er, from 18.78 to 19.05 Ma on the geomagnetic polarity timescale of Gee and Kent. These correlations further confirm the calibration of the latest Arikareean (Ar4) to early Hemingfordian (He1) transition in Nebraska, at the base of chron C5Er, at about 19.05 Ma. The Centenario Fauna occurs at the beginning of the Hemingfordian North American Land Mammal Age, i.e., He1. A broad faunal province existed during the early Hemingfordian that can be recognized across a north-south range of 5000 km throughout North America, with the southernmost limits defined by the Centenario Fauna of Panama.

New early Miocene protoceratids (Mammalia, Artiodactyla) from Panama

ABSTRACT Although Cenozoic protoceratid artiodactyls are known from throughout North America, species referred to the Miocene protoceratine Paratoceras are restricted to subtropical areas of the Gulf Coast and southern Mexico and tropical areas of Panama. Newly discovered fossils from the late Arikareean Lirio Norte Local Fauna, Panama Canal basin, include partial dentitions of a protoceratid remarkably similar to those of Paratoceras tedfordi from Mexico, suggesting a rapid early Miocene colonization of recently emerged tropical volcanic terrains (Las Cascadas Formation). Partial lower dentitions from the overlying shallow marine to transitional Culebra Formation (early Centenario Fauna) are here referred to Paratoceras orarius, sp. nov., based on relatively small size, shallow mandible anterior to p3, and narrow cheek teeth. New early Hemingfordian protoceratine fossils from the upper part of the Cucaracha Formation (late Centenario Fauna) include a partial male skull and several dentitions that, together with specimens previously referred to P. wardi (only known from the Barstovian of Texas), are here referred to Paratoceras coatesi, sp. nov., based on distinctly more gracile cranial ornamentation, relatively longer nasals, a smaller and wider lower p4 (relative to m1), and more bulbous lower premolars. Results from a cladistic analysis of 15 craniodental characters coded for 11 protoceratine species suggests that Paratoceras is a monophyletic clade with its origin in subtropical areas of Central America, spreading into the tropics of Panama during the early Miocene (Arikareean through Hemingfordian North American Land Mammal Ages [NALMAs]), and later inhabiting subtropical areas of the Gulf Coast during the middle—late Miocene (Barstovian through Clarendonian NALMAs).