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Here, 21-million-year-old fossils of a New World monkey from Panama are described, constituting the earliest known evidence for mammalian interchange between North and South America. Early migration of New World monkeys South America and North America were separated by an ocean until about 3.5 million years ago, when the Isthmus of Panama emerged to link the two. A dramatic episode of land animal migration followed, known as the Great American Biotic Interchange. New excavations related to the expansion of the Panama Canal have now resulted in the surprising discovery of a 21-million-year-old fossil of an early Miocene New World monkey — the first fossil monkey found on the North American landmass. This finding supports the idea that over-water dispersals were possible for plants and animals more generally in the Caribbean region during the early Miocene epoch. 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) 1 . 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 2 , 3 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 4 . 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.

Fossils of an insectivorous bat from the early Miocene of Panama are described as a new genus and species, Americanycteris cyrtodon (Chiroptera: Phyllostomidae: Phyllostominae). Americanycteris is a large phyllostomine bat, similar in size to the living species Chrotopterus auritus . Americanycteris cyrtodon can be distinguished from other closely related species by a posteriorly curved p4 and a thick labial cingulum on m1. Americanycteris cyrtodon occurs in two early Miocene vertebrate faunas from Panama. The holotype mandible with p4–m1 and an isolated p3 of A. cyrtodon were recovered from the early Hemingfordian (19–18 Ma) Centenario Fauna, and a mandible with p2 was found in the older late Arikareean (21 Ma) Lirio Norte Local Fauna. A similar large phyllostomine bat is known from the early Miocene Gran Barranca Fauna in Argentina. The presence of early Miocene phyllostomids in both North America and South America confirms the overwater dispersal of bats between the Americas before the late Miocene onset of the Great American Biotic Interchange. Pre-late Miocene chiropteran dispersals between the Americas were previously documented for the Emballonuridae and Molossidae. Although the five endemic New World families in the Noctilionoidea, including Phyllostomidae, were previously thought to be South American in origin, the oldest fossil records of noctilionoids (Mormoopidae and extinct Speonycteridae) are from the early Oligocene of Florida and one of the earliest records of the Phyllostomidae is from the early Miocene of Panama. The currently available fossil records from Panama and Florida suggest a possible North American origin for the Noctilionoidea.

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.

The proboscidean Gomphotherium is reported here from the Alajuela Formation of Panama. Gomphotherium was widespread throughout Holarctica during the Miocene, and the Panama fossil represents the extreme southernmost occurrence of this genus in the New World. Allocation of the Panama Gomphotherium to a valid species is impossible given both the fragmentary material represented and the taxonomic complexity of species assigned to this genus. In North America, Gomphotherium has a relatively long biochronological range from the middle Miocene (∼15 Ma) to early Pliocene (∼5 Ma). Based on morphological comparisons, the Panama Gomphotherium is either middle Miocene, thus representing the earliest-known entry of this genus into Central America, or late Miocene/early Pliocene, which challenges the currently accepted middle Miocene age of the Alajuela Formation as it has been previously reported from Panama.

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.

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).

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.

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.

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).

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.