Late Pleistocene Megafaunal Deposits on the Isthmus of Panama and Their Paleoenvironmental Implications
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Caribbean Journal of Science, Vol. 41, No. 1, 1-13, 2005 Copyright 2005 College of Arts and Sciences University of Puerto Rico, Mayagu¨ez Late Pleistocene Megafaunal Deposits on the Isthmus of Panama and Their Paleoenvironmental Implications GEORGES A. PEARSON Department of Anthropology, University of Kansas, 622 Fraser Hall, 1415 Jayhawk Blvd., Lawrence, KS 66045-7556, USA [email protected] ABSTRACT.—Two megafaunal deposits were discovered next to the villages of La Trinidaíta and Llano Hato during an archaeological survey on the Azuero Peninsula of Panama. The fossil sites were found near other localities first reported in 1957 by Smithsonian paleontologist Lewis Gazin. Bones at La Trinidaíta and Llano Hato rested in tight horizontal clusters at the bottom of ancient creeks and ponds. The remains appear to have been buried rapidly by thick deposits of fine clay when environmental conditions were wetter and cooler than today. Animals identified at La Trinidaíta and Llano Hato include Cuvieronius tropicus, Eremo- therium sp., and some turtles. Radiocarbon-dated charcoal fragments associated with the bones gave con- temporaneous dates of 44,840 ± 700 and 47,040 ± 900 14C yr B.P. These dates indicate that carcasses accumu- lated during the Marine Isotope Stage 3 (MIS3) interstadial. Because other fossils were not found above or below these principal deposits, it is argued that the MIS3 environment was especially favorable for sup- porting large browsers and mixed-feeders and preserving their bones on the landscape. These environmental and taphonomical characteristics likely disappeared as the climate of the lowland Neotropics became drier in response to the following arid glacial advance. During this time, C4 plants dominated the landscape and may have forced gomphotheres and giant ground sloths to abandon the dry Pacific lowlands and follow the rising tree-lines. Such large-scale range re-organization may have caused animals to abandon some regions of lower Central America during the Last Glacial Maximum. KEYWORDS.—Panama, megafauna, MIS3, Cuvieronius, Eremotherium, Rancholabrean, Pleistocene. INTRODUCTION types of armadillos (Holmesina, Glyptothe- rium, Chlamydotherium), ground sloths During the late Pliocene and the Pleis- (Megalonyx, Eremotherium, Glossotherium), tocene, Central America acted as a connect- toxodonts (Mixotoxodon), porcupines, opos- ing corridor that allowed North and South sums, anteaters, and monkeys. Initially, the American animal species to intermingle. dispersal of animals across the Isthmus was This process, called the Great American facilitated by the presence of a continuous Faunal Interchange (Marshall et al. 1982; savanna corridor spanning both continents Stehli and Webb 1985; Marshall 1988; Webb (Webb 1978, 1991). By the end of the Pleis- 1991, 1997), began approximately 3 MYA tocene, however, a more humid environ- with the final emergence of the Panama- ment began to prevail in lower Central nian land bridge and closure of the last ma- America, which culminated in the modern rine corridor between the Pacific and At- Neotropics. With time, the increasingly hot lantic oceans (Coates 1997). Among the and wet expanse between Nicaragua and northern animals that crossed the Isthmus Panama became less of a conduit and more into South America were gomphotheriids of a filter, gradually preventing animal and (Cuvieronius), horses, llamas, deer, peccar- plants from traversing to and from the con- ies, tapirs, and rabbits (Webb 1991). Immi- tinents. Although the emergence of the grants from the south included several Isthmus of Panama was responsible for the terrestrial swapping of animals, very few fossils have been recovered on the land ms. received August 16, 2004; accepted December bridge itself. The first discoveries were re- 21, 2004 ported by Gazin in 1957 who excavated two 1 2 GEORGES A. PEARSON megafaunal localities on the Azuero Penin- Florida Museum of Natural History in sula. The first site, La Coca, near the town Gainesville where the author conducted of Ocú, contained the remains of Eremothe- preliminary identification. Fossils were rium and Mixotoxodon (originally described eventually returned to the Smithsonian as Toxodon, Gazin 1957:346). At the second Tropical Research Institute in Panama locality, El Hatillo, Gazin (1957:347) identi- where they are presently stored. fied the bones of Eremotherium, Equus, Cu- vieronius, Glyptodon, Pseudemys (Trachemys), RESULTS and Glossotherium among others. Here I re- port the discovery of two additional fossil de- La Trinidaíta posits found near Gazin’s original sites. Setting.—La Trinidaíta (98 m a.s.l.) is lo- cated approximately 9 km west of the town METHODS of Pesé (UTM: 17NNU533942873738, NAD 27) (Figure 1). The site was discovered in a In 2001, I conducted an archaeological small topographic depression that drains survey whose main objective was to locate into a small creek and becomes boggy dur- late Pleistocene-early Holocene human oc- ing the rainy season (Figure 2). The bowl is cupations on the Azuero Peninsula (Pear- delineated by a gentle northern slope and son 2002; Pearson and Cooke 2002). Be- eroding sidewalls to the south. Although cause Paleoindian sites sometimes contain the land was in a fallow state at the time of the remains of megamammals (Grayson our study, it had previously served to grow and Meltzer 2002), ancient bone deposits tubers. Prior planting and harvesting ac- were also sought to determine if a human tivities had exposed bones along the shal- presence might be associated with some of low edges of the depression. them. During the reconnaissance, bones of Excavations at La Trinidaíta were ori- extinct megafauna were discovered next to ented perpendicular to the draining axis of the small villages of Llano Hato and La the basin in order to sample a cross-section Trinidaíta. of the sloping deposits.A2m2 test pit (TP1) Both the Llano Hato and La Trinidaíta was placed near the bottom of the bowl localities were excavated following ar- aligned with a narrow trench (TR1) and a chaeological protocols. Using a theodolite, second test pit (TP2, 1 m2) along its slope. horizontal grids of 1 m2 excavation units In addition, several test pits were dug at a were established over the extent of the distance from the main excavation area to work area. These grids were oriented along determine the extent of the fossil-bearing magnetic north or an arbitrary site north. deposit. All bones recovered during the Excavations proceeded by shovels and study were found in TP1, TP2, TR1, and trowels to remove sterile overburden fol- unit N80E122. lowed by wooden shish kebab sticks to ex- Stratigraphy and dating.—Bones at La pose the bone beds. Because fossils were Trinidaíta were first observed on the sur- embedded in a moist and sticky clay ma- face of the northern incline. These fossils trix, sieves were not used in the field. Most were protruding from a 10 cm-thick plow- bones were in a very poor state of preser- zone observed in the stratigraphic profile of vation and had to be treated in the field the trench. Specimens recovered in situ in with a solution of diluted white glue and in TR1 were resting on an ancient creek bed the laboratory with an acetone-based con- composed of rounded cobbles and sand ly- solidant (Acryloid B-72). The absence of hu- ing immediately above the bedrock (Figure man-made artifacts coupled with a suite of 3). Fossils were encased and overlaid by early radiocarbon dates later confirmed fine clay which thickened progressively to- that the deposits represented non-cultural wards the bottom of the basin (from 25 cm paleontological accumulations. Following in TR1 to over a meter in TP1). Although the excavations, bone elements with visible our excavations did not reach the rocky identifying features were shipped to the substrate in TP1, megafaunal remains were LATE PLEISTOCEN MEGAFAUNAL DEPOSITS 3 FIG. 1. Map of Panama and Azuero Peninsula (above) and inset map (below) showing towns (solid squares) and fossil localities (open circles) described in text. found at 85 cm below surface within the consisted of Eremotherium vertebrae. De- fine clay deposit (Figure 4). A charcoal pending on the taxonomic scheme one sample collected approximately 15 cm be- wishes to adopt, late Pleistocene ground low an isolated Eremotherium vertebra in sloth species from Panama belong to either TP1 gave a date of 44,840 ± 700 14C yr B.P. E. laurillardi (Cartelle and De Iuliis 1995) or (Beta-158916, Table 1). This date is consid- to E. rusconi (Guérin and Faure 2000). Cu- ered an average age because the charcoal vieronius tropicus was represented by a sample was recovered under some of the tooth fragment (Figure 6), a complete tibia, fossils in clay that buried the main bone some vertebrae, and a proximal rib. Finally, bed. Several remains were also collected a small section of carapace was ascribed to from TP2 but excavations in this test unit an unidentified terrestrial turtle. Regretta- were incomplete due to in-filling of water bly, it was not possible to classify many during heavy rains. The bone-rich clay was fragmentary specimens (n < 80) and the not observed in units N100E128 and MNI count for this collection must tenta- N115E88-89. tively remain at one individual for each Fossil remains.—At least three genera species. were identified at La Trinidaíta (Table 2, Bones did not show obvious signs of sur- Figure 5). The majority of bone elements face weathering such as root etching, insect 4 GEORGES A. PEARSON FIG. 2. Topographic map of the La Trinidaíta fossil locality, Panama. Test pits and trenches are indicated by solid squares and rectangles respectively. burrows, rodent gnawing, or desiccation Pesé (UTM: 17NNU533587881887, NAD 27). cracks and exfoliation. This evidence sug- The site was discovered in a cow pasture gests that the animals died at the watering on the edge of a small, seasonal stream hole and carcasses were deposited in the (Quebrada El Jobo).A2m2 test pit (TP1) aqueous environment of the creek.