Fossilized turtle bones discovered on a massive, arid plateau called the Altiplano – with a current elevation of 3,500 meters (11,500 feet) and temperatures as low as 4°C (39.2°F) – suggest that this part of the Andes had a much lower elevation back in the Miocene some 13 million years ago. Tortoises and turtles would not have been able to tolerate the cold temperatures associated with today’s high elevations. The findings are published in the Journal of South American Earth Sciences this month.
The town of Quebrada Honda is located in the Eastern Cordillera of the Tarija department of southern Bolivia near the border with Argentina. It’s about 3,500 meters above sea level, and it’s late middle Miocene in age, or 13.18 to 13.03 million years old. Plenty of fossils have been collected at Quebrada Honda, but determining its Miocene elevation hasn’t been straightforward. Previous work with proxies for soil temperature estimate the paleoelevation of the southern Altiplano near Quebrada Honda at 2,600 meters (8,500 feet).
But now, a trio led by Edwin Cadena of Yachay Tech University have analyzed the first ever fossil turtles at Quebrada Honda – the country’s only Miocene turtle remains. A large right scapula-acromion (shoulder bone) and various shell fragments belonged to giant tortoises in the genus Chelonoidis. They resemble today's Galapagos giant tortoises. Meanwhile, a left plastron (or shell) bone belonged to a freshwater, side-necked turtle in the genus Acanthochelys. The length and width suggest it’s a juvenile turtle.
The occurrence of a giant tortoise and a freshwater turtle suggests that the paleoelevation of the area when the fossils were deposited was lower than previously estimated. In fact, the southern Altiplano near Quebrada Honda probably had a maximum elevation of less than 1,000 meters (3,300 feet). The temperatures at a greater elevation would have been beyond the tolerable physiological limits for giant ectothermic reptiles – those who rely on external sources for heat.
Today’s tortoises live in a wide range of habitats spanning tropical to temperate zones on most continents. But when it comes to altitude, they tend to be restricted to areas of low elevation: 0 to 1,200 meters (3,900 feet) above sea level for North American species, below 1,800 meters (5,900 feet) for Asian, European, and African tortoises, and below 950 meters (3,100 feet) for South American Chelonoidis. Additionally, fossils from the smaller, aquatic turtle suggest that the climate near Quebrada Honda was much wetter than today too.
Until now, the Bolivian fossil record of turtles had been restricted to just four localities that date back to the Paleocene, late Oligocene, and late Pleistocene.