Giant turtles all the way down

A Colombian coal mine opens a treasure chest of fossils.

By Liza Lester

Edwin Cadena and his 60-million-year-old giant "coal turtle"  Carbonemys cofrinii, discovered in Cerrejón coal mine, Colombia, South America. Used by permission.

Edwin Cadena and his 60-million-year-old giant “coal turtle” Carbonemys cofrinii, discovered in Cerrejón coal mine, Colombia, South America. Credit, Edwin Cadena. Used by permission.

Edwinn Cadena excavates the fossil "coal turtle" in the Cerrejón coal mine, Colombia.

Edwin Cadena excavates the fossil “coal turtle” in the Cerrejón coal mine, Colombia. Edwin Cadena. Used by permission.

IT was large, that much was obvious. When Edwin Cadena first saw the fossil in 2005, he thought he might be uncovering another specimen of Titanoboa cerrejonensis, the ancient snake he and his colleagues discovered in 2004 on a Smithsonian expedition lead by Carlos Jaramillo, Jason Head, and Jonathan Bloch. But as he slowly picked the rock away, the fossil was revealed as another giant reptile, a ~58-million-year-old contemporary of Titanoboa, and one of the largest freshwater turtles ever found. So big, even the 13-meter, two ton snake probably couldn’t swallow it. He named it Carbonemys cofrinii, “coal turtle.”

Cadena, now a doctoral student with Dan Ksepka at NC State, found his turtle in the La Puente cut in the north zone of Colombia’s Cerrejón mine, one of the largest open-pit coal mines in the world. La Puente is an expanse of naked mudstone, stripped of jungle and a layer of coal. Hot, windy, and full of bones, with distant methane fires blooming off and on from an active face, it looks apocalyptic in photos. It is a treasure chest of fossils.

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We have few fossils from the tropics, where ancient remains lie deep buried under living jungle and tons of soil. It is rare that natural erosion reveals the secrets of the ancient tropics. The coal mine has peeled back the surface for the paleontologists, gratis. Candena has an awareness of his a time-table. Fossil excavation is slow work. He has only a few years to finish projects in progress before active mining resumes.

How did the snakes and turtles get so large? Cadena thinks habitat and ecological interactions drove the gigantism. In addition to giant reptiles, the mine has uncovered the ancient jungle they in lived in, a wealth of plant fossils as important to understanding ancient ecosystems as the exciting megafauna. The fossil plant world is surprisingly familiar. The team has seen precursors of the living jungle, including modern bean, banana, and chocolate plants.

Climate change may also have been a factor. Based on the great size of Titanoboa, Head et al concluded that the mean annual temperature of equatorial South America must have been warmer, in the range of 30–34 °C, consistent with controversial hot Palaeogene climate models. It seems the Colombia of the Palaeocene neotropics was not that unlike today, only more so – hotter, wetter, bigger. Cadena, E., Ksepka, D., Jaramillo, C., & Bloch, J. (2012). New pelomedusoid turtles from the late Palaeocene Cerrejón Formation of Colombia and their implications for phylogeny and body size evolution Journal of Systematic Palaeontology, 10 (2), 313-331 DOI: 10.1080/14772019.2011.569031