Giant snake fossils point to steamy ancient tropical climate.
Palaeontologists have long been searching for additional clues to the evolution of tropical vertebrates in the wake of the dinosaurs' demise roughly 65 million years ago. Sixty-million-year-old rock in the Cerrejón Coal Mine of northeastern Colombia yielded an exciting find — fossils of the largest snake species ever seen. These offered an opportunity to exploit reptile biology for palaeoclimatic clues.
A team of palaeontologists has been studying the mine's rocks for several years. Massive tractors are used deep below the surface to excavate long, thick coal seams that can spontaneously combust. “The mine itself resembles Dante's Inferno, but it is heaven for a fossil collector,” says Jonathan Bloch, a vertebrate palaeontologist at the Florida Museum of Natural History in Gainesville and one of the team that discovered and identified the snakes' giant vertebrae.
Bloch contacted Jason Head, an expert on fossil snakes at the University of Toronto in Mississauga, Canada. Head jumped out of his seat when he saw the specimens. “When you look at fossil snake vertebrae bigger than your hand, you soon realize that their size alone is shedding new light on this past ecosystem,” says Head. Even before they had an accurate estimate of the snake's size, he and his colleagues began to question what it said about past climate.
Plant fossils or ratios of oxygen and carbon isotopes are usually used as proxy data to interpret past climate. But cold-blooded vertebrates add a novel biological indicator because their metabolic rate controls their maximum size at a given temperature. “In this regard, the reptile fossil record is uniquely important and incredibly understudied,” says Head.
However, dating the species wasn't as simple as plugging a number into an equation. Having single fossil vertebrae from a number of snakes, Head had to painstakingly quantify the shapes of individual vertebrae to identify their likely positions, basing his approximations on the 250-bone vertebral column of an extant snake of the same subfamily. Then, he and another colleague plugged their measurements into a mathematical model of vertebral column dimensions and positions in living snakes to estimate the fossil's original size.
Head estimates that the species was 13 metres long, between half a metre and a metre in diameter, and weighed more than a tonne, making it the world's largest snake. The authors named it Titanoboa cerrejonesis — the titanic boa from Cerrejón. From comparisons with the lengths of living species, they calculate that mean annual temperatures of at least 30–34 °C would have been required to support it (see page 715).
Head says that this work supports one of two hotly contested hypotheses about how heat was distributed across the planet when the poles were free of ice. Although some suggest that equatorial climates extended farther into higher latitudes, distributing the heat more widely, this work supports the alternative view that equatorial temperatures were much hotter than those on the rest of the planet.
However, the data also present a conundrum: the temperatures they suggest are above those that modern tropical forests can tolerate. “So, 60 million years ago either the snake or the forest was unlike those we know today,” says Bloch. “This is a window to a time and place that we've never had access to before.”