Some 15,000 years ago, a dusky rattlesnake stretched out and spent its last living moments inside the jawbone of a giant mastodon.

Now researchers are using the remains of this and a few other reptiles found around the fossilised mastodon, discovered in central Mexico, to gain an idea of what the climate looked like in the region near the end of the Pleistocene.

Jose Alberto Cruz at the Meritorious Autonomous University of Puebla in Mexico says the mastodon bones were found in 2019 by villagers who were building a house in the state of Puebla. He and his colleagues were surprised to discover the well-preserved fossil of an 80-centimetre dusky rattlesnake (Crotalus triseriatus) inside the jaw bone of the American mastodon (Mammut americanum).

Since these mastodons were herbivores, Alberto Cruz says it is unlikely the giant elephant-like animal ate the snake. Instead, the reptile was probably using the jawbone of a dead mastodon for shelter when a mudslide led to an untimely end.

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He and his colleagues say that reptile fossils from the Pleistocene are rarely found in Latin America – this is the first dusky rattlesnake found in Mexico. Remains of a brown snake, several anole lizards and an alligator lizard were found in the same sediment layer as the mastodon.

Many of these species are still around today, so the authors compared what we know about the climate in their preferred habitats with other research on palaeoclimate to estimate the rough age of the fossils.

Previous research has suggested that the climate in the past was colder and more humid, says Alberto Cruz. Understanding the range of temperatures these reptiles can survive could help uspredict how they might adapt to ongoing climate change. Today, some of the reptiles found near the mastodon remains aren’t found near the area where these fossils were found.

Journal reference: Quaternary International, DOI: 10.1016/j.quaint.2020.10.058