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The remarkably rare discovery of a fossilised pregnant ground sloth and its fetus has been reported.

at the Pontifical Catholic University of Minas Gerais in Brazil had been exploring an extensive Brazilian cave for 2 hours when he discovered the fossil sloth, from the species Nothrotherium maquinense.

While excavating this fossil, Cartelle says he was surprised when a small bone fell into his hands. 鈥淎s I gained a better understanding of the orientation, location, and stage of [its] development, I concluded, trembling with excitement, that I was face-to-face with something I had never dreamed of finding: a fetus and its mother,鈥� he says.

Extinct ground sloth species of various sizes are known throughout the Americas and the Caribbean. The exact age of the fossil mother and fetus is unknown, but it dates from the late Pleistocene, sometime between 11,700 and 129,000 years ago.

While fossils of very young ground sloths have been found previously, these comprised isolated bones or teeth. No other ground sloth fetus fossil as complete as this has been found before, and none have been found within the mother itself. Only one other fossilised fetus from a land mammal within its mother has been reported before: a type of primitive horse discovered in .

鈥淚t鈥檚 an amazing fossil to find a fetus inside its mother,鈥� says at the Argentine Institute of Snow Research, Glaciology and Environmental Sciences in Mendoza, who worked on the analysis.

The size and position of the fetus, with its head near the mother鈥檚 hind limbs, indicates that the mother was close to giving birth when she died. How she perished is unknown, but the absence of any bite marks or other traces suggest she died naturally, not as a result of predation.

Caves seem to be a common repository for ground sloth fossils, suggesting that some of the animals may have lived within them. Evidence in southern Brazil and in Patagonia, which spans parts of Argentina and Chile, also hints that some may have dug enormous burrows. In this case, the researchers couldn鈥檛 determine whether the mother took shelter in the cave to give birth or whether its carcass was washed into the cave.

Clues from the fetus, however, are more informative. That there was only one fetus suggests that extinct ground sloths, like today鈥檚 sloths, gave birth to only one pup at a time. Permanent teeth rather than milk teeth and wear identified on those teeth mean the fetus ground its teeth in the uterus, which is known to be a habit in modern tree sloths. Its large and well-developed claws and its strong limbs show that the ancient baby sloths may have been able to cling to the back of the mother after birth.