Researchers examining the fossil record spend much of their time unravelling palaeontological puzzles using incomplete skeletons, a scattering of teeth or some intriguing footprints. But in the slowly thawing permafrost of the Siberian tundra, long-expired specimens, from mammoths to bears, regularly turn up in startlingly well-preserved condition. The latest addition to the frozen menagerie is a nearly 28,000-year-old female cave lion (Panthera spelaea) unearthed close to the Semyuelyakh River, complete with whiskers and tack-sharp claws still intact.

An almost perfectly preserved cave lion cub was unearthed in Russia's northeast. Image © Love Dalén/Center for Palaeogenetics/Twitter

Sparta, as the lion cub is called, was likely one or two months old at the time of her death, according to a study published this month in the journal Quarternary. "To my knowledge, this is the best-preserved frozen specimen from the last Ice Age ever found," Love Dalén, an evolutionary geneticist at Stockholm University and one of the study authors told NBC News. "Sparta is in near-perfect condition." The frozen cub was found in 2018 with her teeth, skin, soft tissue, and even organs still in place.

Sparta is the fourth cave lion cub to be discovered hidden beneath the permafrost in northeast Russia. A year prior, and just 15 metres away from the site where Sparta was found, a similar cub was uncovered. Named Boris, the earlier discovery showed more signs of damage, possibly from the permafrost caving in, but the cub was still in remarkably good condition.

Despite the similarities in appearance and the close proximity in which they were found, the cubs are not believed to be related: radiocarbon dating indicates that Boris is around 15,000 years older than Sparta.

Cave lions lived across Europe and Asia during the colder times of the Pleistocene Epoch, before going extinct around 14,000 years ago. Close cousins of the modern African lion, it's unlikely that they actually lived in caves, but rather their name derives from the fact that many of their fossils are found in caves.

Much of what we know about the species comes from ancient art, fossils, tracks and mummified carcasses like those of Sparta and Boris. The ancient cold-loving cats share many similarities with modern lions, but evidence suggests that they lacked manes which raises questions about their social makeup. Boris and Sparta are too young for researchers to determine how their yellowish coats may have developed, but experts suspect that the colour could have faded to a greyer shade as they grew older.

Given the cold climate that cave lions called home, their fur was adapted to handle icy weather. Cave lion fur "has a long thick fur undercoat consisting of strombuliform aeriferous fur hair," the study authors point out. "It covers the body of a cave lion cub evenly and most likely helped cave lion cubs adapt to the cold climate."

The latest find helps fills in some of the gaps in our knowledge of cave lions. Scientists are hoping to sequence the genome of both cubs to learn more about the mysterious species.