Around this time last year, palaeontologists working in Russia revealed an incredible find: two beautifully preserved, frozen cave lion cubs, the most complete remains of this species ever unearthed. Since then, further research has shed light on some amazing details about the life – and death – of these little lions.
The two cubs were affectionately named Uyan and Dina, after a river near the discovery site. They were found last summer, several metres underground in Siberian permafrost deposits in the Uyandina River basin of the Yakutia (Sakha) Republic.
The fossils there, which also include skeletal remains of other Ice Age favourites like woolly mammoths and steppe bison, are understood to be at least 30,000 years old.
Scientific analysis of these incredible fossils, which was presented at this year's meeting of the Society of Vertebrate Paleontology, has been the job of an international team of researchers led by the Department of Mammoth Fauna Studies at the Yakutian Academy of Science in Yakutsk, Russia. The ice that encapsulates the cubs has preserved incredible details, such as their tan-yellow fur and their short ears and tails. With the use of CT scanning, palaeontologists can even examine the inside of the cubs' bodies.
"All this together gives us an idea of how old the lions were," said Olga Potapova, Collections Curator and Manager at the Mammoth Site in Hot Springs, South Dakota. "They're pretty young, probably between one and two weeks old."
Given their age at the time of death, the cubs' bodies were still growing rapidly. Their milk teeth – that is, their baby teeth – hadn't even fully grown in yet, and were still nestled within the gums. In modern-day African lions, these teeth don't erupt until an animal reaches about one month of age. And just like in other baby lions, these cubs were so young their eyes hadn't opened yet. One cub, Uyan, has one eye partially open, but Potapova speculates this was most likely due to the processes that buried the animals.
So how did these cubs come to be buried in an icy grave? It appears the poor babies became fossilised after the collapse of their den. Like living lions, mom may have tucked them away for safety, but some mishap led to a cave-in.
Examination of the cubs' skeletons revealed fatal crushing of the skull and separation of the neck vertebrae. According to Potapova, those injuries indicate the youngsters were likely killed very quickly in the collapse.
Though it spelled obvious disaster for the animals, the cave-in also allowed their bodies to become so well preserved that scientists could even examine some of their internal organs. The skulls were damaged, but the brain of one cave lion was sufficiently intact for researchers to reconstruct its 3D shape, which was found to be quite similar to that of living cats, particularly the closely related African lion.
Organs of the digestive tract were preserved as well. The researchers found that Uyan's stomach was empty, but some food residues were left behind in the intestines. The cub had probably gone just a few hours since its last meal from mom.
Of course, whenever an Ice Age fossil is found in such good shape, scientists get excited about DNA. "What makes [these cubs] exciting is the quality of the specimens," said ancient DNA researcher Leigha Lynch from Oklahoma State University, who wasn't involved in this study. "You have multiple tissue types to work with: teeth and bones, hair and muscle." She explained that different parts of the body may preserve different quality and quantity of DNA, expanding the possibilities of what researchers might find.
Indeed, working alongside renowned molecular biologist Beth Shapiro of the Paleogenomics Laboratory in Santa Cruz, California, the cave lion research team was able to uncover DNA from the mitochondria of the cubs' cells. Not only is this the oldest complete mitochondrial DNA ever found for cave lions, but it's also only the third such case, including DNA extraction from bones and hair earlier this year.
These cubs have a lot in common with living lions, but ongoing research is revealing some notable differences. "We found indications that the cave lions showed a different pattern of development than the African lions," Potapova told me.
As research continues, the team will aim to gather more information about the cave lions' growth, anatomy and genetics. Clearly, these prehistoric felines hold many more secrets inside their tiny frozen frames.
* In addition to the organisations listed above, from Russia, California, and South Dakota, the researchers would also like to acknowledge the work of colleagues at the University of Gröningen in the Netherlands, and the other organisations participating in this research.