LAS VEGAS—Computed tomography (CT) scans of two extraordinarily well-preserved baby woolly mammoths from Siberia have yielded startling new insights into these iconic Ice Age beasts. Previously examinations of the external features of the mammoths suggested that the two creatures were quite similar, exhibiting the same developmental stage and similar age at death. But the new full-body scans—the first ever obtained for largely intact mammoths—tell a different story. Researchers unveiled the new findings on November 5 at the annual meeting of the Society of Vertebrate Paleontology.

Both of the baby mammoths—dubbed Lyuba and Khroma--were recovered from Siberian permafrost, hence their remarkable state of preservation, which includes hair, skin and internal organs. Lyuba, discovered in 2007, died around 42,000 years ago; Khroma, found in 2009, is geologically older, although experts have yet to determine exactly when this individual lived.

Although the two babies look alike on the outside, the CT scans showed dramatic differences in the shape of the skull and limb bones. According to Ethan Shirley of the University of Michigan, lead author of the new study, Khroma's face is wider and more robust than Lyuba's and has a boney structure at the tip of the upper jaw that looks like a mustache; Lyuba, meanwhile, has longer forelimbs.

Although the CT results were surprising, Shirley and his colleagues thought they had an explanation. Physical examination had established that Lyuba was female, and suggested that Khroma was male. So the researchers initially chalked up the newly observed differences in internal anatomy to variation between the sexes. But when they took a closer look at the sex organs in the CT scans, they found features of the soft tissues of the urogenital tract that indicate Khroma was actually a female.

The new results "complicate our view of mammoth development" and suggests that there may be more variability in developmental timing than previously thought, remarks team member Daniel Fisher of the University of Michigan. The differences between the two female baby mammoths could indicate that they belonged to different species. Alternatively, woolly mammoth anatomy may have varied through time and from region to region.

Images courtesy of International Mammoth Committee; CT scans by Ford Motor Company, USA, and Centre hospitalier Emile Roux, Le Puy-en-Velay, France