Computerized tomography (CT) scans of two newborn woolly mammoths that died approximately 40,000 years ago in Siberia are revealing details about the early development of the prehistoric mammals that were previously inaccessible. The X-ray images show that both creatures died tragically after having inhaled mud.
Dubbed 'Lyuba' and 'Khroma,' who died at ages 1 and 2 months, respectively, are the best-preserved baby mammoths ever found. Lyuba's full-body CT scan, which used an industrial scanner at a Ford Motor Corp. testing facility in Michigan, was the first of its kind for any mammoth. University of Michigan paleontologist Daniel Fisher was elated at the finds. "This is the first time anyone's been able to do a comparative study of the skeletal development of two baby mammoths of known age." He added, "This allowed us to document the changes that occur as the mammoth body develops," Fisher said. "And since they are both essentially complete skeletons, they can be thought of as Rosetta Stones that will help us interpret all the isolated baby mammoth bones that show up at other localities."
Fisher directs the U-M Museum of Paleontology at the Ann Arbor-based institution. He is the lead author of a paper published in the Journal of Paleontology, which gives details on 30 previously unpublished CT images.
The paper's 10 authors are from the United States, Russia and France. They include three recent U-M graduates and a collections manager at the U-M paleontology museum.
Lyuba and Khroma lived more than 40,000 years ago and belonged to mammoth herds living roughly 3,000 miles apart. Lyuba was found by reindeer herders in May 2007 in northwest Siberia on the banks of the Yuribei River on the Yamal Peninsula. The baby mammoth was found frozen and partially dehydrated but otherwise appeared to be intact, except for the loss of most of her hair and all of her nails. Inhabitants of Siberia often hunt for buried mammoth remains in the hope of finding valuable fossil ivory.
Khroma was found in 2008 near the Khroma River in far north Yakutia, northeast Siberia. It was found in permafrost in an upright position. Ravens and possibly arctic foxes scavenged exposed portions of her carcass, including parts of the trunk and skull and the fat hump that likely covered the back of her neck. Otherwise, the body was recovered in good condition.
Stringent conditions were followed in the study of the remarkably preserved specimens. Some dissection and limited sampling were permitted, but both specimens were left mostly intact. The CT scans offered a non-destructive visualization and analysis of their anatomy without compromising exhibit potential or options for future analysis.
Lyuba underwent medical CT scans in Japan in 2009 and in Wisconsin in 2010. Because of Lyuba's size (about 110 pounds and slightly smaller than a modern baby elephant), the researchers could not acquire 3-D data from her entire body. It was at Ford Motor Co.'s Nondestructive Evaluation Laboratory in Livonia, Michigan, using a scanner designed for finding flaws in vehicle transmissions, that a complete scan was obtained.
CT scans for Khroma were done at two French hospitals. However, micro-CT scans of teeth from both mammoth calves were conducted at the University of Michigan School of Dentistry. Fisher and colleagues determined on the basis of the dental images that Lyuba died 30 to 35 days after birth and estimated that Khroma lived between 52 and 57 days.
30 years in the making
The researchers used a technique developed by Fisher over the past 30-plus years that involves counting daily growth layers inside the teeth, which are similar to the annual growth rings on a tree to determine its age. The dental studies also indicate that both mammoths were born in the spring.
Scans of Khroma's skull showed she had a brain slightly smaller than that of a newborn elephant, which hints at the possibility of a shorter gestation period for mammoths.
Lyuba's skull is conspicuously narrower than Khroma's, and her upper jawbones are more slender, while Khroma's shoulder blades and foot bones are more developed. These differences may simply reflect the one-month age difference between the calves, or they could relate to the different populations from which the two calves derived.
The researchers refer to both calves as mummies due to the high level of soft-tissue preservation. In addition to fully articulated skeletons, the carcasses held preserved muscle, fat, connective tissue, organs and skin. Khroma even had clotted blood inside intact blood vessels and undigested milk in the stomach.
"These two exquisitely preserved baby mammoths are like two snapshots in time. We can use them to understand how factors like location and age influenced the way mammoths grew into the huge adults that captivate us today," said co-author Zachary T. Calamari of the American Museum of Natural History, who began investigating mammoths as a U-M undergraduate working with Fisher.
Short lives, violent deaths
The CT scans of Lyuba and Khroma show that both died suffocated after inhaling mud. Scans revealed a solid mass of fine-grained sediment blocking the air passages in the middle of Lyuba's trunk. Sediment was also seen in its throat and bronchial passages. If Lyuba had died by drowning rather than suffocation – as some have suggested – then traces of sediment should also have been detected in parts of the lungs beyond the bronchial passages, but that was not the case.
Slightly coarser sediment was found in Khroma's trunk, mouth and throat. However, her lungs were not available for study because they were scavenged before the carcass was recovered. Since both animals appear to have been healthy at the time of death, a "traumatic demise" involving the inhalation of mud and suffocation appears to be the most likely cause of death.
It is believed that Lyuba died in a lake because sediments found in her respiratory tract include fine-grained vivianite, a deep blue iron- and phosphate-bearing mineral that commonly forms in cold, oxygen-poor lake bottoms. Researchers believe that Lyuba may have fallen through a sheet of ice while crossing a lake during the spring melt. If she was struggling to breathe while submerged in a frigid lake, the so-called "diving reflex" may have kicked in during her final moments. The reflex is triggered by cold water contacting the face, initiating physiological changes that enable mammals to stay underwater for extended periods of time. Those changes include a shifting of blood from the extremities to the brain and heart. The blood shift would help explain the presence of small vivianite nodules found on Lyuba's facial tissues during a necropsy. The CT scans revealed vivianite nodules, up to several millimeters in length, inside and on the surface of the skull.
Blood entering Lyuba's brain, due to the mammalian diving reflex, may have provided the iron source for the vivianite nodules, while lactic acid-producing bacteria ate away at her bones after death and possibly liberating the phosphate ions used to make vivianite, Fisher said.
As for Khroma, one scenario places the calf and her mother on a riverbank in the spring. Khroma had been nursing less than an hour before her death, as evidenced by undigested milk found in her stomach during a necropsy. "It looked like you'd just popped the top on a container of yogurt," U-M paleontologist Fisher recalled. "It was that white. It was that smooth. Just fresh, creamy milk from mama mammoth." Perhaps the riverbank collapsed and caused a fall that would account for the fractured spinal column revealed by Khroma's CT scan, as well as the mud she inhaled.