October 17, 2014

Giant Extinct Kangaroos Did Not Hop

Brett Smith for redOrbit.com - Your Universe Online

Modern kangaroos are known for their hopping abilities, but that doesn’t mean we should assume that all ancient relatives of the marsupial used this mode of transportation.

According to a new study from a team of American and Spanish evolutionary biologists, giant now-extinct sthenurine kangaroos didn’t have the physiology to hop around – but instead probably ambled along, occasionally using their tails as a fifth limb.

Published in the journal PLOS ONE, the new study was based on a thorough statistical and biomechanical investigation of the bones of sthenurines and other kangaroos, both extant and extinct. In all, the biologists took almost 100 measurements on each of over 140 kangaroo and wallaby skeletons from many different species.

The scientists discovered many factors that showed stheneurines were essentially unlike the large red and grey kangaroos seen in most modern zoos. On charts of hind limb bones, sthenurines persistently stood apart from their modern relatives.

Image Above:  A variety of anatomical features suggest that sthenurines could put their weight on one leg at a time, an essential capability for walking on two feet. Credit: Brian Regal

The researchers also considered the size of the extinct animals, particularly one species that could have grown to around 550 pounds. The team concluded that there was no way these animals could have hopped around on their hind limbs.

"If it is not possible in terms of biomechanics to hop at very slow speeds, particularly if you are a big animal, and you cannot easily do pentapedal locomotion, then what do you have left?" said study author Christine Janis, professor of ecology and evolutionary biology at Brown University. "You've got to move somehow."

Modern day kangaroos hop at rapid rates and shift to all fours for slow speed travel, occasionally using their tail. This transition calls for a versatile backbone, strong tail, and hands that can support the weight of their entire body. Sthenurines don't seem to have had any of those qualities, the researchers said.

"I don't think they could have gotten that large unless they were walking," Janis said.

The study found that sthenurines probably used bipedal walking as their mode of slow speed locomotion. The scientists discovered numerous types of evidence to support the idea of sthenurines putting their weight on one foot at a time, a prerequisite for walking. The only known instances of walking in living kangaroos are anecdotal accounts of it in tree kangaroos, Janis said.

Previous research has already mentioned other variances, such as the teeth sthenurines had for food browsing, as opposed to grazing, like modern kangaroos.

"People often interpret the behavior of extinct animals as resembling that of the ones known today, but how would we interpret a giraffe or an elephant known only from the fossil record?” Janis said. “We need to consider that extinct animals may have been doing something different from any of the living forms, and the bony anatomy provides great clues.”

The study team said that despite their findings on the animals’ mode of transportation, they still aren’t sure how sthenurines became extinct about 30,000 years ago. Janis speculated that they may have had difficulties escaping human hunters or unable to migrate fast enough to escape changing climate factors.



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