Did That Extinct Mammal Live On Land Or In Water? Look At Its Rib Cage

An index based on the resistance of an animal’s rib cage to gravitational compression can shed light on the lifestyles of extinct creatures.

AsianScientist (Aug. 8, 2016) – Researchers in Japan have developed an index for predicting if a species, existing or extinct, lived its entire life in water. Details were published in the Journal of Anatomy.

Despite the extensive fossil record of mammals, it is often difficult to use fossil data to reconstruct the lifestyles and habitats of extinct species. Although mammals originally evolved as terrestrial organisms, some returned to aquatic lives, which can occur independently. Examples include whales, dolphins and manatees, which never leave the water; and seals and hippopotamuses, which split time between land and water.

Developed by Drs. Konami Ando and Shin-chi Fujiwara of Nagoya University, the index is based on how the ribs must be relatively strong for an animal to walk or crawl over land, but not for it to swim.

The researchers analyzed rib cages in 26 modern-day terrestrial, semiaquatic and exclusively aquatic species—including the killer whale, polar bear and hippopotamus—for resistance to vertical compression. This resistance represents an animal’s ability to support its body weight against gravity while walking or crawling—a trait aquatic organisms do not need.

“We selected mammals with different habitats from a range of taxa and analyzed fossils for which the bones in the thoracic region were well-preserved,” Fujiwara said.

After establishing that the index could correctly classify living species with known habitats and lifestyles, the researchers applied it to extinct groups: Ambulocetus, an early ancestor of whales, and three desmostylian species, which are the kin of elephants and sea cows.

From their index, the researchers could predict that these four extinct mammalian species could not have supported themselves on land. All of these species had retained their four limbs, but showed signs of having been partially or completely aquatic.

The article can be found at: Ando & Fujiwara (2016) Farewell to Life on Land – Thoracic Strength as a New Indicator to Determine Paleoecology in Secondary Aquatic Mammals.


Source: Nagoya University.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

Asian Scientist Magazine is an award-winning science and technology magazine that highlights R&D news stories from Asia to a global audience. The magazine is published by Singapore-headquartered Wildtype Media Group.

Related Stories from Asian Scientist