New analysis of a 100-million-year-old fossil embedded in a rocky cove in Australia suggests echidnas may have evolved from swimming ancestors.
That's essentially unprecedented: Although there are many examples In contrast to land-dwelling mammals adapting to an aquatic environment, it appears that echidnas took a different path and moved towards a more terrestrial existence.
Ever since its identification in the early 1990s, paleontologists have been discussing what type of creature might have left behind the fossilized limb bone. It has been established that this ancient being belonged to the monotreme group, which consists of egg-laying mammalian species rather than those that give birth to live offspring.
Some believe that based on its external appearance, the bone likely came from an ancient terrestrial predecessor of today’s echidnas. Others propose it could have been from a creature adapted for swimming. monotreme that pre-dates both of its living kin, echidnas and platypus.
A new examination of the small humerus bone belonging to the ancient species has been conducted. Kryoryctes cadburyi supports a narrative of origins rooted in water.
Although the outer form of a bone enables direct comparison with related species, aiding in determining evolutionary ties, the inner composition usually provides insights into the creature’s way of life and environmental interactions, says paleontologist Suzanne Hand, who headed the research, is affiliated with the University of New South Wales. .
The inner anatomy doesn’t always reveal what specific type of animal we’re dealing with, but it can provide insights into its habitat and lifestyle.
Peeking inside the fossil with microCT scans, Hand and her team discovered the bone has a thick, heavy wall and a tiny medullary cavity (the hollow space inside bones where red and white blood cells are formed).
Bones like this are seen in semi or fully aquatic mammals, like sea otters, dugongs, and platypus. Like the weight belt scuba divers use to stay underwater, these traits reduce the animal's buoyancy so it can remain underwater with little effort.
The microscopic structure of the fossil Kryoryctes The humerus resembles the inner skeletal makeup found in platypuses, where their dense bones function as ballast, enabling them to effortlessly dive for feeding purposes. says Hand.
In terrestrial environments, heavily built and robust bones pose a drawback—not just due to the increased energy required for their movement, but also since they're more susceptible to breaking. That’s why we do not observe these characteristics in echidnas, whose skeletons feature extremely delicate bone structures.
The team additionally performed phylogenetic analysis, assembling information on where K. cadburyi It sits within the branches of both the mammalian and monotreme evolutionary trees. This discovery confirmed that the species is truly a basal monotreme, having diverged from an ancestral lineage shared with platypuses and echidnas.
The news about echidna evolution is quite peculiar. It indicates that their forebear was an aquatic, digging monotreme that ultimately shifted to a dramatically different terrestrial lifestyle. Several additional hints support this possibility.
"As widely recognized, the duck-billed platypus possesses a bill equipped with numerous highly sensitive receptors capable of detecting minuscule electrical currents produced by their prey," Handusch points out .
Although the echidna’s beak contains fewer sensory receptors, some believe these receptors are vestiges from their ancestral connection to platypuses. Similarly, traces of the platypus-like bill can still be observed in developing echidna embryos.
Also, echidnas possess backward-facing rear feet, aiding them in excavation tasks similar to how platypuses utilize their reversed feet as rudders for navigating through water. This could be the reason behind the echidna’s distinctive abilities. have been known to move from one island to another This particular foot orientation doesn't occur in any other mammals.
Maybe it wasn't the adventurous, seafaring platypus that was stranded on land with the echidnas. Perhaps it was the echidnas themselves who chose to explore uncharted territories.
We’re discussing a semi-aquatic mammal that transitioned from living in water to adapting to land, which is an incredibly uncommon occurrence. However, we believe this transformation took place with echidnas, says Hand.
This study was published in PNAS .