Huge Ur-Lurch with diving belt

Metoposaurs (olive green, below) appeared to be lurking at the bottom—probably to avoid competition from cyclotosaurs (blue). © Illustration: Sudipta Kalita

On the trail of an interesting adaptation: Around 200 million years ago, in what is now Europe, huge amphibians apparently divided up their hunting grounds in the water by adjusting their buoyancy behavior, paleontologists report: While the cyclotosaurs were more likely to be driven upwards, the metoposaurs were held by heavy shoulder bones a lead belt on the bottom so they could lie in wait for prey there. Interestingly, the compactness of the bone structure was similar to that in the breast bones of modern-day manatees, comparisons show.

If we could travel back in time to the Triassic era at the Krasiejόw site in south-western Poland, we would end up in a primeval floodplain populated by bizarre creatures. In addition to the remains of crocodile-like reptiles, paleontologists found fossils of representatives of the so-called temnospondyls there. It was a group of amphibians, some of which were several meters long, that had developed around 300 million years ago. Although they later died out in their special form, they could also have formed the evolutionary basis of an enormously important line of development: "Some researchers believe that today's frogs, toads and salamanders could be descendants of these temnospondyli," says Sudipta Kalita von the University of Bonn.

A questioning look at a structure that seems massive

As part of their study, Kalita and his colleagues have now targeted two species of temnospondyls that coexisted at the site around 225 to 215 million years ago. One is the approximately 3.5 meter long Cyclotosaurus intermedius, the other is the approximately two meter long Metoposaurus krasiejowensis. The questioning gaze of the researchers was particularly directed at the second of the two giant amphibians. Because Metoposaurus differed from other Temnospondylus species in one characteristic: What was it about its particularly massive shoulder bones? These heavy-looking structures raised the suspicion that they might have reduced buoyancy in the water to allow the animal to stay comfortably at the bottom.

But even the opposite could have been the case: "A large bone does not necessarily have to be heavy," says co-author Dorota Konietzko-Meier from the University of Bonn. "The density of the bone is crucial for understanding a particular adaptation," explains the amphibian expert. In order to gain insights into the buoyancy properties and thus into the way of life, the team examined the internal structure of the bones of Metoposaurus and Cyclotosaurus. This made comparisons possible between the two original amphibians and with animal species living today. To do this, the researchers cut open elements of the shoulder girdle and made thin sections of the structures. In addition to microscopic examinations, these sections were also scanned so that pixel counting software could automatically record the percentage compactness of the bone structures.

Heavy bones like a manatee

As the researchers report, in the case of Metoposaurus, there was a strikingly dense bone mass – in both adult and juvenile fossils. In comparison, the structures of Cyclotosaurus appeared rather porous. In the case of Metoposaurus, comparisons with modern-day creatures then confirmed that the heavy breast bones probably served to regulate buoyancy: "The compactness of the bone bears a striking resemblance to that of the breast bones of modern manatees," reports Kalita. This trait is known to allow these mammals to comfortably submerge in their shallow coastal habitat and eat sea grass underwater. Otherwise they would have to use a lot of muscle energy to compensate for the buoyancy of paddling movements.

This is apparently why the primeval giant amphibian also had its heavy shoulder area: like the lead belt of divers, this structure kept the predators at the bottom of the primeval waters so that they could lurk there motionless and without effort. Presumably, the animal only surfaced to gasp when needed. The researchers conclude that an interesting specialization is now emerging in the ecosystem of the time: While Cyclotosaurus and the crocodile-like phytosaurs (shown in the picture above right) tended to hunt in the surface area, Metoposaurus waited on the bottom for prey to appear in front of its broad muzzle swam.

Source: University of Bonn, specialist article: Journal of Anatomy, doi: 10.1111/joa.13755

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