Paleontologists have examined the shoulder girdle bones of an extinct species of temnospondyl amphibian called Metoposaurus krasiejowensis.
Metoposaurus krasiejowensis lived in what is now Poland between 225 and 215 million years ago (Triassic period).
This ancient creature is particularly known for its large and thick dermal pectoral girdle.
In addition, a larger temnospondyl species, Cyclotosaurus intermedius, and crocodile-like reptiles known as phytosaurs co-existed with Metoposaurus krasiejowensis.
“Some researchers believe that modern frogs, toads and salamanders could be descendants of these temnospondyli,” said Dr. Sudipta Kalita, a researcher with the Institute of Geosciences at the University of Bonn.
“Metoposaurids are strikingly different from other temnospondyl species because of their large rounded skull tops, massive shoulder bones and small pelvic girdle with small legs.”
In 1913, the German paleontologist Eberhard Fraas first proposed the role of metoposaurids’ pectoral girdle bones as weights for attaining negative buoyancy.
A similar strategy is used by manatees, which use the weight of their breast bones to submerge themselves in their shallow coastal habitat to graze on seagrass underwater.
“A big bone doesn’t have to be heavy. The density of the bone being crucial for understanding such an adaptation,” said Dr. Dorota Konietzko-Meier, a researcher in the Department of Paleontology at the University of Bonn.
“Like lead weights in diving, heavy bones facilitate descent. Otherwise, a lot of muscle energy would have to be used to compensate for resisting buoyancy through paddling movements while diving.”
“Eberhard Fraas’ conjecture raises the question of whether the shoulder bones of metoposaurids were actually heavy.”
In the study, the authors examined the internal microstructure of Metoposaurus krasiejowensis’ bones to look whether there was actually a lot of bone mass there.
They focused on two questions: did the shoulder bones contribute to its underwater bottom-dwelling lifestyle? and did young and old metoposaurids occupy different niches at different water depths?
To test these hypotheses, the researchers cut open the two giant elements of the shoulder girdle: the clavicles and the interclavicle, a bone located between the paired clavicles.
They took very thin sections from these bones and examined them under a microscope. The sections were then scanned and converted into black-and-white images.
Using these data, a pixel-counting software calculated the percentage of compactness.
As a bone grows in size, so do the pores inside nullifying the added weight due to bone growth. These pores provide blood and oxygen supply to the bones when the animals are alive.
However, the researchers found that the interclavicles of Metoposaurus krasiejowensis still contained a lot of bone, even in the largest of specimens.
“This suggests that the compactness of the bone at some locations within the interclavicles bears a striking resemblance to the compactness of the thoracic bones of modern manatees,” Dr. Kalita said.
“The interclavicles helped with sinking and allowed Metoposaurus krasiejowensis to lead an underwater bottom dwelling lifestyle.”
“This interpretation supports the conjecture of Fraas and later paleontologists who described Metoposaurus krasiejowensis as a bottom dwelling ambush predator,” Dr. Konietzko-Meier said.
“Moreover, young and old metoposaurs did not live at different depths of the waters, but in the same underwater ecosystem close to the substrate,” added Dr. Elżbieta Teschner, a researcher at the University of Opole.
“Considering the heavy interclavicles, Metoposaurus krasiejowensis only surfaced to catch its breath. It then slowly sank into the depths to wait for prey.”
“Unlike the metoposaurid, Cyclotosaurus may have lived closer to the water surface like modern crocodiles and alligators.”
The team’s paper was published in the Journal of Anatomy.
Sudipta Kalita et al. To be or not to be heavier: The role of dermal bones in the buoyancy of the Late Triassic temnospondyl amphibian Metoposaurus krasiejowensis. Journal of Anatomy, published online September 27, 2022; doi: 10.1111/joa.13755
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