Scientists have discovered the fossilized remains of a 410-million-year-old bony fish that questions the established knowledge about the evolution of shark' sublime design. It was a prehistoric cousin of both sharks and other animals with bony skeletons.
The unearthing of the bony skull fossil suggests that the lighter cartilaginous skeletons of sharks may have evolved from bony ancestors instead of the other way round. Dr. Martin Brazeau, lead researcher of the study, said, "It was a very unexpected discovery."
"Conventional wisdom says that a bony inner skeleton was a unique innovation of the lineage that split from the ancestor of sharks more than 400 million years ago, but here is clear evidence of bony inner skeleton in a cousin of both sharks and, ultimately, us," he explained.
Finding New Answers In Mongolia
The skeleton of a shark is made up entirely of cartilages, whose density is nearly half of bones and makes them lighter. It is known that cartilaginous skeletons evolved before bony ones. However, it was believed that sharks diverged from other animals in the evolutionary tree and held on to their cartilage-based skeletons while others developed skeletons comprising of bones, including human beings.
Several fossils of early fishes have been excavated across the US, Europe and Australia. More recently, fossilized fish remains have been found in South America and China as well. An international team lead by the Natural History Museum, Imperial College London and scientists in Mongolia, decided to exhume fossils in Mongolia, as the rocks in the region were characteristic of the said period and had remained untouched.
The team unburied the partial skull, which also included the braincase, of a fish that was 410 million years old. Named Minjinia turgenensis, the new species belongs to an inclusive category of fishes known as 'placoderms'. The widely held belief is that sharks and all other 'jawed vertebrates'—animals with mobile jaws and backbones—evolved from these creatures.
Questioning Evolution Theory
During the development of fetuses, bony vertebrates and humans have skeletons that are cartilaginous—similar to sharks. However, they are replaced by 'endochondral' bone—solid bones that our skeletons are built of after birth—at a crucial phase of development. What is interesting is that no placoderm has been discovered with endochondral bone so far. However, the skull pieces of M. turgenensis were "wall-to-wall endochondral" or entirely bony.
Though the authors of the study remain vigilant about avoiding excessive generalization based on a single fossil sample, it is possible that they may uncover similar ancient bony fishes from the materials collected in Mongolia. The scientists, nevertheless, say that supporting evidence could offer more fascinating insights into the evolution of sharks.
"If sharks had bony skeletons and lost it, it could be an evolutionary adaptation. Sharks don't have swim bladders, which evolved later in bony fish, but a lighter skeleton would have helped them be more mobile in the water and swim at different depths. This may be what helped sharks to be one of the first global fish species, spreading out into oceans around the world 400 million years ago," concluded Dr. Brazeau.