Analysis of a fossilized shark skull that was found in South Africa in the 1980s has revealed that the mysterious ghost shark, a group of elusive prehistoric fish that has existed before the time of the dinosaurs, is descended from a shark-like fish that lived 280 million years ago.
Bizarre-Looking Deep-Sea Fish
Scientists had difficulty fitting the ghost shark, also known as chimaera, into the evolutionary tree of life because of its strange appearance. The deep-sea fish characterized by wing-like fins and large eyes does not look much like any of the creatures that live today.
By analyzing the shark skull that has chimaera-like traits, however, scientists now have an idea where to best fit the elusive marine animal.
Analysis Of Ancient Shark Skull Using Micro CT Scan
In a new study, Michael Coates, from the University of Chicago Medical Center, and colleagues revealed that ghosts sharks descended from the ancient fish known as Dwykaselachus oosthuizeni, which belongs to a family of prehistoric sharks called symmoriiforms that emerged 300 million years ago.
Coates and colleagues used micro computed-tomography scanner to look more closely at the ancient shark skull, which allowed them to produce a virtual 3D image of the ancient creature's skull and the braincase, where the brain originally sat.
Analysis revealed that the ancient shark was just as strange as the modern-day chimaeras. The braincase structures of the D. oosthuizeni, as well as the nostrils, major cranial nerves, and the inner ear resemble those that are seen in ghost sharks that live today.
Shared Characteristics Of Chimaera And D. Oosthuizeni
In modern day sharks and rays, the skull's cartilage roof is open at the front, but this roof is closed in both the ghost shark and the D. oosthuizeni. The characteristics of the tubes and ducts that have the inner ear's semicircular canals are also shared. The large eyes were likewise alike.
"Internally, the morphology exhibits otherwise characteristically chimaeroid specializations, including the otic labyrinth arrangement and the brain space configuration relative to exceptionally large orbits," the researchers wrote in their study, which was published in Nature.
Large Eyes Adapted To Dark Conditions In Deep Waters
Researchers think that the large orbits of the D. oosthuizeni may have been an adaptation to dimly lit waters, which allowed them to hunt for fish in dark conditions, suggesting that the symmoriiforms have adapted to light-limited conditions such as those in deep waters.
Coates said that their findings hinted where the chimaeras should be in the evolutionary tree of life. The scans showed that the D. oosthuizeni diverged from symmoriiform sharks before it evolved into the modern-day ghost sharks. The researchers said that the D. oosthuizeni was an early chimaera.
"Chimaeras are ancient specialists, now anchored within a large and very distinctive group of early shark-like fishes that thrived in the late Paleozoic era," Coates said. "We now have a glimpse of the preconditions from which modern chimaeras evolved, suggesting that the large eyes of these early sharks predisposed chimaeras for low-light, deep-sea habits."
