Researchers studying the skull of a 409-million-year-old fish reveal that this lobe-finned predator is closely related to coelacanths, a rare fish that’s alive today but declining. The findings, published in Science Advances last week, will help researchers understand the origins of the first animals to walk on land.
Lobed-finned fishes like coelacanths and lungfish are more closely related to amphibians, reptiles, and mammals than they are to ray-finned fishes, which include fishes we normally think about (like salmon or clownfish). Coelacanths and lungfish are two of the major groups of animals that make up the class called sarcopterygians. The other major group of living sarcopterygians is the tetrapods – all four-limbed terrestrial animals, including those who lost their limbs and those who’ve returned to a life at sea. Modern, or “crown”, sarcopterygians are very different from stem sarcopterygians, which include gnarly animals like Psarolepis. But because transitional species are rare, the origins of the crown group are a mystery.
There is, however, an enigmatic group of fish called onychodonts that may possess characteristics of both stem and crown sarcopterygians. They swam the Devonian seas some 400 million years ago with their protruding fins – which contain muscles and bones that eventually helped animals adapt to life on land. The well-ossified skull of Qingmendous, an onychodont from China, was discovered just a few years ago. Up until then, the only fossil specimens of this extinct species had poorly ossified braincases.
Using high-resolution CT scanning, a team led by Jing Lu and Min Zhu from the Chinese Academy of Sciences imaged the internal structures of the Qingmendous braincase and then reconstructed a virtual cranial endocast. Qingmenodus, they discovered, bridges the morphological gap between stem sarcopterygians and coelacanths: It has similarities with stem sarcopterygians in one part of the braincase and exhibits coelacanth-like neurocranial features in another.