A creature from the era of land conquest: paleontologists present a “missing link” in the evolution of prehistoric fish to the first land vertebrates. The fossil, about 380 million years old, provides insights into how the vertebrate hand once emerged from fins. The results show that the development of the characteristic basic structure of these limbs began with the fish.
It was one of the most momentous developments in the history of evolution: some primeval fish gradually produced special fin structures that eventually enabled them to move ashore. At some point, limbs formed from it, which we would call hands and feet. This is how the first land vertebrates (tetrapods) came into being, which also included humans. Fossils indicate that land vertebrates first appeared around 374 million years ago – literally. However, some older fossils already suggested that the roots of their characteristic limb structures go deeper into the history of development.
“Missing Link” presented
For some time now, research into the early history of evolution has focused on fossils of beings called Elpistostegalia. They lived in the middle and late earth ages of the Devonian – 393 to 359 million years ago. The anatomy of these fish already shows some parallels to that of the land vertebrates. So far, however, an important piece of the puzzle has been missing: there were no Elpistostegalia fossils that showed the complete anatomy of the pectoral fins. But that has now changed: “We are now presenting the fossil of a complete specimen of a tetrapod-like representative of Elpistostegalia, which provides new information on the development of the vertebrate hand,” says co-author John Long from Flinders University in Adelaide.
The fossil comes from a locality in Canada, where a flat sea stretched millions of years ago. There, the fish named by the researchers Elpistostege watsoni probably lived in the coastal area. Elpistostege was probably the largest predator in its habitat and snapped its tooth-studded mouth for fish, the fossils of which were also discovered at the site. As part of their study, the scientists meticulously exposed the 1.57-meter-long specimen and, above all, examined the animal’s well-preserved pectoral fin. Computer tomography was used to make even the finest details visible. For the first time, they were able to show exactly how the front fins were structured at the beginning of the development into land vertebrates.
Basic structures of the arm, hand and fingers
As they report, an upper arm bone, a forearm bone, hand bone and especially finger structures emerged. “This is the first time that we have clearly seen fingers embedded in a fin with fins. The finger-like structures in the fin resemble the finger bones in the hands of most animals today, ”says Long. “This finding confirms the origin of the finger structures in fish and shows that the structure pattern of the vertebrate hand emerged shortly before the fish left the water,” said the paleontologist.
“The development of these finger-like structures is related to the development of the ability of these fish to carry their weight in shallow water or for short trips to the country. The increased number of small bones in the fin gave more flexibility to better distribute the weight over the fin, ”explains the study’s first author, Richard Cloutier from the Université du Québec in Rimouski. “Other features that we found in relation to the structure of the humerus also show common features with the properties of early amphibians,” said the paleontologist.
The now revealed characteristics of Elpistostege watsoni also gave an impression of how this creature once looked and how it could have behaved. These ideas formed the basis for a short animated film developed by the researchers. It shows how Elpistostege watsoni swims through the water, hops onto a rock and straightens its front limbs. Finally, Cloutier emphasizes: “Elpistostege is not necessarily our direct ancestor, but it is the best example of one of the transitional beings between the fish and the tetrapods,” said the paleontologist.
Source: Flinders University, Technical article: Nature, doi: 10.1038 / s41586-020-2100-8