study of late fish fossils in Devonian approximately 375 million years ago, detailing the evolution of fins that began to transform into limbs suitable for walking on land. University of Chicago This new study by paleontologists, published in the Proceedings of the National Academy of Sciences, uses CT scans to examine the shape and structure of fin rays that are still trapped in surrounding rocks. These imaging tools allow researchers to construct a digital three-dimensional model of the entire fin of the fish pod Tiktaalik and its close relatives in fossil records for the first time. These models can then be used to infer how fins work and change as they evolve into limbs.
At this critical transition phase, most of the research on fins has focused on huge, unique skeleton and cartilage fragments corresponding to our upper arms, forearms, wrists and fingers. The researchers tracked how these bones, known as "endoskeletons", transformed into recognizable arms, legs, and fingers in tetrapods, namely tetrapods. The slender rays and spines on the fins form the second equally important "dermal" skeleton, which also undergoes evolutionary changes during this period. These fragments are often overlooked because when animals become fossils, they may shatter, or because fossil makers deliberately remove them to expose the larger bones of the endoskeleton.
dermal rays formed most of the surface area of many fins, but disappeared completely in the earliest organisms with limbs. "We are trying to understand the overall trends and evolution of dermal bones before all other changes occur and mature limbs evolve. If you want to understand how animals evolved to use fins in this history, this will be an important data set. The ancient fins in
3D simulations
Stewart and colleagues studied three late Devonian fishes with primitive characteristics of tetrapods: the lizard (Sauripterus Taylori), the real dragon (Eusthenopteron Foordi) and the Tiktaalik Roseae, which was discovered by a team of senior authors of the new study and a paleontologist at the University of Chicago. Lizards and Eusthenopterons are considered to be completely aquatic and use their pectoral fins to swim, although they may have been able to support themselves at the bottom of lakes and streams. Tiktaalik may have been able to support most of its weight with its fins.
might even use them to venture out of the water for short trips through shallows and mudflats. By looking at the entire fin of Tiktaalik, you can get a clearer understanding of how it supports itself and moves. The fin has a palm that can be placed flat on the muddy bottom of rivers and streams. Stewart and Shubin collaborated with undergraduate student Ihna Yoo and Dr. Justin Lemberg, another researcher in Shubin's lab, to scan the fossils while they were also wrapped in rocks. The 3D model is then reconstructed using imaging software so that they can move, rotate and visualize the dermal bones as if it were completely extracted from the surrounding material. The
model shows that the fin rays of these animals have been simplified, and the overall size of the fin mesh is smaller than that of the predecessor. Surprisingly, the top and bottom of the fins also became asymmetric. Fin rays are actually formed from pairs of bones. For example in Eusthenopteron, the dorsal or apex fin is slightly larger and longer than the ventral or bottom fin. Tiktaalik's dorsal rays are several times larger than the abdominal rays, which suggests that there are stretched muscles on the underside of the fin, like the fleshy bottom of the palm to help support its weight. This provides further information to let us understand how animals like Tiktaalik use fins in this transformation.
Animals go from swimming freely, controlling the surrounding water flow with their fins to adapting to pushing the water surface open at the bottom of the water. Stewart and colleagues also compared the dermal bones of live fish such as sturgeon and lungfish to understand the patterns seen in fossils. Some of the same asymmetric differences were found at the top and bottom of the fins, suggesting that these changes played a greater role in fish evolution.This gave the researchers more confidence, and another set of data suggests that these patterns are real, broad and important to fish, not only in fossil records, as it is related to fin-to-limb transitions, but also on a wider scale, the function of fins.
Boke Park｜Research/From: University of Chicago Medical Center
Reference journal Proceedings of the National Academy of Sciences
DOI: 10.1073/pnas.1915983117
Boke Park｜Science, science, scientific research, popular science
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