When tetrapods (four-legged backbones) began to move from water to land around 390 million years ago it promoted the rise of berries, birds, mammals, and all present-day terrestrial animals, including humans and some aquatic spines such as whales. and dolphins.
The earliest tetrapods came from their fish ancestors in the Devonian period and are more than twice as old as the oldest dinosaur fossils. They resembled a cross between a giant salamander and a crocodile and were about 1-2 meters long, had gills, net legs and tail wings, and were still strongly attached to water. The short arms and legs on each hand and leg had up to eight fingers and appeared to be predators, weeping in shallow water waiting for prey to approach.
Scientists know how fish wings changed to tetrapod limbs, but there are still controversies about where and how the earliest tetrapods used their organs. And, while many comments have been suggested, very few studies have rigorously tested them using the fossil record.
In a paper published 22 January in Advances in science an international team of researchers studied three-dimensional digital models of bones, joints, and wing muscles and limbs of two extinct early tetrapods and closely related fossil fish to reveal how function changed the forelimb as the wings changed into members. The research led by Julia Molnar, Associate Professor at the New York College of Technology of Osteopathic Medicine and Stephanie Pierce, Thomas D. Cabot Associate Professor of Organic Biology and Evolution at Harvard University, found three distinct levels of action in the transition from wings to organs. , and that these early tetrapods had a very distinctive pattern of muscle levers that did not look like a fish wing or modern tetrapod limbs.
To recreate what members of the earliest known tetrapod, Molnar, Pierce and co-authors John Hutchinson (Royal College of Medicine), Rui Diogo (Howard University), and Jennifer Clack (Cambridge University) had to do determine which muscles were present in the fossil animals. Challenging activity in that muscles are not retained in fossils, and the muscles of fresh fish are completely different from the muscles of tetrapod limbs. The team spent several years trying to answer the question, how exactly did the few simple muscles of growth become dozens of muscles that perform all sorts of functions in a tetrapod arm? ?
“Determining which muscles were present in a 360-million-year-old fossil took several years of work just to get to the point where we could begin to develop highly complex skeletal muscle models. build, “Pierce said. “We need to know how many muscles were present in the fossil animals and where they were attached to the bones so that we could test how they worked.”
They constructed three-dimensional musculoskeletal models of the inner pectoral wing Eusthenopteron (a tetrapod closely related fish that lived in the late Devonian about 385 million years ago) and forelimbs two early tetrapods, Acanthostega (365 million years old living near the end of the Late Devonian period) and Pederpes (348-347 million years living early in the Carboniferous period). For comparison, they also constructed similar models of the pectoral wings of live tetrapods (coelacanth, lungs) and forelimbs of live tetrapods (salamander, lizard).
To find out how the wings and limbs worked, the researchers used computer software that was originally developed to study human locomotives. This method has recently been used to study locomotion in human ancestors and also dinosaurs such as T. rex, but never in something as old as an early tetrapod.
Handling the models in the software, the team was able to measure two functional features: the maximum range of motion of the joint and the ability of the muscles to move the end joints or limbs. Both measurements would reflect the trade value in the locomotor system and allow the researchers to test assumptions of action in extinct animals.
The forelimbs team found all terrestrial tetrapods that went through three distinct levels of action: a “benthic fish” level that was similar to today’s lung fish, an “early tetrapod” level unlike an extinct animal, and a “crown tetrapod” level with the features of both lizards. and salamanders.
“The fin from Eusthenopteron they had a fish-like lung pattern, which is one of the closest relatives of a tetrapod, “Pierce said.” But the early tetrapod limbs showed more similarities than either. modern fish or tetrapod. “
“Maybe that’s what surprised me,” Molnar said. “I thought Pederpes, and perhaps Acanthostega, falls well within the range of the modern tetrapod. But they created their own unique browser that didn’t look like a modern tetrapod ball or fish shield. In the middle they weren’t a smack dab but had their own collection of features that perhaps reflected the unique environment and behavior. “
The results showed that early tetrapod organs were more suited to movement than weight. In the water, animals use their arms to move to move themselves forward or backward allowing the water to support their body weight. Moving on land, however, the animal must work against gravity and push down with their organs to support their body mass.
That doesn’t mean that early tetrapods weren’t able to move on land, but instead didn’t move like a live tetrapod today. Their locomotive style may have been unique to those animals that were still heavily water-related, but also landed, where there were few opportunities for invertebrates but very few. competition or fear from predators.
“These results are encouraging because they independently support a study I published last year using completely different fossils and methods,” Pierce said. aimed at the upper arm bone, an early tetrapod had the potential for ground movement but may not have been very good at it. “
The researchers are closer to the evolutionary reconstruction of earth movement, but more work is needed. They plan to model the next organ to examine how the four limbs worked together. It was said that early tetrapods used their forelimbs for movement, but today’s tetrapods get most of their moving power from the rear end.
“We look forward to looking for any evidence of a shift from a forelimb-driven locomotion to an end-to-end drive locomotive, like a modern tetrapod,” Molnar said. on the forelimb and hind arm together that could tell more about the transition from water to land and how tetrapod came to take control of the land.
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Author’s article and details
JL Molnar, JR Hutchinson, R. Diogo, JA Clack, SE Pierce. Evolution of forelimb skeletal muscle activity over the transfer of fish to tetrapod. Advances in science 7, eabd7457.
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