Ancient sea creatures pioneered the gallop

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When horses or cheetahs feel the need for speed, they adopt a gait known as a gallop. Linked rabbits. Some aquatic animals crawl using forms of locomotion that rest on their fins, called crutches or rowboats. All of these moves are special – the timings of the left and right footsteps or flippers are not equally spaced.

Not all animals rely on these types of movements to move quickly. However, so-called asymmetrical gaits are not new to vertebrates, scientists have reported on March 8 in the Journal of Experimental Biology. The researchers analyzed observations of movements from hundreds of species and concluded that irregular gaits may have first appeared in ancient fish-like animals even before vertebrates made the journey to land. The results suggest that different groups of animals gained and lost the ability to use asymmetrical gaits throughout vertebrate history.

Although it is “more or less” known that land animals evolved independently of these movements, “the idea that this ability is ancient for jawed fish is relatively new and intriguing,” John Hutchinson, professor of evolutionary biomechanics at the Royal Veterinary College Hawkshead Campus in Hatfield, England, who was not involved in the research said in an email.

When an animal walks or trots, it moves its limbs in a smooth, regular pattern known as the symmetrical gait. To travel faster, many animals can adopt asymmetrical gaits, says Eric McElroy, professor of biology at the College of Charleston in South Carolina and co-author of the findings. A classic example is the gallop of a horse.

In a canter, all four feet hit the ground at different and unevenly spaced times, McElroy and collaborator Michael Granatosky, of the New York Institute of Technology at Old Westbury, wrote in the paper. Mammals aren’t the only gallopers; some crocodilians have also been observed using this gait.

Gazelles can achieve another type of asymmetrical gait called pronking, which involves leaping into the air and landing on all four feet simultaneously. Toads and rabbits use leaping or semi-leaping gaits, in which both hind legs touch the ground at the same time. Mudskippers, sea turtles and some seals move their front flippers simultaneously in a “crutch” gait move on land. Some rays and other fish emerge, simultaneously moving their pelvic fins to move along the seabed.

There are also vertebrates that do not appear to use asymmetrical gaits, including lizards, salamanders, platypus, hedgehogs, lorises, and elephants.

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To understand when these gaits first appeared, McElroy and Granatosky looked at the ratios of symmetrical and asymmetrical movements in 308 different species of gnathostomes, or jawed vertebrates. The vast majority of modern vertebrates belong to this group, with the exception of hagfishes and lampreys.

The researchers used computer models to study four different evolutionary scenarios. In one, the common ancestor of gnathostomes had the ability to move asymmetrically, and its descendants could lose this ability but not regain it. Another model assumed that the trait could only be acquired, implying that the ancestor of the gnatosome did not have an asymmetrical gait.

In the third model, asymmetric gaits appeared and disappeared at approximately equal rates in the family tree. The fourth model removed the rate constraint. This allowed the organisms “to have very rapid evolution of asymmetric gaits and very slow losses of asymmetric gaits,” says McElroy. “They can be very different in terms of the speed of evolution.”

He and Granatosky found this fourth scenario to be the most likely, based on how asymmetrical gaits are distributed among modern vertebrates. They calculated that the gnathostome ancestor had about a 75% chance of using some sort of asymmetrical gait.

This fish-like creature probably inhabited shallow coastal seas 400 to 450 million years ago, about 25 to 100 million years before some of its descendants invaded land. The gnathostome ancestor may have used its fins to crutch itself or throw itself on the seabed, McElroy says, noting that many fossils of early jawed vertebrates resemble modern-day fish that use these movements such as rays and stingrays. rays.

The team also determined that the ancestor that gave rise to modern mammals likely had the ability to move asymmetrically, unlike the ancestors of amphibians and lizards.

It is unclear why asymmetrical gaits have been lost in some vertebrate groups. Elephants can be too big to gallop without putting dangerous pressure on their bones. Some animals, such as lorises and many turtles, may never move fast enough to require asymmetrical gaits.

Lizards can dash very quickly without moving their limbs asymmetrically. “I’ve never seen a lizard gallop, and it’s weird that they don’t and that suggests some kind of neuromuscular restraint,” McElroy says. “I’d like to take a closer look at this to really understand that lizards just aren’t able to do this, or is this something they do very, very rarely, or certain groups of them can do this. and haven’t you just studied these groups?”

The 308 living animals the researchers examined represent only a fraction of the roughly 69,000 species of vertebrates. This, Hutchinson said, could have skewed the results of the scan.

Sightings of modern fish species were relatively rare, acknowledges McElroy. “Fish will tend to have a greater effect on the [evolutionary] reconstructions because they are the oldest species,” he says, and represent “both a limitation and an area for future discovery.” Accounting for the locomotion of extinct vertebrates would also improve these estimates, but little is known about how they moved.

Despite these limitations, Hutchinson said, “the study is valuable in that it synthesizes a lot of data with good, scalable tools and will prompt further investigation of the questions it raises.”

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