[Animals] When Did Animals First Lose Their Legs? We Finally Have the Answer

[#Wittels-]

Judging by the snakes, the evolutionary solution to limblessness is a success. Now, a fossil found in Illinois offers the oldest known legless reptile, and suggests it could be an ancient condition.

Artist's impression of Micraroter recumbirostran microsaur, a contemporary of Nagini, but still has forelimbs while Nagini does not.

 

Locomotion is good, but sometimes, losing one’s leg seems to confer an evolutionary benefit. This is evidenced by the fact that limblessness has evolved over 60 times just in squamates (scaled reptiles). It has evolved in other groups too, though never in mammals.

Inquiring minds wonder when all this began, and how. Did all four limbs disappear at the same time? The front ones first? Back ones first? Did it happen the same way in all limbless animals? What exactly is that on a python’s bottom, insofar as it has one?

In short, did our very, very distant ancestors possess an adaptive ability to lose legs?

That could be a yes. A new paper published in Nature identifies the earliest-known complete loss of legs among amniotes. Meet Nagini mazonense, an early reptile that lived in Carboniferous-era Illinois at least 307 million years ago. This animal, a type of microsaur, had an elongated snakelike body and had lost its front legs, but not its back ones. Though those hind limbs seem somewhat reduced.

 

Not only did Nagini have no forelimbs whatsoever: It had even lost its pectoral girdle, the bones to which the arms would have connected, if there had been any.

This is the earliest known complete limb loss detected in an amniote, say Arjan Mann of the Smithsonian Institution, Jason Pardo of the University of Calgary, and Hillary Maddin of Carleton University.

All four-legged animals are tetrapods, meaning they evolved from fish. Amniotes are a subset of tetrapods. Amniotes include all animals that develop immersed in amniotic fluid (even inside the egg) such as mammals, birds and reptiles. Fish and amphibians are not amniotes.

 

A viper, an example of the lack of legs as a good evolutionary solution

 

Remarkably, Nagini's fossilization in shale at Francis Creek was good enough to preserve well-defined impressions of its soft tissue. These impressions support paleontologists' interpretation that the animal's body was serpentine, Mann explains.

He adds that in practically the same place and from the same time, he and other paleontologists found another microsaur, Joermungandr bolti, with a similar body pattern: lengthening and shortening of limbs, but it still had limbs. Including the forelimbs.

Nagini is the first amniote known to lose legs, as far as we know, but it's not the first tetrapod to do so. That honor goes to extinct amphibians called aisopods that lived in the Carboniferous and Early Permian in Europe and North America and some were quite large, up to a meter long.

 

The discovery of complete loss of forelimbs in Nagini, a creature at or near the base of Amniota, argues that limb loss is an ancient choice among amniotes. When considered together with the lack of limbs in early amphibians like the isotopod that lived at the same time, it is possible that the feature may be even more entrenched and ancestral to Tetrapoda, the team write.

Vestigial snake legs? Not quite

Technically, Nagini is a recumbirostrant molgophid, which is believed by some, but not all, paleontologists to be an ancient reptile. Their evolutionary pattern of losing their front legs first is the same as that of snakes.

In other parts of the animal kingdom, the legless lost their hind legs first, the authors explain. An example is the Mexican burrowing lizard known as Bipes, po[CENSORED]rly called axolotl or mole lizard. They still have vestigial front legs.

 

The burrowing Mexican lizard Bipes, po[CENSORED]rly called ajolotes or mole lizards, still have vestigial front feet

 

So one conclusion is that the evolutionary serpentine pattern of losing the front legs first may not be as rare among the amniote set as previously thought.

It bears adding that studying leglessness in ancient animals is tricky: One is looking for fossils of things that are not there as well as exquisite preservation of the being’s vertebral column. Also, if it had vestigial limbs, being small and delicate they might not preserve well. Nagini however had completely lost its pectoral girdle, which says it all.

But why would ancient animals lose legs anyway? Running is good, whether to hunt or evade being eaten, so why has this pattern of leglessness on land evolved time and again – in reptiles, mind you (and some salamanders and another amphibian, the caecilian)?

 

A snake's mating spurs. No they are not vestigial legsCredit: Dawson
Note that the limb loss is accompanied by elongation, as exemplified by snakes and caecilians, and that there are zero mammals that went this route. That could indicate that losing limbs is an inherent privilege that developed within the squamate set but not a potential solution for all tetrapods and one would assume it has to do with niche capture, for instance, burrowing through sand and dirt.

In separate work, paleontologists believe they have identified limblessness in early snakes in correlation with starting to burrow for shelter and to hunt.

 

But a lot of mammals do that too – take the naked mole rat, the vole and the badger. Legless, they are not. The authors suggest that the absence of serpentine forms in mammals, frogs and turtles may be due to evolutionary constraints within these groups.

Finally, remember we said snakes lose their front legs first? There had been a theory out there that the boa constrictors, anacondas and pythons have vestigial hind legs nestled in their rear-sector muscle.

They do not. Experts reassure us all, what you’re seeing, if you’re looking there, are mating spurs.

Mating spurs are fun. Giant snakes utilize this clawlike feature while moseying along the ground, to help them along when need be, when climbing trees, and when courting – to caress and stimulate the lady snake. We are not making this up. It’s in Herpetological Review.

 

Link: https://www.haaretz.com/science-and-health/.premium-when-did-animals-first-lose-their-legs-we-finally-have-the-answer-1.10703606