Quirks & Quarks

Fish fingers and bilateral symmetry — new fossils shed light on critical stages of evolution

Recent discoveries give clues about two critical stages in the evolution of modern complex animals

Recent discoveries give clues about two critical stages in the evolution of modern complex animals

Artistic representation of the first land dwelling animal, Elpistostege watsoni (Katrina Kenny)

Originally published on May 23, 2020.

A tiny 555-million year old worm the size of a grain of rice discovered in the Australian outback is the earliest animal ever found with bilateral symmetry — a front and back with openings at each end connected by a gut, and left and right symmetry.

"If you think about everything from insects to us, those [worms] have that same basic body plan," said Scott Evans from the University of California, Riverside, who was part of the team that made the discovery. 

And right here in Canada, two different research teams describe fossils from 200 million years later that straddle a key evolutionary transition: the transformation of fins into limbs that occurred when our ancestors, the earliest vertebrate animals, moved onto land. 

A template for complex animal body types

Evans said that they were surprised to actually find remains of a creature, dubbed Ikaria wariootia. They had previously only suspected its existence based on the burrows it left behind.

Artist's rendering of Ikaria wariootia, the first fossilized evidence of bilateral symmetry (Sohail Wasif/UCR)

"It just seemed like the conditions that we would need to preserve that organism were gonna be so rare that we would never see it, and even if we could see it, that we wouldn't be able to recognize it," Evans told Quirks & Quarks host Bob McDonald

This creature is the first glimpse into where complex body shapes we see in animals today, from crustaceans to mammals, likely originated. 

Will Fish getting a 'fin-hold' on land

Another key event in the evolution of animals is when early fish species developed the ability to walk on land.

Neil Shubin from the University of Chicago told Quirks & Quarks' McDonald about three fish fossils he studied that he discovered in Ellesmere Island. He says together show most of the evolutionary steps it took for our fish ancestors to trade in their fins for limbs. 

Artist’s reconstruction of Neil Shubin's three fish fossil species. From top to bottom: Sauripterus taylori, Eusthenopteron foordi, and Tiktaalik roseae. The one on the bottom showed all the characteristics of being able to hoist itself up on its fins to get around in mudflats. (Alex Boersma)

He said the star of the three fossil species is Tiktaalik roseae, with both lungs and gills, and fins with wrist and elbow-like bones. It's fin has a palm-like end which he thinks was capable of supporting the fish to do a "kind of a pushup where the animal could walk about." 

But did it live in the water or on land? Shubin said "that's the million dollar question and our answer is both."

Fossil evidence of the first land-dwelling animal

Three thousand kilometres south of Ellesmere Island, where Shubin found his three fish fossils, a Canadian-led team found what might be the next evolutionary step — the very first land-dwelling "tetrapod' or four-limbed animal. 

Richard Cloutier from the University of Quebec in Rimouski said the fossil he found, Elpistostege watsoni, looks much like Shubin's Tiktaalik fossil. It has eyes on the top of its head, and a slender body with one extra special feature: the first finger-like digits in the history of vertebrates.

Fossil reveals early ‘fish fingers’ from what may be the first land-dwelling animal, Elpistostege watsoni (John Long/Richard Cloutier’s lab)

"We can consider an animal to be a tetrapod if that animal has digits like what we have in humans. So using that specific definition, then we can say that Elpistostege is not the last fish before tetrapod, but in fact it's the first Tetrapod."

Produced by Sonya Buyting and Mark Crawley. Written by Sonya Buyting.


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