Around 300 million years ago, in the early Permian Period, life on land was a cauldron of strange experiments.

Species leaving the waterlogged swamps of the Carboniferous Period were beginning to conquer dry land for the first time, forming the earliest fully-terrestrial vertebrate ecosystems.

And many of the traits the were experimenting with became such key parts of our modern world that its har to imagine life on land without them.

Think strong limb bones that can support big bodies, eggs with shells that can be laid away from water, and even just eating plants.

All of these were radical fo their time, but are now seen in many species today.

But theres one trait from the Early Permian that was the complete opposite: it was extremely common back then, but its almost totally absent in living species today.

And its arguably the first truly bizarre trait in the history of vertebrate life on land.

A trait that, despite over a century of thought, we still struggle to explain.

This is a tale about sails, an the enigma of why, for a brief but critical phase in the history of life, sporting a big biological billboard was all the rage.

The mystery of sails began i the late 1800s, with one of the most prolific paleontologists in history, Edward Drinker Cope (he was also a jerk).

Cope noticed a series of long, bony spines emerging from the vertebrae of many ancient Permian beasts that hed been describing from fossil sites mostly in Texas.

These included the now-famou Dimetrodon the worlds first terrestrial apex predator, as well as Edaphosaurus one of the worlds first big herbivores.

They belonged to a collection of species he called pelycosaurs, a group of synapsids vertebrates who were mor closely related to mammals than to reptiles that dominated life on land in the Early Permian.

And while Cope was understandably puzzled by the gigantic sails that they sported, this was a man who infamously didnt hesitate to give his opinions.

And his guess as to their function was a simple one: sails were for sailing.

He argued in 1886 that the lon spines probably acted like the masts of a ship, holding a membrane that coul catch the wind, allowing these animals to cruise around ancient Permian lakes.

This hypothesis was worth a shot, I guess, but it didnt hold water Mainly because, for Dimetrodon an Edaphosaurus, water didnt hold them these were animals built for life on land, not for swimming.

And since Copes time, the mystery has only deepened.

As weve unearthed more pelycosaurs and reconstructed their family tree in ever-greater detail, it has become clear just how common sails were during this slice of deep time.

For one, Dimetrodon and Edaphosaurus didnt inherit their sails from a common ancestor, they evolved them independentl right around the same time As did another pelycosaur named Echinerpeton, a littl earlier than the others, in the Late Carboniferous Period.

And, as well as evolving a least three times independently in pelycosaurs, sails also emerged in unrelated species that lived alongside them, like the tiny amphibian Platyhystrix.

Sailed species seem to have been nearly everywhere across the Late Carboniferous and Early Permian, but these structures then disappeared from the Permian fossil record as mysteriously as they arrived.

And while a few other vertebrates evolved sail-y structures again in the hundreds of millions of years since a Spinosaurus here, a basilisk lizard there weve never seen another period where theyre so common.

Weve also never seen the re-emerge on our synapsid side of the family tree despite this being where they firs appeared and reached their peak prevalence.

So what could have been s great about having a sail that pushed it to evolve over and over again in this one short window of time, in species that occupied totally different ecological niches?

Especially seeing as the wouldve had to devote a lot of energy and resources to growing such a large and complex structure.

There must have been some intense, widespread selective pressure behind it, right?

Right?

Well, one early idea was that it might have been a defense against predators, making individuals look bigger, mor intimidating, or just harder to chomp.

Now this would make sense for the species that had to worry about the threat of predation.

But it kinda falls apart when you bring later, larger Dimetrodon species into the picture.

The largest of these were apex predators that weren't preyed on by any other species, yet they had the biggest sails of all!

So if sails played a defensive role against predation, why would animals that didn't have any predators still go to the effort of sprouting them?

And, being an ambush predator, having a massive sail that advertised your presence wouldve also been pretty unhelpful in Dimetrodons day-to-day life.

So if they came with som significant tradeoffs, and they weren't actually for defense, then what were they used for?

Well, beginning in the mid-20th century, many researchers favored whats called the ‘thermoregulatory hypothesis as a stronger explanation.

This hypothesis essentially proposes that sails helped these ancient animals better regulate their body temperatures.

By angling it toward the sun, the large surface area of the sail would have helped the warm up faster in the mornings.

And during hotter parts of the day when they needed to cool down, sails could release heat like giant biological radiators.

This explanation became popular in the scientific community because, in theory at least, it makes a lot of sense.

Thermoregulation is a pretty universal challenge.

So if this was what sails wer being used for, it wouldn't be surprising that multiple lineages evolved them convergently, even in totally different ecological niches.

Especially seeing as, this far back, species weren't able to self-regulate their body temperature like many can today.

But as time has gone by, thi hypothesis has run into its own issues, too.

Newer research from the 2010 onward, for example, looked at the microscopic details of pelycosaur sail bones a high resolution, to gain better insights into how they actually worked.

These are the bones that I actually got to excavate and hold in my hands when we were working on location i west Texas, for “Eons: Life and Death on Pangea”.

Its one of the most amazing experiences Ive ever had.

Anyway, if their main function was thermoregulation, wed expect to see evidence of a complex network of blood vessels wrapped around the bones.

This would help circulate bloo around the sail and then drain it back to the rest of the body.

But the researchers didnt see evidence of this at all.

There simply werent enough blood vessels running across the sails for thermoregulation to actually work very well.

Plus, having a sail might have actually made some aspects of thermoregulation harder.

When cooling down, for example, species often seek shade by entering crevices or burrows something thats much harder to do when youve got a huge sail that youve got to somehow fit into those shady spots with you.

So instead, the focus has now shifted to alternative explanations, like maybe sails had more of a social and sexual function.

These animals might have used their sails as a means of communication and species-recognition, for example.

And this could have bee especially useful at this time because, as far as we can tell, Early Permian species weren communicating much using sound.

Ears and eardrums had not evolved yet, and those vertebrates are essentially thought to have been functionally deaf to most frequencies of airborne sounds.

Maybe being visually loud wa their solution to this problem.

So gigantic sails may have been the Permian equivalent of peacocks tails elaborate structures used for things lik signaling health and fitness to potential mates, intimidating rivals, and other forms of social interaction.

And sexual selection in particular could explain why those animals developed such huge and costly structures that had no other obvious benefits in day-to-day life.

When traits are sexually selected, its often an evolutionary flex for them to be unwieldy and inconvenient it proves that individuals are fit and healthy enough to survive an thrive in spite of having them!

And if the sails of the Lat Carboniferous and Early Permian were indeed a result of this process, that woul make them among the very first and most extreme sexually selected traits in the entire history of terrestrial vertebrates.

But heres the thingwe still dont know for sure.

And perhaps we never will.

For over a hundred years, scientists have been trying and failing to pin the sail on a purely utilitarian function.

Maybe the answer has nothing to do with utility, though.

Maybe they were just really sexy?

Perhaps truly understanding th meaning of the sail would first require us to understand the mind of a pelycosaur looking for love.

And maybe that's somethin humans just were never meant to know.