The Weird Trait That Took Over the Permian

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Big news Eons fans! We made a tv show! Eons: Life and Death on Pangea is our most  ambitious project yet, and it’s now available! Check out the link in the description for details,  and we’ll tell you more later

in the episode. 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 they were  experimenting with became such key   parts of our modern world that it’s hard  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 for their time,  but are now seen in many species today. But there’s one trait from the

Early  Permian that was the complete opposite:   it was extremely common back then, but it’s  almost totally absent in living species today. And it’s 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, and 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 in 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 he’d been  describing from fossil sites mostly in Texas. These included the now-famous Dimetrodon –  the world’s first terrestrial apex predator,   as well as Edaphosaurus – one of  the world’s first big herbivores.

They belonged to a collection of  species he called pelycosaurs,   a group of synapsids – vertebrates who  were more 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 didn’t  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 long spines probably  acted like the masts of a ship, holding a   membrane that could catch the wind, allowing these  animals to cruise around ancient Permian lakes. This hypothesis was worth a shot,  I guess, but it didn’t

hold water… Mainly because, for Dimetrodon and Edaphosaurus,   water didn’t hold them – these were animals  built for life on land, not for swimming. And since Cope’s time, the  mystery has only deepened. As we’ve 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 didn’t  inherit their sails from a common ancestor,   they evolved them independently  right around

the same time… As did another pelycosaur named Echinerpeton,   a little earlier than the others,  in the Late Carboniferous Period. And, as well as evolving at 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 – we’ve never seen  another period where they’re so common. We’ve also never seen them re-emerge  on

our synapsid side of the family   tree – despite this being where they first  appeared and reached their peak prevalence. So what could have been so 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 they would’ve  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, more  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   would’ve also been pretty unhelpful  in Dimetrodon’s day-to-day life. So if they came with some 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   what’s 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  them 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 were 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, this hypothesis  has run into its own issues, too. Newer research

from the 2010s onward, for  example, looked at the microscopic details of   pelycosaur sail bones at 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 in west Texas,  for “Eons: Life and Death on Pangea”. It’s one of the most amazing  experiences I’ve ever had. Anyway, if their main function  was thermoregulation, we’d expect   to see evidence

of a complex network of  blood vessels wrapped around the bones. This would help circulate blood around the sail  and then drain it back to the rest of the body. But the researchers didn’t  see evidence of this at all.

There simply weren’t 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 that’s much  harder to do when you’ve got a   huge sail that you’ve 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 been especially useful  at this time because, as far as

we can tell,   Early Permian species weren’t  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 was 

their solution to this problem. So gigantic sails may have been the Permian  equivalent of peacocks’ tails – elaborate   structures used for things like 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,  it’s often an evolutionary flex for   them to be unwieldy and

inconvenient  – it proves that individuals are fit   and healthy enough to survive and  thrive in spite of having them! And if the sails of the Late Carboniferous  and Early Permian were indeed a result of   this process, that would

make them  among the very first – and most   extreme – sexually selected traits in the  entire history of terrestrial vertebrates. But here’s the thing…we still don’t 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 the  meaning of the

sail would first   require us to understand the mind  of a pelycosaur looking for love. And maybe that's something humans  just were never meant to know. We are so excited to tell you about our new  tv series, Eons: Life

and Death on Pangea. We’ve been working on this for the  last few years, and it’s finally ready. The six-part series, set in the Permian Period,  tells the story of the formation of Pangea when   life -- unlike anything seen

today -- was  flourishing, but it was a time that ended   in tragedy, with the most catastrophic  mass extinction in our planet’s history. It’s free on PBS platforms  including the app. Check out   the link in the description  for more

on where to watch. We also gotta sail-abrate  this month’s Eontologists! Addie, Amanda Seabock, Annie &  Eric Higgins, Benjamin Birdsall,   Carl Woelfel, Jackie Scott-Ralston,  Jake Hart, John Davison Ng, JuanM,   Melanie Lam Carnevale, Nico Robin, Raphael  Haase, Tony Dai, Victor

Angkico, and S.T.E.V.E. Become an Eonite at patreon.com/eons and you   can get fun perks like a monthly  digital puzzle of Eons paleo art! That sounds really fun. And as always thanks for joining me  in the Annie and Eric Higgins

studio.   Subscribe at youtube.com/eons  for more ancient adventures. Ears and eardrums had not evolved yet,  and those vertebrates are essentially   thought to have been functionally deaf  to most frequencies of airborne sounds. And that makes me wonder, like, what  did

the Permian world sound like? If no one on Earth could hear,  what did Earth sound like? If you write that script, Farhan, I want to  host it, because I want to know the answer. Okay? Legit.