The collapse

A couple of weeks ago, I wrote about the spike in atmospheric oxygen concentration -- by some estimates, rising to 35% -- during the Carboniferous Period, triggered by explosive growth of forests, and allowing arthropods like insects, arachnids, and millipedes to grow to enormous sixes.

The good times, though (for them at least), were not to last.  Around three hundred million years ago, there was a sudden drop in oxygen and rise in carbon dioxide.  This triggered rapid climatic shifts that resulted in the Late Carboniferous Rain Forest Collapse, which saw a major alteration from the swamp-dwelling plants and animals at the height of the period to species that could tolerate the dry heat that was to persist throughout the next period, the Permian.  (This set up the rise of reptiles, which would see their peak in the dinosaurs of the Mesozoic.)

Artist's depiction of the mid-Carboniferous swamps (ca. 1887) [Image is in the Public Domain]

The source of the excess carbon dioxide was very likely volcanic.  Besides the fact that lava can contain dissolved gases (mostly carbon and sulfur dioxide), the heat of the eruptions may have caused the oxidation of the plentiful limestone and coal deposits formed during the earlier lush, wet part of the period -- a precursor of the much bigger disaster that was in store fifty million years later, when at the end of the Permian, the Siberian Traps erupted and tore through a huge amount of the sequestered carbon, causing widespread global anoxia and climate change, and the largest mass extinction ever.

By some estimates, ninety percent of life on Earth died.

But the rain forest collapse at the end of the Carboniferous was bad enough.  A study that came out this week in Proceedings of the National Academy of Sciences found the anoxia/hypoxia hit the oceans the hardest, where the oxygen levels rapidly dropped by between four and twelve percent, with a commensurate rise in dissolved carbon dioxide.  When carbon dioxide dissolves in water, it produces a weak acid -- carbonic acid -- lowering the pH.  Organisms that make their shells out of calcium carbonate, like mollusks, brachiopods, and corals, literally dissolved.

You ready for the kicker?

The study's estimate of the rate of carbon dioxide release during the Late Carboniferous Rain Forest Collapse is a hundred times smaller than the rate we're putting carbon dioxide into the atmosphere today through burning fossil fuels.

"This is a huge discovery, because how do you take an ocean sitting under an atmosphere with much more oxygen than today and permit this?" said Isabel Montañez of the University of California - Davis, senior author of the study.  "The message for us is, 'Don't be so sure that we can't do this again with our current human-driven release of carbon dioxide.'"

The problem is, the current administration is in the pockets of the fossil fuel industry, and is doing their level best to pretend this isn't happening, and to discredit anyone who says it is.  Worse, actually; they've cancelled funding for any scientific research about climate.

Because apparently "la la la la la la not listening" is now considered wise political policy.  This, despite warning signals like the eastern half of the United States sweltering this past week under the most extreme heat wave we've had in over fifty years.

So I'm expecting studies like the one released this week by Montañez et al. to receive exactly zero attention from the people who actually could work toward addressing this situation.  It brings to mind a quote from Upton Sinclair, uttered almost a century ago: "It is difficult to get a man to understand something when his salary depends on his not understanding it."

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Creepy crawlies

Whenever we have a wet summer -- not an uncommon occurrence in our rainy climate -- we have a plague of little pests trying to get into our house.

They're called millipedes, slinky guys maybe a couple of centimeters long, with lots of legs (not a thousand, though).  They're completely harmless; they don't bite like their cousins the centipedes do, and if you poke at them, they coil up into a ball.  So I guess they're really more of a nuisance than an actual problem.  They don't even damage anything, the way mice can.  Mostly what they seem to do is get in through every crack and crevice (there are lots of these in a big old house like ours), look around for a while, then curl up and die.

[Image licensed under the Creative Commons Totodu74, Anadenobolus monilicornis 03, CC BY-SA 3.0]

So I don't like them, and I wish they stayed outside, but in the grand scheme of things they're no big deal.  Imagine, though, if they were bigger.

A lot bigger.

Recently, paleontologists announced the discovery on a beach in Northumberland, England, of a millipede fossil from the Carboniferous Period.  It's been dated to the middle of the period, about 326 million years ago.  It looks a bit like the millipedes I see trundling across my basement floor in summer.

Only this one was 2.6 meters long (approximately the length of a Mini Cooper), a half a meter across, and weighed something on the order of fifty kilograms.

It's been named Arthropleura, and holds the record as the largest-known arthropod in Earth's history.  Nothing is known for sure about its behavior; if it's like the rest of millipedes, it was a scavenger on leaf detritus, but there's no way to know for certain.  Given its size, it could well have been a lot more dangerous than the ones we have around now.  To paraphrase the old joke about five-hundred-pound gorillas:

Q: What does a fifty-kilogram millipede eat?

A: Anything it wants.

Those of you who are (like me) biology nerds may be frowning in puzzlement at this point.  How on earth could an arthropod get so big?  Their size is limited by the inefficiency of their respiratory system (not to mention the weight of their exoskeletons).  Most arthropods (millipedes included) breathe through pairs of holes called spiracles along the sides of the body.  These holes open into a network of channels called tracheae, which bring oxygen directly to the tissues.  Contrast that with our system; we have a central oxygen-collecting device (lungs), and the hemoglobin in our blood acts as a carrier to bring that oxygen to the tissues.  It's a lot more efficient, which is why the largest mammals are a great deal bigger than the largest arthropods.  (So, no worries that the bad sci-fi movies from the 50s and 60s, with giant cockroaches attacking Detroit, could actually happen.  A ten-meter-long cockroach not only wouldn't be able to oxygenate its own tissues fast enough to survive, it couldn't support its own weight.  It wouldn't eat Detroit, it would just lie there and quietly suffocate.)

So how could there be such ridiculously enormous millipedes?

The answer is as fascinating as the beast itself is.  As the temperature warmed and rainfall increased after the previous period (the Devonian), it facilitated the growth of huge swaths of rain forest across the globe.  In fact, it's the plant material from these rain forests that produced the coal seams that give the Carboniferous its name.  But the photosynthesis of all these plants drove the oxygen levels up -- by some estimates, to around 35% (contrast that to the atmosphere's current 21% oxygen).  This higher oxygen level facilitated the growth of animals who are limited by their ability to uptake it -- i.e., arthropods. (At the same time, there was a dragonfly species called Meganeura with a seventy-centimeter wingspan.  And unlike millipedes, these things were carnivores, just as modern dragonflies are.)

Eventually, though, the system was unsustainable, and a lot of the rain forests began to die off in the Late Carboniferous, leading to a drier, cooler climate.  However, remember the coal seams -- by that time a huge percentage of the carbon dioxide that had fed the photosynthesis of those rain forests was now locked underground.  The fuse was lit for a catastrophe.

Fast forward to the end of the next period, the Permian, 255 million years ago.  What seems to have happened is a series of colossal volcanic eruptions that created the Siberian Traps, a basalt deposit covering most of what is now Siberia.  The lava ripped through the coal seams, blasting all that stored carbon into the atmosphere as carbon dioxide.  The temperature in the late Permian had been cool and dry, and the spike of carbon dioxide created a commensurate spike in the temperature -- as well as a huge drop in oxygen, used up by the burning coal.  The oxygen concentration seems to have bottomed out at around twelve percent, just over half of what it is now.  The extra carbon dioxide dissolved into ocean water, dropping the pH, and the increasing acidity dissolved away the shells of animals who build them out of calcium carbonate -- e.g. corals and mollusks.

Wide swaths of ocean became anoxic, acidic dead zones.  The anaerobic organisms began to eat through all the dead organic matter, churning out more carbon dioxide and another nasty waste product, sulfur dioxide (which gives the horrible smell to rotten eggs, and is also an acidifier).  The result: an extinction that wiped out an estimated ninety percent of life on Earth. In short order, a thriving planet had been turned into a hot, dead, foul-smelling wasteland, and it would take millions of years to recover even a fraction of the previous biodiversity.

Of course, at highest risk would be the big guys like our friends Arthropleura and Meganeura, and the Earth hasn't seen giant arthropods like this since then.  Today, the largest arthropod known is the Japanese spider crab (Macrocheira), topping out at around twenty kilograms -- but crabs and other crustaceans have gills and an oxygen carrier called hemocyanin, so they can boost the efficiency of their respiratory system somewhat over their terrestrial cousins.  The largest insect today is the African Goliath beetle (Goliathus), at about a tenth of a kilogram.  And in today's atmosphere, it's at a pretty significant disadvantage.  They may look big and scary, but in reality, they're slow-moving, harmless creatures.  Kind of a beer can with six legs, is how I think of them.

So that's today's look at creepy-crawlies of the past.  In my opinion it's just as well the big ones became extinct. The last thing I need is having to shoo a fifty-kilogram millipede out of my basement.

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The forest primeval

New from the "I Thought I'd Heard Everything" department, we have: a warning that you should look out for a specific kind of tree, because if you see one, you have slipped through a portal in space-time.

The tree is a Lepidodendron, and the good thing about it is at least it's pretty distinctive-looking:

[Image licensed under the Creative Commons Tim Bertelink, Lepidodendron, CC BY-SA 4.0]

So it's unlikely you'd mistake it for anything else.  Here's one account (of many) that have shown up all over social media, especially Reddit and TikTok:

I was on a hike in central Pennsylvania with some friends, and went off from the others to explore.  I grew up not far from there and know the area pretty well, but after about a half-hour things started looking weird.  The area is kind of rocky and hilly, but the path I was on kept heading down, and soon I was in a swampy terrain I'd never seen before.  I spent a lot of time outdoors as a kid and I know the kind of trees that grow there, and I'd never seen ones like this.  Tall and skinny, kind of like a stretched-out pine tree, but the bark was weird, with a pattern like the scales of a fish.  There were other plants, too, but I didn't recognize a single one.  Something about the place "felt wrong," like I'd stumbled into somewhere I wasn't supposed to be.  By this time I was completely freaked out.  I tried to retrace my steps, but the undergrowth was really thick with these strange-looking plants of all kinds.  Eventually I found my way to drier ground, and pretty soon found the path again.  Now all around me I saw maples and oaks and hickories, just ordinary trees, and the weird out-of-place feeling disappeared.  After another fifteen minutes of walking I found my friends again, and everything turned out okay, but to this day I can't let go of the feeling that I had a narrow escape from being lost forever.

The guy said he looked for pics online of "weird skinny trees like pine trees" and eventually found one that was an exact match to what he'd seen.

You guessed it.  The Lepidodendron.

The problem with all this is that the Lepidodendron has been extinct for over 250 million years.

They had their heyday in the Carboniferous Period, and in fact are only (very) distantly related to pines; the extant plants most closely related to the Lepidodendron are club mosses, most commonly found in the understory of deep, undisturbed forests.  And at least the unnamed storyteller got the place and climate right; a lot of rocks in Pennsylvania are of Carboniferous age, and it was in general a hot, humid, rainy period of Earth's history.

The thing is, though, if the people who say they've seen Lepidodendrons actually have wandered through a fold in the space-time continuum and found themselves back in the Carboniferous Period, it wouldn't be apparent only because they'd see strange scaly trees and be calf-deep in mud.  If you were suddenly transported to the mid-Carboniferous, (1) it would be absolutely unambiguous, and (2) you'd be damn lucky to last fifteen minutes.  The temperatures were an average of ten degrees Celsius warmer than they are today, with oxygen levels at around 30% (as compared to today's 21%).  The higher oxygen favored the evolution toward larger size in animals that are limited by the efficiency of their respiratory system -- most notably arthropods.  In those same swamps where you'd find Lepidodendron trees, you'd find the dragonfly Meganeura, with a 75-centimeter wingspan; the 2.6-meter-long millipede Arthropleura; and the 70-centimeter-long scorpion Pulmonoscorpius.  If that's not bad enough, you'd have to avoid being eaten by the three-meter-long, sixty-kilogram predatory reptile Sphenacodon, which came equipped with a long row of big, nasty, pointy teeth.

Sphenacodon ferox skull in the Field Museum of Chicago [Image licensed under the Creative Commons Smokeybjb, Sphenacodon ferox 1, CC BY-SA 3.0]

So as interesting as it is, the Carboniferous Period is not a place you'd want to go in your time machine.

Anyhow, I got curious about why all of a sudden people are seeing an obscure genus of extinct trees in downtown Harrisburg or wherever, so I did some digging.  After wading through a bunch of accounts of the "YES I SAW ONE OMG I WAS IN ANOTHER DIMENSION AND IT WAS SOOOO SCARY" type, I found out that the whole thing started three years ago when someone posted the following on Reddit:

You should know two things about this, though; (1) the person who posted this originally is an actual paleontologist, and (2) for fuck's sake, he meant it as a joke.  It didn't get much traction beyond a few har-de-hars from people who were fossil enthusiasts until fall of last year, when a TikToker with the handle @jese2063 posted images of spooky trees that look vaguely like Lepidodendrons, with an equally creepy-sounding soundtrack and scary text about how if you see one, you've gone back in time and are in horrific danger.  (It's hard to tell whether he believed it himself; my sense is not, but I have an unfortunate habit of giving people the benefit of the doubt when they don't deserve it.)

In any case, that opened the floodgates.  @jese2063's video got over 4.5 million views, and now there are hundreds of similar claims, many of them from people like the Pennsylvanian hiker who said they'd actually visited the Carboniferous Period and lived to tell the tale.

The problem is, like with @jese2063, it's difficult to discern how many of these are true believers, and how many are simply adding their contributions to a growing Carboniferous creepypasta.  I have nothing against scary fiction -- after all, I've written my fair share of it -- but you have to wonder if some of these people are deadly serious.

I mean, the benefit of the doubt only goes so far.

In any case, that's the latest frightening thing to look out for.  If you're ever in, say, Scotland, and suddenly you find yourself in a hot fern-filled rainforest, now you'll be prepared.  Can't honestly tell you what to do about it, however.  Just enjoy looking around for fifteen minutes until you're eaten by a Sphenacodon or attacked by enormous millipedes, I guess.

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The forest primeval

A couple of days ago my friend and fellow writer Andrew Butters sent me a link about a new fossil discovery.  He hastened to point out that he'd thought of me because he knows I'm interested in paleontology, not because my name comes to mind when he thinks about antiquated relics.  There may have been a touch of local pride involved as well, because the discovery was made in his home province of New Brunswick, Canada.

There's no doubt that it was a remarkable find.  The fossil was unearthed in a quarry near the town of Norton, where there is a layer of 350-million-year-old sandstone deposited when Atlantic Canada was a tropical swamp.  This was near the beginning of the Carboniferous Period, a time when the Earth was significantly warmer and wetter than it is now.  The high temperatures and carbon dioxide levels boosted plant growth, causing the oxygen levels to rise to an estimated 35% (compared to today's 21%).  This had the effect of allowing animals that had previously been held back by their inefficient respiratory systems, especially arthropods, to become huge, leading to millipedes 2.5 meters long and dragonflies with 75-centimeter wingspans.

The plants, though, were also pretty different from what we've got around now; by far the most common ones today are flowering plants, which make their first unequivocal appearance in the fossil record about 130 million years ago, in the early Cretaceous Period.  So when the New Brunswick sandstone was being deposited, the first flowering plants were still 220 million years in the future.

The dominant plants back then were distantly related to today's ferns, horsetails, and club mosses, although as you'll see we don't have anything left that looks much like what you'd see in a typical Carboniferous forest.  But the fossil discovered in the Canadian quarry was bizarre even compared to a lot of the strange vegetation from that time -- and has drawn comparisons to something you might see in a book by Dr. Seuss.

"What it really does look like is one of those truffula trees from The Lorax," said Olivia King, of St. Mary's University in Halifax, who was part of the team that discovered the fossils.  The most remarkable thing about the fossil, though, is its amazing state of preservation.  It's thought to have been entombed in an upright position when a landslide caused part of the bank of a lake to collapse.  The entire tree was dragged down to the bottom of the lake and buried in sediment, where it's lain for 350 million years -- and now has been excavated carefully to reveal what it looked like when it was alive.  Dubbed Sanfordiacaulis densifolia, after quarry owner Laurie Sanford who has been instrumental in allowing the investigation to progress, it had a mop of spiky leaves topping a spindly trunk:

A reconstruction of Sanfordiacaulis densifolia [Image credit: artist Tim Stonesifer]

With the acceleration of plant growth during that time period, there was extreme competition to access light, leading to dramatic increases in plant height and canopy size.  Eventually, there were club mosses (genus Lepidodendron) fifty meters tall -- taller than the oaks and maples in an average hardwood forest.

A sampler of ancient trees, from Robert Gastaldo et al., Current Biology.  [Note: "Mississippian" and "Pennsylvanian" are two divisions of time usually lumped together as the Carboniferous Period.]

Of course, the good times -- at least if you were a weird club moss tree or a 2.5-meter-long millipede -- couldn't last forever.  Around 305 million years ago, the climate turned from hot and humid to cool and arid, probably because by that time the plants had locked up so much atmospheric carbon dioxide underground -- what would eventually become our coal deposits -- that the greenhouse effect decreased and the temperature fell.  In essence, the plants sowed the seeds of their own destruction, and in the Carboniferous rain forest collapse that followed, the enormous forests and many of the animals that depended upon them became extinct.

It also set the fuse for the largest mass extinction ever.  All that organic matter sequestered underground was tinder just waiting to burn, and when the Siberian Traps erupted 252 million years ago, the lava ripped through a huge chunk of the Carboniferous coal and peat, using up oxygen (dropping it to an estimated 12%) and dumping that excess carbon dioxide back into the atmosphere.  The temperature spiked, the oceans became anoxic, and something like 95% of life on Earth became extinct.

But at the point that the Sanfordiacaulis tree was growing in what would become New Brunswick, that cataclysmic event was still a hundred million years in the future.  Think about what a thrill it'd be to get to wander amongst those bizarre forests, so unlike anything we have today.

Even if it meant dodging enormous dragonflies.

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The orphan

Whenever the topic here at Skeptophilia takes a paleontological turn, I always seem to focus on bizarre creatures very far removed from the iconic T. rexes and stegosauruses that typically grace the covers of children's books on prehistoric life.

I'm not sure why the charismatic dinosaurs hold less of a fascination for me than weird critters like spiky Hallucigenia (which means "causes hallucinations" in Greek, and is such an anomaly that the earliest fossil reconstructions were upside down), Opabinia (with its five eyes and vacuum-hose mouth), and Anomalocaris (Greek for "abnormal shrimp" -- abnormal indeed, with mouthparts like a pair of serrated scissors).

So given my penchant for appreciating the odd, I suppose it's no shock that my attention was immediately grabbed by a new piece of research about a creature nicknamed "the Alien Goldfish."  More surprising, though, is that I'd never even heard of this thing.

Named Typhloesus ("blind eater"), this peculiar animal was discovered in Carboniferous-age rocks in Bear Gulch, Montana, in 1973.  Early studies were confounded by the discovery within one of the fossils of hard, toothlike structures that turned out not to be from the animal itself but from its last meal, a conodont fish.  Once that was cleared up, it rapidly became obvious that Typhloesus was an "orphan" -- a species which, like its weird contemporary Tullimonstrum, wasn't closely related to anything we know of.

Typhloesus pursuing a conodont

The recent study, though, found that a close examination of Typhloesus fossils showed the animal had a radula -- a rasping, toothlike organ found in mollusks, especially gastropods (snails).  So if the research team's conclusions are correct, Typhloesus was a free-swimming, shell-less gastropod -- perhaps a little like modern nudibranchs.

Not everyone is convinced, though.  There are other animals who have evolved radula-like structures, and the placement of this orphan species in Phylum Mollusca is still tentative at best.  Mark Purnell, of the Centre for Palaeobiology at the University of Leicester, who was not involved in the current research, said, "It is still a very strange animal.  [The researchers] have found some tantalizing new information, but it is far from being a slam-dunk case in terms of definitely knowing what this weird thing is."

It's fascinating to contemplate how different the world was back then -- during the Carboniferous Period there were nearly worldwide rainforests of "trees" more closely related to today's ferns and club mosses, which drove the oxygen concentration of the atmosphere so high (35%, by some estimates, as compared to the current 21%) that animals like arthropods got huge.  There were predatory dragonflies with seventy-centimeter wingspans, and millipedes two and a half meters long and a half a meter wide, weighing an estimated fifty kilograms.

And in the oceans, there were some creatures so strange they've yet to be placed into the animal family tree with any real certainty.  It certainly brings home the truth of the final words of Charles Darwin's The Origin of Species, that evolution continually produces "endless forms most beautiful and most wonderful."

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Creepy crawlies

Whenever we have a wet summer -- not an uncommon occurrence in our rainy climate -- we have a plague of little pests trying to get into our house.

They're called millipedes, slinky guys maybe a couple of centimeters long, with lots of legs (not a thousand, though).  They're completely harmless; they don't bite like their cousins the centipedes do, and if you poke at them, they coil up into a ball.  So I guess they're really more of a nuisance than an actual problem.  They don't even damage anything, the way mice can.  Mostly what they seem to do is get in through every crack and crevice (there are lots of these in a big old house like ours), look around for a while, then curl up and die.

[Image licensed under the Creative Commons Totodu74, Anadenobolus monilicornis 03, CC BY-SA 3.0]

So I don't like them, and I wish they stayed outside, but in the grand scheme of things they're no big deal.  Imagine, though, if they were bigger.

A lot bigger.

Just last week, paleontologists announced the discovery on a beach in Northumberland, England, of a millipede fossil from the Carboniferous Period.  It's been dated to the middle of the period, about 326 million years ago.  It looks a bit like the millipedes I see trundling across my basement floor in summer.

Only this one was 2.6 meters long (approximately the length of a Mini Cooper), a half a meter across, and weighed something on the order of fifty kilograms.

It's been named Arthropleura, and holds the record as the largest-known arthropod in Earth's history.  Nothing is known for sure about its behavior; if it's like the rest of millipedes, it was a scavenger on leaf detritus, but there's no way to know for certain.  Given its size, it could well have been a lot more dangerous than the ones we have around now.  To paraphrase the old joke about five-hundred-pound gorillas:

Q:  What does a fifty-kilogram millipede eat?

A:  Anything it wants.

Those of you who are (like me) biology nerds may be frowning in puzzlement at this point.  How on earth could an arthropod get so big?  Their size is limited by the inefficiency of their respiratory system (not to mention the weight of their exoskeletons).  Most arthropods (millipedes included) breathe through pairs of holes called spiracles along the sides of the body.  These holes open into a network of channels called tracheae, which bring oxygen directly to the tissues.  Contrast that with our system; we have a central oxygen-collecting device (lungs), and the hemoglobin in our blood acts as a carrier to bring that oxygen to the tissues.  It's a lot more efficient, which is why the largest mammals are a great deal bigger than the largest arthropods.  (So, no worries that the bad sci-fi movies from the 50s and 60s, with giant cockroaches attacking Detroit, could actually happen.  A ten-meter-long cockroach not only wouldn't be able to oxygenate its own tissues fast enough to survive, it couldn't support its own weight.  It wouldn't eat Detroit, it would just lie there and quietly suffocate.)

So how could there be such ridiculously enormous millipedes?

The answer is as fascinating as the beast itself is.  As the temperature warmed and rainfall increased after the previous period (the Devonian), it facilitated the growth of huge swaths of rain forest across the globe.  In fact, it's the plant material from these rain forests that produced the coal seams that give the Carboniferous its name.  But the photosynthesis of all these plants drove the oxygen levels up -- by some estimates, to around 35% (contrast that to the atmosphere's current 21% oxygen).  This higher oxygen level facilitated the growth of animals who are limited by their ability to uptake it -- i.e., arthropods.  (At the same time, there was a dragonfly species called Meganeura with a seventy-centimeter wingspan.  And unlike millipedes, these things were carnivores, just as modern dragonflies are.)

Eventually, though, the system was unsustainable, and a lot of the rain forests began to die off in the Late Carboniferous, leading to a drier, cooler climate.  However, remember the coal seams -- by that time a huge percentage of the carbon dioxide that had fed the photosynthesis of those rain forests was now locked underground.  The fuse was lit for a catastrophe.

Fast forward to the end of the next period, the Permian, 255 million years ago.  What seems to have happened is a series of colossal volcanic eruptions that created the Siberian Traps, a basalt deposit covering most of what is now Siberia.  The lava ripped through the coal seams, blasting all that stored carbon into the atmosphere as carbon dioxide.  The temperature in the late Permian had been cool and dry, and the spike of carbon dioxide created a commensurate spike in the temperature -- as well as a huge drop in oxygen, used up by the burning coal.  The oxygen concentration seems to have bottomed out at around twelve percent, just over half of what it is now.  The extra carbon dioxide dissolved into ocean water, dropping the pH, and the increasing acidity dissolved away the shells of animals who build them out of calcium carbonate -- e.g. corals and mollusks.

Wide swaths of ocean became anoxic, acidic dead zones.  The anaerobic organisms began to eat through all the dead organic matter, churning out more carbon dioxide and another nasty waste product, sulfur dioxide (which gives the horrible smell to rotten eggs, and is also an acidifier).  The result: an extinction that wiped out an estimated ninety percent of life on Earth.  In short order, a thriving planet had been turned into a hot, dead, foul-smelling wasteland, and it would take millions of years to recover even a fraction of the previous biodiversity.

Of course, at highest risk would be the big guys like our friends Arthropleura and Meganeura, and the Earth hasn't seen giant arthropods like this since then.  Today, the largest arthropod known is the Japanese spider crab (Macrocheira), topping out at around twenty kilograms -- but crabs and other crustaceans have gills and an oxygen carrier called hemocyanin, so they can boost the efficiency of their respiratory system somewhat over their terrestrial cousins.  The largest insect today is the African Goliath beetle (Goliathus), at about a tenth of a kilogram.  And in today's atmosphere, it's at a pretty significant disadvantage.  They may look big and scary, but in reality, they're slow-moving, harmless creatures.  Kind of a beer can with six legs, is how I think of them.

So that's today's look at creepy-crawlies of the past.  In my opinion it's just as well the big ones became extinct.  The last thing I need is having to shoo a fifty-kilogram millipede out of my basement.

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Neil deGrasse Tyson has become deservedly famous for his efforts to bring the latest findings of astronomers and astrophysicists to laypeople.  Not only has he given hundreds of public talks on everything from the Big Bang to UFOs, a couple of years ago he launched (and hosted) an updated reboot of Carl Sagan's wildly successful 1980 series Cosmos.

He has also communicated his vision through his writing, and this week's Skeptophilia book-of-the-week is his 2019 Letters From an Astrophysicist.  A public figure like Tyson gets inundated with correspondence, and Tyson's drive to teach and inspire has impelled him to answer many of them personally (however arduous it may seem to those of us who struggle to keep up with a dozen emails!).  In Letters, he has selected 101 of his most intriguing pieces of correspondence, along with his answers to each -- in the process creating a book that is a testimony to his intelligence, his sense of humor, his passion as a scientist, and his commitment to inquiry.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]