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Some 400 million years ago, in the early Devonian Period, the world was a very different place.
Sea levels were high, and the continents were surrounded by vast, shallow seas, full of
exotic creatures like rugose corals, crinoids, and jawless fish.
And in what’s now Utah, one such fish met an end that was as grim as it was typical.
The jawless fish, known as Lechriaspis, was killed by three distinct puncture wounds to
the head -- puncture wounds made by crushing claws.
The culprit was an apex predator that dominated the seas of the late Silurian and early Devonian:
the sea scorpion.
Sea scorpions thrived for 200 million years, coming in a wide variety of shapes and sizes,
from Alkenopterus, which was about the size of a paper clip, to the terrifying Jaekelopterus,
which was 2 and a half meters long!
But in time, these able hunters would meet some stiff competition.
In the Late Devonian, giant armored jawed fish -- the placoderms -- would come to dominate
these seas and force other ocean-dwellers to adapt, flee, or perish.
These formidable predators would give the period its nickname: The Age of Fish.
And as a result of this competition, sea scorpions would eventually diverge into two distinct
lineages, each with its own unique strategy for dealing with the rise of the fish.
They developed a number of adaptations -- from those crushing claws that snagged the hapless
Lechriapsis, to flattened tails for swimming, as well as primitive lungs and strange, filter-feeding
appendages.
And some of them adapted by getting so big that they still hold the record as the largest
arthropods of all time.
They would even make the giant, historic leap from the water onto dry land.
But unlike their other arthropod relatives that we share the world with today, for the
sea scorpions, that move would prove to be the beginning of the end.
Scientists know “sea scorpions” by their proper, more accurate but also somewhat less
cool name: eurypterids
And eurypterids probably aren’t ancestral to the arachnids we know as scorpions today.
Sea scorpions and true scorpions are actually sister clades that both belong to a Subphylum
known as Chelicerata, an ancient group of arthropods that first appeared in the oceans
of the Cambrian Period.
By the time the Silurian Period rolled around, tens of millions of years later, the two clades
had diverged, each adapting a different way of breathing that ultimately set them down
very different evolutionary paths.
In the lineage of true scorpions, external respiration organs called “book-gills”
were adapted into internal “book-lungs”.
These lungs allowed the ancestors of today’s arachnids to become some of the first animals
ever to colonize land.
Sea scorpions, meanwhile, retained their book-gills, which are similar to what horseshoe crabs
use to breathe today.
And this kept them dependent on water—mostly.
But they also developed a second respiratory system, known as a “kiemenplatten”, which
functioned as a rudimentary lung and allowed them to take short trips onto land.
Because of these features, some researchers believe that kiemenplatten allowed sea scorpions
to mate and lay their eggs on land, or even to develop an amphibious lifestyle.
But, as history would show, this adaptation would prove to be a limitation, as well as
an advantage.
In 2015, the earliest-known eurypterid, Pentecopterus decorahensis, was discovered rocks in Iowa
that date back 467 million years.
It had compound eyes, a two-section body, and six pairs of appendages.
The first pair, by the mouth, would have been used for feeding, while the other five were
used for moving around.
Sea scorpions would soon come to be defined by how they these appendages adapted, splitting
off into two distinct suborders with two very different evolutionary strategies.
The first suborder to appear was Stylonuria.
And its first known member appears just nine million years after Pentecopterus, in what’s
now Wales.
This species was a humble little water monster just under 20 centimeters long, and it scuttled
along on ten legs, scavenging for food at the bottom of the shallow seas.
Like all of the Stylonurines that would follow, Brachyopterus used its appendages for walking
and stuck to a strategy of scavenging and filter feeding on the seafloor, in order to
stay out of the way of fish, which were on the rise.
But, during the Late Ordovician, a second suborder of sea scorpion emerges in the fossil
record — the Eurytperina.
The earliest-known eurypterine is a little species dated to 445 million years ago, discovered
in the shales of South Africa.
And these sea scorpions had a pair of appendages that were more like paddles, which allowed
them to swim, a lifestyle that put them in direct competition with jawless fish and cephalopods.
The newfound mobility of this group allowed it to diversify throughout the Silurian, and
fast, as they were able to colonize new areas and take advantage of new niches.
Plus, by competing with fish and cephalopods, they put themselves under selective pressure,
and launched an evolutionary arms race with other swimmers.
This led to much greater diversity in this suborder, which is probably why 75% of all
known sea scorpion species -- and more than 95% of all the specimens ever found -- belong
to Eurypterina.
But as the Silurian gave way to the Devonian, a dangerous new competitive pressure entered
the seas — the armored, jawed fish known as the placoderms.
So, after reaching their peak diversity at the very start of the Devonian Period, the
number of Euryterine species begins to drop in the fossil record, likely because many
simply couldn’t compete with this new powerhouse predator.
But the eurypterines that did survive managed to stage a sort of evolutionary rally, getting
bigger, faster, and stronger in the Devonian to cope with seas filled with placoderms that
were doing the same.
So, we probably have placoderms to thank for the nightmare fuel that followed, a family
of giant eurypterines that persisted for nearly 40 million years.
These are known as the pterygotids.
They were some of the only sea scorpions to cross the open ocean, exploring the seas between
the continents of Euramerica and Gondwana.
And several aspects of their anatomy indicate that they were active, powerful predators.
They had thin, light-weight exoskeletons that helped them move faster, and flattened tails
called telsons that gave them extra swimming power.
Their eyes faced forward and had thousands of lenses, giving them keen eyesight for spotting
prey.
And they caught that prey with mouth appendages called chelicerae that ended in large, raptor-like
claws.
These claws were so robust that they often preserved better than the thin exoskeleton,
and sometimes they’re the only parts that remain in fossils of pterygotids.
Many pterygotids reached over 1 meter in length, but the king of them all was Jaekelopterus
— the largest arthropod that has ever lived, maxing out at 2.5 meters long.
It was described from a claw found in Germany in 2007 that’s nearly half-meter long, with
tooth-like serrations that are about the size of a tiger’s canine teeth!
But despite their best efforts, these invertebrates couldn’t keep up with placoderms, and eurypterines
lost over 50 percent of their diversity within the first 10 million years of the Devonian.
By the end of the Devonian, all of the giant marine eurypterines had gone extinct.
Only one swimming genus -- the tiny, widely dispersed Adelophthalmus -- remained.
But while the dramatic rise and fall of Eurypterina had been happening out at sea, the slow and
steady Stylonuria were still hanging on.
In contrast to their cousins the eurypterines, the stylonurines remained relatively unaffected
by the rise of the fish.
While the eurypterines were busy competing with fish, stylonurines moved inland, to brackish
and freshwater ecosystems where there were fewer placoderms and less competition.
By the time of the Carboniferous Period, about 360 million years ago, Gondwana and Euramerica
had come together, allowing the stylonurines to spread out over the new supercontinent.
Now, the Carboniferous is most famous for giant arthropods like 2 meter long millipedes
and griffenflies with wingspans larger than that of some birds.
But if you were to hang around the muddy banks of a river, you might just spot the last of
the giant sea scorpions — a big weirdo called Hibbertopterus.
This was a very different sort of creature from the swimming, predators that had come
before.
It was basically a big, tank-like roomba, slowly combing the sediment at the bottoms
and banks of swamps and rivers with spiny sweep-feeding appendages.
A big trackway discovered in Carboniferous rock in Scotland has been attributed to Hibbertopterus.
And these tracks suggest that, while it wasn’t as long as Jaekelopterus, it was probably
heavier, with thicker legs to support it on land.
Scientists don’t really know why hibbertopterus got so big, but the size increase was gradual,
indicating that it might have been to better adapt to the species’ new freshwater home.
That’s because chelicerates have blood that has a salinity similar to that of sea water.
So a larger body size and thicker exoskeleton would have helped insulate hibbertopterus,
to maintain its blood salinity in freshwater and even in the open air.
Hibbertopterus died out in the Carboniferous, but other Hibbertopterids persisted for millions
of years.
The final eurypterid, Campylocephalus permianus, disappeared at the end of the Permian Period,
during the biggest extinction event in Earth’s history.
The end-Permian extinction, also called The Great Dying, was probably triggered by massive
volcanic eruptions in Siberia that warmed the climate and killed off more than 96% of
marine species and 70% of species on land.
Campylocephalus was among the many casualties, and with its demise, the 210 million year
age of eurypterids was brought to an end.
While sea scorpions ultimately succumbed the evolutionary pressure of powerful new vertebrates
and mass extinction events, for a big window of time, they were the most fearsome and diverse
arthropods around.
And they show us how different evolutionary paths can have very different outcomes.
By laying low and avoiding competition, stylonurines stuck around millions of years longer, while
eurypterines were far more diverse and plentiful at their peak.
In the end, their undoing might have been their inability to fully colonize the next
frontier -- dry land, where there was less competition and many more niches to fill.
While the sea scorpions couldn’t make the transition, it was made successfully by the
eurypterids’ sister-group: the arachnids.
Maybe if they’d developed a way to leave the water sooner, sea scorpions would still
be around.
Today, just three classes of Chelicerata remain: On land, there are the arachnids, like spiders,
ticks, and true scorpions.
And in the seas there are the sea spiders and the primordial-looking horseshoe crabs
-- living reminders of the watery origin of the chelicerates and the reign of the sea
scorpions.
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