Some 400 million years ago, in the early Devonian Period, the world was a very different place.

0:00:05.510,0:00:10.240 Sea levels were high, and the continents were surrounded by vast, shallow seas, full of 0:00:10.240,0:00:14.090 exotic creatures like rugose corals, crinoids, and jawless fish.

0:00:14.090,0:00:19.260 And in what’s now Utah, one such fish met an end that was as grim as it was typical.

0:00:19.260,0:00:24.270 The jawless fish, known as Lechriaspis, was killed by three distinct puncture wounds to 0:00:24.270,0:00:27.980 the head -- puncture wounds made by crushing claws.

0:00:27.980,0:00:33.480 The culprit was an apex predator that dominated the seas of the late Silurian and early Devonian: 0:00:33.480,0:00:34.969 the sea scorpion.

0:00:34.969,0:00:40.610 Sea scorpions thrived for 200 million years, coming in a wide variety of shapes and sizes, 0:00:40.610,0:00:45.809 from Alkenopterus, which was about the size of a paper clip, to the terrifying Jaekelopterus, 0:00:45.809,0:00:47.620 which was 2 and a half meters long!

0:00:47.620,0:00:51.899 But in time, these able hunters would meet some stiff competition.

0:00:51.899,0:00:56.780 In the Late Devonian, giant armored jawed fish -- the placoderms -- would come to dominate 0:00:56.780,0:01:01.339 these seas and force other ocean-dwellers to adapt, flee, or perish.

0:01:01.339,0:01:05.320 These formidable predators would give the period its nickname: The Age of Fish.

0:01:05.320,0:01:10.370 And as a result of this competition, sea scorpions would eventually diverge into two distinct 0:01:10.370,0:01:14.920 lineages, each with its own unique strategy for dealing with the rise of the fish.

0:01:14.920,0:01:19.990 They developed a number of adaptations -- from those crushing claws that snagged the hapless 0:01:19.990,0:01:25.730 Lechriapsis, to flattened tails for swimming, as well as primitive lungs and strange, filter-feeding 0:01:25.730,0:01:26.730 appendages.

0:01:26.730,0:01:31.950 And some of them adapted by getting so big that they still hold the record as the largest 0:01:31.950,0:01:32.950 arthropods of all time.

0:01:32.950,0:01:38.130 They would even make the giant, historic leap from the water onto dry land.

0:01:38.130,0:01:42.640 But unlike their other arthropod relatives that we share the world with today, for the 0:01:42.640,0:01:49.500 sea scorpions, that move would prove to be the beginning of the end.

0:01:49.500,0:01:53.909 Scientists know “sea scorpions” by their proper, more accurate but also somewhat less 0:01:53.909,0:01:55.670 cool name: eurypterids 0:01:55.670,0:02:00.979 And eurypterids probably aren’t ancestral to the arachnids we know as scorpions today.

0:02:00.979,0:02:05.670 Sea scorpions and true scorpions are actually sister clades that both belong to a Subphylum 0:02:05.670,0:02:10.639 known as Chelicerata, an ancient group of arthropods that first appeared in the oceans 0:02:10.639,0:02:12.069 of the Cambrian Period.

0:02:12.069,0:02:16.769 By the time the Silurian Period rolled around, tens of millions of years later, the two clades 0:02:16.769,0:02:21.280 had diverged, each adapting a different way of breathing that ultimately set them down 0:02:21.280,0:02:23.659 very different evolutionary paths.

0:02:23.659,0:02:27.889 In the lineage of true scorpions, external respiration organs called “book-gills” 0:02:27.889,0:02:30.560 were adapted into internal “book-lungs”.

0:02:30.560,0:02:34.889 These lungs allowed the ancestors of today’s arachnids to become some of the first animals 0:02:34.889,0:02:36.750 ever to colonize land.

0:02:36.750,0:02:41.330 Sea scorpions, meanwhile, retained their book-gills, which are similar to what horseshoe crabs 0:02:41.330,0:02:43.120 use to breathe today.

0:02:43.120,0:02:46.139 And this kept them dependent on water—mostly.

0:02:46.139,0:02:50.970 But they also developed a second respiratory system, known as a “kiemenplatten”, which 0:02:50.970,0:02:55.629 functioned as a rudimentary lung and allowed them to take short trips onto land.

0:02:55.629,0:02:59.709 Because of these features, some researchers believe that kiemenplatten allowed sea scorpions 0:02:59.709,0:03:04.010 to mate and lay their eggs on land, or even to develop an amphibious lifestyle.

0:03:04.010,0:03:09.099 But, as history would show, this adaptation would prove to be a limitation, as well as 0:03:09.099,0:03:10.099 an advantage.

0:03:10.099,0:03:16.840 In 2015, the earliest-known eurypterid, Pentecopterus decorahensis, was discovered rocks in Iowa 0:03:16.840,0:03:20.340 that date back 467 million years.

0:03:20.340,0:03:24.939 It had compound eyes, a two-section body, and six pairs of appendages.

0:03:24.939,0:03:28.980 The first pair, by the mouth, would have been used for feeding, while the other five were 0:03:28.980,0:03:30.680 used for moving around.

0:03:30.680,0:03:35.469 Sea scorpions would soon come to be defined by how they these appendages adapted, splitting 0:03:35.469,0:03:40.099 off into two distinct suborders with two very different evolutionary strategies.

0:03:40.099,0:03:43.060 The first suborder to appear was Stylonuria.

0:03:43.060,0:03:48.709 And its first known member appears just nine million years after Pentecopterus, in what’s 0:03:48.709,0:03:49.760 now Wales.

0:03:49.760,0:03:54.829 This species was a humble little water monster just under 20 centimeters long, and it scuttled 0:03:54.829,0:03:59.419 along on ten legs, scavenging for food at the bottom of the shallow seas.

0:03:59.419,0:04:04.489 Like all of the Stylonurines that would follow, Brachyopterus used its appendages for walking 0:04:04.489,0:04:09.329 and stuck to a strategy of scavenging and filter feeding on the seafloor, in order to 0:04:09.329,0:04:12.660 stay out of the way of fish, which were on the rise.

0:04:12.660,0:04:16.959 But, during the Late Ordovician, a second suborder of sea scorpion emerges in the fossil 0:04:16.959,0:04:18.660 record — the Eurytperina.

0:04:18.660,0:04:24.730 The earliest-known eurypterine is a little species dated to 445 million years ago, discovered 0:04:24.730,0:04:26.110 in the shales of South Africa.

0:04:26.110,0:04:30.790 And these sea scorpions had a pair of appendages that were more like paddles, which allowed 0:04:30.790,0:04:36.190 them to swim, a lifestyle that put them in direct competition with jawless fish and cephalopods.

0:04:36.190,0:04:40.161 The newfound mobility of this group allowed it to diversify throughout the Silurian, and 0:04:40.161,0:04:45.150 fast, as they were able to colonize new areas and take advantage of new niches.

0:04:45.150,0:04:50.070 Plus, by competing with fish and cephalopods, they put themselves under selective pressure, 0:04:50.070,0:04:53.290 and launched an evolutionary arms race with other swimmers.

0:04:53.290,0:04:58.760 This led to much greater diversity in this suborder, which is probably why 75% of all 0:04:58.760,0:05:04.040 known sea scorpion species -- and more than 95% of all the specimens ever found -- belong 0:05:04.040,0:05:05.040 to Eurypterina.

0:05:05.040,0:05:09.980 But as the Silurian gave way to the Devonian, a dangerous new competitive pressure entered 0:05:09.980,0:05:13.690 the seas — the armored, jawed fish known as the placoderms.

0:05:13.690,0:05:18.160 So, after reaching their peak diversity at the very start of the Devonian Period, the 0:05:18.160,0:05:23.030 number of Euryterine species begins to drop in the fossil record, likely because many 0:05:23.030,0:05:26.360 simply couldn’t compete with this new powerhouse predator.

0:05:26.360,0:05:31.390 But the eurypterines that did survive managed to stage a sort of evolutionary rally, getting 0:05:31.390,0:05:35.940 bigger, faster, and stronger in the Devonian to cope with seas filled with placoderms that 0:05:35.940,0:05:37.100 were doing the same.

0:05:37.100,0:05:41.760 So, we probably have placoderms to thank for the nightmare fuel that followed, a family 0:05:41.760,0:05:46.690 of giant eurypterines that persisted for nearly 40 million years.

0:05:46.690,0:05:48.090 These are known as the pterygotids.

0:05:48.090,0:05:53.560 They were some of the only sea scorpions to cross the open ocean, exploring the seas between 0:05:53.560,0:05:56.530 the continents of Euramerica and Gondwana.

0:05:56.530,0:06:00.840 And several aspects of their anatomy indicate that they were active, powerful predators.

0:06:00.840,0:06:05.820 They had thin, light-weight exoskeletons that helped them move faster, and flattened tails 0:06:05.820,0:06:09.110 called telsons that gave them extra swimming power.

0:06:09.110,0:06:14.570 Their eyes faced forward and had thousands of lenses, giving them keen eyesight for spotting 0:06:14.570,0:06:15.570 prey.

0:06:15.570,0:06:20.010 And they caught that prey with mouth appendages called chelicerae that ended in large, raptor-like 0:06:20.010,0:06:21.010 claws.

0:06:21.010,0:06:25.650 These claws were so robust that they often preserved better than the thin exoskeleton, 0:06:25.650,0:06:29.750 and sometimes they’re the only parts that remain in fossils of pterygotids.

0:06:29.750,0:06:34.640 Many pterygotids reached over 1 meter in length, but the king of them all was Jaekelopterus 0:06:34.640,0:06:39.380 — the largest arthropod that has ever lived, maxing out at 2.5 meters long.

0:06:39.380,0:06:45.280 It was described from a claw found in Germany in 2007 that’s nearly half-meter long, with 0:06:45.280,0:06:49.010 tooth-like serrations that are about the size of a tiger’s canine teeth!

0:06:49.010,0:06:54.030 But despite their best efforts, these invertebrates couldn’t keep up with placoderms, and eurypterines 0:06:54.030,0:06:58.810 lost over 50 percent of their diversity within the first 10 million years of the Devonian.

0:06:58.810,0:07:03.250 By the end of the Devonian, all of the giant marine eurypterines had gone extinct.

0:07:03.250,0:07:08.530 Only one swimming genus -- the tiny, widely dispersed Adelophthalmus -- remained.

0:07:08.530,0:07:13.830 But while the dramatic rise and fall of Eurypterina had been happening out at sea, the slow and 0:07:13.830,0:07:16.750 steady Stylonuria were still hanging on.

0:07:16.750,0:07:21.680 In contrast to their cousins the eurypterines, the stylonurines remained relatively unaffected 0:07:21.680,0:07:23.200 by the rise of the fish.

0:07:23.200,0:07:27.740 While the eurypterines were busy competing with fish, stylonurines moved inland, to brackish 0:07:27.740,0:07:32.260 and freshwater ecosystems where there were fewer placoderms and less competition.

0:07:32.260,0:07:37.830 By the time of the Carboniferous Period, about 360 million years ago, Gondwana and Euramerica 0:07:37.830,0:07:42.580 had come together, allowing the stylonurines to spread out over the new supercontinent.

0:07:42.580,0:07:47.690 Now, the Carboniferous is most famous for giant arthropods like 2 meter long millipedes 0:07:47.690,0:07:50.760 and griffenflies with wingspans larger than that of some birds.

0:07:50.760,0:07:55.690 But if you were to hang around the muddy banks of a river, you might just spot the last of 0:07:55.690,0:08:00.000 the giant sea scorpions — a big weirdo called Hibbertopterus.

0:08:00.000,0:08:03.840 This was a very different sort of creature from the swimming, predators that had come 0:08:03.840,0:08:04.840 before.

0:08:04.840,0:08:09.680 It was basically a big, tank-like roomba, slowly combing the sediment at the bottoms 0:08:09.680,0:08:14.120 and banks of swamps and rivers with spiny sweep-feeding appendages.

0:08:14.120,0:08:20.050 A big trackway discovered in Carboniferous rock in Scotland has been attributed to Hibbertopterus.

0:08:20.050,0:08:24.520 And these tracks suggest that, while it wasn’t as long as Jaekelopterus, it was probably 0:08:24.520,0:08:27.550 heavier, with thicker legs to support it on land.

0:08:27.550,0:08:33.479 Scientists don’t really know why hibbertopterus got so big, but the size increase was gradual, 0:08:33.479,0:08:37.910 indicating that it might have been to better adapt to the species’ new freshwater home.

0:08:37.910,0:08:42.640 That’s because chelicerates have blood that has a salinity similar to that of sea water.

0:08:42.640,0:08:47.770 So a larger body size and thicker exoskeleton would have helped insulate hibbertopterus, 0:08:47.770,0:08:52.430 to maintain its blood salinity in freshwater and even in the open air.

0:08:52.430,0:08:57.190 Hibbertopterus died out in the Carboniferous, but other Hibbertopterids persisted for millions 0:08:57.190,0:08:58.190 of years.

0:08:58.190,0:09:03.170 The final eurypterid, Campylocephalus permianus, disappeared at the end of the Permian Period, 0:09:03.170,0:09:05.920 during the biggest extinction event in Earth’s history.

0:09:05.920,0:09:10.580 The end-Permian extinction, also called The Great Dying, was probably triggered by massive 0:09:10.580,0:09:16.279 volcanic eruptions in Siberia that warmed the climate and killed off more than 96% of 0:09:16.279,0:09:20.060 marine species and 70% of species on land.

0:09:20.060,0:09:25.870 Campylocephalus was among the many casualties, and with its demise, the 210 million year 0:09:25.870,0:09:28.260 age of eurypterids was brought to an end.

0:09:28.260,0:09:32.870 While sea scorpions ultimately succumbed the evolutionary pressure of powerful new vertebrates 0:09:32.870,0:09:38.110 and mass extinction events, for a big window of time, they were the most fearsome and diverse 0:09:38.110,0:09:39.960 arthropods around.

0:09:39.960,0:09:44.720 And they show us how different evolutionary paths can have very different outcomes.

0:09:44.720,0:09:50.131 By laying low and avoiding competition, stylonurines stuck around millions of years longer, while 0:09:50.131,0:09:53.920 eurypterines were far more diverse and plentiful at their peak.

0:09:53.920,0:09:57.970 In the end, their undoing might have been their inability to fully colonize the next 0:09:57.970,0:10:03.090 frontier -- dry land, where there was less competition and many more niches to fill.

0:10:03.090,0:10:07.650 While the sea scorpions couldn’t make the transition, it was made successfully by the 0:10:07.650,0:10:09.960 eurypterids’ sister-group: the arachnids.

0:10:09.960,0:10:13.840 Maybe if they’d developed a way to leave the water sooner, sea scorpions would still 0:10:13.840,0:10:14.840 be around.

0:10:14.840,0:10:20.130 Today, just three classes of Chelicerata remain: On land, there are the arachnids, like spiders, 0:10:20.130,0:10:21.710 ticks, and true scorpions.

0:10:21.710,0:10:25.900 And in the seas there are the sea spiders and the primordial-looking horseshoe crabs 0:10:25.900,0:10:30.960 -- living reminders of the watery origin of the chelicerates and the reign of the sea 0:10:30.960,0:10:30.980 scorpions.