Chapter 5: Life in Transition Part 1
It is now the middle of the Devonian Period, 380 million years ago. The continents have continued to shift as underneath them, the Earth's crust is always on the move. While Gondwana remains the largest continent on Earth, another supercontinent has now joined it. To the north the continents of Avalonia, Laurentia, and Baltica have now formed a new supercontinent around the Equator known as Euramerica. The Earth at this time is also going through a time of global warming.
These rising temperatures have led to sea levels rising exponentially creating inland seas. These inland seas are also covered in massive amounts of Coral Reefs that dot the coastlines of Earth. Reefs of this scale have not been seen since the Ordovician and yet they have now returned arguably greater than ever. One of the largest reef systems can be found in a part of North Eastern Gondwana. This section of Gondwana will one day become Northern Australia and yet at this time it is underwater.
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The reef itself is teeming with life, the warm tropical sun lit waters feeding an expansive ecosystem. Massive amounts of microscopic plankton thrive in these waters. The corals themselves feed on these plankton. The corals that make up these reefs are not actually the stone corals of today; instead they are a now extinct group known as horn corals. The Devonian is a prime time for these corals and they thrive in an abundance.
They are not the only reef builders, however, for a group of sponges, known as Stromatoporoids have also evolved to form great reefs of their own. These sponges are direct competitors of the horn corals due to their similar ecological role. They too are filter feeders and they are a major component of this reef system.
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Living in these reefs themselves is an enormous variety of animals, in particular fish. As the Devonian period has gone on many of the vertebrate's competitors have dwindled in number and this combined with their already growing diversity in the Silurian and at the start of the Devonian has allowed them to become the dominant family of animals. The absolutely staggering diversity of fish in the Devonian seas has even given the period its nickname, The Age of The Fish. Directly above the Corals we can find one type of these fish, meet Mimipsicis. Mimipsicis is relatively small by fish standards and it relies primarily on its speed and agility to evade predation from the larger fish in these waters.
One of the most distinctive features of Mimipicis is its family lineage for it is part of a class of fish known as the Actinopterygians or ray finned fish. Ray finned fish stand out from lobe finned fish thanks to their fins. While lobe finned fish have strong muscular fins anchored to a large bone, the ray finned fish have long webs of skin that connect to smaller bone structures. In the modern age ray finned fish are by far the most common, however, in the Devonian they are far outnumbered by both lobe finned fish as well as the armored placoderms that share the seas with the. One such other fish can be found on the sea bed, meet Bothriolepis, a genus of armored placoderm.
Bothriolepis is a very successful genus, made up of multiple different species all of whom vary in size. Bothrolepis itself also stands out for being able to be found all across the planet and from a purely numbers standpoint they are easily one of the most successful genus of fish in the entire world. Despite its massive success from a numbers standpoint Bothriolepis is quite low in the food chain, for they spend their time sifting through the muck looking for small organisms to feed on. This combined with its relatively small size makes Bothriolepis a prime target for larger fish. To combat this Bothriolepis has thick armor around its head making it quite well protected from predators.
Fish aren't the only animals that thrive in these shallow reefs nor are they the only type of animal that have increased in diversity, for cephalopods have also expanded their numbers and new forms have evolved to fill the waters. The most notable new member to the cephalopods are the Ammonoids. Ammonoids are coil shelled cephalopods that are similar to the Tarphycerids of the Ordovician in appearance despite not being especially closely related. Ammonoids themselves evolved in the Early Devonian and they have since then become extremely common throughout the Earth's many seas and oceans. One of the most common Ammonoids in this reef is the genus Tornoceras.
Tornoceras itself is part of an order within the Ammonoids known as Goniatites. Goniatites themselves are extremely common at this time and they will be an important part of ocean ecosystems for the rest of the Paleozoic Era. Tornoceras itself mainly feeds on small fish and other creatures, while also attempting to avoid larger predators in the reef. Unfortunately for these Tornoceras one such predator has just spotted them in this case another type of placoderm, unlike Bothriolepis, however, this one is a predator. Meet Eastmanosteus calliaspis, a 5 foot long predator.
Eastmanosteus itself comes equipped with powerful shearing jaws that can slice right through most prey, even Tornoceras is in trouble once an Eastmanosteus lays its eyes on it. The Eastmanosteus swims towards the shoal. The shoal begin to scatter, as the Eastmanosteus swims right towards one of them. The Tornoceras tucks itself into its shell in a last ditch effort to save itself, but it is no use. The Eastmanosteus grabs its prey and cracks right through the shell with its powerful jaws, it then begins to feed.
After eating its fill the Eastmanosteus begins to look for a place to rest. As it rests it will digest its meal. Eastmanosteus itself is another very successful genus, with many different species, in fact Eastmanosteus calliaspis isn't even the largest one, with some other species within the genus getting up to lengths of 10 feet.
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Some animals prefer to avoid predators altogether by hiding, take Gogonasus, a type of lobe finned fish. Gogonasus spends its time hiding in between the crevices of the reef, the corals and sponges forming excellent hiding spots. One of the most interesting aspects of Gogonasus is its fins. Even by lobe finned fish standards the fins of Gogonasus are strong allowing it to walk along the seafloor. Believe it or not, Gogonasus is actually a very important animal for the evolution of land vertebrates or Tetrapods as they have been named by scientists.
For one thing it has lungs as well as gills allowing it to breathe air should the ocean's Oxygen supply dwindle, this is a similar adaptation to what its close relatives the lungfish have. It's more important evolutionary feature can be found in its fins, for within them are the precursors to the radius and the ulna bones that Tetrapods will evolve in the future for better maneuverability on dry land. These bones are notably perfect for bearing weight which will be important for when some of Gogonasus relatives make the leap to the land. Despite having characteristics of the future Tetrapods, Gogonasus is not actually a Tetrapod itself, nor is it the ancestor of true Tetrapods. Instead Gogonasus is part of the Clade Tetrapodomorpha; these include Tetrapods and animals related to them such as Gogonasus.
In Spite of its advanced features, Gogonasus still sits relatively low on the food chain, at about a foot long it isn't a very impressive animal in terms of size and in fact it could make a good meal for some of the other predators within its environment. Take this Gogoselachus for example. Gogoselcahus is part of a Class of Fish known as Chondricthyans, a group that will one day include the most infamous of the modern oceans marine predators, sharks. Chondrichthyans stand out for their skeletons which are made out of cartilage rather than bone like most fish. This design allows them to be faster and more agile than bony fish like Gogonasus.
The Gogonausus quickly freezes; this is its only real effective defense, for the Gogoselachus would easily outswim it in open water. Luckily for the Gogonasus the Gogoselachus doesn't notice it and instead it keeps on swimming. For now at least the Gogonasus has escaped death, but there are no guarantees in these waters that you will remain safe for long and it must stay vigilant.
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Not all lobe finned fish sit at the bottom of the food chain; some have attained quite large sizes and have become apex predators. Take Onychodus jandemarrai, at 10 feet long it is the largest fish to live in this reef. Onychodus is another worldwide genus made up of many different species and they patrol shallow seas across the globe. Believe it or not Onychodus is actually distantly related to Gogonasus despite the two taking very different evolutionary paths. Onychodus itself is an ambush predator and its powerful fins allow it to crawl on the sea bottom and find hiding spots within the coral. From here it will wait for unsuspecting fish to pass by.
In this case it has its eyes set on a group of Bothriolepis. The Onychodus bursts out from hiding and grabs the unsuspecting placoderm in its jaws. From here the Onychodus makes use of its jaws. In comparison to the rigid jaws of most fish, Onychodus jaws are quite flexible, allowing it to swallow larger prey than other fish whole. The Bothriolepis goes down headfirst into the stomach of the Onychodus.
Soon the Bothriolepis is inside of the Onychodus stomach, such a large meal will last for a few days at least. For the time being the Onychodus finds a nice place to rest while its food digests in its stomach.
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A few weeks pass and life continues on as normal for the creatures of the reef. The group of Mimipicis patrol the edge of the reef looking for prey. One of them spots something on the seafloor and it swims in to investigate. The creature is an Adelophthalmus waterstoni, a type of Eurypterid. Eurypterids have been going through some hard times throughout the Devonian, they have lost their position as top predator to the fish and yet they still continue to survive even through all of this.
Despite its small size Adelophthalmus has done quite well for itself all things considered it can be found throghuout the globe and is destined to live on for almost 100 more million years, of course it sits quite low on the food chain and as a result it is prey for predators like Mimipicis. The Adelophthalmus attempts to swim away, but in open water, its chances of escape are slim. One of the Mimipicis grabs the unlucky arthropod as more join in and they proceed to tear it apart. Even seemingly inconspicuous fish like Mimipicis can be voracious predators to anything unlucky enough to be on their menu.
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Elsewhere further in the reef we find another type of fish, meet Materpiscis, a type of small placoderm. Materpiscis specializes in feeding on corals and brachiopods, its powerful jaws making short work of their armored shells. Right now, however, this Materpicis is not worried about finding food for it is doing something else. The Materpiscis finds a secluded spot and starts to convulse. This particular Materpiscis is pregnant, but rather than lay eggs like most fish, Materpiscis gives birth to live young.
The advantage of this is that the young can better defend themselves from predators increasing their chances of survival, however, it also leads to longer pregnancies and a far more arduous birthing process. After a few minutes the tail of the baby pops out and soon so does the rest of its body. Immediately it looks to its mother for protection. Rather than leaving their young defenseless, Materpiscis mothers keep close watch over their babies. Anything foolish enough to get between her and the baby will face her wrath. This will last for a time and once the baby is old enough it will be forced to strike out on its own.
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Meanwhile emerging from a hole in the coral is the Gogonasus from earlier. The Gogonasus is looking for suitable prey, due to its small size it is restricted to hunting smaller creatures. Luckily for Gogonasus there are plenty of young fish hiding in the rocks, it just has to find them. Suddenly a shadow cast over the Gogonasus causing it to slink away into a small crevice. Swimming directly above the Gogonasus is a hungry Eastmanosteus.
The large fish is once again on the hunt for suitable prey. As it swims over the Gogonasus, the latter freezes in place. Luckily for the Gogonasus even if the Eastmanosteus did see it, the chances of it squeezing between the coral is extremely low. Finally the bigger fish swims off allowing the Gogonasus to get back to what it was doing.
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Elsewhere we find the Mimipicis school on the lookout for more prey, it seems that the Adelophthalmus has not satisfied them. Unfortunately for them something else has them in its sights. Watching nearby is a small group of hungry Gogoselachus. Gogoselachus will sometimes form mobs when hunting, however, these mobs are very loosely connected and unorganized; they are all here by pure happenstance. The Gogoselachus swim right at their prey, causing the Mimipicis to scatter. The Gogoselachus quickly pick off the slower individuals.
Some aggressively fight for the ownership of carcasses. These fights often lead to tug o wars that often end in the fish being torn in half. The blue water soon turns red as blood fills the water, this only further agitates the Gogoselachus causing them to grow even more aggressive. Some of the smaller Gogoselachus are snatched up by larger individuals in the confusion. Finally after a time the chaos begins to fizzle out.
Body parts of dead fish fall to the seafloor and some scavengers are quick to take notice. Adelophthalmus swarm the bodies feeding on the flesh of fish. A tail of a Gogoselachus ends up falling between the coral and our Gogonasus is quick to take advantage of the easy meal. The Gogonasus begins to feed on the tail of the Gogoselachus tearing off chunks with its jaws. It seems that the Gogonasus has won out today at least.
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A few days later we find the Onychodus emerging from its lair once again on the hunt for prey. The large fish quickly finds a spot in the coral to stage an ambush. From here it waits for something to come by. It could be days before something pops up so patience is key. Thankfully for the Onychodus, it has plenty of patience.
A few hours pass and something begins to approach the hidden Onychodus. Emerging out of the blue is the Eastmanosteus. The Onychodus begins to ready itself for the attack, even the powerful Eastmanosteus is on the menu for Onychodus. The Eastmanosteus continues to get closer and closer blissfully unaware of the danger. Soon it gets within striking range and the Onychodus bursts out from hiding with impressive speed.
The Eastmanosteus doesn't stand a chance. The Onychodus quickly begins to gulp down the struggling fish and soon it swallows it whole. It seems that the Onychodus has scored another successful meal, but all is not well for within the Onychodus, the Eastmanosteus is still alive and thrashing around. The danger of Onychodus feeding style is that with larger and stronger fish they can often bite off more than they can chew. Soon the struggling Eastmanosteus ruptures an organ.
The blow fatally wounds the Onychodus. Soon the former terror of the reef is nothing but a lifeless carcass floating to the ocean floor, felled by its own appetite. It isn't long before hungry scavengers begin to ascend on the carcass. Nature is seldom wasteful and the Onychodus body will be used by other denizens of the reef. Adelophthalmus are among the first to arrive on the scene and they quickly begin to feed on their prey.
They are soon joined by Gogoselachus, the hungry opportunists quick to take advantage of the bounty. Finally the Gogonasus arrives once the Gogoselachus have eaten their fill and it too begins to feed. After eating its fill it slinks off back into the reef. It then finds a nice hiding spot to rest. While Gogonasus is just a lowly scavenger some of its relatives are already begging to conquer new frontiers.
Soon some of the Tetrapodomorphs will conquer a frontier that no vertebrate has gone before but to see that we must move forward in time.
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It is now 375 million years ago. We have moved further north and are now in what will one day become Greenland, while Greenland in the present day is a cold inhospitable island in the Northern Hemisphere, at this time it straddles the Equator and is connected to the rest of the supercontinent of Euramerica. This area also receives plenty of rain which when coupled with the warm climate creates the perfect breeding ground for life. The first thing one may notice is the fact that we are in a forest, plants have been undergoing a massive revolution throughout the Devonian and it has now hit its climax, no longer restricted to the waters edge thanks to the evolution of seeds, plants can now form vast forests across the globe, for the first time in Earth's History it is a green and blue world. Perhaps the most notable planet in these forests, is the massive Archaeopteris, one of the first trees. Trees like these have more or less replaced the Prototaxites that were once the largest organisms on land, at up to 85 feet in height and 5 feet in width they dwarf the giant fungus that they have replaced.
Archaeopteris itself can be found all across the globe, a true success story. Other smaller plants have also evolved like ferns, these carpet the ground in droves. At this time in Earth's History there are no large land herbivores to feed on, allowing them to spread far and wide throughout the globe. There are, however, smaller creatures that live in these forests. Millipedes and Scorpions have continued to diversify, the first providing the perfect environment for them to thrive.
Another group that has become successful are the Trigonotarbids. Trigonotarbids closely resemble spiders, however, they are not spiders themselves. They lack venom and the ability to produce silk, despite this they are quite ferocious carnivores hunting down smaller creatures in the leaf litter.
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To find larger animals in these parts one must first explore the myriad of water sources that dot the forest. A variety of rivers and streams cut through the forest and there are also many lakes and ponds. Unlike the salty oceans, these areas are instead made up of freshwater and the creatures here must deal with a variety of unique challenges. Space here is far more limited forcing animals closer together, on top of this algal blooms are common as dead plant matter falls into the water. The water is also murky and filled with logs that make movement difficult.
Only the most tenacious creatures can survive here. Despite these challenges, many creatures call these waterways home, one such pond is home to many different animals. At the bottom of this pond are Bothriolepis. Bothriolepis is notable for its ability to survive in both fresh and saltwater, an ability that has allowed it to inhabit pretty much any aquatic ecosystem. These Bothriolepis are currently combing the pond bed attempting to find food.
Doing this is risky, however, as some predators like to hide in the mud. As the group combs through the sediment, one is suddenly grabbed causing the rest of them to flee. The attacker is a Laccognathus embryi a type of lobe finned fish. At up to 6 feet in length Laccognathus is a formidable predator, its dull coloration allowing it to hide effortlessly under the mud while it waits for passing prey, the Bothriolepis never stood a chance. The Laccognathus begins to tear the Bothriolepis apart.
The armored head, however, isn't the most edible thing in the world so it sticks to the torso and tail of the Bothriolepis. By the time it's finished there isn't much left of the Bothriolepis.
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Laccognathus isn't the only large predator in this pond for there is another creature, one that is perhaps the most important in the transition of vertebrates from water to land, meet Tiktaalik roseae. Tiktaalik is part of a smaller clade within the lobe finned fish called Elpistostegelia, a group that will one day include us humans. Tiktaalik itself is an early member of this group and it as well as other early members have been nicknamed fishapods for having features of both early lobe finned fish and later Tetrapods. Like a fish Tiktaalik has gills and fish-like scales, however, it also has a neck like a Tetrapod plus lungs, similar to earlier Tetrapodomorphs like Gogonasus. It also has a strong rib cage for bearing weight perfect for when it is in shallow water.
The most distinctive feature of Tiktaalik, however, are its fins. Like Gogonasus the fins are strong and can bear weight, but in Tiktaaliks case they are far stronger than those of Gogonasus, this allows Tiktaalik to maneuver through the pond effortlessly pushing branches and other debris out of the way. In the future many of these current adaptations will be used to help Tetrapods become fully terrestrial, but for now Tiktaalik is more worried about finding dinner. At up to 9 feet long Tiktaalik is the biggest creature in the pond and by extension it is the apex predator, not even Laccognathus can pose a threat. This also means that pretty much everything is on the menu.
The Tiktaalik moves into shallow water, where it spots an interesting sight. These are Quikqtania, a close relative of Tiktaalik at 2.5 feet long; they are quite a bit smaller. Quikqtania are notably more adapted for aquatic life than Tiiktalik but believe it or not they are not actually ancestors of Tiktaalik instead they split off from Tiktaaliks lineage extremely recently in evolutionary terms and returned to a more fully aquatic lifestyle with fins that are far less adapted to bearing weight. Unfortunately for Quikqtania there is no lost love between them and Tiktaalik and they are very much on the menu. The Tiktaalik swims towards the group causing them to panic and scatter.
In the murky water it can be hard to see, luckily for Tiktaalik its neck allows to shake its head around and feel out where the Quikqtania are. Finally the Tiktaalik grabs one and with one flick of its head half of the Quikqtania is in its mouth. A snapping bite finishes off the Quikqtania. The Tiktaalik then begins to thrash its head from side to side with the Quikqtania in its jaws. This splashes water everywhere and soon rips its prey in half making it easier to swallow.
After finishing its meal the Tiktaalik swims off having satisfied its hunger. Little does Tiktaalik know life is about to get a lot harder for it and all of the other inhabitants of this pond.
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A few months later and it is now the dry season. The rivers that flow through the forest have receded greatly since the wet season ended. This has left many creatures stranded in water holes. Space is contentious in these water holes and in some cases whole groups of fish are wiped out, as oxygen is sucked out by bacteria during algal blooms choking animals to death. Even those not doomed to be killed by algal blooms must deal with the burning heat of the sun.
As the sun beats down on water holes it begins to dry out the water, many fish like Bothriolepis are doomed to cook to death in the sun. Luckily for the creatures of the pond the water is deep enough to ensure that it will not completely dry up, however, space is still becoming limited as the shallow parts of the water begin to recede. On top of this algal blooms are still a problem and they are slowly beginning to choke out life in the pond. Bothriolepis are hit hardest by this, as they only have gills once the oxygen has been removed from the water they will slowly suffocate in the stagnant water. This is not the case for some fish, however, Laccognathus for example with its ability to breathe both air and water can simply swim up to the surface and take breaths of air.
Tiktaalik also has this ability, as does Quikqitania. For now it seems that these animals may just survive but there is another problem. As the other fish die the entire food web of this pond is being disrupted, something which could doom the other fish here, something has to be done if they are to survive. Our Tiktaalik begins to take action. It swims to the shore and begins to do something incredible, it begins to climb onto land.
The fins of Tiktaalik not only support in the water but also out of it, which when combined with a strong ribcage to deal with the pressures of gravity allows Tiktaalik to climb onto the shore. The trek Tiktaalik must embark on is an arduous one as it is still more adapted for life in the water, but it is one it must take if it is to survive. Tiktaalik isn't the only one Laccognathus has similar adaptations allowing it too to come ashore. Unfortunately for the Quikqitania their fins can not support their weight on land unlike their relatives meaning that they will slowly die from starvation in the pond. But this is not the case for Tiktaalik.
It drags itself slowly but surely through the undergrowth spooking millipedes and other small invertebrates as it goes. Finally it finds what it is looking for, the river. The Tiktaalik drags itself into the water and begins to swim. Not long after it the Laccognathus also makes it and it too dives in. Luckily for the two fish this river is safe even from the drought and it contains plenty of prey, a massive score for the two pioneers.
The Tiktaalik begins to explore its new home happily swimming through the water, there are also others of its kind here meaning it can potentially find a mate once breeding season comes and continue the success of its species.
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Tiktaalik is a true pioneer among the animals of the Devonian and its ability to walk on land allows it to face many of the planet's challenges, however, even it will struggle to survive what is soon to come. About 372 million years ago an event known as the Kellwasser Event occurred, the 2nd of the big 5 mass extinctions in Earth's history. As terrestrial plants continue to expand in size and number they begin to take in Carbon Dioxide and give off Oxygen causing the temperature to drop. This is combined with a period of anoxic conditions in the ocean, soon whole communities all over the globe are beginning to be snuffed out. Horn corals and stromatoporoids massively decline in number, unable to deal with changes in temperature. This causes entire reef ecosystems to be wiped out in short order destroying valuable habitat for animals like Brachiopods, ammonoids and placoderms.
In freshwater ecosystems, fishapods like Tiktaalik are unable to survive the harsh changes and they too are wiped out in short order. By the end of the event 75% of all genera have been wiped out and this is just the beginning for the rest of the Devonian will be a time of great climatic change will minor extinction events causing frequent devastation to Earth's ecosystems, despite this life will carry on. Notably while many creatures have died out others have survived and they are now poised to deal with the rest of the Devonian's challenges, and in the case of creatures like Tiktaalik, while the extinction may have been too much for them other more advanced tetrapodomorphs are destined to take their place in the great story as the vertebrates continue their march to land.
Trivia
This episode is mainly based off of the Gogo Reef and Fram Formations.
The title is a reference to not just life moving on to land, but also this series transitioning itself from aquatic to terrestrial settings. This episode is also a two parter with the next chapter taking place at the end of the Devonian and that's all I will say without giving any spoilers.
I originally planned on having a scene where the Onychodus tried and failed to catch a Gogoselachus, but I ultimately cut it as I couldn't find a good place to put it in.
Originally I was going to have the Gogonasus hunt down a young Mimipicis hiding in the reef, but I cut that scene for pacing reasons.
Materpiscis originally had more page time including a scene where the mother attacks the Gogonasus and fights off a Gogoselachus, but these were cut once again for pacing reasons.
The scene where the Onychodus kills itself trying to swallow a Eastmanosteus is based off an actual fossil of a Onychodus with a placoderm of some kind in its stomach, I'm not sure what kind of placoderm is actually was but, Eastmanosteus was a good enough size to where I could see an Onychodus biting off more than it can chew.
Originally the Laccognathus was going to die during its trek to find water, but I found that outcome too predictable, plus it leaned into the idea that Tiktaalik is somehow a superior genus which was not the intention.
Originally Holoptychius was going to appear, but I replaced it with Quikqtania as the latter is a lot more interesting needless to say.