fossils

When monsters ruled the water: The Age of Fishes

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Vasika
Udurawane

Writer
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Julio
Lacerda

Staff Artist and Writer
It was a time when the oceans and rivers of the world were ruled by aliens with armored bodies and slicing bony blades in their mouths. Welcome to the Devonian, the Age of Fishes, when these creatures were at the height of their strangeness.

Fish are the most numerous vertebrates on Earth today, outnumbering the others 10 to one. There was, however, a time when they were much more diverse. We call it the Devonian, the so-called Age of Fishes. From about 419 to 358 million years ago, the evolution of fish had gone into overdrive. Even by this time though, the fish were already pretty much an established animal group. They had already evolved relatively recognizable forms more than 200 million years earlier, from some of the first vertebrate ancestors.


These backboned pioneers had come a long way since their beginnings as minor parts of the fauna like Pikaia, one of the forefathers of all vertebrates. Soon, the earliest fish had begun to look somewhat like their future descendants, at least superficially. The standard body plan was already present. Most of these early fish lacked jaws. They breathed through gills supported by loop-like arches of bone or cartilage and had a lateral line system along their bodies to sense vibrations in the water.


There are still many types of fish that have no jaws, like lampreys and hagfish which have both been called “living fossils” by some. The extinct jawless fish are placed under an umbrella group known as ostracoderms, and were all slow swimmers with armored heads. They had fleshy mouths to siphon up tiny living things from the sand on the seabed. It is these armored forequarters that get preserved in the fossil record, and we currently have a variety of strange jawless forms starting at more than 480 million years ago, far before the Devonian even began.


However, jawed forms began appearing around 419 million years ago, with the appearance of the first cartilaginous and bony fish. Their appearance was during the later Silurian period, and the rocks of China shows us the best remains. Some of the earliest bony fish were lobe-finned fish, the same group as coelacanths and lungfishes. Then there were the so-called spiny sharks, a short-lived group of small shark-like fish with immovable spines running along their bodies. All jawed fish had to modify their first gill arches – a series of bony loops supporting the gills – thus turning them into their lower jaws.


Both sharks and bony fish evolved from a single common ancestor. We have fossils of one of these fish, – or at least its closest relative – and the find is quite a revolutionary one. It is known as Janusiscus. It dates back to the very start of the Devonian, hailing from Siberia. It may not be the true common ancestor, but this is as close as it gets. Its secrets were revealed by high-powered CT scans of its tiny skull. Janusiscus’ body has been restored as being somewhat shark-like. This seems like a stretch, considering that it is known from a partial skull roof but it is reasonable. This little sliver of skull is made of bone. The blood vessels that carried blood to the brain though, had left scars in the bone and they were most similar to those of sharks.


This leads to the new theory that the common ancestor of the two did not have cartilage. Instead, Janusiscus rewrites the story of fish as a whole, putting bone into the skeleton of the very first ancestor of two of the most prolific groups of fish today. Sharks are certainly not primitive animals, instead evolving over the last 400 million years. We know that to get to the way they are now, they had to lose their bony skeletons altogether and replace it with a framework of light but strong cartilage.


A separate family of bony fish developed their jaws independently of the others. These strangers are known as placoderms. Placoderm fish developed their jaws at around the same time as these other fish, and the earliest examples hail from Silurian China. But the feature that made the placoderms famous was their armored head and neck.


Entelognathus


The first of the placoderms is known as Entelognathus. It is beautifully preserved thanks to its armored shield, while the rest of its body was left clean of any plating, instead having a thickened tail similar to a cross between a shark and a eel. The stage was set for jaws to take over, and once the Devonian began, placoderm evolution went berserk. The spawn of Entelognathus had conquered the seas and rivers of the world with their armored jaws, taking a number of varied and sometimes monstrous forms. This is particularly true when face to face with the huge Dunkleosteus, seen in our main image and gallery.


This is the archetype in armored marine monsters. Dunkleosteus lived in the Late Devonian, 360 million years ago. It is the Cleveland Shale member of the greater Ohio Shale Formation that has given us the largest and most complete fossils, and the best-known is a skull. This rock bed of black shale preserves an anoxic seabed environment devoid of any reef-building organisms. No corals or sponges have been found in this area. The few invertebrates we have found here include the superficially bivalve-like albeit unrelated brachiopods and a few ammonites, the coil-shelled mollusks related to the modern nautilus. This sea floor drops down to at least 100 meters.


Dunkleosteus belonged to a group of placoderms that had specialized in an active lifestyle. Some were herbivores and filter-feeders while the majority were predatory. They lacked teeth but instead had a set of bony plates that sliced past each other in the jaws. Dunkleosteus had an incredibly rapid bite with a force much stronger than a lion. It could exert a force of 7,400 Newtons when its blades closed down on its prey. Traditional restorations of Dunkleosteus show it with its jaws agape, blades showing. Recently though, it has been shown with lip-like tissue over its mouth. This, coupled with the bullet-shaped head of the animal, presents an orca-like profile. It was far more hydrodynamic this way, and it only needed to expose its blades when making a kill.


Whatever its face was like, Dunkleosteus was the top predator of its time. While hardly any remains from the animal’s hindquarters have been found, estimates from its smaller relatives have helped us to paint a picture of a terrifying animal. It rivaled a great white shark in size, and was just as deadly. As an adult it spent its time preying on animals much smaller than itself, though. On the menu were other placoderms and lobe-finned fish, most of which were also relatively slow-moving. But the evolution of the jawed fish was still not done yet. There were plenty of others that lacked any armor and had skeletons made of cartilage. They were the first proper sharks.


Cladoselache


Cleveland Shale fossils show that Dunkleosteus coexisted with several of these early sharks. Many were barely a meter long at the time but they had set the standard for their descendants. Even then, around 360 million years ago they were fast, streamlined and toothy predators, with cartilaginous skeletons. Of course, there was still a long way to go. One of these early sharks was an animal called Cladoselache. The body fossils of this shark are so well preserved here due to the lack of oxygen around the shallow seabed. Shark cartilage is a rarity due to its brittleness compared to bone. But under the right conditions, shark body fossils will crop up. It had all the classical shark features, with some obvious differences. For one, the largest oceanic sharks today dwarf it in size. It was, however a highly successful fish that ranged the Devonian seas far and wide. A number of Cladoselache species probably preyed upon smaller fish, while in turn dodging massive predatory placoderms and lobe-finned fish.


Another small shark of the time was Stethacanthus. It was similar to Cladoselache and was even more successful, lasting far beyond this time period. It lived in a wide range of habitats, with numerous species ranging from half a meter in length to over four times this size in later individuals. The Cleveland Shale preserves Stethacanthus fossils. The largest are present in the much later Bear Gulch, over forty million years after the Late Devonian.


Stethacanthus is also known as the “ironing-board shark” for its strangely-shaped dorsal fin. It was flat-topped, with little spines, almost in the shape of an anvil. This may have been a display structure. Even the top of its head was sprinkled in little spines. These are not bony spines. Rather they are hardened denticles, the little spiny projections that cover a shark’s body instead of scales.


This so-called “spine-brush complex” has been a mystery to scientists for decades. It certainly caused drag and made Stethacanthus a slow mover, so it was a detriment to hunting prey. It may have had a more subtle purpose. Sharks today exhibit a range of bodily gestures, and Stethacanthus possibly used body language to communicate with either enemies or members of its own kind. Despite its relative slowness, it was still a predator to be reckoned with in shallow waters. In comparison to its neighbors though, it would still be an underdog.


Bothriolepis


A better term for the Devonian may be the “Age of Placoderms” since it is they that truly dominated the waters of the world. One of the most common and well-known of all placoderms is the rather inoffensive Bothriolepis. It lived all over the world, with fossils ranging from Australia to Europe to North America. These animals got between thirty and a hundred centimeters in total length, and were all bottom-feeding animals that stuck to the seabed. This fish’s diet consisted of detritus and algae, and on a diet like this they could survive almost anywhere. Remains are as common in freshwater as well as in marine deposits. They certainly had jaws, but these were on the undersides of their armored heads. Bothriolepis’ jaw was in two separate parts which could function independently.


It is easy for one to reconstruct this animal with a good degree of confidence, and apply that to other placoderms. The tail was probably scaleless, and more flexible than the heavy armored head and neck of the animal. Even the innards and gills of Bothriolepis have been found. While we do not know of its breeding habits, more recent fossils in Australia show us that placoderms in general were live-bearing fish. This probably holds true for both Bothriolepis and even the mighty Dunkleosteus itself.


A more startling piece of evidence suggests that placoderms had fewer young than most live-bearing fish. The young were often large, as displayed by fossil embryos. This suggests that they may even have cared for their offspring just like mammals do. This form of reproduction is known as K-strategy, in which an organism invests more care in a small number of offspring. Hard as it may be to imagine Dunkleosteus and its armored kin as caring mothers, it seems the placoderms were more complex in their behaviors than anyone expected.


Materpiscis


The Gogo Reef Formation of Australia preserves an abundant array of Middle Devonian fish. The top predators here include massive lobe-finned monsters and big placoderms with mouths full of slicing blades. Certainly the best known are the latter. We have fossils of mothers that died in childbirth and even the first signs of penetrative sex in vertebrates. Death in coitus is common in the fossil record. Mating placoderms show us that they used an organ similar to a shark’s during copulation. Male placoderms had a pair of claspers to hold onto their mates, just like a male shark. The most famous fossil from Gogo is Materpiscis. This is a small placoderm with a snub nose and crushers in its mouth for processing hard-shelled food. Fossils of mother Materpiscis show that they nurtured their young via umbilical cord, just as in some sharks today.


Doryaspis


Despite the advent of the jaw, jawless fish were still not out of the picture. In fact, the profusion of placoderms and sharks had no effect on their survival and instead they kept going quite strongly. The ostracoderms had hardly changed, and were still sifting little morsels of food out of the water. One of the weirder ones was Doryaspis. This animal was about as armored as its cousins. Its fins too were relatively inflexible, with only its tail left free for swimming. The queerest feature of Doryaspis is an elongated snout, which it used to uncover its meals. Despite the existence of bottom-feeding placoderms, it and its jawless relatives thrived, even if for a short while.


The Devonian came and went with the placoderms. They arrived on the scene just as the period began, and went at its end, leaving no descendants. A mass extinction at the end of the period wiped out almost 50 percent of all life on Earth. It may have caused by either an increase of underwater volcanism, or an imbalance of oceanic oxygen. This event began over 375 million years ago when the latter stages of the Devonian were in full swing. With it, the strange armored fish were gone, never to return.


Reef-building corals were destroyed, and so were a number of other seabed-dwelling invertebrates. The fish that did survive were the lobe-finned fish, the sharks which went from strength to strength, and the ray-finned fish. The latter group consists of all other fish in the world today, the most diverse group of vertebrates in history. Their dynasty began in the Devonian, and they would soon dominate the world’s waters while the sharks would be the live-bearing marine giants of the future. The lobe-fins though, did something further.


Tiktaalik


By the Late Devonian, there were already forests on dry land. Trees soared into the air for the first time, and the early ancestors of insects and arachnids played our life’s dramas in the undergrowth. They were the unopposed lords of the land, living under the shadow of gargantuan horsetail and clubmoss plants. But they would soon find opposition in the form of the first fish to crawl onto land. Tiktaalik is one lobe-finned fish that made it. It was discovered in the Fram Formation of Ellesmere Island, around 375 million years old.


It was vaguely crocodilian in shape, with long toothy jaws and eyes positioned atop its head. Tiktaalik was a predator, and a powerful one at that, at 2.7 meters long as suggested by footprints. In today’s world it would be a sizeable and intimidating freshwater hunter. By virtue of both size and habits, it was similar to an alligator gar. Both it and the gar are big predators with tough scales and a lung-like organ as well as mere gills. They both hunted in shallow, oxygen-deficient water in swamps and bayous.


Tiktaalik is known as a “fishapod” in popular culture. When up on land, it would have been similar to a mudskipper. These fish also come onto land, propped up by their fins. Tiktaalik though, being a lobe-finned fish had fleshier fins than a mudskipper. It even had a functional wrist, elbow and shoulder configuration. These fins were also incredibly powerful, strong enough to carry it over mudflats and through shallow water full of algae. This was just the first step in the age of the tetrapod vertebrates. While the Age of Fishes would never return, it gave rise to half-fish creatures on leg-like fins that were the beginning of the vertebrate conquest of Earth.

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Vasika
Udurawane

Writer


http://sulc.us/4cmru
http://www.eartharchives.org/articles/when-monsters-ruled-the-water-the-age-of-fishes/