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While the subject of “prehistoric fish” is enormous, much more so than any of the other top ten lists made to date, the amount of prehistoric fish that are well known to the general public is quite slim. Therefore, it did not seem wise to create separate lists pertaining to taxonomic classes. While this resulted in the following list being extremely competitive, it is better in this writer’s view than multiple lists that are boring due to the fact that only their top five or six entries are significant.

10th - Leedsichthys
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Leedsichthys is possibly the largest fish ever, and certainly the largest known bony fish. It is estimated that the largest Leedsichthys could have grown up to sixteen meters. However, not all of them were this massive, and individuals above fourteen meters in length were probably not common. Leedsichthys is thought to have been a filter feeder like the largest extant fish and used a gill basket to extract food from the ocean.
9th - Stethacanthus
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Stethacanthus is one of the most bizarre-looking Chondrichthys ever. Its most notable feature is the anvil-like dorsal fin, the purpose of which is still the subject of debate. Fossils ascribed to Stethacanthus have been found in Asia, Europe, and North America. Not only was this genus’ geographic distribution wide, its temporal distribution was too. Earlier fossils hail from the Late Devonian, and Stethacanthus species appear to have survived into the later stages of the Carboniferous. 
8th - Hybodus
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While Hybodus does not catch the untrained eye in the same way Stethacanthus does, this prehistoric shark is fascinating to paleo ichthyologists. Hybodus sported two sets of teeth. Some appear to have been adapted for gripping slippery prey, while the others were more suited to crushing shells. This paints a picture of Hybodus as a generalistic opportunist, a strategy that served the species well. First appearing during the Permian period, Hybodus survived into the later stages of the Cretaceous. During this time, the genus became quite successful, and specimens have been found worldwide.
7th - Dunkleosteus
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This list would certainly be incomplete without Dunkleosteus. Dunkleosteus is one of the largest and most impressive known placoderms, and certainly the most famous. Rather than having teeth, it seems to have relied on bony “plates” attached to the skull, which would have been able to withstand tremendous force, a necessity given the fact that the bite of Dunkleosteus is estimated to have been around 750 kilograms at the highest point. This came from its incredible ability to open and close its mouth at immense speeds. These predatory adaptations suggest that Dunkleosteus was the apex predator of the Late Devonian, a time when many of the potential prey items such as ammonites or other placoderms would have warranted such measures. Like Hybodus, Dunkleosteus also had a wide geographic distribution, as fossils found in Africa, Europe, and North America have been ascribed to this genus.
6th - Leptolepis
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Sometimes the most important creatures are the most “ordinary” in terms of appearance. Leptolepis represents one of the earliest teleost fish, an infraorder that today represents 96% of all extant fish species. Fossils attributed to this genus are common, and they hail from locations around the world. Not only that, Leptolepis fossils have been found in both fresh and salt water deposits. Like several of the other entries on this list, the temporal distribution of Leptolepis is quite impressive. Older fossils may be found in Early Triassic formations and younger ones are present up until the Early Cretaceous. 

5th - Haikouichthys

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Thousands of these fish have been found in Early Cambrian rocks, and many of the specimens are preserved quite well. Their morphology is quite different from modern fish, but they do seem to have gills and fins. Perhaps the most significant trait of Haikouichthys are its bones. The creature sports what appears to be a skull. This has caused researchers to believe it to be an early craniate. 
4th - Tiktaalik
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The discovery of Latimeria chalumnae in 1938 was used by creationists to “shoot down” evolution. Prior to this, lobe-finned fishes were thought to have evolved into tetrapods, however, the fact that some still retained their basic exterior morphology seemed to show that such a transition did not happen. However, the discovery of Tiktaalik illuminated the truth. While technically classified as a lobe-finned fish, studies of the bones of Tiktaalik have shown that its limbs could be used to support itself in shallow water. The reality is that some lobe-finned fishes did not make the transition whereas others did. Tiktaalik is arguably the most significant milestone in the debate over the early evolution of tetrapods. In life, Tiktaalik probably lived in swampy environments, possibly moving through streams and ponds, or even occasionally crossing land to get into a new aquatic location.
3rd - Cladoselache
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Cladoselache has contributed phenomenally to our understanding of early shark evolution. Multiple fossils from the Late Devonian have been attributed to this genus, and, just as importantly, some are excellently preserved. Cladoselache demonstrates some characteristics similar to those of modern sharks, specifically the mackerel sharks, probably the most famous extant shark family. Analysis of its teeth and stomach contents has revealed that this shark’s primary means of consuming prey was probably by swallowing them. 
2nd - C. megalodon
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Number two is rather unusual because instead of being a taxonomic genus, it is just a single species. Nevertheless, the study and popular representation pertaining to C. megalodon has made it so its position on this list was never in question. Found worldwide in deposits ranging from the Oligocene to the Pliocene, this prehistoric shark is represented mostly by teeth. Nevertheless, vertebrae are also occasionally found, and these have been used to construct an image of what this super predator was like in life. It seems to have been a somewhat specialist predator of whales, as wounds on fossils of prehistoric whales have suggested that they were caused by immense force, which most believe was a result of a megalodon impacting them from below. One of the megalodon’s most famous features is its large bite force, which many researchers think would have been higher than any other animal currently known. Aside from the interesting predatory behavior of this shark, its size has also garnered it fame in popular media, and it is among the most iconic of all prehistoric creatures. 
1st - Coelacanth
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Like number two, number one is rather taxonomically odd, in this case, because it consists of an entire taxonomic order. Nevertheless, it did not seem fitting to split the Coelacanthiformes. First appearing in the Early Devonian, they are the second-oldest entry on this list. While they were not part of the tetrapod lineage, they did survive for a remarkable amount of time and diversify and adapt in many fascinating ways. One famous example is Mawsonia, a Cretaceous Coelacanth that could have grown up to six meters in length. Coelacanths appear to have lived in a wide variety of environments, and their fossils have been found around the world. While their basic outward appearance has not changed dramatically, especially when compared to their tetrapod relatives, their evolution, especially pertaining to internal organs and physiology, has continued. Coelacanths survived the enormous Permian-Triassic extinction event, which seems to have hit most aquatic species particularly hard. They also survived the Mesozoic, but seemed to have gone extinct with the non-avian dinosaurs at the end of the Cretaceous. Still, this would make them by far the entry on this list that had survived the longest, which is quite impressive considering it contains creatures such as Hybodus and Leptolepis. However, in 1938, living Coelacanths were caught off the coast of Africa, showing that, while they were not present in the known Cenozoic fossil record, they had, in fact, persisted. Today, we know at least one other species of living Coelacanth inhabits the world’s oceans. The Coelacanth is not only famous among paleontologists and paleontology enthusiasts, but has become a “rallying point” in the world of cryptozoology as proof that some animals thought to be extinct could still be alive. However, we should remember that most Coelacanth species are extinct, and we should not use them as a weak analogy as they are not the same as other orders.

 

Honorable mention

Helicoprion - Helicoprion is a bizarre Chondrichthyan predator with unique eating apparatus which baffled researchers until recently. Still, however, not enough material is known to earn Helicoprion a spot on this list. 

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