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New Study Says We Were Wrong About Giant Armored Fish That Terrorized Devonian Sea

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In a recent study published in PeerJ Life & Environment, Russell Engelman, a PhD student at Case Western Reserve University, attempts to address a longstanding issue in paleontology: determining the size of Dunkleosteus and other late Devonian arthrodire placoderms.

These extinct fishes had armor covering their head and part of their torso, but like sharks, the rest of their skeleton was made of cartilage, which means that most of their body did not preserve when they became fossilized.

Engelman’s study focused on whether the oral dimensions of extant sharks accurately predict the body size of Dunkleosteus terrelli (Placodermi: Arthrodira), which has been a persistent problem in the field of paleontology.

Earlier estimations of the size of Dunkleosteus terrelli were primarily based on the creature’s mouth and jaws. However, these methods were never validated to determine if they accurately estimated the size of placoderms. This study aimed to assess the efficacy of these methods by utilizing data from modern sharks and other fishes to determine if they accurately predicted body size in Dunkleosteus terrelli and smaller arthrodire placoderms known from complete remains. Because these smaller species are known from complete remains, they could be utilized to examine if the previous methods reliably predicted body size in arthrodires.

“Length estimates of 5–10 m have been cited for Dunkleosteus for years,” Engelman points out, “but no one seems to have checked these methods statistically or tested if they produce reliable or reasonable results in arthrodires.”

The study revealed that the size of the mouth of sharks did not correspond to the body size of arthrodires accurately. Complete arthrodires always had larger mouths compared to sharks at the same body length, resulting in body length estimates of complete arthrodires being 2-2.5 times greater than their actual size. Dunkleosteus had an exceptionally larger mouth even among arthrodires, which also raises doubts about using the mouth and jaw parts of smaller species to estimate the size of this Devonian giant.

Previous length estimations of Dunkleosteus were found to be biologically illogical, resulting in an unrealistically small head and hyper-elongated torso if applied to the known dimensions of fossils. These proportions were even longer than those seen in most eels, contradicting a previous study that suggested a shorter body more similar to pelagic sharks. Moreover, the implied long shape of Dunkleosteus in earlier studies would have made the fish’s gills too small relative to its body, leading to suffocation. While estimates based on mouth dimensions suggested extreme proportions for other arthrodires, no other species exhibited such characteristics, implying that previous length estimations for Dunkleosteus are highly unlikely.

Paleontology Shocker: Giant Armored Fish Of Devonian Seas Were Not As Big As We Thought
Paleontology Shocker: Giant Armored Fish Of Devonian Seas Were Not As Big As We Thought

The study concludes that the size of a shark’s mouth cannot accurately predict the length of arthrodires, and most previously estimated sizes of this group’s large members are overstated. This conclusion supports the findings of a previous study by the same author. Arthrodires, unlike sharks, have larger mouths relative to their body length, and their mouth widths are more comparable to those of predatory catfish.

“Dunkleosteus has often been assumed to function like a great white shark,” Engelman adds, “but as we learn more about this fish it might be more accurate to describe it as a mix of shark, grouper, viperfish, tuna, and piraiba [a type of giant predatory Amazonian catfish, well known to fans of Animal Planet’s River Monsters]”

However, while it may be disappointing that these giant Devonian fish were not as large as previously believed, the study highlights the importance of recognizing their large mouths. As the apex predators of the Devonian period, accurately estimating the body length and proportions of arthrodires is crucial for reconstructing their life habits and the overall ecology of the Devonian. Despite frequently being compared to sharks in reconstructions, the study notes that the large mouths of arthrodires suggest they could attack much larger prey relative to their body size than living sharks. This finding suggests that while arthrodires have often been reconstructed based on comparisons with sharks, the two may have behaved more differently than previously thought.

“Mouth size is probably the biggest factor in determining the largest prey a fish can eat,” Engelman adds, “the results of this study suggest arthrodires were hitting far above their weight class.”

Source: 10.7717/peerj.15131

Image Credit: RUSSELL ENGELMAN

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