Giant, swimming mouths: oral dimensions of extant sharks do not accurately predict body size in Dunkleosteus terrelli (Placodermi: Arthrodira)
by: Russel Engelman
Summarized by Marcella Funes is currently a biological science major at Binghamton University. She is currently a senior and plans to extend her academic studies at Binghamton University for her masters in biological sciences. Once concluded with her degrees, she hopes to do behavior research on animals. In her free time, Marcella enjoys checking out the animals at local aquariums and zoos!
What data were used? This study primarily focused on the extinct fish Dunkleosteus terrelli, a giant armored fish from the Late Devonian Period. This extinct fish is part of a class of fish known as placoderms, meaning “plate skin” fish. Along with their armored skin, these fish had a jointed neck, putting them in the extinct order, called “arthrodires”. Scientists focused on the measurements of the upper jaw perimeter, mouth width, mouth length, and head length of D. terrelli. These specific measurements were collected from the four Dunkleosteus terrelli specimens held at the Cleveland Museum of Natural History (Ohio, USA). These measurements were then compared to the measurements of 985 existing fish species, 180 of which were sharks. Comparative measurement data was also collected from multiple smaller taxa of fossil arthrodires.
What was the hypothesis being tested? Scientists sought out to provide evidence that the previous proposed body lengths for the Dunkleosteus terrelli were inaccurate. The current predicted body length measurements were established by using the relationship between the upper jaw perimeter of the D.terrelli and body length of existing sharks. However, scientists believed that this method is unreliable, and they sought to test the accuracy of this method.
Methods: Researchers collected measurements from skeletons of the extinct D. terrelli. These measurements were then compared to measurements of existing shark species. Using this comparative analysis, researchers were able to tell how reliable the body size predictions of D. terrelli are. Since there are only skeletons of D. terrelli, the measurements of existing sharks are often used to fill in the gaps and predict the possible body size of the armored giant. To conclude that their findings were statistically significant, researchers also ran statistical tests. These tests showed the validity of using shark mouth and body length dimensions to predict body size in D. terelli.
Results: Scientists found that the current body length estimates of D. terrelli are not accurate for many reasons. The first reason being stated that extinct arthrodires, such as the D. terrelli, have proportionally larger mouths for their body size. Sharks, on the other hand, typically have a proportionate mouth to body size. The shark-based model also caused extreme overestimates of D. terrelli body size. The shark-based model estimated the body length of D. terrelli to be 5.3-8.8 meters– this is now known to be an overestimation. These overestimated body sizes can be observed in Figure 1. Based on the head lengths of D. terrelli found in this study, the body length was more accurately predicted to be around 2.69 meters.
Why is this study important? Previously, there had been many debates on the accurate body length of arthrodires such as the Dunkleosteus terrelli. Being a staple of the Devonian Period, having accurate measurements gives better insight into how this apex predator species hunted. Since D. terrelli did not have any teeth, they utilized their jaws to shear their prey. Accurate measurements of their jaws can display how these apex predators utilized their powerful jaws. With these measurements, future research can be done about their jaw force as well how they used their mouths as a vacuum for prey.
Broader Implications beyond this study: This study can cause scientists to revisit previous size estimates and hopefully better understand the marine ecosystem of the Devonian Period. D. terrelli were at the top of the food chain during the Devonian Period. These apex predators would eat through other thick shelled placoderms, suggesting possible cannibalism. Along with placoderms, soft- bodied prey was also a part of their diet. With better full body measurements further research can be done into the eating habits, as well into changes to the jaws through growth of D. terrelli.
Citation: Engelman, Russel. 2023. Giant, swimming mouths: Oral dimensions of extant sharks do not accurately predict body size in Dunkleosteus terrelli (Placodermi: Arthrodia). PeerJ. https://peerj.com/articles/15131/