Paleo Safaris: The Late Devonian Sea

Ohio, USA, 370 million years ago

Dawn rises over a sea 370 million years ago. This is the Devonian period, and what will become the American States of Ohio and New York are submerged beneath salty waves. There are no birds in the sky, and no whales, dolphins or seals cruising the waters. There are not even any turtles or sea snakes. In fact, the marine reptiles and pterosaurs that co-existed with the dinosaurs will not be seen for another 130 million years! As a result, the ecosystem of this time is quite unique and very different to today! So let us take a plunge, and see what creatures lurk beneath the waves!

Ctenacanthus, a “shark” that is distinguished by its comb spines
Image Credit: James St. John, https://www.flickr.com/photos/jsjgeology/34272706675

The first animal we spot is a fish, for even this far back fish still rule the waters. But not just any fish, but what at first glance seems to be a shark! Sharks are an incredibly successful group, unfairly known by far too many people as large, blood thirsty monsters, and have a hugely diverse and fascinating history. Today, they come in a wide variety of different species, from the slow cruising, long living Greenland Shark; to the peaceful bottom dwelling Nurse Shark; to powerful predators like the Great White Shark. But it was during the Devonian period that sharks and shark like fish first evolved. Our fishy friend is a Ctenacanthus, whose name means “comb spine” due to a set of spines that protrude from its fins. These spines possess tubercles on them, giving them a superficially comb-like appearance. The Ctenacanthus cruises through the sea, using its keen sense of smell to direct it towards its next meal, ignoring the weird suction feeling…..

CRUNCH!!!

A bite of between 6,000 to 7,400 newtons almost snaps the shark in two! From out of the depths, a mighty predator uses its jaws to manoeuvre the lifeless Ctenacanthus, and before you can process what has happened it swallows it whole in a big gulp. Satisfied with its meal, the giant male Dunkleosteus closes its jaws behind a thick pair of lips and swims away. It will be back.

Dunkleosteus. Our ambush hunter from the depths!
Image Credit: Entelognathus, https://commons.wikimedia.org/wiki/File:Dunkleosteus_terrelli.png

This Devonian leviathan is 8 metres long and weighs almost three and a half tonnes. The bite inflicted on the poor Ctenacanthus is among the most powerful of any animal that has ever lived, and certainly the strongest ever delivered by a fish! He is part of the Placoderm class of fishes, a now extinct group characterized by hard armor plating covering their head and upper body. Placoderms are even theorized by some palaeontologists to include the distant ancestors of the vertebrates that would first crawl onto land. Therefore, at a time even earlier than this your ancestors may have been smaller and more peaceful variants of Dunkleosteus! Dunkleosteus’ size is almost unmatched by any animal at this time in earth’s history, with only the filter feeding Placoderm Titanichthys potentially eclipsing it. Dunkleosteus is not only one of the largest animals of the Devonian period, but also one of the largest animals that has ever existed up to this point in earth’s history.

Another fish that can be found in these seas is Cladoselache. Just like Ctenacanthus, this man-sized fish was another early relative of the shark family. However, Cladoselache possesses a few features that are very un-shark like! For one it had a short, rounded snout, lacked claspers (which are important structures used in reproduction) and was not quite as “scaly” as other sharks, with scales being restricted to the edges of the eyes, mouth and fins. But just like sharks it possessed two dorsal fins (though with a spine in front of each fin), paired pectoral fins at the front of the body, powerful jaw muscles, and it would’ve eaten almost any animal it came across. Using its excellent vision, one Cladoselache pinpoints a golden opportunity appearing into view, a school of ray finned fish. Cladoselache is built for speed, and powerful strokes of its tail allow it to dart in and out of the school, snatching fish using jaws full of branch shaped cusped teeth and swallowing them whole. In defense the school sticks together, co-ordinating their movements and sheer numbers to avoid and confuse the shark. But more Cladoselache join the hunt, attracted by the bounty on offer. There is no real pack hunting strategy here, but the corralling of one shark pushes some of the fish into the oncoming opportunistic jaws of another. The action intensifies into a frenzy of activity, with multiple Cladoselache darting quickly in and out of the school, grabbing all the fish they can. After a few hours the school has diminished in size, and the Cladoselache have quenched their hunger. The sharks once again go their separate ways, returning to their never-ending patrolling the deep blue sea.

Cladoselache. It hunting shoals of fish may be speculative, but it certainly would’ve been a fast hunter of the Late Devonian Sea!
Image Credit: Nobu Tamura, http://spinops.blogspot.com/2015/10/cladoselache-fyleri.html

The Dunkleosteus is not interested in mere fish schools, and is content to cruise the water column, looking for a rocky haven to rest in for the night. All in all, he’s had a very good day; he has successfully hunted and fed himself whilst avoiding serious injury. This sounds like a simple task, but predators of all shapes and sizes often fail in their hunts, and accidents can happen! As a result, it is great to not waste precious energy on an unsuccessful one. On a different day the Ctenacanthus might have been more aware of the danger and could have narrowly escaped the Dunkleosteus’ mighty jaws. Despite this, the Dunkleosteus feels weirdly uneasy. Maybe it’s the strange suction feeling he’s only just noticed…

CRUNCH!!

From out of the depths another Dunkleosteus plunges its sharp plated teeth into the soft tail of the big male. The attack devastates it, scything through muscle and blood vessels, and cripples the males swimming capabilities. Slowed down, and loosing strength rapidly, he is helpless as the newcomer rounds for another attack. The newcomer targets the body, and his 2nd bite cleaves through the male, stabbing into his internal organs. The sudden violent trauma, blood loss and punctured organs are too much for the male and his day, and life, are ended in the worst possible way. Dunkleosteus are opportunistic and eat anything they can sink their teeth into, including members of their own species. Even our male has gone after smaller Dunkleosteus on more than one occasion. After the second Dunkleosteus is done with his meal. Ctenacanthus gather to pick what is left before the body drifts down to the oxygen poor sea floor. Now that the scary predator is just a carcass, he is a lot more inviting for these opportunistic shark relatives! Eventually the Ctenacanthus go as far down as they dare, and after they leave the rest of the male comes to a rest on the muddy bottom. The oxygen quantity is poor here, so the Ctenacanthus don’t follow, and over time the armored head is buried by the movement of murky sediment which, in conjunction with the lack of oxygen, will shield it from further breaking and decomposition. 370 million years from now future vertebrates, in the form of humanity, will unearth the males giant head, where through careful study, they visualize how he looked and may have lived his life!

With the males passing, we end our journey through the Late Devonian Sea.

References/Further Reading

• The inspiration for this Paleo Safari came from rereading “Evolution: The Story of Life” by Douglas Palmer & Peter Barrett. In particular this story first came to mind from reading pages 80-81; “A Giant of the Devonian Deep” with the beautiful illustration of Dunkleosteus and Cladoselache done by Peter Barrett.

Palmer, Douglas, Barrett, Peter, “A Giant of the Devonian Deep”, “Evolution: The Story of Life”, octopus publishing group, 2009, pg 80-81

Carr 2010, a paper about the Paleoecology of Dunkleosteus, showing is preferred habitat and effect on the ecosystem both in life and in death

Carr, R. K. (2010). Paleoecology of Dunkleosteus terrelli (Placodermi: Arthrodira). Kirtlandia, 57, 36-45.

Anderson & Westneat 2009 paper examining the feeding of Dunkleosteus using a biomechanical model. This allowed them to calculate its staggeringly powerful bite force

Anderson, P., & Westneat, M. (2009). A biomechanical model of feeding kinematics for Dunkleosteus terrelli (Arthrodira, Placodermi). Paleobiology, 35(2), 251-269. doi:10.1666/08011.1

A Cleveland Museum of Natural History article on Cladoselache fossils found in the area, their anatomy, lifestyle and how they were preserved.

Cleveland Museum of Natural History, “INTRODUCING CLEVELAND’S TOOTHIEST SHARK”, Cleveland Museum of Natural History, www.cmnh.org, https://www.cmnh.org/science-news/blog/august-2019/introducing-cleveland%E2%80%99s-toothiest-shark

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