The rise and fall of the armored fishes

what we can learn from the doom of a lost kingdom

I got my start in fish history by studying a sort of lost kingdom of fishes. The stereotype is that finding a lost kingdom requires climbing a high mountain or traversing some deep jungle. However, I found traces of one among the fossiliferous limestone of Michigan’s lower peninsula. This kingdom arose over 400 million years ago with the spread of some of the first fishes with jaws. These fishes are called the placoderms (from the Greek for “plated skin”) for the bony plates covering their heads and bodies, which are first found in 430 million year old rocks. Placoderms are one of the best flash in the pan stories of science, illustrating that even the most prominent groups of organisms can be wiped from the face of the Earth in the geological blink of an eye.

I began studying placoderms by coincidence. Although the stereotypical paleontologist is pictured in a remote desert, my first experiences with fossils was along my parent’s midwestern gravel driveway. I grew up smack dab in the middle of the lower peninsula of Michigan, where 10,000 years ago massive ice sheets swept down from Canada and carved the Great Lakes. Glaciers are essentially giant abrasive pads, grinding and crushing rock with their immense weight and the rocks accumulated on their leading edge. Geologists call the macerated rock they leave behind till or gravel, which my grandparents found useful to pave their driveway long after the glaciers melted. Inspired by paleontology television and movies, I scoured our driveway gravel for fossils. I soon learned that the glacial till was full of fossil corals, shells, and other strange animals beyond my identification skills. I was lucky that the glaciers had scraped through rock from the Devonian period, when Michigan was was covered by a shallow, tropical sea teeming with life. Although it may seem odd that a state known for cold weather and fresh water would have tropical, marine fossils, we have to remember that over millions of years the Earth’s surface has changed radically. Over 380 million years ago the majority of North America was covered by a shallow ocean, located far closer to the equator than today. The Devonian world map is practically unrecognizable, with continents primarily concentrated in the southern hemisphere and largely flooded by shallow, tropical seas. The plants and animals of the Devonian pre-date most of the groups we are familiar with today. There were no mammals, no flowers, and no dinosaurs, but there were fishes. Ho boy, there were a lot of fishes.

I learned from other fossil hunters that the remains of the Devonian reefs are concentrated in a ring around the northern part of Michigan’s lower peninsula, and that the best collecting was to be had in the limestone quarries and strip mines around Alpena, Rogers City, and Petoskey. The fossil bearing rocks were laid down ~380 million years ago, formed from the skeletons of the animals that lived in massive reef complexes. The glaciers scraped away the younger rocks, exposing the reef at the surface. Thanks to the kindness of my parents (I wasn’t old enough to drive), I found a bounty of fossils at a variety of state parks, limestone quarries, and an abandoned construction project (whose location I have to keep secret). These fossils had avoided the misfortune of being mashed up by glaciers, meaning I had a larger and better preserved sample of the Devonian reefs than my parent’s driveway. The rarest fossils were fragmentary armor plates from placoderms, which I accumulated with time, patience, and hard work. Around this time, I continued a grand tradition in paleontology of bothering further along people in the field by emailing a young dinosaur paleontologist for advice. I told him that I was trying to decide if I should study feathered dinosaurs or the fish I was finding. To this day. I am deeply grateful that he told me to study the fish. This not only started me on the path to writing this article (first draft written on a dark summer night on a fossil dig in Arizona), but gave me the kick in the butt needed to explore a small corner of a lost kingdom of fishes.

Placoderms are a beautiful and ultimately tragic chapter in the history of fishes. Placoderms are known as the “armored fishes”, but they are not alone in bearing armor, which is a common adaptation in the animal world. From armadillos to armored catfishes, a wide variety of animals develop bony coverings to defend themselves from predators. The general evolutionary trend in fishes, however, has been away from heavy armor. Most fishes today have lighter, less armored scales than their evolutionary predecessors. Placoderms are on the opposite extreme, evolving interlocking bony plates that are most commonly found as isolated pieces in fossil assemblages. In addition to bearing heavy armor, placoderms are the first group of jawed fishes to rise to prominence. We take jaws for granted today, because almost every vertebrate on Earth has then (with the exception of lamprey and hagfish). However, when placoderms came on to the scene in the Late Silurian, jaws were a novel evolutionary trait. Jaws gave placoderms the ability to feed more voraciously and on a wider variety of prey than their jaw-less predecessors. The placoderms I found in Michigan include approximately a dozen species from the hundreds that have been named from every continent. Almost two centuries of research on these fossils paint a picture of a major group of fishes with stunning anatomical and ecological diversity. The smallest placoderms, called the ptyctodonts, were bottom-feeders with thick, rounded teeth resembling the business end of a meat tenderizer for crushing invertebrate shells. Most placoderms, including my favorite called Protitanichthys rockportensis, a species endemic (unique) to Michigan, had sharp, blade-like jaws for biting other fishes. Predatory placoderm forms like Protitanichthys would have been voracious predators of other, smaller armored fishes. We only have fossils of the thick armor plates from Protitanichthys, meaning its exact size is unknown. In life, Protitanichthys would have looked like a bulldog in a suit of armor, with beady little eyes, blades for teeth, and a long, streamlined tail like a shark. Although predatory forms like Dunkleosteus after often cited as being 20 or 30 feet long, recent statistical analyses suggest that predatory placoderms would have been at most 13 feet (~4m ) in length. However, the title of the largest placoderm belongs to Titanichthys, which likely exceeded 5 m (~16 feet) in length and, similar to the largest animals in today’s oceans (baleen whales), filtered small animals out of the water with a wide, toothless mouth. Despite their great diversity, fossils of placoderms are only found in Devonian aged rocks, meaning they are completely extinct. They are analogous to a lost kingdom or city of humans. Their fossils give a glimpse into a mighty past, now lost to history.

Restoration of Protitanichthys rockportensis by Megan Sodano.

Although the placoderms are the clear frontrunners of the Devonian, they were not the only fishes around. Early relatives of sharks swam alongside placoderms. These sharks are not what we see in aquariums today. Rather, they belong to more ancient groups that have long since gone extinct, but would be recognizable as sharks. The ray-finned fishes, represented today by over 30,000 species, also lurked in the shadows of placoderms. The first ray-finned fishes lived at the margins, widespread but not abundant or diverse. These early species were small and display very little diversity in their anatomy. A lone exception to this trend is Tegeolepis, a formidable man-sized animal with massive fangs and a powerful body, hallmarks of a swift, voracious predator. Other Devonian ray-finned fishes were also predators, but rarely surpassed a foot in length. Aquatic lobe-finned fishes, which today include only lungfish and coelacanths, were at their heyday in the Devonian. The earliest fishes lobe-finned fishes (which today include you and I, more on this in a future installment) were taking their first steps on to land in the Devonian. Additionally, a wide array of marine lungfishes and stranger predatory forms were prominent components of global faunas. Strange fishes with many spiny fins called the acanthodians were also lurking about, although we know very little about them because we rarely find more than their spines and scales. Finally, a variety of fishes without jaws were important parts of global faunas. These animals, including the lamprey-like conodonts, are extremely abundant in the fossil record due to their mouth being lined with dozens of tooth-like structures, which probably functioned to process prey similar to living jawless fishes. This wide diversity of fishes is why the Devonian is often referred to in books and the scientific literature as the “age of fishes”. At no other point in Earth’s history have so many disparate groups of fishes existed at the same time. Although there are far more species of fish in today’s waterways, almost all of them belong to a single group (more on this in future installments). So while fish are certainly more abundant now than in the Devonian, in an important respect they are far less diverse. Up until this point we have traced the rise of fishes from small, soft-bodied worm-like organisms to cosmopolitan groups of large, bony animals occupying a wide variety of ecological roles. this process culminated in the high tide of group diversity in the Devonian, when all the major groups of fishes co-existed. From here on out, fish diversity on the whole will go down. We will trace how some groups went completely extinct, how some waxed and waned to near extinction, and how a few rose to extreme success in the modern world. Let us begin with the fall of the placoderms.

Despite their global presence and wide diversity of form, each and every placoderm is extinct. No placoderm fossils are found past the Devonian, and they did not leave behind any descendants. Out of the nearly forty thousand species of fish on Earth today, none are placoderms. It is difficult to reconcile the clear success of the placoderms with their apparent demise. How is it possible that such a successful group went completely extinct? The answer lies in mass extinction, an Earth system phenomenon that will come into play in fish history many more times. Mass extinction is a scientific term for doom, for the near annihilation of life and ecosystems on a global scale that has happened with frightening speed in the history of life.

The placoderms thrived in shallow, tropical Devonian seas and warm river systems, which had persisted as stable global environments for tens of millions of years. Over 50,000 years, a geological blink of an eye, this world was unmade. The doom began approximately 358 million years ago, when global sea levels rose dramatically, coupled with algal blooms and a rise in temperature. It is counterintuitive that a spike in biological activity can cause so much damage, but the principle is similar to an algal bloom in a small pond or lake. The photosynthesizing algae rapidly grow and remove large quantities of CO2 from the air to fuel their metabolism. As they die, aerobic (or oxygen metabolizing) bacteria multiply rapidly as they consume the dead algae, using up enormous amounts of dissolved oxygen in the process. In their quest to endlessly multiply, these bacteria doom themselves and the life around them, using up all the precious oxygen and creating an environment hostile to almost all forms of life. The process of environmental destruction is marked by contemporaneous Devonian deposits of organic rich black shale from around the world, a harrowing record of the death of the shallow, tropical seas placoderms depended upon. Chemical study of one such black shale in the Holy Cross Mountains of Poland by Paul Myrow and colleagues in 2014 reveals the chemical signatures of green sulphur bacteria, single-celled organisms that photosynthesize in environments without oxygen to produce acidic compounds. Their presence heralds highly acidic oceans without oxygen, which would have killed all placoderms and other organisms that breathe oxygen. The placoderms were not alone in their suffering. We estimate that 70% of all vertebrate species went extinct in less than a million years, a stunning loss of biodiversity that took millions of years for the Earth to replace. Crucially, once a species is extinct it cannot be revived. Therefore, despite millions of years of success on a global scale, the placoderms were wiped out in a rapid succession of changes to the environment they depended upon. Today, they can only be found in museum drawers or in entombed in rock.

358 million years later we humans named this geological series of events the Hangenberg crisis, one of a half dozen mass extinctions we recognize in the fossil record from the last 500 million years. Although the initial cause of the Hangenberg crisis is a matter of scientific debate, the most likely explanations are an extra-terrestrial impact or a massive increase in volcanic activity. No matter the cause, the Hangenberg crisis and other mass extinctions are the evolutionary equivalent of flipping the table and spilling the board game all over the floor, overturning tens of millions of years of evolutionary history to create abundant opportunity in the form of space. Among the survivors are the sharks and ray-finned fishes, along with a handful of the strange, spiny acanthodians, the jawless conodonts, and lobe-finned fishes (our earliest land-walking ancestors among them). These lucky few survivors emerged into a far more empty but rapidly recovering biosphere, ripe with opportunity for aggressive expansion. In future installments we will see how the Hangenberg set the stage for critical aspects of how fish diversity is structured today, and dive into how mass extinctions work.

For now, I think we should take a moment to reflect on a broader lesson the placoderms teach us. We humans are a widespread and successful species in our own, extremely narrow slice of Earth history. We depend upon an Earth with water we can drink, air we can breathe, and soil that can raise the food we eat. All of these conditions are temporary, fragile states of a dynamic and shifting Earth system. If we are not careful, unchecked greed for growth and wealth could take the Earth out of the narrow zone we depend on for life. There is no reason why we cannot have the same fate as the placoderms, if we aren’t careful.

Sources and recommended reading:

Becker, R.T., Marshall, J.E.A., Da Silva, A.C., Agterberg, F.P., Gradstein, F.M. and Ogg, J.G., 2020. The Devonian period. In Geologic time scale 2020 (pp. 733-810). Elsevier.

Boyle, James, and Michael J. Ryan. "New information on Titanichthys (Placodermi, Arthrodira) from the Cleveland Shale (Upper Devonian) of Ohio, USA." Journal of Paleontology 91, no. 2 (2017): 318-336.

Engelman, Russell K. "A Devonian fish tale: a new method of body length estimation suggests much smaller sizes for Dunkleosteus terrelli (Placodermi: Arthrodira)." Diversity 15, no. 3 (2023): 318.

Kaiser, Sandra Isabella, Markus Aretz, and Ralph Thomas Becker. "The global Hangenberg Crisis (Devonian–Carboniferous transition): review of a first-order mass extinction." Geological Society, London, Special Publications423, no. 1 (2016): 387-437.

Long, J., 2010. The rise of fishes: 500 million years of evolution. Chapter 4: “Armored fishes and fishes with arms”. The Johns Hopkins University Press. Baltimore, Maryland.

Myrow, Paul M., Jahandar Ramezani, Anne E. Hanson, Samuel A. Bowring, Grzegorz Racki, and Michał Rakociński. "High‐precision U–Pb age and duration of the latest Devonian (Famennian) Hangenberg event, and its implications." Terra Nova 26, no. 3 (2014): 222-229.

Stack, Jack, and Lauren Sallan. "An examination of the Devonian fishes of Michigan." PeerJ 6 (2018): e5636.