When diving sites here in Midcoast Maine, we often encounter various marine flora and fauna. Some of the life we see in Penobscot Bay looks downright pre-historic to me! That got me thinking what it might have been like to dive in earlier, more ancient oceans.

My interest was further piqued when I heard about a stretch of time in Earth’s history around 400 million years ago, referred to as the “Age of Fishes.” Also called the Devonian Period, it lasted from 415 to 355 million years ago, a period spanning about 60 million years. How cool would it have been to dive those seas, seeing what kind of marine life they held?

So, during this Age of Fishes was the entire planet covered in water? Well, not quite. The land areas that did exist were basically the Gondwana supercontinent to the south, the continent of Siberia to the north, and in between, the newly formed smaller continent of Euramerica. In the Devonian Period, the two land masses that would later become North America and Europe had recently collided, creating a new formation we know today as the Appalachian Mountains.

During this period on land there was significant adaptive dispersal of life, such as the rise to dominance of vascular plants. Many became seed bearers at this time with evolving leaves and true roots. But the age and true action of the Devonian was defined by what took part in the ocean, hence the Age of Fishes.

Water covered approximately 85% of the Devonian globe, compared to about 70% today. There is little evidence of the existence of any ice caps back then, Devonian climate is thought to have been quite warm and equitable, warmer than present.

Sea levels in the Devonian were therefore generally high, although levels did tend to fluctuate, rise (transgress) and fall (regress). Some changes in sea levels were accompanied by short periods where black shales and limestones were deposited, which made the waters anoxic, or oxygen depleted. This is thought to have contributed to large extinctions of marine species by the end of the period.

It is called the Age of Fishes due to extensive evolutions of major groups of fish during this period. It was their true Golden Age. Molluscan groups were well-represented, and the first ammonite species appeared. These were squid-like creatures with spiral shells, predators that moved through the water by jet-propulsion, making them one of the more commonly found fossils of the Devonian. They are now extinct; the nautilus is their closest living relative.

Marine bivalves, such as clams and oysters, diversified greatly at this time, especially near shore. The first freshwater bivalves also appeared toward the end of the Devonian.

Late Devonian reef developments in Western Australia suggest a near tropical environment. Studying coral growth lines suggests that the Devonian year was longer than our current year, lasting about 400 days. Even its lunar cycle was 30.5 days, one day longer than it is now.

Many groups of Devonian fish diversified substantially at this time, such as placoderms. When fish first started to develop, they had no jaws, and their support structure was made of cartilage. The next development was fish with jaws, gills and paired fins. Devonian placoderms were the first fish to exhibit all three characteristics.

This fish fossil is from the collection of Charles H. Lagerbom. Audrey C. Lagerbom

Placoderms became the beasts of the Devonian waters; the largest — known as Dunkleosteus — could reach almost 29 feet in length and weigh 4.5 tons! Placoderms were heavily armored; in fact, this class of armored prehistoric fish came to dominate the marine environment. Their tough exterior also helped them leave behind good representation in the fossil record.

Sharks are thought to be descendants of placoderms, but somehow lost the ability to form the boney outside armor. They were also unable to form bones on the inside, so instead were supported by cartilage. Because of these cartilage skeletons, very little fossil evidence from the Devonian has survived. However, they did leave behind their teeth.

The first abundant genus of shark, Cladoselache, appeared in oceans during this time. But they were not the big brutes we know today. These little guys were only a foot long, with very large eyes and specialized jaws.

Fish without jaws also appeared at this time. Jawless fishes had no upper or lower jaw which could open and close. Instead, their mouth was disc-like, with a rasping tongue. There are not many species around today, but they were much more diverse during this Age of Fishes.

Lobe-finned fishes, ancestors to our amphibians, also made their appearance at this time. Early cartilaginous (Chondrichthyes) and bony fishes (Osteichthyes) also become widespread. Lungfishes (Dipnoi), coelacanths, and rhipidistians also appeared. The last group is thought to be the precursor to the four-footed amphibians we know today, as well as higher groups of vertebrates. Which leads us back to the land, literally!

Throughout the period there appear to have been long spans of widespread hypoxic or anoxic sedimentation in Devonian seas. This has been linked to some possible significant mass extinction events during the Devonian, all associated with some animal anomaly in the marine strata.

As oceans experienced reduced dissolved oxygen levels, they caused widespread extinction of many species. This was followed by periods of other species’ diversification and proliferation, as descendants of surviving critters quickly occupied those abandoned environments and habitats.

The Late Devonian extinction started about 375 million years ago and killed off all placoderms and trilobites, save for a few species of the order Proetida. All fossil agnathan fishes, except for psammosteid heterostraci, also died off. The most important group to be affected by this extinction event were the great Devonian reef-builders.

When a second strong pulse of mass extinctions hit, it pretty much closed out the Devonian era. Known as the Late Devonian extinction, this was one of five major extinction events in the pre-history of our planet, way more drastic and far-reaching than the better-known Cretaceous extinction event which ended the dinosaurs.

Reasons for the Late Devonian extinction and end of the Age of Fishes are still not known. Explanations for how it happened remain speculative, although some scientists lean toward the possibility of an asteroid strike. An impact crater 40 miles in diameter known as the Siljan Ring in central Sweden, may be the culprit. It has been dated to about 377 million years ago.

Regardless, the Devonian Period seemed to be one of the hey-days for fishes and the oceans. It set the course for what we have today. Even though it ended badly, I would still like to have strapped on a scuba tank and gone for a dive in those warm waters, perhaps encountering one of those dinky sharks or maybe a placoderm as big as a London bus!

Charles Lagerbom teaches AP U.S. History at Belfast Area High School and lives in Northport. He can be contacted at clagerbom@rsu71.org. He is author of “Whaling in Maine,” available through Historypress.com.