The Fish Kingdom
Published in The EarthSphere Blog. Feature Image: Devonian Transformation (by ArcheanArt)
Prologue
We wrapped up the Silurian Period in our last Forgotten Origins article, and now we take a peek at life in the Devonian.
Passage into the Devonian
The Devonian Period spans 60 million years, from 419.2 to 358.9 million years ago — Earth was hot, and sea levels were high. Today, oceans cover 71 percent of Earth’s surface, but during the Devonian, the planet was a blue jewel circling the sun with 85 percent of its surface underwater. Shallow seas abounded, creating vast marine ecosystems for life to exploit. The period is sometimes called the Age of Fishes. Our water-bound friends were the apex of animal life as evolution created a host of new marine species. But while fish were ruling the planet, some four-legged vertebrates finally snuck from the ocean, virtually unnoticed, and crawled out onto dry land.
The end of the Silurian and the beginning of the Devonian Period was mostly a footnote in the annals of geological history. Extinctions in the fossil record separate all major periods of the Paleozoic, but these extinction events come in all shapes and sizes. Species loss at the end of the Silurian was minor compared with many other extinctions, and probably caused by changes in the climate and the environment. There was death but not outright devastation.
Mother nature sometimes takes on the specter of a mad, passionate artist, wiping the canvas clean out of frustration and painting a new creation. At other times she’s gentler, smudging over and repainting portions of the canvas but leaving the gross composition intact. The change from Silurian to Devonian was the latter of the two. But during the Devonian, she painted a dizzying array of new fish onto the canvas.
At Sea
Most animal life during the Devonian remained in the shallow seas and coastal waters, so evolution worked with what was available. Armored fish, along with jawed fish, flourished and new species appeared. The two seem to go hand in hand. Once your neighbors develop jaws, you surely want some armor to avoid being clamped down on for dinner. Scaly armor on the outside keeps the insides where they belong.
Sometime around the middle of the Devonian, a group of fish arose called Chondrichthyes. These were precursors to sharks with a cartilaginous skeletal framework. Today this class of fish also includes skates, rays, and chimeras.
Nature was also busy perfecting the use of coiled shells in the Early Devonian, and a variety of ammonites appeared. Similar to today’s nautilus, these ancient predators used their coiled shells for protection and mobility. A tube called the siphuncle runs through the chambers of the nautilus’ coiled shell, allowing the release of fluids and gases to maintain neutral buoyancy and float in the water column. The siphuncle in a nautilus runs through the center of its shell chambers. But the ammonites’ tube ran along the outer edge of its shell. I speak of them in the past tense because ammonites are no longer with us.
But perhaps the most interesting development in the Devonian fish world occurred within a now-extinct group of fish called Rhipidistia. Fossil remains indicate they were bony fish with a lobe-finned anatomy. Their fins extended out from two distinct pairs of ventral and dorsal lobes. Not too much imagination is needed to extrapolate these lobes into budding legs. Detailed analysis of their fossil remains supports the theory that these fish evolved into the first four-legged tetrapods to venture onto land.
Life on the Move
In parallel to the Rhipidistia, lungfish also appeared in the Devonian. Their evolutionary trick of living for extended periods out of water and surviving on atmospheric oxygen also offered another part of the solution for land-based animal life. Insects may have been the first animals on land during the Silurian, but four-legged life was not far behind. By the end of the Devonian, animals gained a strong foothold on Earth’s continents. The earliest of these tetrapod invaders were probably salamander-like amphibians.
Earth’s continents were on the move also. The mighty supercontinent of Pangea was consolidating. As Laurasia and Gondwanaland moved closer together, the last of the Iapetus Ocean was subducted. The collision of these two massive continents resulted in large-scale mountain building, producing the early Appalachian Mountains and continuing the rise of the Caledonides in Britain and Scandinavia.
The success of life in the Devonian resulted from the interplay of climate, geology, and biology. A warm climate and associated high sea levels provided long-lasting shallow seas. Over 6,000 feet of limestones and shales were deposited in what is now Utah, and in Europe, extensive marine deposits covered areas from England to Russia. The period also saw some of the most prolific coral reef growth in geological history.
Animal life on the planet surged ahead, but plants also made great strides. Vascular plants and the first true trees appeared on the evolutionary stage. The ramifications of this explosion in plant life may have been more significant for Earth’s history than the changes wrought in the animal kingdom.
(Next time — The rise of Devonian trees)
Sources:
Silurian Period (Source: Cal Poly Humboldt Natural History Museum)
Rhipidistia (Source: Britannica)
Devonian Period (Source: InfoPlease)
Devonian Period (Source: National Geographic)