12.6: Middle Paleozoic Life - The Silurian and Devonian

Evolutionary Innovations: Vertebrate Diversification and Life on Land

After the End-Ordovician Mass Extinction, life bounced back during the Silurian period. The period’s major evolutionary event was the development of jaws from the forward pair of gill arches in bony fishes. Hinged jaws allowed fish to exploit new food sources and ecological niches. This period also included the start of armored fishes, known as the placoderms. In addition to fish and jaws, Silurian rocks provide the first evidence of terrestrial or land-dwelling plants and animals. The first vascular plant, Cooksonia, had woody tissues, pores for gas exchange, and veins for water and food transport. Insects, spiders, scorpions, and crustaceans began to inhabit moist, freshwater terrestrial environments.

Figure \(\PageIndex{1}\): The armor-plated fish (placoderm) Bothriolepis panderi from the Devonian of Russia.

The Age of Fishes

Throughout the Cambrian and into the Silurian, jawless fish (agnathans) and armored jawless fish (ostracoderms) diversified. Jaws in vertebrates likely evolved from the gills in ostracodermi, and the spiny sharks (acanthodians) displayed the first jaws in the early Silurian. Other jawed fish, including armored fish (placoderms) and bony fish (osteichthyes) appeared during the Silurian and radiated during the Devonian. The oldest cartilaginous fish (chondrichthyes), i.e. sharks and rays, radiated in the earliest Devonian.

Figure \(\PageIndex{2}\): Spindle diagram of fish diversity through time. Vertical length of shapes indicate geologic range of species while the width represents diversity. (Creative Commons Attribution-Share Alike 3.0 Unported license.)

The Devonian Period is called the Age of Fishes due to the rise in plated, jawed, and lobe-finned fishes. The lobe-finned fishes, which were related to the modern lungfish and coelacanth, are important for their eventual evolution into tetrapods: four-limbed vertebrate animals that can walk on land. The first lobe-finned land-walking fish, named Tiktaalik, appeared about 385 million years ago and serves as a transition fossil between fish and early tetrapods. Though Tiktaalik was clearly a fish, it also possessed stronger limbs, including hip-like structures on the hind limbs, a neck that allowed for swiveling motions, and large eye sockets on the top of the head. Several other fossils from the Devonian are more tetrapod-like than fish-like but were not fully terrestrial, as the first fully terrestrial tetrapods would not evolve until the early Carboniferous period.

Figure \(\PageIndex{3}\): Several different types of fish and amphibians that led to walking on land.

Invertebrate Animals on Land

Invertebrate groups diversified significantly throughout the Paleozoic oceans, increasing the competition for resources. As time went by, there was an increase in ecological tiering above and below the seafloor, as animals evolved forms that allow for deeper burrowing and higher extension away from the sediment-water interface. It was only a matter of time until life found a way to exploit the vast expanse of geographic space on land.

This transition to land by invertebrates is thought to have occurred during the early Paleozoic, but it was not instantaneous. Trace fossils from millipedes have been found from the Late Ordovician, and by the Silurian and Devonian, spiders, scorpions, centipedes, and silverfish all found their way onto land. Aquatic invertebrates would have had an advantage due to their exoskeletons, which allowed for protection and moisture retention as these organisms adapted first to freshwater ecosystems and then to dry land. It is not surprising, then, that arthropods are the first aquatic invertebrate group to venture into the terrestrial realm.

Out of all of the invertebrate phyla existing at the time, arthropods had the body plan with the greatest evolutionary flexibility: in particular, lots of segmented appendages that were highly specialized for different types of movement such as walking, burrowing, swimming, and feeding. Aquatic arthropods take in oxygen via book gills (a structure that looks similar to folded or overlapping sheets of paper) which are easily visible on horseshoe crabs you might find along the East Coast of the United States today.

Figure \(\PageIndex{4}\): Modern horseshoe crab showing specialized appendages and book gills. (Creative Commons CC0 1.0 Universal Public Domain Dedication)

The first land arthropods would have developed internal book lungs from these structures, which we see in modern arachnids, like spiders and scorpions. Around the Late Silurian to Early Devonian, terrestrial fossils of tiny spider-like arthropods appeared, often found in association with plant fossils, scorpions, and myriapods (i.e. millipedes, centipedes, and eventually insects). In time, some groups of arthropods, including insects, developed tracheae, a system of tube-like structures that directly deliver oxygen from external pores in the exoskeleton to tissues of the body. And so, the rest of us animals have come to live in a bug’s world.

Plants on Land

Like the first animals, the first multicellular plants evolved in the ocean: marine green and red algae that were likely derived from photosynthetic cyanobacteria during the Neoproterozoic. The first land plants evolved around the early Ordovician, when fossil evidence of the first bryophytes are found. Bryophytes include plants like mosses, hornworts, and liverworts, which are commonly found in moist areas. They do not rely on a root system for uptake of water and nutrients, rather taking these in via their leaves, and they reproduce via spores. Later in the Ordovician, the first true vascular plants (trachaeophytes) evolved, and by the Devonian, this group looked like modern plants: 1) green with waxy leaves (containing cutin), 2) possessing a lignin-reinforced stem (for structural support), 3) with roots that conducted water and nutrients through their tissues, and 4) using seeds for reproduction.

Figure \(\PageIndex{5}\): Moss (a bryophyte) with spore releasing structures. (Creative Commons Attribution-Share Alike 4.0 International license.)

Imagine land on Earth before plants! It would have been lots of bare rock, very much like deserts or areas after volcanic eruptions today. The first liverworts and mosses to colonize land would have begun to significantly impact physical and chemical weathering of terrestrial landscapes. Plants can assist in the development of soils from bedrock as they extract nutrients, retain water, and produce organic matter. Later, root systems of vascular plants could wedge into cracks in rock in search of water, physically creating more surface area for physical and chemical weathering to occur. However, given enough time and spread of populations, plants and their roots can also add stability to slopes, lead to the development of new ecological environments (forests, swamps), and provide new resources (leaves, seeds) for other life forms to put to evolutionary use.

Figure \(\PageIndex{6}\): Diagram showing the phylogeny of major plant groups. (Creative Commons Attribution-Share Alike 4.0 International license.)

Gymnosperms were the dominant group of vascular, seeded plants that developed during the Devonian; this group includes seed ferns and eventually conifers, cycads, and ginkgos and are characterized by the ability to reproduce via wind-blown pollen released from cones. The most common type of plant on Earth today - the flowering plants (angiosperms) - would not evolve until the Mesozoic.

Late Devonian Mass Extinction

Toward the end of the Devonian, things started getting rough for organisms on Earth, but unlike other mass extinctions, this one took a while. The Late Devonian Mass Extinction was spread out over more than 20 million years, maybe as many as 25 million. It’s not called the “end”-Devonian, for that reason: it took a while. It got worse, and then a little better, and then worse again. The two biggest die-offs occurred at 374 Ma (the Frasnian-Famennian stage boundary, or Kellwasser Event) and 359 Ma (the end-Devonian, or Hangenberg Event).

Figure \(\PageIndex{7}\): Dunkleosteus, a predatory placoderm from the Devonian that disappeared from the fossil record after the Hangenberg Event. (Public domain.)

By the end of this extinction event, 99% of reefs had been destroyed, the diverse fish world had been severely cut back, and many marine groups had been severely pruned or eliminated altogether. Among reef-builders, the tabulate and rugose corals got slammed. Conservative estimates put the carnage at a 40% loss of taxa. The atrypid brachiopods, for instance, were a diverse tropical group of articulate brachiopods that were extinguished. Taking over their shallow-sea habitat after the extinction were silica-skeleton sponges, which otherwise prefer cooler, deeper water.

Figure \(\PageIndex{8}\): Atrypa reticularis, from Devonian strata in Germany. (CC-BY; Dwergenpaartje photo, via Wikimedia.)

One of the first clues that things went awry in the Late Devonian is the proliferation of black shale in the rock record. Black shale is mudrock that is rich in organic carbon; if muddy sediment is deposited under low-oxygen or anoxic (no oxygen) conditions, any organic carbon will be buried in the sedimentary record, giving its host rock a dark gray or black color. This is good evidence that the shallow ocean experienced prolonged and repeated periods of anoxic conditions throughout the Late Devonian.

Figure \(\PageIndex{9}\): Watkins Glen, near Ithaca, New York, is a gorge cut into Devonian black shale of the Genesee Group. (CC-BY; Photograph by Barbara Friedman, via Flickr.)

Why was so much of the ocean anoxic in the late Devonian? The primary suspect in the case of Devonian anoxia and extinction is the proliferation of trees. Yes, you read that right: trees. During the Devonian, Earth’s land surfaces saw the growth of the first forests. The trees in these forests had roots, and those roots probed down into the ground, helping break up rocks below, and thereby releasing their nutrients at an accelerated rate. Organic acids produced by the trees probably helped accelerated chemical weathering, too.

Some of these liberated nutrients were washed away by streams and rivers, ending up in the oceans. Today, agricultural use of phosphorus and nitrogen fertilizers produces a similar kind of nutrient-rich runoff, which, when it reaches the ocean, accelerates the rate of reproduction of algae in a process called eutrophication. When these supercharged populations of algae die, they die in large numbers; the rotting of billions of dead algae cells consumes all the available oxygen in the water, creating anoxic “dead zones”. Any fish swimming into such a dead zone must turn around immediately, or suffocate to death. (Sadly, this happens regularly today in places like the Gulf of Mexico, where the Mississippi River efficiently delivers nitrogen and phosphorus washed off farm fields in the central United States.)

Figure \(\PageIndex{10}\): Eutrophication of the Potomac River adjacent to a wastewater input. (CC-BY; Photo by Alexandr Trubetskoy.)

Once the supply of oxygen is exhausted, remaining unrotted dead algae rain down on the seafloor, resulting in a lot of black, organic-rich shale. You may have heard of fracking, the process by which humans use high-pressure fluids to shatter organic-rich rocks deep underground, allowing natural gas to flow out. The targets for those fracking operations are mostly Devonian black shales. The Bakken Shale in North Dakota and the Marcellus Shale in western Pennsylvania are enticing targets for natural gas exploration because of events that occurred during the Late Devonian Mass Extinction.

Key Terms

• anoxic - having no oxygen
• arthropods - a phylum of invertebrate animals characterized by a segmented body, a hard exoskeleton, and jointed appendages
• eutrophication - the over-enrichment of water by nutrients, primarily phosphorus and nitrogen, causing excessive algae growth and oxygen depletion
• tetrapods - a vertebrate animal with four limbs