The Devonian period spans from 416 to 359.2 million years ago. The Devonian period is named for Devonshire, England where rocks of this age were first studied (USGS). Many groups continue their adaptive radiations resulting in many first appearances in both aquatic and terrestrial environments.

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Primary Producers & Reefs

The dominant primary producers in the oceans continue to be cyanobacteria, green and red algae (Knoll, Summons, Waldbauer, and Zumberge, 2007, p. 148). Tabulate corals and stromatoporoids continue to be the primary builders of reef systems (Stanley, 1987, p. 75) and (Webb, 2001, p. 175). Michigan’s state stone is the Petoskey Stone. Petoskey Stones are fossils of colonial rugose corals that grew in the Devonian seas. Many Petoskey Stones represent the colonial coral Hexagonaria percarinata. Some sources identify the coral making up the Petoskey stones as the Prismatophyllum genus. Petoskey stones were given their cobble shape from the action of Pleistocene glaciers.

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Plants

The Devonian is often called the “age of fishes”, but could just as easily be named the “age of plants”. Plants with specialized cells to transport water and nutrients evolved during the Silurian. Early in the Devonian these vascular plants were simple, small, stick-like structures like Asteroxylon, Cooksoni, and Swadonia (formerly Psilophyton). Pertica quadrifaria was one of the first simple plants with no leaves or roots. Stems functioned as the photosynthetic organs of this plant and reproductive structures holding spores lay at the end of branching stems. Pertica quadrifaria is the state fossil for Main. By the end of the Devonian complex plants had evolved creating the first forests and well-developed soil ecosystems (Kenrick & Davis, 2004, p. 34).

The adaptive radiation of plants during the Devonian would see the evolution of leaves, roots, wood, and primitive seeds. The first leaves were spike-like as found on Baragwanathia. Branching and webbing may have led to the development of fern-like fronds found in plants such as Rhachophyton.

 

The appearance of wood represents the evolution of the cambium. Cambium tissue allows a plant to grow in girth to produce trunks. The evolution of roots and cambium were critical in creating the tree form (Kinrick and Davis, 2004, p. 66) Archaeopteris could be referred to as the first modern tree (Murphy, 2006, Archaeopteris page). Archaeopteris made up a significant portion of the canopy of early forests. The trunk (Callixylon) of this tree was constructed of conifer-like wood, while the branches were adorned with fern-like fronds (Archaeopteris). The underside of the fronds had sac-like sori that contained spores for reproduction. It is easy to understand why the fern-like fronds and conifer-like wood of this progynmosperm were considered to be separate organisms until the fossils connecting the foliage and wood were discovered. Archaeopteris is a true missing link between fern-like plants and conifer-like plants.

Pteridosperms or seed ferns are the earliest plants with seeds. The fronds of Pteridosperms were identical to ferns, but bore seeds instead of spores. The term pteridosperm is descriptive but, misleading as these plants were really early gymnosperms (Cleal & Thomas, 2009, p. 139). The seeds of these early plants were small and had an incomplete integument or seed coat (Kenrick & Davis, 2004, pp. 44-45). The first seeds also had slight wing-like structures indicating that they were wind dispersed. Seeds not only protect the embryo, but also provide nutrients, a vehicle for dispersal, and the possibility of delayed growth. Seeds also represent a change in reproductive strategy in which an alternation between plants that produce spores (sporophytes) and plants that produce sperm and eggs (gametophytes) is abandoned. In seed plants the gametophyte is no longer an entire plant; it is reduced to pollen and ovules. Seedless plants require water to carry the sperm to the eggs, while seed plants use pollen as a vehicle to carry the sperm to the egg. The evolution of the seed is an important adaptation for dry conditions.

 

The evolution of trees and forests produced a new carbon dioxide sink. Forest lowered carbon dioxide levels in the air through the process of photosynthesis, which in turn would lower global temperatures. As roots evolved during the Devonian they increased in depth and had a great impact on the weathering and development of soils (Kenrick & Davis, 2004, pp 50-51). Roots weather calcium and magnesium silicates allowing carbon dioxide from the air to form calcium and magnesium carbonates. These carbonates help to form limestone and dolomite. The evolution of plants created new ecosystems on the land and in the soil for terrestrial organisms to exploit. Plants impacted the soil, atmosphere, rock cycle, and oceans through photosynthesis and the action of roots.

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The Rhine Chert

The Rhine Chert near the Aberdeenshire villiage of Rhynie in Scotland is early Devonian (396 Ma). Plants and animals of this area lived near a sinter terrace. The area was periodically flooded with silica rich solution from hot springs and geysers (Selden & Nudds, 2004, p. 52 and Kenrick & Davis, 2004, p. 24). Some of the thick chert deposits preserve plants in their life position (Kerp, 2002, p. 24). Organisms were permeated with silica before any cellular decay could occur. Five of the seven land plants described are true vascular plants (Tracheophytes) with tracheids for water conduction. All of the land plants have cuticles, stoma, intercellular spaces, and vascular strands with lignin. Rhine plants are branching stick-like structures growing off rhizomes and reaching only knee height. Among the plants are found algae, fungi, and the oldest known lichen. The fauna of the Rhine Chert is comprised of crustaceans, trigonotarbid arachnids, the first mites, collembolans, euthycarcinoids, and myriapods. Like other early land faunas carnivores and detritivores dominate the Rhine, while herbivores were absent or rare (Kenrick & Davis, 2004, p. 28). True herbivores, which do not make their appearance until the Carboniferous, require a symbiotic relationship between bacteria and fungi in their gut to digest plant material (Selden & Nudds, 2004, p. 57).

 

The Hunsrück Slate

The Hunsrück Slate in Bundenbach, Germany is lower Devonian (390 million years ago). Both the Hunsrück Slate and the Burgess Shale represent marine benthic communities living above a muddy seabed at a depth of less than 200m. All of the fish groups except the chondrichthyans (which had not yet evolved) are represented with agnathans and placoderms being the most common. Among the Echinoderms the starfish, brittlestars, and crinoids are the most common. Polychaete worms are a rare find and represent the Annelids. Among arthropods, trilobites are common, while crustaceans, and chelicerates are rare. Sponges, corals, brachiopods, and mollusks are also found in the Hunsrück, but are not numerous. Small pieces of plant material washed out to sea along with fish coprolites, burrows, and tracks are also preserved (Selden & Nudds, 2004, pp. 37-46).

 

Gilboa, New York

Near Gilboa, New York, Devonian aged limestones, shales, and sandstones provide windows into several different habitats. The first appearance of trees and a forest are found in the fossils of Gilboa (385 million years ago). Stumps with roots stretching out into a paleosol (fossil soil) have been preserved as sandstone casts at a site known as Riverside Quarry. The stumps were given the name Eospermatopteris and are now known to be a cladoxylopsid. Cladoxylopsids are thought to be ancestors of ferns and horsetails. Recently, two Gilboa trees with trunk and branching crowns were found lying prostrate. The trunk of the trees was identified as a cladoxylopsid of the genus Wattieza (Stein et al, 2007, pp 904-907). Illustrations combining Wattieza with Eospermatopteris give paleontologist an idea of what this Gilboa tree looked like in real life. Carbonized foliage at Riverside Quarry preserves arboresent and herbaceous lycopods, such as Lepidosigillaria and Leclercqia. Progymnosperms like Aneurophyton can also be found. In the Devonian, Riverside Quarry was a swampy forest near a coastline. A second location near Gilboa, known as Brown Mountain, is a lagerstatten, which preserves important animal fossils in what was once a tangled mass of Leclercqia (a herbaceous lycopod) and mud, which was located in a deltaic environment. Fossil foliage at Brown Mountain includes lycopods, progymnosperms, and cladoxylopsids. Animals preserved in the tangled Leclercqia mud mass at Brown Mountain include: eurypterids, scorpions, trigonotarbids (spider-like organisms), the first known spider, the first known pseudoscorpions, millipedes, and centipedes. A third site, called South Mountain, represents a deltaic environment like that found at Brown Mountain. Fossil foliage at South Mountain is a mix of lycopods, progymnosperms, and cladoxylopsids. Two types of fish are found at South Mountain, placoderms and acanthodians. Gilboa, like other Devonian aged fossil ecosystems, represents a primitive trophic structure that lacks herbivory; only predator-detritivore food chains existed at this time (Nudds & Selden, 2008, pp. 95-113).

Mass Extinction

The mass extinction that occurred in the Late Devonian affected mainly the marine environment with terrestrial plants escaping the crises. It is estimated that up to 75% of marine species and 50% of marine genera were lost (Prothero, 2004, p. 90). Brachiopods, trilobites, conodonts, ammonoid, corals, and stromatoporoids were hit hard. Reef building communities were decimated. Tabulate corals and stromatoporoids would never again be major reef builders after the Devonian crises. The rest of the Paleozoic would see very little reef building. Reef building would recover in the Mesozoic with the appearance of modern corals (Stanley, 1987, pp. 78-79). The Devonian crisis seems to be correlated with cooling. Coral reefs were in decline as cold water glass sponges expanded. Shallow, warm water marine species declined. Freshwater fish that were adapted to seasonal environments survived, while warm water marine fish experienced heavy extinction. As these shallow warm water species declined the stromatolites had a small resurgence in reef building. There is evidence of glaciation and lower sea levels. Both the Ordovician and Devonian cooling events may be tied to the movement of Gondwanaland over the South Pole (Stanley, 1987, pp. 86-89). The cooling event may also explain Late Devonian carbon and oxygen isotope anomalies. A severe cooling would trigger a massive overturn within the ocean. This overturn would bring deep ocean water to the surface. The deep ocean water is nutrient rich, but cold and oxygen poor. The Devonian crises lasted for 4 million years (Prothero, 2004, p. 90).