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.

Marine Invertebrates

Echinoderms, brachiopods, mollusks, rugose corals, and bryozoans continue to flourish. Sponges and tabulate corals continue to be important in reef building. Some of the best brachiopods come from Devonian sediments in Kentucky. The "generic" brachiopod is the state fossil for Kentucky. Trilobites are present, but show a decline in numbers during the Devonian. Phacops rana is a trilobite with wonderful compound eyes containing calcite lenses. Phacops rana is Pennsylvania's state fossil. Ammonoids make their first appearance in the Devonian and represent an important evolutionary trend in the Cephalopods. Ammonoids look like Nautiloids, but can be distinguished by looking at the structure of their shells. The sutures joining the Ammoniod chambers are folded into complex patterns. This suturing may have strengthened the shell. The ammonoid siphuncle is near the outer margin of the whorls. The septae curve towards the body chamber in Ammonoids.

Fish

Fish undergo such a great adaptive radiation during the Devonian that it is often referred to as “the age of fish.” Cartilaginous fish and bony fish (ray-finned, lobe-finned, and lungfish) make their first appearance early in the Devonian.

The class of armored jawed fish Placodermi becomes well established in the early Devonian. In fact they were the dominant fish group during this time (Prothero, 1998, p. 345). Placoderms possessed articulated bony plates over their head and shoulder regions. A neck joint allowed placoderms to lift the anterior portion of their head shield. A cartilaginous skeleton supported the placoderm body. Placoderms had scales or skin covering the rest of their body and tail. Placoderms had a long dorsal fin and a heterocercal tail (an asymmetrical tail in which the dorsal lobe is extended due to an upward flexion of the spine). Placoderms had paired pectoral fins and were the first vertebrates to evolve paired pelvic fins (Benton, 2005, p. 55). The majority of placoderms did not have true teeth, their head shield and mandibular shield were modified into sharp-edged cutting surfaces forming broad dental plates. Most placoderms were small fish reaching lengths of 15 cm; however, a few species reached lengths of 4 to 10 meters. Placoderms lived in both marine and freshwater environments. Placoderms were one of the first groups of fish to evolve jaws. Although placoderms reached their greatest diversity during the Devonian they did not survive the extinction event at the end of this period.

The class Chondrichthyes includes the cartilaginous fish, today represented by sharks, skates, rays, and ratfish. Fish in this class have skeletons made of cartilage strengthened by granules of calcium carbonate in a mosaic pattern. A thin layer of bone covers the cartilage and the skin is covered with teeth-like scales. The paired fins are reinforced with rays of cartilage. The jaws contain bony teeth. Finally, males have “clasping” pelvic fins to aid in sperm transfer during copulation. Sharks diversified into many forms throughout the Devonian and Carboniferous after which they decline in the Permian. Sharks make a rebound during the Mesozoic. In the Jurassic modern groups of sharks evolve (Dixon, 1988, p. 28).

The class Osteichthyes includes the bony fish, represented by ray-finned, or actinopterygians and the lobe-finned, or sarcopterygians. Bony fish now account for more than half of all vertebrate species. Lobe-finned fish include the ancestors of all land vertebrates. Osteichthyes proved to be a very successful vertebrate class. Fish in this class have a skeleton made of bone. Ray-finned fish possess parallel bony rays that support and stiffen each fin. The fin does not have any internal muscles; muscles in the body of the fish move it. The first primitive ray-finned fish (Palaeoniscids) had thick bony articulated scales, a single dorsal fin near the posterior end and an asymmetric tail. Ray-finned fish evolved to live in both marine and freshwater environments. The first lobe-finned fish appear in the early Devonian. The pectoral and pelvic fins of sarcopterygians are long, fleshy, muscular lobes. Each fin has articulated bones, muscles, and a fan of bony rays. The bones in the fins can be related to the bones found in the limbs of terrestrial vertebrates. The muscles can move each fin independently. The sarcopterygians are divided into two groups the lobe-finned fishes and the lungfishes. Today only the coelacanth and 6 species of lungfish represent the sarcopterygians. The best fish candidate as ancestor to the first amphibians and all tetrapods would be a member of the Rhipidistia, a group of lobe-finned fish. The characteristics shared by these lobe-finned fish and the first tetrapods are remarkable. The skulls of both share the same bone arrangement. The foreleg and fore limbs have bones equivalent to the humorous, radius and ulna. The vertebrae and the vertebral body of both creatures share a similar form (Johnson & Stucky, 1995, p. 46). Panderichthys lived during the Devonian and had a tetrapod-like head and paired muscular fins. Fish like Panderichthys were the ancestors to tetrapods, air breathing terrestrial vertebrates (Plamer, 1999, p 79).

Fish to Tetrapod

The first terrestrial vertebrates or tetrapods (Superclass Tetrapoda) appear during the Devonian period. Tetrapods include vertebrates with four toe-bearing legs, or descendants of such a vertebrate. The word tetrapod is used to refer to vertebrates other than fish. Fossil evidence suggests that lobefinned fish, such as Eusthenopteron (Order Osteolepiforemes) and Panderichthyes (Order Panderichthyida) are among the closest sarcopterygian relatives of tetrapods (Benton, 2005, p. 80). In fact, Eusthenopteron, Panderichthyes, Tiktaalik, Acanthostega, and Ichthyostega make up a series of fossils used to model the transition from aquatic lobe-finned fish to fully four-legged tetrapods.

Prothero (2007) explores the transitional features of these fossils and how they help us to better understand the evolution of tetrapods from lobefinned fish (pp. 222-230). Eusthenopteron is a Devonian aged lobefinned fish. The pelvic and pectoral bones in this fish are homologous to tetrapod limb bones. The skull bones of Eusthenopteron are the same as early tetrapods.

Panderichthyes from the Late Devonian had a flattened body with a straight tail. This "fishibian" had a skull and brain case similar to early tetrapods with upward facing eyes. In fact, the skull of Panderichthyes was classified as a tetrapod until its fishlike body was discovered. Panderichthyes had labyrinthodont-like teeth, which characterize later tetrapods. Panderichthyes possessed both lungs and gills. This fish had lost its dorsal and anal fins, but retained pectoral and pelvic foot-like lobed fins.

Tiktaalik from the Late Devonian (375 million years ago) is nicknamed the "fishapod" by its discoverer Neil Shubin. Tiktaalik roseae like other fish had gills, scales, and fins. However, the fins were weight bearing possessing both wrist bones and finger-like bones. Tiktaalik also possessed tetrapod characteristics. The head of Tiktaalik ("large freshwater fish"-taken from the Inuktitut language) was flat with eyes positioned on the top of the skull (Ridley, 2009, p. 70). The pectoral girdle was separate from the skull forming a neck and allowing the head to turn. The ribs were designed to support the body and allow breathing. The fact that Tiktaalik had spiracles on its skull and a more robust ribcage indicates this organism had lungs as well as gills. The appearance of tetrapod characteristics in a fish that existed 12 million years before the first tetrapod is very significant (Tiktaalik website, 2008).

Acanthostega is a basal tetrapod from the Upper Devonian (365 million years ago) that shows characteristics intermediate between lobe-finned fish and the first fully terrestrial tetrapods. Acanthostega had gills, fins, and a lateral line for sensing vibrations, but possessed lungs, spine, and limbs like a tetrapod. Studies of Acanthostega's limbs suggest that they were used for swimming or crawling along the bottom.

Ichthyostega from the Late Devonian is yet another basal tetrapod that represents a transition between fish and amphibians. Ichthyostega possessed limbs that were longer and more tetrapod-like than Acanthostega. Ichthyostega still possessed a tail, gill slits, and a lateral line for sensing movement, but it also had the limbs and spine of a tetrapod. Acanthostega and Ichthyostega represent important transitional fossils linking fish and amphibians, but are not classified as amphibians.

Early amphibian-like fossils are referred to as basal tetrapods. Acanthostega and Ichthyostega from Greenland are the best known Devonian-aged basal tetrapods. Ichthyostega and Acanthostega measured around 1 meter in length. As we have already mentioned, both had fully developed limbs and limb girdles, but still retained features associated with their aquatic life. These animals had a gill apparatus as shown by their opercular bones. Flanges on their ribs indicate they also had lungs. They possessed a strong fin tail used for swimming. Evidence of lateral lines can be seen on their skulls and a possible otic notch on the back of the skull may indicate the presence of an eardrum (Prothero, 1998, p. 362). Interestingly, it was recently discovered that Acanthostega had eight toes and Ichthyostega had seven. Evidently, the standard pentadactyl (five-fingered) condition in tetrapods developed later.

Acanthostega and Ichthyostega are found in sedimentary deposits that indicate they lived in meandering rivers that flowed through forests of lycopods and ferns (Benton, 2005, pp. 82-85). They probably lived in water choked with vegetation. Being able to swim and step over vegetation would be a real adaptive advantage. Basal tetrapods and amphibians underwent a great adaptive radiation during the Carboniferous.

Amphibians

Traditionally, the term amphibian has been used to refer to all tetrapods that are not amniotes (reptiles, birds, and mammals). Thus, under the traditional definition the first appearance of amphibians would be in the Devonian. However, it is now clear that this is a paraphyletic term. The class Amphibia now refers to present-day amphibians and their extinct sister groups. The extinct sister groups of amphibians appear in the Carboniferous and modern representatives of Amphibia appear in the Triassic.

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.

Insects

Although there is evidence for insect-like organisms in the Silurian the first true insect appears in the Devonian. Springtails belong to the order Collembola but are no longer considered insects. Springtails are found in the Rhine Chert. Bristletails (order Archaeognatha) are the most primitive living insects. Bristletails make their first appearance during the Devonian (Grimaldi & Engel, 2005, p. 148). The first insects were wingless, underwent incomplete metamorphosis and did not organize into social groups.

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).

Grimaldi, D. & Engel, M.S., (2005). Evolution of the Insects. New York: Cambridge University Press.

Johnson, K.R. & Stucky R.K. (1995). Prehistoric Journey: A History of Life on Earth. Boulder, Colorado: Roberts Rinehart Publishers.

Kenrick, P. & Davis, P. (2004). Fossil Plants. Washington: Smithsonian Books.

Kerp, H. (2002). The Rhynie Chert: The Oldest and Most Completely Preserved Terrestrial Ecosystem. In Dernbach, U. & Tidwell, W.D. Secrets of Petrified Plants: Fascination from Millions of Years (pp. 23-27). Germany: D’ORO Publishers.

Knoll, Summons, Waldbauer, and Zumberge. (2007). The Geological Succession of Primary Producers in the Oceans. In Falkowski, P.G. Knoll, A.H. [Eds] Evolution of Primary Producers in the Sea. (pp. 133-163). China: Elsevier Academic Press.

Murphy, D. (2006). Devonian Times: See: http://www.devoniantimes.org/index.html

Nudds J.R. & Selden P.A. (2008). Fossil Ecosystems of North America: A Guide to the Sites and Their Extraordinary Biotas. Chicago: The University of Chicago Press.

Palmer, D. (1999). Atlas of the Prehistoric World. New York: Discovery Books.

Prothero, D.R. (2007). Evolution: What Fossils Say and Why It Matters. New York: Columbia University Press.

Ridley, M. (2009). The Darwin Bicentennial Part II: Modern Darwins. National Geographic, February 2009, Vol. 215, No. 2.

Stanley, S.M., (1987). Extinction. New York: Scientific American Books.

Stein, W.E., Mannonlini, F, VanAller Hernick, L., Landing, E. & Berry, C.M. (2007). Giant cladoxylpsid trees resolve the enigma of the Earth's earliest forest stumps at Gilboa. Nature, vol 446: pp. 904-907.

Selden P. & Nudds, J. (2004). Evolution of Fossil Ecosystems. Chicago: The University of Chicago Press.

Tiktaalik roseae (2008): http://tiktaalik.uchicago.edu/

Webb, G.E. (2001). Biologically Induced Carbonate Precipitation in Reefs through Time. In Stanley, G.D. Jr. [Ed] The History and Sedimentology of Ancient Reef Systems (159-203). New York: Kluwer Academic/Plenum Publishers.