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The Devonian Portal

A map of Earth as it appeared 390 million years ago during the Middle Devonian Epoch

The Devonian (/dəˈvoʊni.ən, dɛ-/ də-VOH-nee-ən, deh-) is a geologic period and system of the Paleozoic era during the Phanerozoic eon, spanning 60.7 million years from the end of the preceding Silurian period at 419.62 million years ago (Ma), to the beginning of the succeeding Carboniferous period at 358.86 Ma. It is the fourth period of both the Paleozoic and the Phanerozoic. It is named after Devon, South West England, where rocks from this period were first studied.

The first significant evolutionary radiation of life on land occurred during the Devonian, as free-sporing land plants (pteridophytes) began to spread across dry land, forming extensive coal forests which covered the continents. By the middle of the Devonian, several groups of vascular plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants (pteridospermatophytes) appeared. This rapid evolution and colonization process, which had begun during the Silurian, is known as the Silurian-Devonian Terrestrial Revolution. The earliest land animals, predominantly arthropods such as myriapods, arachnids and hexapods, also became well-established early in this period, after beginning their colonization of land at least from the Ordovician Period.

Fish, especially jawed fish, reached substantial diversity during this time, leading the Devonian to be called the Age of Fishes. The armored placoderms began dominating almost every known aquatic environment. In the oceans, cartilaginous fishes such as primitive sharks became more numerous than in the Silurian and Late Ordovician. Tetrapodomorphs, which include the ancestors of all four-limbed vertebrates (i.e. tetrapods), began diverging from freshwater lobe-finned fish as their more robust and muscled pectoral and pelvic fins gradually evolved into forelimbs and hindlimbs, though they were not fully established for life on land until the Late Carboniferous. (Full article...)

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Selected Devonian Article

Partially reconstructed D. terrelli skull and trunk armor (specimen CMNH 5768), Cleveland Museum of Natural History

Dunkleosteus is an extinct genus of large arthrodire ("jointed-neck") fish that existed during the Late Devonian period, about 382–358 million years ago. It was a pelagic fish inhabiting open waters, and one of the first vertebrate apex predators of any ecosystem. Fossils of Dunkleosteus have been found in the United States, Canada, Poland, Belgium, and Morocco.

Dunkleosteus consists of ten species, some of which are among the largest placoderms ("plate-skinned") to have ever lived: D. terrelli, D. belgicus, D. denisoni, D. marsaisi, D. magnificus, D. missouriensis, D. newberryi, D. amblyodoratus, D. raveri, and D. tuderensis. However, the validity of several of these species is unclear. The largest and best known species is D. terrelli, the type species.

Since body shape is not known, various methods of estimation put the living total length of the largest known specimen of D. terrelli between 4.1 to 10 m (13 to 33 ft) long and its weight around 1–4 t (1.1–4.4 short tons). However, current evidence suggests length of less than 5 metres (16 ft) was more likely, with the most recent and extensive studies on the body shape and size of D. terrelli producing estimated lengths of approximately 3.4 metres (11 ft) for typical adults and 4.1 metres (13 ft) for exceptionally large individuals of this species. It has been suggested that Dunkleosteus could quickly open and close its jaw, possibly creating suction like modern-day suction feeders, and had an extremely high bite force, but both claims have recently been questioned. (Full article...)

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Selected Devonian land plant article

Collage of modern lycophytes. Upper left: Lycopodium clavatum (Lycopodiales, Lycopodioideae) Lower left: Huperzia serrata (Lycopodiales, Huperzioideae) Top right: Isoetes japonica (Isoetales) Right centre: Selaginella tamariscina Lower right: Selaginella remotifolia Selaginellales

The lycophytes, are a clade of vascular plants that reproduce with spores. When broadly circumscribed, they include the clubmosses. They are sometimes placed in a division Lycopodiophyta or Lycophyta or in a subdivision Lycopodiophytina. They are one of the oldest lineages of extant (living) vascular plants; the group contains extinct plants that have been dated from the Silurian (ca. 425 million years ago). Lycophytes were some of the dominating plant species of the Carboniferous period, and included the tree-like Lepidodendrales, some of which grew over 40 metres (130 ft) in height, although extant lycophytes are relatively small plants.

The scientific names and the informal English names used for this group of plants are ambiguous. For example, "Lycopodiophyta" and the shorter "Lycophyta" as well as the informal "lycophyte" may be used to include the extinct zosterophylls or to exclude them. (Full article...)

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The Traverse Group is a geologic group in Michigan, Indiana and Ohio comprising middle Devonian limestones with calcareous shale components. Its marine fossils notably include Michigan's state stone, the Petoskey stone (the extinct coral Hexagonaria percarinata), among other corals and records of ancient marine life. A range of trilobites has also been found in the Traverse Group.

The Traverse Group outcrops in Emmet and Charlevoix counties along the northwestern shore of Michigan's lower peninsula. Its formations are Gravel Point, Charlevoix Limestone, Petoskey, and Whiskey Creek. The Gravel Point Formation consists of a lithographic gray to brown limestone with shale beds up to 0.5 meters thick; it also includes chert nodules and bioherms (fossilized reef mounds). The Charlevoix Limestone is a mildly argillaceous limestone with interbedded coquina. The Petoskey Formation is an arenaceous limestone named for its locale (Petoskey, Michigan), and contains the eponymous Petoskey stones. The Whiskey Creek Formation is a limestone.

The Traverse Group formed as a shallow carbonate shelf during the Devonian period (~419 to 359 Ma), when the most recent supercontinent, Pangea, was just beginning to take shape. (Full article...)

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Selected Devonian fish article

Lepidaspis serrata ("serrated scaly shield") is an extinct heterostracan jawless fish from Early Devonian Canada. Its scientific name refers to the fact that the armor is composed of hundreds of tiny scales with serrated edges. Although it is regarded as a heterostracan, its exact placement within Heterostraci is regarded as incertae sedis, as the fact that it is currently impossible to discern where its dorsal and ventral plates (which are made up of a mosaic of the aforementioned tiny scales) end or begin. Some experts regard it as being descended from the basal heterostracans of the Silurian. (Full article...)

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Selected Devonian invertebrate

Orchesella cincta

Springtails (class Collembola) form the largest of the three lineages of modern hexapods that are no longer considered insects, the others being Protura and Diplura. Although the three lineages are sometimes grouped together in a class called Entognatha because they have internal mouthparts, they do not appear to be any more closely related to one another than they are to insects, which have external mouthparts. There are more than 9,000 species worldwide.

Springtails are omnivorous, free-living organisms that prefer moist conditions. They do not directly engage in the decomposition of organic matter, but contribute to it indirectly through the fragmentation of organic matter and the control of soil microbial communities. The word Collembola is from the Ancient Greek κόλλα kólla meaning 'glue' and ἔμβολος émbolos meaning 'peg'; this name was given due to the existence of the collophore, which was previously thought to stick to surfaces to stabilize the creature.

Early DNA sequence studies suggested that Collembola represent a separate evolutionary line from the other Hexapoda, but others disagreed. This seemed to be caused by widely divergent patterns of molecular evolution among the arthropods. The adjustments of traditional taxonomic rank for springtails reflect the occasional incompatibility of traditional groupings with modern cladistics: when they were included with the insects, they were ranked as an order; as part of the Entognatha, they are ranked as a subclass. If they are considered within Hexapoda, they are elevated to full class status. (Full article...)