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Devonian is a time interval from 416 to 359 Ma in the Palaeozoic era. Is was named after the type area of Devon (SW England). Most of its stratigraphy (stages of the Devonian) were however established in the fossiliferous marine deposits of teh Ardennes (Belgium). Its base is characterized by Monograptus uniformia a graptolite, and its top by Gattendorfia an ammonoid cephalopd.

Invertebrates of the Devonian period are essentially of types established in the Ordovician (505-440 Ma). The first spiders, millipedes and insects appear in the Devonian, but the Devonian is called the "Age of Fishes" since freshwater and marine varieties proliferate.

Evidence from the Devonian supports the idea that the present continents were being united in a single continental mass called "The Old Red Sandstone Land Mass" uniting Gondwana, Laurentia and Baltica. Laurentia and Baltica were sutured together through the Caledonian orogeny during the early to mid-Devonian. During the mid- to late-Carboniferous Gondwana collided with Laurasia (ie Laurentia+Baltica) during the Variscan Orogeny.

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As continents gathered together in low latitude regions (see reconstruction above), large warm epicontinental sea opened new ecological niches rapidly colonized by a range of marine species amongst them: Gastropods, Brachiopods, Corals, Cephalopods and Fishes.

Sea levels were high worldwide, and much of the land lay under underneath shallow seas, where tropical reef organisms flourished.   Elsewhere a huge deep ocean covered the rest of the planet.  The presence of widespread reefs and evaporite deposits indicate the climate was warm and mild world-wide, and generally dry.  It was a "greenhouse" age.

Types of marine life: In Devonian seas, sponges were represented by newly evolved siliceous forms, many of which were similar to the modern Venus flower basket.  The association between algae, sponges, and corals that began in the Ordovician continued, with flourishing reefs thriving in the warm shallow seas.   During this time not only the hylaesponges, rugose and tabulate corals (shown below) but also the brachiopods reached their zenith in number and diversity.  The spiriferid brachiopods were particularly abundant.  Among molluscs, while gastropods, bivalves, and nautiloids continue with little change from the Silurian, the first ammonoids mark the beginning of an important new phase of molluscan evolution. Trilobites were generally on the decline, but a few groups remained abundant, and some giant forms evolved, such as the huge spiny Terataspis grandis (30 to 60 cm).  The increase in swimming predators (such as new forms of fish and cephalopods) may have contributed to the trilobite decline.The Devonian saw the rapid evolution diversification of fish, especially the Placodermi, primitive sharks, Sarcopterygii (lobe-finned fish and lungfish) and Actinopterygii (conventional bony fish or ray-finned fish).  So pronounced is this evolutionary radiation that the Devonian has been called "the age of Fish".

Terrestrial life: Many arthropods, including eurypterids, arachnids (spiders and their kin) and primitive wingless insects invaded the land.  Towards the end of the period the first fish-like tetrapods move ashore.  Seed-bearing plants (Gymnosperms) also appeared during the latest Devonian.  Seeds mean a freedom from dependence on moist habitats for reproduction, and allowed plants to expand into drier areas.

There is a major mass extinction during the late Devonian (the so-called Frasnian-Famennian event). The  tabulate-stromatoporoid reefs disappear completely, with corals so seriously decimated that extensive reef building did not happen until the Triassic with the evolution of a new group of reef-building corals, the scleractinians. Brachiopods, trilobites and primitive fish groups either were either diminished or completely killed off, as were many planktonic and nektonic (floating and swimming) animals. The planktonic graptolites and enigmatic tentaculites die out and trilobites are much reduced.  Tropical taxa were the most severely affected. The effect on terrestrial ecosystems was not as marked.

Various causes have been suggested. Global cooling tied to Gondwanan glaciation has been proposed as the cause of the Devonian extinction, as it was also suspected of in the case of the terminal Ordovician extinction. Support for this hypothesis comes from the fact that the forms of marine life most affected by the extinction were the warm water to tropical ones. Another hypothesis is that environmental sea-level and climatic change in conjunction with an extraterrestrial impact (comet/asteroid) caused a global cooling. There are several impact sites known to be of the right potential age to have been involved in this extinction. But neither the glaciation or the impact hypothesis is unequivocally supported by the available data.

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NB: The classification of fossils follows the classification of living organisms:

I- Prokaryotes (cell without nuclei) -

I.1 Archaeabacteria (also called Archaea) old bacteria found in extreme environments like hot springs.
Eubacteria also kown as true bacteria including the kind that cause infections.

2- Eukaryotes (cell with nucleis) - they are divided in four kingdoms:

2.1 Protista: Unicellular and simple multicellular organisms such as algae (such as diatoms) and forams.
2.2 Fungi: Such as mushrooms and yeast.
2.3 Plantae: Tress, grass, fern, etc.
2.4 Animalia: Sponges, corals, snails, dinosaurs, ants, people.

Each kingdoms is divided into Phyllums (Chordata, Arthropoda, Mollusca, etc.), each Phyllum into Class (Mammallia. Cephalopoda, Gasteropoda...), each Class into Order (Primates...), each Class into family (Hominidae...), each family into Genus (Homo), each Genus into Species (Homo sapiens).

Gasteropoda, as Pelecypoda and Cephalopoda, is a class of the phyllum Mollusca. Gasteropods are characterized by a spiral shell that does not contain internal chambers.


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Brachiopods are marine animals that, upon first glance, look like clams, but unlike bivalve brachiopods shell is made of two inequal halves. They are also quite different from clams in their anatomy, and they are not closely related to the molluscs. As the phyllum Mollusca, Brachiopods is a phyllum of Lophotrochozoa, therefore Brachiopods are lophophorates, and so are related to the Bryozoa and Phoronida.

Although they seem rare in today's seas, they are actually fairly common. However, they often make their homes in very cold water, either in polar regions or at great depths in the ocean, and thus are not often encountered. There are about 300 living species of brachiopods. Despite their relative obscurity today, brachiopods have a long and rich paleontological history. During the Paleozoic era, they were extremely abundant. They diversified into a number of different morphologies and even participated in the build-up of ancient reefs. At the end of the Paleozoic, they were decimated in the worst mass extinction of all time, the Permo-Triassic event. Their numbers have never been as great since that time.

Xystostrophia and Anthiris


Spinnela Yassensis from the Yassensis limestones.

Spinela and crinoides.


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Cephalopod (Cambrian to present), a member of the Cephalopoda, a class of higly organized marine molluscs of which squid, octopus, cuttlefish and nautilus are living representatives. Their squeleton, when there is one, is made of chambered univalve shell, composed mainly of aragonite. The shell can be internal or external.

ORTHOCERATOIDEA- The members of this subclass are known for the particular ornamentation on their shells.  Their orthoconic or cyrtocone (slightly curved) shell have rings, grooves, and/or color.  Secondary deposits in the shell are well developed.  They entered the fossil record in the Ordovician and left in the Carboniferous.  They are the probable ancestor to other cephalopoda.


Orthoceras from the Yassensis limestone (Good Hope Resort).

AMMONOIDEA- Their shells are straight to tightly spiraled and contain internal chambers. At their surface the ammonite shells display complex sutures that varies from species to species.  Like the bactritoids, they have have a bulbous protoconch and a marginal siphuncle.  They entered the fossil record in the Devonian and left in the Cretaceous at the famous K/T boundary.

Ammonoidea from the Yassensis limestone (Good Hope Resort).

NAUTILOIDEA-These were the first cephalopods to appear in the fossil record.  They appeared in the Late Cambrian and quickly expanded.  The only surviving Nautiloidea are members of the genus Nautilus (picture in heading of page).  The members of the subclass Nautiloidea have orthoconic or coiled shells.  Many of the straight Nautiloidea secreted deposits in their older chambers to make their shells neutrally buoyant.

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Corals belong to class Anthozoa.
Rugose Corals - (Middle Ordovician to Upper Permian) The outer skin of the corallum, the epitheca, may be transversely wrinkled, hence the name "rugose". They range from simple solitary to complex colonial types. Unlike tabulata, rugose coral were not reef builders.

Cystiphiyllum (Solitaris)

Solitary and on the right Cystiphyllum conifollis-

Below Metriophyllum from the Warroo limestone.

Solitary from the Yassensis limestones (Good Hope Resort)


Hexagonaria, tabulate section perpendicular and parallel to corallium.

Tabulate corals - Extinct group (Lower Ordovician to Permian) of colonial corals characterized by slender corallites with prominent tabulae and reduced or absent septa (dividing-walls). Tabulate corals were reef builders.

Structure and polishes slab of favosite-

Thamnopora (the one on the top left is adjacent to a brachiopod). The bottom two pictures shows Thamnopora from the Yassensis limestone.


Favosite from Yassensis limestones.

Syringopora from the Yassensis limestones.



Stromatopora characterized by prominent mamelons with opening of astrorhizal canals at their summits.


Receptaculies- Receptaculites was long assigned to sponges, but it considered to represent the fossil remains of calcareous algae. They lived from the lower Ordovician through the Permian. Specimens found are usually globular to platter shaped, nd measure from a few cm to over half a meter across. The surface is covered by rectangular plates arranged in intersecting sets of clock-wise and counterclock-wise patterns.

Receptaculites from the Receptaculites limestones in the Taemas serie. View in section (top two pictures,) view from the top (bottom two pictures)

Crinoids: While the crinoids were the dominant echinoderm of the Paleozoic with more than 6,000 described species, there are only 600 or so species living today. The taxon Crinoidea was established in 1821 by J. S. Miller pulling the stalked crinoids out of the starfish group Stellarides. Crinoid morphology reveals their echinoderm ancestry; they have the characteristic pentameral symmetry, calcareous plates, and the peculiar water vascular system with its associated ambulacral grooves and tube feet.
Stalked crinoids, or "sea lilies" as they look like flower but are actually animals. Their stalks consist of numerous circular plates stacked on top of each other. Crinoids lived attached to the bottom, and filtered food particles from the currents flowing past them.

The earliest fossil crinoid may have been Echmatocrinus, from the famous Burgess Shale of the middle Cambrian; some paleontologists, however, do not feel that Echmatocrinus was a true crinoid. By the beginning of the Ordovician many groups of echinoderms flourished, especially the crinoids. The crinoids were the most abundant group of echinoderms from the early Ordovician to the late Paleozoic, when they, along with the rest of the echinoderms, nearly went extinct during the Permo-Triassic extinction.

Crinoide ossicules from the Yassensis limestone (Next two pictures), and a view from the crinoidal limestone.

Bryozoans: Bryozoans, or "moss animals," are aquatic organisms, living for the most part in colonies of interconnected few to millions of individuals. A few to many millions of these individuals may form one colony. Some bryozoans encrust rocky surfaces, shells, or algae. Bryozoan colonies range from millimeters to meters in size, but the individuals that make up the colonies are rarely larger than a millimeter. Colonies may be mistaken for hydroids, corals, or even seaweeds.

Each individual, or zooid, is enclosed in a sheath of tissue, the zooecium, that in many species secretes a rigid skeleton of calcium carbonate. Each zooid in the electron micrograph is less than a millimeter long and has a single opening, the orifice. Through this opening, the lophophore, a ring of ciliated tentacles centered on the mouth, protrudes to capture small food particles. The lophophore can be retracted very rapidly by specialized retractor muscles, and the opening closed by a doorlike operculum, visible on some of the zooids.

The oldest known fossil bryozoans, including representatives of both major marine groups, the Stenolaemata (tubula bryozoans) and Gymnolaemata (boxlike bryozoans), appear in the Early Ordovician. It is plausible that the Bryozoa existed in the Cambrian but were soft-bodied or not preserved for some other reason; perhaps they evolved from a phoronid -like ancestor at about this time.

The stenolaemate bryozoans quickly radiated in the early Paleozoic and are very characteristic fossils of Paleozoic rocks, sometimes making substantial contributions to the formation of reefs, calcareous shales, and limestones. They included forms with robust skeletons, such as the trepostome Hallopora; such forms were common in shallow-water habitats that today are dominated by corals. There were also forms with delicate, branching fanlike skeletons such as the fenestrates pictured below. With the exception of one order of stenolaemates, the Tubuliporata or Cyclostomata, all of these Paleozoic bryozoan lineages were severely impacted in the Permian extinction.

Polypora and Rhombopora.

Trilobites: These have segmented shell that is divided across its width into three parts. They are a type of Arthropods.

Phacops: A trilobite exclusively found in the Devonian.

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