Sphenacodon is an extinct genus of synapsid that lived from about 300 to about 280 million years ago during the Late Carboniferous and Early Permian periods. Like the related Dimetrodon, Sphenacodon was a carnivorous member of the Eupelycosauria family Sphenacodontidae. However, Sphenacodon had a low crest along its back, formed from blade-like bones on its vertebrae instead of the tall dorsal sail found in Dimetrodon. Fossils of Sphenacodon are known from New Mexico and the Utah-Arizona border region in North America. Researchers recognize two species: Sphenacodon ferox and Sphenacodon ferocior. Sphenacodon ferocior can be up to 40% larger in overall size compared to Sphenacodon ferox. In addition, the dorsal spines in Sphenacodon ferocior are proportionately 45% taller than in Sphenacodon ferox; the recent discovery of a nearly complete skull of Sphenacodon ferox has helped clarify other distinctions between the two species, including the number of teeth in certain parts of the jaws and the size of the indented notch between the maxillary and premaxillary bones in the upper jaw.
The two species occur together in some formations, but Sphenacodon ferox survived into the Early Permian. Sphenacodon and Dimetrodon have been found in different geographical areas that were separated by the ancient Hueco Seaway that penetrated equatorial Pangaea during the Early Permian and "covered much of southern New Mexico and parts of West Texas". Sphenacodon is known from the west in New Mexico and Utah, Dimetrodon is known from the east in Texas and Oklahoma in more deltaic environments. However, the species Dimetrodon occidentalis is found in New Mexico; each genus would have been an apex land predator in its region and preyed on amphibians and early synapsids and diapsids. Sphenacodon appears to have died out before about 280 million years ago during the Wolfcampian; the genus Dimetrodon survived until about 270 million years ago. Such large sphenacodontid predators were replaced by therapsids, the group of synapsids that includes the direct ancestors of mammals; the skull of Sphenacodon is similar to that of Dimetrodon.
It is narrow from side to side and vertically deep, with an indented notch at the front of the maxillary bone in the upper jaw. The upper and lower jaws are equipped with an array of powerful teeth, divided into sharp pointed "incisors", large stabbing "canines", smaller slicing back teeth; the orbit is set high and far back with a single opening behind and below the eye, a characteristic of synapsids. Body proportions are similar to Dimetrodon, with a large head, short neck, robust trunk short front and hind limbs, a tapering tail that makes up about half the animal's entire length. However, the tops of the neural spines along the back bone are strikingly different in each genus. In Dimetrodon, the neural spines develop into long, cylindrical projections that support a tall vertical dorsal sail that ends near the base of the tail. In Sphenacodon, the neural spines are enlarged but retain a flat-tipped, blade-like shape along the back and tail, form a crest rather than a tall sail. There is evidence for strong epaxial muscles along the base of the raised neural spines in both Sphenacodon and Dimetrodon helping to stiffen and strengthen the backbone for walking and for lunging at prey by restricting side-to-side flexing motion.
A recent study of the structure of the neural spines on Sphenacodon confirms that the upper parts were not encased in a thick muscular hump and instead protruded above a layer of muscle to form a low dorsal crest. Finds of sphenacodontid specimens in which postmortem distortion of the body caused the dorsal spines to overlap suggests that the spines were not connected by hard or tough tissue; the possible function of a low, skin-covered crest in Sphenacodon is debated. A thermoregulatory role seems unlikely, although the taller crest in Sphenacodon ferocior is allometrically larger than in S. ferox. Recent research has favored a display role for the tall sails in Edaphosaurus. Both Sphenacodon and Dimetrodon have been depicted with their short limbs splayed outward at 90 degrees from the body in a wide pushup position and with the tail dragging on the ground, similar to modern lizards and crocodiles. A sprawling stance is typical for Sphenacodon and Dimetrodon skeletons as mounted in museums.
However, trackways called Dimetropus that match the foot configuration of large sphenacodontids show animals walking with their limbs brought under the body for a narrow, semi-erect gait without tail or belly drag marks. Such clear evidence for a more efficient upright posture suggests that important details about the anatomy and locomotion of Sphenacodon and Dimetrodon may not be understood; some well preserved narrow Dimetropus tracks found in parts of the Prehistoric Trackways National Monument in New Mexico match the smaller size of Sphenacodon, a genus known from skeletal fossils in the state, but could come from a small Dimetrodon. The American paleontologist O. C. Marsh named Sphenacodon in 1878, based on part of a lower jaw bone found in the redbeds of northern New Mexico by fossil collector David Baldwin. In his short description of the jaw, Marsh cited the back teeth as characteri
Calamites is a genus of extinct arborescent horsetails to which the modern horsetails are related. Unlike their herbaceous modern cousins, these plants were medium-sized trees, growing to heights of more than 30 meters, they were components of the understories of coal swamps of the Carboniferous Period. A number of organ taxa have been identified as part of a united organism, which has inherited the name Calamites in popular culture. Calamites refers only to casts of the stem of Carboniferous/Permian sphenophytes, as such is a form genus of little taxonomic value. There are two forms of casts; the most common is an internal cast of the hollow void in the centre of the trunk. This can cause some confusion: firstly, it must be remembered that a fossil was surrounded with 4-5 times its width in vascular tissue, so the organisms were much wider than the internal casts preserved. Further, the fossil gets narrower as it attaches to a rhizoid, a place where one would expect there to be the highest concentration of vascular tissue.
However, because the fossil is a cast, the narrowing in fact represents a constriction of the cavity, into which vascular tubes encroach as they widen. Further organ genera belonging to sphenophytes include: Arthropitys Astromyelon Annularia and Asterophylites; the trunks of Calamites had a distinctive bamboo-like appearance and vertical ribbing. The branches and cones were all borne in whorls; the leaves were needle-shaped, with up to 25 per whorl. Their trunks produced secondary xylem; the vascular cambium of Calamites was unifacial, producing secondary xylem towards the stem center, but not secondary phloem. The stems of modern horsetails are hollow or contain numerous elongated air-filled sacs. Calamites was similar in that its stems were hollow, like wooden tubes; when these trunks buckled and broke, they could fill with sediment. This is the reason. Calamites reproduced by means of spores, they are known to have possessed massive underground rhizomes, which allowed for the production of clones of one tree.
This is the only group of trees of their period known to have a clonal habit. This type of asexual reproduction would allow them to spread into new territory, help to anchor them in the unstable ground along rivers and in newly deposited delta sediments; the rhizomes of Calamites look quite similar to the stems in most cases, but have nodes that get progressively closer together as they approach the apical area. Calamites come in a variety of different "form genera". One type, Calamites suckowi, is distinguishable from other Calamites forms by its prominent, swollen nodes and wide-spaced longitudinal ribs. Another example, Calamites cisti, has much smaller nodes and the ribs are closer together. In addition, the distance between successive node lines on a Calamites suckowi specimen is much wider than the diameter. In other forms like Calamites cisti, the opposite is true or the specimen is just wider than the diameter. However, the value of these form taxa is limited; the distance between nodes, for example, is variable, an intercalary meristem means that this distance varied as the organisms grew.
The genus Calamites is placed in the family Calamitaceae in the plant class Equisetopsida in the fern allies division Pteridophyta. The Calamitaceae became extinct in the Lower Permian, a time which saw the origin and diversification of the herbaceous genus Equisetum, the only living sphenophyte genus. Archaeopteris How large are the giant horsetails? Sphenophyta: Fossil Record Stewart, W. N. Paleobotany and the Evolution of Plants. Cambridge University Press, Cambridge. Davis and Kenrick, Paul. Fossil Plants. Smithsonian Books, Washington, D. C.. ISBN 1-58834-156-9
Limnoscelis is a genus of large reptile-like diadectomorph from the Early Permian of North America. Contrary to other diadectomorphans, Limnoscelis appear to have been carnivorous. Though the post cranial skeleton is similar to the early large bodied reptiles like pelycosaurs and pareiasaurs, the digits lacked claws, the bones of the ankle bones were fused like in other reptile-like amphibians; this would not allow them to use their feet in traction, but rather as holdfasts, indicating Limnoscelis hunted slow moving prey. Bringing Fossils To Life: An Introduction To Paleobiology by Donald R. Prothero
Dimetrodon is an extinct genus of synapsids that lived during the Cisuralian, around 295–272 million years ago. It is a member of the family Sphenacodontidae; the most prominent feature of Dimetrodon is the large neural spine sail on its back formed by elongated spines extending from the vertebrae. It walked on four legs and had a tall, curved skull with large teeth of different sizes set along the jaws. Most fossils have been found in the southwestern United States, the majority coming from a geological deposit called the Red Beds of Texas and Oklahoma. More fossils have been found in Germany. Over a dozen species have been named since the genus was first described in 1878. Dimetrodon is mistaken for a dinosaur or as a contemporary of dinosaurs in popular culture, but it became extinct some 40 million years before the first appearance of dinosaurs. Reptile-like in appearance and physiology, Dimetrodon is more related to mammals than to modern reptiles, though it is not a direct ancestor of mammals.
Dimetrodon is assigned to the "non-mammalian synapsids", a group traditionally called "mammal-like reptiles". This groups Dimetrodon together with mammals in a clade called Synapsida, while placing dinosaurs and birds in a separate clade, Sauropsida. Single openings in the skull behind each eye, known as temporal fenestrae, other skull features distinguish Dimetrodon and mammals from most of the earliest sauropsids. Dimetrodon was one of the apex predators of the Cisuralian ecosystems, feeding on fish and tetrapods, including reptiles and amphibians. Smaller Dimetrodon species may have had different ecological roles; the sail of Dimetrodon may have been used to stabilize its spine or to heat and cool its body as a form of thermoregulation. Some recent studies argue that the sail would have been ineffective at removing heat from the body due to large species being discovered with small sails and small species being discovered with large sails ruling out heat regulation as its main purpose; the sail was most used in courtship display with methods such as threatening rivals or showing off to potential mates.
Dimetrodon was a sail-backed synapsid. Most Dimetrodon species ranged in length from 1.7 to 4.6 metres and are estimated to have weighed between 28 and 250 kilograms. The largest known species of Dimetrodon is D. angelensis at 4.6 metres and the smallest is D. teutonis at 60 centimetres. The larger species of Dimetrodon were among the largest predators of the Early Permian, although the related Tappenosaurus, known from skeletal fragments in younger rocks, may have been larger at an estimated 5.5 metres in total body length. Although some Dimetrodon species could grow large, many juvenile specimens are known. A single large opening on either side of the back of the skull links Dimetrodon with mammals and distinguishes it from most of the earliest sauropsids, which either lack openings or have two openings. Features such as ridges on the inside of the nasal cavity and a ridge at the back of the lower jaw are thought to be part of an evolutionary progression from early tetrapods to mammals; the skull of Dimetrodon is compressed laterally, or side-to-side.
The eye sockets are positioned far back in the skull. Behind each eye socket is a single hole called an infratemporal fenestra. An additional hole in the skull, the supratemporal fenestra, can be seen; the back of the skull is oriented at a slight upward angle, a feature that it shares with all other early synapsids. The upper margin of the skull slopes downward in a convex arc to the tip of the snout; the tip of the upper jaw, formed by the premaxilla bone, is raised above the part of the jaw formed by the maxilla bone to form a maxillary "step." Within this step is a diastema, or gap in the tooth row. Its skull was more built than a dinosaur's; the size of the teeth varies along the length of the jaws, lending Dimetrodon its name, which means "two measures of tooth" in reference to sets of small and large teeth. One or two pairs of caniniforms extend from the maxilla. Large incisor teeth are present at the tips of the upper and lower jaws, rooted in the premaxillae and dentary bones. Small teeth are present around the maxillary "step" and behind the caniniforms, becoming smaller further back in the jaw.
Many teeth are widest at their midsections and narrow closer to the jaws, giving them the appearance of a teardrop. Teardrop-shaped teeth are unique to Dimetrodon and other related sphenacodontids, help distinguish them from other early synapsids; as in many other early synapsids, the teeth of most Dimetrodon species are serrated at their edges. The serrations of Dimetrodon teeth were so fine; the dinosaur Albertosaurus had crack-like serrations, but, at the base of each serration was a round void, which would have functioned to distribute force over a larger surface area and prevent the stresses of feeding from causing the crack to spread through the tooth. Unlike Albertosaurus, Dimetrodon teeth lacked adaptations that would stop cracks from forming at their serrations; the teeth of D. teutonis lack serrations, but still have sharp edges. A study in 2014 shows; the smaller species, D. milleri, had no serrations. As prey grew larger, several Dimetrodon species started developing serrations on their teeth and increasing in size.
For instance, D. limbatus had enamel serrations that helped it cut through flesh (which were sim
Camptosaurus is a genus of plant-eating, beaked ornithischian dinosaurs of the Late Jurassic period of western North America. The name means'flexible lizard'. Camptosaurus is a heavily built form, with robust hindlimbs and broad feet, still having four toes. Due to the separate status of Uteodon it has become problematic which material from the Morrison belongs to Camptosaurus; the specimens with certainty belonging to Camptosaurus dispar, from Quarry 13, have been recovered from deep layers dating to the Callovian-Oxfordian. The largest fragments from strata indicate adult individuals more than 7.9 metres long, 2 metres at the hips. The Quarry 13 individuals are smaller though, they have been described as reaching 6 meters in 785 -- 874 kg in weight. In 2010 Gregory S. Paul gave an lower estimate: a length of five metres and a weight of half a tonne. Earlier reconstructions, such as those by Marsh and Gilmore, were based on the skull of Theiophytalia and display an incorrect, more rectangular profile.
The skull was in fact triangular with a pointed snout, equipped with a beak. Its teeth were more packed in the jaw compared to other Morrison euornithopods. Museum curator John Foster describes them as having "thick median ridges on their lateral sides and denticles along their edges," these features were similar to, but "more developed" than those in Dryosaurus. Camptosaurus teeth exhibit extensive wear, which indicates that individuals in the genus had a diet of tough vegetation. On September 4, 1879 William Harlow Reed in Albany County, Wyoming found the remains of a small euornithopod; that same year Professor Othniel Charles Marsh described and named the find as Camptonotus, or "flexible back", from Greek κάμπτω, "to bend" and νῶτον, "back", in reference to the presumed flexibility of the sacral vertebrae. The holotype was a partial skeleton; the genus was renamed Camptosaurus by him in 1885 because the original name was in use for a cricket. In 1879, Marsh named C. dispar for material he received from his collectors at Quarry 13 near Como Bluff, Wyoming in the Morrison Formation and C. amplus based on the holotype YPM 1879, a foot found by Arthur Lakes at Quarry 1A.
The foot was shown to have belonged to Allosaurus. Throughout the 1880 and 1890s, he continued to receive specimens from Quarry 13 and in 1894 named two additional species: C. medius and C. nanus, based in part on size. Charles W. Gilmore named two additional species, C. browni and C. depressus in his 1909 redescription of the Marsh specimens. In the Morrison Formation, Camptosaurus fossils are present in stratigraphic zones 2-6. In 1980, Peter Galton and H. P. Powell in their redescription of C. prestwichi, considered C. nanus, C. medius and C. browni to be different growth stages or different gender of the larger C. dispar, therefore only C. dispar was a valid species. They considered a skull, YPM 1887, in 1886 referred to C. amplus by Marsh confirmed by Gilmore, to belong to C. dispar as well. Gilmore had used this skull to describe the skull of Camptosaurus, but the specimen was shown by Brill and Carpenter not to belong to Camptosaurus. In 2007, they put it into its own species, Theiophytalia kerri.
Camptosaurus depressus was recovered from the Lakota Formation near the town of Hot Springs, South Dakota. It was described by Charles Gilmore in 1909 based on the holotype and only known specimen USNM 4753, a fragmentary postcranium, by the "narrowness or depressed nature of the ilia". Carpenter and Wilson referred this species to Planicoxa, as P. depressa, on the basis of similarities between its ilium and the holotype ilium of Planicoxa venenica. However, McDonald and colleagues, McDonald found that the horizontal postacetabular process of C. depressus is more a product of distortion. Therefore, McDonald put it into Osmakasaurus. An additional species, Camptosaurus aphanoecetes, was named by Carpenter and Wilson in 2008 for specimens from Dinosaur National Monument, it differs from C. dispar in the lower jaw, shorter neck vertebrae, straighter ischium ending in a small "foot" among other features. An analysis by Andrew McDonald and colleagues in 2010 suggested that like C. aphanoecetes is more related to more advanced iguanodonts.
It has been moved to the new genus Uteodon. While Marsh was describing Camptosaurus species in North America, numerous species from Europe were referred to the genus in the late 19th and early 20th centuries: C. inkeyi, C. hoggii, C. leedsi, C. prestwichi, C. valdensis. C. inkeyi consists of fragmentary material, a dentary and articular from Upper Cretaceous rocks of the Haţeg Basin in Romania. It is certainly a rhabdodontid and is no longer considered valid. C. valdensis is a dubious dryosaurid, based on the holotype and only known specimen NHMUK R167, a poorly preserved left femur lacking the distal end. It is thus difficult to compare it to other dryosaurids, including the contemporary Valdosaurus canaliculatus. C. leedsi is a valid dryosaurid, moved to the new genus Callovosaurus. C. hoggii was named Iguanodon hoggii by Richard Owen in 1874 and was moved to Camptosaurus by Norman and Barrett in 2002. It has since been transferred to the genus Owenodon; the remaining European species Camptosaurus prestwichii was recovered from Chawley Brick Pits, Cumnor Hurst in Oxfordshire in England.
The fossil was found. It was described by Hulke in 1880 as Iguanodon prestwichii, and
Archaeopteryx, meaning "old wing", is a genus of bird-like dinosaurs, transitional between non-avian feathered dinosaurs and modern birds. The name derives from the ancient Greek ἀρχαῖος meaning "ancient", πτέρυξ, meaning "feather" or "wing". Between the late nineteenth century and the early twenty-first century, Archaeopteryx had been accepted by palaeontologists and popular reference books as the oldest known bird. Older potential avialans have since been identified, including Anchiornis and Aurornis. Archaeopteryx lived in the Late Jurassic around 150 million years ago, in what is now southern Germany during a time when Europe was an archipelago of islands in a shallow warm tropical sea, much closer to the equator than it is now. Similar in size to a Eurasian magpie, with the largest individuals attaining the size of a raven, the largest species of Archaeopteryx could grow to about 0.5 m in length. Despite their small size, broad wings, inferred ability to fly or glide, Archaeopteryx had more in common with other small Mesozoic dinosaurs than with modern birds.
In particular, they shared the following features with the dromaeosaurids and troodontids: jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes and various features of the skeleton. These features make Archaeopteryx a clear candidate for a transitional fossil between non-avian dinosaurs and birds. Thus, Archaeopteryx plays an important role, not only in the study of the origin of birds, but in the study of dinosaurs, it was named from a single feather in 1861, though that feather would prove to be non-avian. That same year, the first complete specimen of Archaeopteryx was announced. Over the years, ten more fossils of Archaeopteryx have surfaced. Despite variation among these fossils, most experts regard all the remains that have been discovered as belonging to a single species, although this is still debated. Most of these eleven fossils include impressions of feathers; because these feathers are of an advanced form, these fossils are evidence that the evolution of feathers began before the Late Jurassic.
The type specimen of Archaeopteryx was discovered just two years after Charles Darwin published On the Origin of Species. Archaeopteryx seemed to confirm Darwin's theories and has since become a key piece of evidence for the origin of birds, the transitional fossils debate, confirmation of evolution. In March 2018, scientists reported that Archaeopteryx was capable of flight, but in a manner different from that of modern birds. Most of the specimens of Archaeopteryx that have been discovered come from the Solnhofen limestone in Bavaria, southern Germany, a lagerstätte, a rare and remarkable geological formation known for its superbly detailed fossils laid down during the early Tithonian stage of the Jurassic period 150.8–148.5 million years ago. Archaeopteryx was the size of a raven, with broad wings that were rounded at the ends and a long tail compared to its body length, it could reach up to 500 millimetres with an estimated mass of 0.8 to 1 kilogram. Archaeopteryx feathers, although less documented than its other features, were similar in structure to modern-day bird feathers.
Despite the presence of numerous avian features, Archaeopteryx had many non-avian theropod dinosaur characteristics. Unlike modern birds, Archaeopteryx had small teeth, as well as a long bony tail, features which Archaeopteryx shared with other dinosaurs of the time; because it displays features common to both birds and non-avian dinosaurs, Archaeopteryx has been considered a link between them. In the 1970s, John Ostrom, following Thomas Henry Huxley's lead in 1868, argued that birds evolved within theropod dinosaurs and Archaeopteryx was a critical piece of evidence for this argument. For instance, it has a long ascending process of the ankle bone, interdental plates, an obturator process of the ischium, long chevrons in the tail. In particular, Ostrom found that Archaeopteryx was remarkably similar to the theropod family Dromaeosauridae. Specimens of Archaeopteryx were most notable for their well-developed flight feathers, they were markedly asymmetrical and showed the structure of flight feathers in modern birds, with vanes given stability by a barb-barbule-barbicel arrangement.
The tail feathers were less asymmetrical, again in line with the situation in modern birds and had firm vanes. The thumb did not yet bear a separately movable tuft of stiff feathers; the body plumage of Archaeopteryx is less well documented and has only been properly researched in the well-preserved Berlin specimen. Thus, as more than one species seems to be involved, the research into the Berlin specimen's feathers does not hold true for the rest of the species of Archaeopteryx. In the Berlin specimen, there are "trousers" of well-developed feathers on the legs. In part they are firm and thus capable of supporting flight. A patch of pennaceous feathers is found running along its back, quite similar to the contour feathers of the body plumage of modern birds in being symmetrical and firm, although not as stiff as the flight-related feathers. Apart from that, the feather traces in the Berlin specimen are limited to a sort of "prot
The Devonian is a geologic period and system of the Paleozoic, spanning 60 million years from the end of the Silurian, 419.2 million years ago, to the beginning of the Carboniferous, 358.9 Mya. It is named after Devon, where rocks from this period were first studied; the first significant adaptive radiation of life on dry land occurred during the Devonian. Free-sporing vascular plants began to spread across dry land, forming extensive forests which covered the continents. By the middle of the Devonian, several groups of plants had evolved leaves and true roots, by the end of the period the first seed-bearing plants appeared. Various terrestrial arthropods became well-established. Fish reached substantial diversity during this time, leading the Devonian to be dubbed the "Age of Fishes." The first ray-finned and lobe-finned bony fish appeared, while the placoderms began dominating every known aquatic environment. The ancestors of all four-limbed vertebrates began adapting to walking on land, as their strong pectoral and pelvic fins evolved into legs.
In the oceans, primitive sharks became more numerous than in the Late Ordovician. The first ammonites, species of molluscs, appeared. Trilobites, the mollusc-like brachiopods and the great coral reefs, were still common; the Late Devonian extinction which started about 375 million years ago affected marine life, killing off all placodermi, all trilobites, save for a few species of the order Proetida. The palaeogeography was dominated by the supercontinent of Gondwana to the south, the continent of Siberia to the north, the early formation of the small continent of Euramerica in between; the period is named after Devon, a county in southwestern England, where a controversial argument in the 1830s over the age and structure of the rocks found distributed throughout the county was resolved by the definition of the Devonian period in the geological timescale. The Great Devonian Controversy was a long period of vigorous argument and counter-argument between the main protagonists of Roderick Murchison with Adam Sedgwick against Henry De la Beche supported by George Bellas Greenough.
Murchison and Sedgwick named the period they proposed as the Devonian System. While the rock beds that define the start and end of the Devonian period are well identified, the exact dates are uncertain. According to the International Commission on Stratigraphy, the Devonian extends from the end of the Silurian 419.2 Mya, to the beginning of the Carboniferous 358.9 Mya. In nineteenth-century texts the Devonian has been called the "Old Red Age", after the red and brown terrestrial deposits known in the United Kingdom as the Old Red Sandstone in which early fossil discoveries were found. Another common term is "Age of the Fishes", referring to the evolution of several major groups of fish that took place during the period. Older literature on the Anglo-Welsh basin divides it into the Downtonian, Dittonian and Farlovian stages, the latter three of which are placed in the Devonian; the Devonian has erroneously been characterised as a "greenhouse age", due to sampling bias: most of the early Devonian-age discoveries came from the strata of western Europe and eastern North America, which at the time straddled the Equator as part of the supercontinent of Euramerica where fossil signatures of widespread reefs indicate tropical climates that were warm and moderately humid but in fact the climate in the Devonian differed during its epochs and between geographic regions.
For example, during the Early Devonian, arid conditions were prevalent through much of the world including Siberia, North America, China, but Africa and South America had a warm temperate climate. In the Late Devonian, by contrast, arid conditions were less prevalent across the world and temperate climates were more common; the Devonian Period is formally broken into Early and Late subdivisions. The rocks corresponding to those epochs are referred to as belonging to the Lower and Upper parts of the Devonian System. Early DevonianThe Early Devonian lasted from 419.2 ± 2.8 to 393.3 ± 2.5 and began with the Lochkovian stage, which lasted until the Pragian. It spanned from 410.8 ± 2.8 to 407.6 ± 2.5, was followed by the Emsian, which lasted until the Middle Devonian began, 393.3± 2.7 million years ago. During this time, the first ammonoids appeared. Ammonoids during this time period differed little from their nautiloid counterparts; these ammonoids belong to the order Agoniatitida, which in epochs evolved to new ammonoid orders, for example Goniatitida and Clymeniida.
This class of cephalopod molluscs would dominate the marine fauna until the beginning of the Mesozoic era. Middle DevonianThe Middle Devonian comprised two subdivisions: first the Eifelian, which gave way to the Givetian 387.7± 2.7 million years ago. During this time the jawless agnathan fishes began to decline in diversity in freshwater and marine environments due to drastic environmental changes and due to the increasing competition and diversity of jawed fishes; the shallow, oxygen-depleted waters of Devonian inland lakes, surrounded by primitive plants, provided the environment necessary for certain early fish to develop such essential characteristics as well developed lungs, the ability to crawl out of the water and onto the land for short periods of time. Late DevonianFinally, the Late Devonian started with the Frasnian, 382.7 ± 2.8 to 372.2 ± 2.5, during which the first forests took shape on land. The first tetrapods appeared in the fossil record in the ensuing Famennian subdivisi