| Actinolepidae Temporal range: Early Devonian | |
|---|---|
 | |
| reconstruction of Actinolepis | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | † Placodermi |
| Order: | † Arthrodira |
| Suborder: | † Actinolepidoidei |
| Family: | † Actinolepidae |
| Genera | |
Actinolepidae is an extinct family of placoderm fishes which lived during the Early Devonian period. They are considered to be among the most primitive of the arthrodires, and are widely accepted to be phylogenetically basal to the group. [1]
The bodies of Actinolepids are wide and flat, suggesting that most members of this family were benthic fish. Their jaws were comparatively underdeveloped in comparison to the more robust-jawed arthrodires that would come after them, such as Dunkleosteus and Coccosteus , indicating that it likely subsisted primarily on smaller, softer-bodied animals such as mollusks or worms instead of larger, tougher prey animals. [2]
The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago (Mya), to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era. As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by a few million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when up to 60% of marine genera were wiped out.
Tetrapods are four-limbed vertebrate animals constituting the superclass Tetrapoda. It includes extant and extinct amphibians, sauropsids and synapsids.
Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, including the presence of lobed fins with a well-developed internal skeleton. Lungfish represent the closest living relatives of the tetrapods.
Cephalaspidomorphs are a group of jawless fishes named for Cephalaspis of the osteostracans. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys, because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day. They are the closest relatives of jawed fishes, who emerged from within them and they would survive if the jawed fish are included.
Placodermi is a class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devonian period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were among the first jawed fish; their jaws likely evolved from the first of their gill arches.
Dunkleosteus is an extinct genus of large armored, jawed fishes that existed during the Late Devonian period, about 382–358 million years ago. It consists of ten species, some of which are among the largest placoderms to have ever lived: D. terrelli, D. belgicus, D. denisoni, D. marsaisi, D. magnificus, D. missouriensis, D. newberryi, D. amblyodoratus, and D. raveri. The largest and most well known species is D. terrelli, which grew up to 8.79 m (28.8 ft) long and 4 t in weight. Dunkleosteus could quickly open and close its jaw, like modern-day suction feeders, and had a bite force of 4,414 N at the tip and 5,363 N at the blade edge. Numerous fossils of the various species have been found in North America, Poland, Belgium, and Morocco.
Ctenurella is an extinct genus of ptyctodont placoderm from the Late Devonian of Germany. The first fossils were found in the Strunde valley in the Paffrather Kalkmulde.
Arthrodira is an order of extinct armored, jawed fishes of the class Placodermi that flourished in the Devonian period before their sudden extinction, surviving for about 50 million years and penetrating most marine ecological niches. Arthrodires were the largest and most diverse of all groups of Placoderms.
Tiktaalik is a monospecific genus of extinct sarcopterygian from the Late Devonian Period, about 375 Mya, having many features akin to those of tetrapods. It may have grown up to 2.5 m (8.2 ft) in length.
Groenlandaspis is an extinct genus of arthrodire from the Late Devonian. Fossils of the different species are found in late Devonian strata in all continents except eastern Asia. The generic name commemorates the fact that the first specimens of the type species were found in Greenland.
Coccosteus is an extinct genus of arthrodire placoderm from the Devonian period. Its fossils have been found throughout Europe and North America. The majority of these have been found in freshwater sediments, though such a large range suggests that they may have been able to enter saltwater. The largest specimens were about 40 centimetres (16 in), although the average length was 20 to 24 centimetres.
Antiarchi is an order of heavily armored placoderms. The antiarchs form the second-most successful group of placoderms after the arthrodires in terms of numbers of species and range of environments. The order's name was coined by Edward Drinker Cope, who, when examining some fossils that he thought were armored tunicates related to Chelysoma, mistakenly thought that the orbital fenestra was the opening for the mouth, or oral siphon, and that the opening for the anal siphon was on the other side of the body, as opposed to having both oral and anal siphons together at one end.
Onychodus is a genus of prehistoric lobe-finned fish which lived during the Devonian period. It is one of the best known of the group of onychodontiform fishes. Scattered fossil bones of Onychodus were first discovered in 1857, in North America, and described by John Strong Newberry. Other species were found in Australia, England, Norway and Germany showing that it had a widespread range.
The Gogo Formation in the Kimberley region of Western Australia is a Lagerstätte that exhibits exceptional preservation of a Devonian reef community. The formation is named after Gogo Station, a cattle station where outcrops appear and fossils are often collected from, as is nearby Fossil Downs Station.
Dicksonosteus is an extinct genus of basal arthrodire, placoderm fish, which lived during the Early Devonian period of Spitsbergen, Norway. Dicksonosteus was once considered an actinolepid, but is now grouped within the family Phlyctaeniidae.
Incisoscutum is an extinct genus of arthrodire placoderm from the Early Frasnian Gogo Reef, from Late Devonian Australia. The genus contains two species I. ritchiei, named after Alex Ritchie, a palaeoichthyologist and senior fellow of the Australian Museum, and I. sarahae, named after Sarah Long, daughter of its discoverer and describer, John A. Long.
The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, and small mostly armoured fish known as ostracoderms, first appeared. Most jawless fish are now extinct; but the extant lampreys may approximate ancient pre-jawed fish. Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans.
The evolution of tetrapods began about 400 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes. Tetrapods are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals. While most species today are terrestrial, little evidence supports the idea that any of the earliest tetrapods could move about on land, as their limbs could not have held their midsections off the ground and the known trackways do not indicate they dragged their bellies around. Presumably, the tracks were made by animals walking along the bottoms of shallow bodies of water. The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear. They are areas of active research and debate among palaeontologists at present.
Kujdanowiaspis is an extinct genus of actinolepid placoderm from the Early Devonian of Nyrkiv, Ukraine and Poland. As an actinolepid, it is among the most basal of all placoderms. Kujdanowiaspis is only known from many well-preserved fragmentary head shield and skull fossils. After revising the genus in 2010, Dupret left three species within the genus: K. buczacziensis, K. podolica and possibly also K. zychi.
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