Phylum: Chordata (vertebrates and relatives, including fish, mammals, birds,
reptiles and amphibians)
Life
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Opisthokonta >
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Bilateria >
Deuterostomia
Chordates are characterized by having a reinforced rod
running down their back called a notocord. During embryonic development of
vertebrates, which make up the vast majority of chordates, this notocord becomes
incorporated into the vertebral column.
Classification
Chordata |
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Tunicata (tunicates, sea squirts and salps)[= Urochordata] |
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Cephalochordata (lancelets, amphioxus)
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Craniata Craniata are chordates in
which the dorsal nerve chord is expanded in the front to form a brain,
which is enclosed within a skull. |
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Hyperotreti (hagfishes) |
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Vertebrata (vertebrates) |
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Hyperoartia
(lampreys) Previously included in the "Agnatha"
(jawless fish). |
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# Euconodonta (conodonts)
Conodont animals were eal-like primitive jawless vertebrates that
lived 520 to 190 million years ago. For many years they were only known
from fossilised tooth-like structures (termed conodonts) and their true
identity was only revealed once fossilised soft body parts were
discovered in association with the conodonts. Conodont fossils are found
worldwide and in southern Africa are know from the Soom Shale in the
Cedarberg (Durand 2005).
Conodonts were previously included in the "Agnatha"
(jawless fish). |
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# Pteraspidomorphi Fossil jawless
fish with large, median ventral and dorsal dermal plates forming most of
the head armour, and with a scaly tail. The only fin they had was the
caudal fin. They also often had oak leaf-shaped tubercles on the dermal
plates. They lived as bottom dwellers in shallow seas and possibly also
in freshwater, from the Early Ordovician to the Late Devonian (470 to
370 million years ago). Are there fossil records from
southern Africa? Information mainly from
article by Philippe
Janvier in Tree
of Life.
Have fossil pteraspidomorphs been found in southern
Africa?
Pteraspidomorphs were previously
included in the "ostracoderms" ("shell-skinned") within the "Agnatha"
("jawless" fish). |
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# Thelodonti Fossil jawless fish with
the surface of the body made up entirely of minute scales each scale
with a growing base and special processes to anchor it in the dermis.
The group is possibly polyphyletic. Complete specimens are rare in the
fossil record - the presence of this group is usually only indicated by
the fossilised scales. Thelodonts have been recorded in the fossil
record from the Lowermost Silurian to the Late Devonian (430 to 370
million years ago). Information mainly from
article by Philippe Janvier
in Tree
of Life; see also
Wikipedia.
Have fossil thelodonts been found in southern Africa?
Thelodonts were previously included
in the "ostracoderms" within the "Agnatha" (jawless fish). |
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# Anaspida Fossil, jawless fish with
a somewhat slender and laterally compressed body, with gill openings
behind the eyes in a slanting row. There was a distinctive tri-radiate
spine (sometimes more than one) behind each series of gill openings.
They had hypocercal tail fins in which the lower lobe was longer than
the upper, with the notocord dipping down into the lower lobe. They had
an anal fin and paired fins ventrally. The dermal skeleton did not form
a large headshield but instead was made up of a combination of bony
plates and scales. There was a central hole in the top of the
head. In lampreys a similar-looking hole called the nasohypophysial
opening links to paired olfactory sacs and is used for smelling.
Anaspids lived in the Silurian, from 430 to 410 million years ago. They
lived in marine coastal areas. Fossils of whole animals are rare with
the best specimens coming from Norway and Scotland. Information
mainly from article
by Philippe Janvier in Tree of Life.
Have fossil anaspids been found in southern Africa?
Anaspids were previously included
in the "ostracoderms" within the "Agnatha" (jawless fish). |
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# Galeaspida Fossil, jawless,
bottom-dwelling fish with large oval to horseshoe-shaped head shield,
ventral mouth and a large, opening in the top of the head that linked to
the phyrynx and gill chambers and was presumably used for breathing and
smelling. There were no fins other than the tail fin and the body was
covered in minute scales. They hold the record in vertebrates for the
largest number of gill openings, up to 45 in some species. They lived in
the Silurian and Devonian periods (430 to 370 million years ago).
Galeaspid fossils have only been recorded from China
(including Tibet) and northern Vietnam.
Galeaspids were previously included
in the "ostracoderms" within "Agnatha" (jawless fish).
Information from
article by
Philippe Janvier in Tree of Life. |
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# Pituriaspida Fossil, jawless fish
with strongly developed head shield that extended forward as a rostral
process. Occurred in the early to middle Devonian (about 390 million
years ago), where they probably occurred in marine, delta-like
environments.
Fossils have been recorded only from Queensland,
Australia.
Previously
included in the "ostracoderms" within "Agnatha" (jawless fish). |
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# Osteostraci (osteostracans, cephalaspids)
One of the major groups of fossil, jawless fish, similar to the
Galeaspida and Pituriaspida in having a large head shield. The group is
characterised mainly by depressions in the head shield covered with
platelets and linked to the inner ear. These depressions are regarded as
having housed electrical organs and/or to have had a sensory function.
Another characteristic of the group is a leaf-shaped, horizontal fin
positioned beneath the caudal fin, possibly a modified anal fin. They
had a nasohypophysial opening in the middle of the top of the head,
similar to lampreys and probably also used for smelling. They had
paired fins. Individuals ranged in length from 4 cm to about 1 m, but
were usually about 20-40 cm long. They lived from the Early Silurian to
the Late Devonian (430 - 370 million years ago) at the bottom of shallow
marine seas, possibly also occurring in freshwater.
Fossils have been found in the North America, Europe, Siberia, and
Central Asia (north of the Tian-Shan mountains).
Previously
included in the "ostracoderms" within "Agnatha" (jawless fish).
Information from article by
Philippe
Janvier in Tree of Life. See also
Wikipedia. |
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Gnathostomata
(jawed vertebrates) Evolved from a jawless
fish-like ancestor through the incorporation of the first gill arch into
the margin of the mouth, thus enabling the fish to bite. |
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# Placodermi (armored jawed vertebrates)
Placoderms were heavily armoured fish with jaws, and were the earliest
evolved group of gnathostomes (jawed vertebrates) . The head and trunk
shields were separate and articulated in most species, so that the skull
could tilt up, thus enabling the mouth to open more widely. The backbone
consisted of a notochord with Y-shaped vertebral elements, often
cartilaginous, on each side. Individuals ranged in size from 10 cm to
about 9 metres in length and ranged from filter-feeders to carnivores.
Placoderms were living from the early Silurian Period to the end of the
Devonian, with the greatest radiation in the Early Devonian.
According to Brazeau (2009), placoderms may be paraphyletic, along with
acanthodians.
In southern Africa, placoderms have been recorded from Early Devonian
(in the Lower Bokkeveld shales) and from Late Devonian strata (Durand
2005).
Information mainly from
Palaeos website. |
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# Acanthodii
(acanthodians)
Originated in the Silurian and went extinct in the Permian. Evidence
presented by Brazeau (2009) suggests strongly that acanthodians are
paraphyletic in that both Chondrichyes and Osteichthyes probably
orginated from within this group as it is currently defined.
Fossil
remains of acanthodians (spines, scale impressions, skull remnants,
rarely articulated skeletons) have been recorded from Devonian and
Carboniferous shallow marine, estuarine and lake deposits in South
Africa (Durand 2005). |
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Chondrichthyes (sharks, rays, chimaeras)
Cartilaginous fishes include the living sharks, rays, and chimaeras, and
have true upper and lower jaws, a sensory snout that overhangs the mouth
and nostrils on the underside of the head, teeth in conspicuous
transverse rows or in fused tooth plates that are replaced from inside
the mouth, no bony plates on the head, scales in the form of small,
toothlike dermal denticles or placoid scales, fins without bony fin
rays, and a simplified internal cartilaginous skeleton without bone. The
Chondrichthyes originated as far back as the late Ordovician, about 461
million years ago and have been roaming our seas ever since although the
early forms were very different from the ones alive today. |
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Osteichthyes (bony fish) The skeleton
is made up almost entirely of bone. |
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Actinopterygii (ray-finned fish) |
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Sarcopterygii (lobe-finned fishes and
four-legged vertebrates) The lobe-finned fish include
the still living lungfish and coelacanths and it was also within this
group that the four-legged vertebrates (tetrapods) evolved, including the major
groups that are still around today, namely the
amphibians,
reptiles
(including
birds), and
mammals. |
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Publications
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Brazeau MD. 2009. The braincase and jaws of a Devonian
'acanthodian' and the origin of modern gnathostomes. Nature 457:305-308
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Durand JF. 2005. Major African contributions to Palaeozoic
and Mesozoic vertebrate palaeontology. Journal of African Earth Sciences
43: 53-82.
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Friedman, M. and Brazeau, M.D. 2010. A reappraisal of the
origin and basal radiation of the Osteichthyes. Journal of Vertebrate
Paleontology 30:36-56.
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Young GC. 2010. Placoderms (armored fish): dominant
vertebrates of the Devonian Period. Annual Review of Earth and Planetary
Sciences 38: 523-550. DOI: 10.1146/annurev-earth-040809-152507
Text by Hamish Robertson
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