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biodiversity explorer

the web of life in southern Africa

Phylum: Chordata (vertebrates and relatives, including fish, mammals, birds, reptiles and amphibians)

Life > Eukaryotes > Opisthokonta > Metazoa (animals) > 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

 
 

Tunicata (tunicates, sea squirts and salps)[= Urochordata]

 

Didemnum molle (CL Griffiths )
 

Cephalochordata (lancelets, amphioxus)

 

 
 

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.

 
   

Hyperotreti (hagfishes)

 

 
   

Vertebrata (vertebrates)

 

 
     

Hyperoartia (lampreys)

Previously included in the "Agnatha" (jawless fish).

 
     

# 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).

 
     

# 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).

 
     

# 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).

 
     

# 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).

 
     

# 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.

 
     

# 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).

 
     

# 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.

 
     

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.

 
       

# 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.

 
       

# 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).

 
       

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.

       

Osteichthyes (bony fish)

The skeleton is made up almost entirely of bone.

 
         

Actinopterygii (ray-finned fish)

 

 
         

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.

Latimeria chalumnae (Coelacanth)

Publications

  • Brazeau MD. 2009. The braincase and jaws of a Devonian 'acanthodian' and the origin of modern gnathostomes. Nature 457:305-308

  • Durand JF. 2005. Major African contributions to Palaeozoic and Mesozoic vertebrate palaeontology. Journal of African Earth Sciences  43: 53-82.

  • 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.

  • 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