Pteraspidomorphi is an extinct
class of early
jawless fish
Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present ( cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animals, cyclosto ...
. They have long been regarded as closely related or even ancestral to jawed vertebrates, but the few characteristics they share with the latter are now considered as
basal traits for all
vertebrates
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...
.
Characteristics
Pteraspidomorphs are characterized by their massive dermal head armour having large, median, ventral and dorsal
plates or shields.
[ Janvier, Philippe (1997]
Pteraspidomorphi
''The Tree of Life Web Project''.
The fossils show extensive shielding of the head. Many had
hypocercal tails in order to generate lift to increase ease of movement through the water for their armoured bodies, which were covered in dermal bone. They also had sucking mouth parts and some species may have lived in fresh water.
Most pteraspidomorphs were marine, but lived very near to the shore, in lagoons and deltas. Some groups are thought to have been fresh water-dwelling. They were certainly bottom-dwellers, as shown by traces of abrasion of the ventral surfaces of their headshields.
Classification
Pteraspidomorphs have been first regarded as related to
bony fishes, then to
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s, then ancestral to
hagfish
Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, ...
es, and finally as the closest jawless relatives of the
gnathostome
Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living ...
s.
This last theory was based on the fact that they seem to have a paired olfactory organ and a sensory-line pattern which is quite similar to that of the gnathostomes. These characteristics are, however, likely to be general for either the vertebrates or, at any rate, for the ensemble of all
ostracoderm
Ostracoderms () are the armored jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic (excluding jawed fishes) (may also be polyphyletic if anaspids are closer to cyclostomes) an ...
s and the gnathostomes. Other ostracoderms, such as the
Galeaspida
Galeaspida (from Latin, 'Helmet shields') is an extinct taxon of jawless marine and freshwater fish. The name is derived from ''galea'', the Latin word for ''helmet'', and refers to their massive bone shield on the head. Galeaspida lived in shallo ...
are now known to have a paired olfactory organ. Current phylogenetic analysis using a large number of characteristics now place pteraspidomorphs as the sister-group of all other ostracoderms and the gnathostomes.
Phylogeny
Based on the work of Mikko Haaramo.
See also
*
Ostracoderm
Ostracoderms () are the armored jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic (excluding jawed fishes) (may also be polyphyletic if anaspids are closer to cyclostomes) an ...
References
External links
Pteraspidomorphiat Tolweb
at Palaeos
{{Taxonbar, from=Q134673
Furongian first appearances
Late Devonian animals
Late Devonian extinctions
Prehistoric fish classes