Pteraspidomorphi

From Palaeos

Craniata
`--o Vertebrata
   |?- †Myllokunmingia fenjiaoa
   |?- †Haikouichthys ercaicunensis
   |--Conodonta
   `--+--Hyperoartia
      `--+--Anaspida
         `--+--Pteraspidomorphi 
            `==Thelodonti 
               |--+--Cephalaspidomorphi
               |  |--Galeaspida
               |  `--Pituriaspida
               `--o Gnathostomata
                  |--Placodermi
                  |--Chondrichthyes
                  `--o Teleostomi
                     |--Acanthodii
                     `--Osteichthyes
                        `--Tetrapoda
                           `--o Amniota
                              |--Sauropsida
                              `--Synapsida

[edit] Introduction

Pteraspidomorphi is an extinct subclass of early jawless fish. The fossils show extensive shielding of the head. Maybe some species even lived in fresh water. They are considered to be the most primitive of vertebrates.

[edit] Overview

The Pteraspidomorphi are sometimes referred to as Heterostraci, although that term is more properly applied to the specialised Silurian and Devonian forms. They are an extinct clade of jawless fish, and were the most abundant and diverse vertebrates of the Silurian. They are distinguished by having an exoskeleton or bony shield composed of several plates, usually a dorsal (upper or back) shield, a ventral (lower or belly) shield, branchial (gill) plates, and a number of smaller plates around the areas of the mouth (oral plates) and eyes. These plates consist of a mostly acellular form of bone, called aspidine, which is believed to be the ancestral condition for the dermal or skin/exoskeleton bone of all bony vertebrates. The body is covered by many scales, each of which has ornament matching the type seen on the larger dermal bones. In all of the more derived forms, there is only one, common gill opening on each side.

The Pteraspidomorphi include the earliest known vertebrates. The oldest certain remains date to the Early or Middle Ordovician, although the disputed and fragmentary Anatolepis probably goes back to the late Cambrian. In any case, by the middle and late Ordovician, there were several different lineages evolving in isolation different parts of the world, as this map indicates.

These Ordovician forms, the Arandaspida, Astraspida and Eriptychiida, were formerly grouped with the Silurio-Devonian Heterostraci, but are rather more primitive. These early types share some unique features with heterostracans, such as the presence of large median dorsal and ventral plates, but do not possess common external branchial openings.

By the start of the Silurian period these lineages had died out, possibly as another result of the mass extinction at the end of the Ordovician. They were replaced by the Heterostraci. This latter group underwent an evolutionary radiation, dividing into a number of lineages and reaching their peak during Late Silurian and Early Devonian times, when a variety of different types evolved and flourished, from mud-eating bottom-dwellers to free-swimming filter-feeders. All had the characteristic head shield, which could grow throughout the life of the animal.

By the middle Devonian the Pteraspidomorphi went into decline, with only a single family of giant (by agnath standards) flattened bottom-dwellers, the psammosteids, continuing almost until the end of the Devonian, the last and also the largest of the armoured jawless fish.

[edit] Basic Information

Loosely synonymous with "Diplorhina". Probably defined as Anatolepis + Psammolepis.

Range: Early Ordovician (or Late Cambrian) to Late Devonian.

Phylogeny: Vertebrata ::: "Thelodontia" + Pteraspidomorphi : Anatolepis + (Arandaspida + Heterostracomorphi).

[edit] Characters

Characters: 10-200 cm; head very long (40-50% body length); paired nasal capsules, and ?openings; solid anterior plates, with at least a major oblong median dorsal and ventral plates and usually a median rostral plate above mouth; rostra and cornua common; pineal opening (psammosteids?); brain with two semicircular canals; some show internal impressions of arcualia on notochord; body tends to be ventrally flattened (numerous exceptions); mobile tail, generally fan- or pad-like; tail hypocercal; no paired appendages; no unpaired fins except caudal; sensory line system present; no cellular bone (but see [H73]); possible traces of calcified cartilage in in some basal lineages; trilaminate aspidine; grebeshki ornamentation on dermal plates (lost independently several times); largely marine nearshore.

[edit] Phylogeny

o †Pteraspidomorphi Goodrich, 1909
  |-- †Anatolepis
  `--+--o †Arandaspida Ritchie & Gilbert-Tomlinson, 1977
     |  |?- †Pircanchaspis rinconensis Erdtmann et al., 2000
     |  |--o †Sacabambaspis Gagnier, Blieck & Rodrico, 1986 [Sacabambaspidae]
     |  |  |-- †S. sp. [Young, 1997] [Ordovician, Central Australia]
     |  |  `-- †S. janvieri Gagnier, Blieck & Rodrico, 1986
     |  `--o †Arandaspididae Ritchie & Gilbert-Tomlinson, 1977
     |     |--o †Arandaspis Ritchie & Gilbert-Tomlinson, 1977
     |     |  |-- †A. sp. [Young, 1997] [Ordovician, Central Australia]
     |     |  `-- †A. prionotolepis Ritchie & Gilbert-Tomlinson, 1977
     |     `--o †Porophoraspis Ritchie & Gilbert-Tomlinson, 1977
     |        |-- †P. sp. [Young, 1997] [Ordovician, Central Australia]
     |        `-- †P. crenulata Ritchie & Gilbert-Tomlinson, 1977
     `--o †Heterostracomorphi Lankester, 1868
        |-- †Astraspis desiderata Walcott, 1892 [taxonomic history] [Astraspida Berg, 1940: Astraspidiformes Berg, 1940: Astraspidae Eastman, 1917]
        `--+-- †Eriptychius americanus Walcott, 1892 [taxonomic history] [Eriptychiida: Eriptychiidae Tarlo, 1962:]
           `--o †Heterostraci Lankester, 1868
              |?-o †Aserotaspis
              |  `-- †A. canadensis Dineley & Loeffler, 1976
              |--o †Lepidaspis
              |  `-- †L. serrata Dineley & Loeffler, 1976
              |?- †Kallostrakon
              |?- †Natlaspis
              |?- †Oniscolepis
              |?- †Tesseraspis
              `--+--o †Traquairaspidiformes
                 |  `--o †Traquairaspididae
                 |     |--o †Phialaspis
                 |     |  `-- †P. symondsi
                 |     |-- †Traquairaspis [Lophopiscis, Lophaspis, Orthaspis, Phialaspis, Yukonaspis]
                 |     `--o †Weigeltaspis
                 |        |-- †W. godmani Tarlo
                 |        `-- †W. brotzeni Tarlo, 1964
                 `--+--o †Tolypelepidida Denison, 1964 [Cyathaspididae: Tolypelepidinae Denison, 1964]
                    |  `--o †Tolypelepididae Denison, 1964 [Tolypelepidinae Denison, 1964]
                    |     |-- †Asketaspis interstincta Dineley & Loeffler, 1976
                    |     |-- †Athenaegis chattertoni Soehn & Wilson, 1990
                    |     `--o †Tolypelepis Pander, 1856 [Tolypaspis]
                    |        |-- †T. undulata Pander, 1856
                    |        |-- †T. lenzi Dineley & Loeffler, 1976
                    |        `-- †T. leopoldensis Loeffler & Jones, 1977
                    `--+-- †Cyathaspidiformes
                       `-- †Pteraspidiformes Berg, 1940

[edit] References

• Halstead, LB (1973), The heterostracan fishes. Biol. Rev. 48: 279-332. [H73]

[edit] Links

• Pteraspidomorphi
• Pteraspidomorphi
• Gagnier
• Pteraspidomorphi
• Comparative Anatomy Topic 3 - Fishes
• Faktaside om lungefisk (Norwegian)
• Class Pterapisdomorphi
• Life of the Ordovician
• FISH 00
• Paleozoic Fossils UK (extraordinary images on this site!).

[edit] Credits

BILL 061113 (introduction, phylogeny) MAK000112 (overview) ATW040316 (basic information, characters, links)