Arandaspida

From Palaeos.org

o Vertebrata
`--o Pteraspidomorphi
   |--Arandaspida
   `--o Heterostracomorphi
      |--Astraspida
      `--Heterostraci
         |--o Cyathaspidiformes
         |  |--Cyathaspidida
         |  `--Amphiaspidida
         `--o Pteraspidiformes
            |--Pteraspididae
            `--Psammosteida
               |--Drepanaspis
               `--Psammosteidae

Information

Arandaspida: Arandaspis, Sacabambaspis, possibly Porophoraspis. Medium-sized (12-14 cm) very primitive, teardrop-shaped marine fishes.

Range: Early to Late Ordovician of Australia & South America.

Phylogeny: Pteraspidomorphi :: Heterostracomorphi + Arandaspida.

Characters: Two large, thin (<1 mm) plates on head (dorsal & ventral); ventral plate convex, dorsal plate flattened; main plates ornamented by drop-shaped or lobate tubercles; eyes in extreme anterior; solid sclerotic ring present; nostrils (?) just medial to orbits on embayment in anterior surface just dorsal to mouth; mouth lined ventrally with numerous small scales; possible lingual or velar apparatus; paired pineal (2?) and parapineal (4?) openings on anterior of dorsal plate (which may be endolymphatic ducts instead); probably at least ten gill slits with smaller plates on sides; posterior body & tail may have been laterally compressed; body scales highly elongated and arranged in large chevrons (pointing anteriorly) on each side; no ridge scales; no paired or unpaired fins except caudal (poorly known); caudal fin probably pad-shaped and diphycercal; well-developed small lateral lines on both plates and medioventral surfaces; little or no endoskeleton; scales and plates trilaminar, with basal laminar layer, cancellous central layer (with double walls) and aspidine or cellular bone tubercles. No growth or fusion of scales.

References

• Janvier, P (1996), Early Vertebrates, Oxford, 393 pp.

Links

• Fischmarkt Bremerhaven (German)
• Class Pterapisdomorphi
• Terre et Vie - Exposition permanente (French)
• New Page 1 - (large image of arandaspid fragment)]
• ORDOVICICO (in Spanish, a very nice summary of the Ordovician context of these fishes - pdf)
• Boletín de Publicación mensual de (Spanish translation of a National Geographic article on Sacabambaspis)
• life-5-b-Xystridura (exemplary short, encyclopedia-like entry with image of Sacabambaspis)
• Gli Agnati(Italian).

Credits

ATW060209 (Palaeos com).