Reevaluation of the caudal skeleton of some actinopterygian fishes: II. Hiodon, Elops, and Albula

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Abstract
The vertebral centra of Hiodon, Elops, and Albula are direct perichordal ossifications (autocentra) which enclose the arcocentra as in Amia. An inner ring of ovoid cells forms in late ontogeny from the intervertebral space inside the autocentrum. The chordacentrum is reduced or completely absent in centra of adult Elops, whereas it forms an important portion of the centra in adult Hiodon. The posterior portion of the compound ural centrum 3+4+5 is partially (Hiodon) or fully formed by the chordacentrum (Elops, Albula). The haemal arches and hypurals are fused medially by cartilage or bone trabecles of the arcocentrum with the centra, even though they appear autogenous in lateral view in Elops and Albula. The composition of the caudal skeleton of fossil teleosts and the ontogeny of that of Hiodon, Elops, and Albula corroborate a one-to-one relationship of ural centra with these dorsal and ventral elements. The first epural (epural 1) of Elops relates to ural centrum 1, whereas the first epural (epural 2) of Hiodon and Albula relates to ural centrum 2. In Albula, the first ural centrum is formed by ural centrum 2 only. With 4 uroneurals Hiodon has the highest number within recent teleosts. Juvenile specimens of Hiodon have eight, the highest number of hypurals within recent teleosts; this is the primitive condition by comparison with other teleosts and pholidophorids. Reduction of elements in the caudal skeleton is an advanced feature as seen within elopomorphs from Elops to Albula. Such reductions and fusions occur in osteoglossomorphs also, but the lack of epurals and uroneurals separates most osteoglossomorphs (except Hiodon) from all other teleosts.