Morphological variation in the first vertebra among acanthomorph fishes – a guide for identifying fossil centra from microvertebrate sites

Innumerable fossil fish specimens have been recovered from fossil microvertebrate sites – areas in which small, isolated elements of the skeleton from multiple individuals have been amassed. The accumulated skeletal remains, often the result of transportation by moving waters, provide an important window on the fauna that was present in a wider area during a broader period of time than that represented by individual articulated specimens. Although microvertebrate localities provide important records of taxa, the disarticulated condition of the fossil elements can cause difficulties for taxonomic identification. This is particularly true for fish, which are most commonly represented in microfossil sites by vertebral centra; however, fish centra are notoriously difficult to identify at lower taxonomic levels, partly because of the vast numbers of living fish and lack of comparative collections. Arguably, the most distinctive vertebral centrum among all fish is the first abdominal centrum of Acanthomorpha. In acanthomorphs, in contrast to all other fishes, the first centrum bears two distinct (separate left and right) facets for articulation with the exoccipitals, which are normally positioned more or less dorsolateral to the articular facet for the basioccipital, forming a unique tri-partite morphology. We here document the first centrum of numerous acanthomorph fishes, and assess the morphologies for taxonomic or phylogenetic consistencies that would allow us to identify isolated centra to a particular acanthomorph group. Features we document include: whether the neural arch is fused to the centrum (as in the paracanthopterygians Lota lota and Percopsis omiscomaycus) or autogenous (most acanthopterygians); whether the left and right facets for the exoccipitals meet in the midline (e.g., Boops boops) or are widely separated (e.g., percids and scorpaenids); and the bone texture on the centrum which may be an anastomosed network forming many small spaces (e.g., Scomber spp.) or a more solid network with no spaces (e.g., Channidae), among others. From our examination of specimens, we note several unique features among the taxa examined: the first centrum of Monopterus alba is opisthocoelus and the first neural arch has processes extending posteriorly; the first centrum of Leiognathus equula has ventral process extending from the facets for articulation with the exoccipitals; there are anterior accessory processes on the neural arch of Parachanna obscura; the transverse processes of Chlorurus sp. and Scarus vetula extend from the ventral area of the centrum and form long triangular processes; and the neural arch of Siganus guttatus has ventrolateral processes that extend to the level of the ventral surface of the centrum. We also find that the overall morphology of the first centrum is conservative within a family, indicating that in many cases fossil material may be identified at least to the family level.

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