Pseudopetalichthyida
From Palaeos
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Fossil_range: Early Devonian |
The Pseudopetalichthyida is a group of extinct, skate-like fishes known only from rare fossils in Lower Devonian strata in Hunsrück, Germany. Like Stensioella heintzi and the Rhenanida, the pseudopetalichthyids had armor made up of a mosaic of tubercles. Like Stensioella heintzi, the pseudopetalichthyids' placement within Placodermi is suspect. However, due to a gross lack of whole, uncrushed, articulated specimens, there are no other groups that the pseudopetalichthyids could be, for a lack of a better word, pigeonholed into.
On the other hand, according to anatomical studies done on the crushed specimens that have been found, those experts who do regard the Pseudopetalichthyida as placoderms consider them to be a group more advanced than the ptyctodonts. And as such, the pro-placoderm experts consider Pseudopetalichthyida to be the sister group of the Arthrodira + Phyllolepida + Antiarchi trichotomy and the Acanthothoraci + Rhenanida dichotomy.
The best known, and aptly named species is Pseudopetalicthys problematica. Some experts have suggested that Paraplesiobatis heinrichsi is the same species as P. problematica, but, due to the specimens of the latter species being crushed in a different position than the specimen of the former species, the anatomies of the two forms appear different (Moy-Thomas & Miles). However, this problem will be resolved only with more specimens. The anatomy of the specimens of Nessariostoma granulosa are sufficiently different from the other two pseudopetalichthyids to be recognized as a separate species. Nessariostoma granulosa bears a superficial resemblence to Stensioella heintzi, and it was this resemblence that originally lead to the pseudopetalichthyids' grouping within Stensioellidae.
Links
Mikko's Phylogeny Archive [1]
References
Long, John A. The Rise of Fishes: 500 Million Years of Evolution Baltimore: The John Hopkins University Press, 1996. ISBN 0-8018-5438-5
Moy-Thomas, J. A. & Miles, R. S. Paleozoic Fishes (2nd ed)Chapman and Hall, London, 1971.
