Rhenanida

Rhenanida ("Rhine (fish)") was an order of primitive, lightly armored (relatively speaking) placoderms. Unlike most other placoderms, the rhenanids' armor was made up of a mosaic of unfused scales and tubercles. This "mosaic" corresponds to the plates of armor in other, more advanced placoderms, suggesting that the ancestral placoderm had armor made of unfused components, as well.

All rhenanids were flattened, ray-like, bottom-dwelling predators or shellfish-eaters that lived in marine environments.

Evolution

The rhenanids were once presumed to be the most primitive, or at least the closest to the ancestral placoderm, as their armor was made up of a mosaic of tubercles, as opposed to the solidified plates of "advanced" placoderms, such as antiarchs and arthrodires. Through comparing the skull anatomies of Jagorina pandora with those of antiarchs, the rhenanids are considered to be the sister group of the antiarchs (together with their respective Acanthothoracid relatives).

Presence in the fossil record

Rhenanida's fossil record is very sparse, with most fossils being isolated tubercles that are identified as being similar to the tubercles from the armor of Gemuendina stuertzi, the most well-known rhenanid. Given the rhenanids' worldwide distribution, this paupacity probably did not reflect a scarcity of living individuals (when they were alive), but reflects the fact that rhenanid armor disintegrated into isolated fragments, and scattered soon after the owner's demise. Most fossils of rhenanids are from the Early Devonian, primarily in the United States and Germany. Asterosteus stenocephalus is known from Mid Devonian Germany and eastern United States, and Jagorina pandora is known from Upper Devonian Germany and Morocco. The youngest known (and last) species of rhenanid was Bolivosteus chacomensis, of the Upper Devonian Malvinokaffric Fauna of Western Gondwana, in what is now Bolivia, South America.

Species

There are four recognized species of rhenanids, in four genera, Asterosteus stenocephalus, Gemuendina stuertzi, Jagorina pandora, and Bolivosteus chacomensis.

A fifth genus, Ohioaspis, is of questionable status, as the first specimens were ichthyoliths that were originally described as being tubercles from a new species of Asterosteus. Later examinations these tubercles have led to the formation of two camps of experts, one of which that believe the three recognized species of Ohioaspis were rhenanids, while the other suggests that they were actually some sort of ostracoderm agnathans.

Timeline of genera

References

• Paleos Rhenanida

References

• Janvier, Philippe. Early Vertebrates Oxford, New York: Oxford University Press, 1998. ISBN 0-19-854047-7
• Long, John A. The Rise of Fishes: 500 Million Years of Evolution Baltimore: The Johns Hopkins University Press, 1996. ISBN 0-8018-5438-5
• Sepkoski, Jack (2002). "A compendium of fossil marine animal genera (Placodermi entry)". Bulletins of American Paleontology 364: p.560. http://strata.ummp.lsa.umich.edu/jack/showgenera.php?taxon=564&rank=class. Retrieved 2011-05-20.