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Gnathostomata | `--Acanthodii etc (if traditional placement) | Placodermi |?==Acanthothoraci |--Rhenanida `--+--Antiarchi `--+--+--Petalichthyida | `--Ptyctodontida `--+--Arthrodira `==Acanthodii |--Chondrichthyes `--Osteichthyes
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. We know the basic bodyplan of the rhenanids due to the whole fossils of Gemuendina stuertzi, and the basic skull anatomy due to the skulls of Jagorina pandora.
The Rhenanida were once thought to be, or at the very least, close to the ancestral placoderm, primarily on account of their piecemeal mosaic armor. Now, though, they are, coupled with their acanthothoracid ancestor, Radotina kosorensis, considered to be the sister taxon of the Antiarchs, due to similiarities of skull anatomies.
The paupacity of the rhenanids' fossil record does not necessarily reflect an actual scarcity of individuals, but, represents the poor ability of individuals to fossilize, as most fossils being isolated tubercles, called "ichthyoliths," that are identified as being rhenanid tubercles due to their similarities to the tubercles from the armor of Gemuendina stuertzi, the most well-known rhenanid. Most fossils of rhenanids are from the Early Devonian, primarily in the United States and Germany. The youngest known (and last) species of rhenanid was Bolivosteus chacomensis, of the Mid Devonian Malvinokaffric Fauna of Western Gondwana, in what is now Bolivia, South America.
There are four recognized species of rhenanids, in four species, Asterosteus stenocephalus, Gemuendina stuertzi, Jagorina pandora, and Bolivosteus chacomensis.
A fifth genus, Ohioaspis, is of questionable status, as the first specimens were originally described as being tubercles from a new species of Asterosteus, while later examinations 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. Stanton060924, modified MAK061002
|Gemuendina. Early Devonian of southeast Euramerica. Length 30 cm|
Rhenanida: Bolivosteus, Gemuendina, Jagorina.
Range: lwD(?) or mD-upD(?) of Eur & NAm.
Phylogeny: Placodermi::: Antiarcha + *.
Characters: Overall appearance very ray-like. Greatly flattened, jaw quite ray-like; gnathal plates with denticles; denticles on palatoquadrate (??); single skull roof plate and suborbitals; dorsal orbits and nostrils; single gill slit; small medial dorsal thoracic plate present; pectoral fin like rays, supported by wide basal and jointed radials; pectoral fins, body and most of head covered in small platelets. Probably marine.
Links: geologic; Gemuendina stuertzi.
References: Janvier (1996); Long (1995). ATW010626.
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