Rhenanida
Encyclopedia
Rhenanida 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.
, 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.
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 ichthyolith
s 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
agnatha
ns.
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
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
Batoidea
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...
-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 stuertziGemuendina stuertzi
Gemuendina stuertzi was an early placoderm species of the order Rhenanida, of the seas of Early Devonian Germany. Gemuendina resembled a scaly ray with a pair of staring eyes, a pug-nose, and an upturned mouth. This leads virtually all artists who reconstruct it to give the creature a quizzical,...
, 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
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
, 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
Morocco
Morocco , officially the Kingdom of Morocco , is a country located in North Africa. It has a population of more than 32 million and an area of 710,850 km², and also primarily administers the disputed region of the Western Sahara...
. The youngest known (and last) species of rhenanid was Bolivosteus chacomensis, of the Upper Devonian Malvinokaffric Fauna of Western Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
, in what is now Bolivia, South America.
Species
There are four recognized speciesSpecies
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of rhenanids, in four genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
, Asterosteus stenocephalus, Gemuendina stuertzi
Gemuendina stuertzi
Gemuendina stuertzi was an early placoderm species of the order Rhenanida, of the seas of Early Devonian Germany. Gemuendina resembled a scaly ray with a pair of staring eyes, a pug-nose, and an upturned mouth. This leads virtually all artists who reconstruct it to give the creature a quizzical,...
, Jagorina pandora, and Bolivosteus chacomensis.
A fifth genus, Ohioaspis, is of questionable status, as the first specimens were ichthyolith
Ichthyolith
An ichthyolith is any disarticulated remains of a fish found in the fossil record, most often a scale, denticle or tooth....
s 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
Ostracoderm
Ostracoderms are any of several groups of extinct, primitive, jawless fishes that were covered in an armor of bony plates. They belong to the taxon Ostracodermi, and their fossils are found in the Ordovician and Devonian Period strata of North America and Europe...
agnatha
Agnatha
Agnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes....
ns.