Banjo shark

Taxobox
name = Banjo shark


status = LC | status_system = IUCN3.1
regnum = Animalia
phylum = Chordata
classis = Chondrichthyes
ordo = Rajiformes
familia = Platyrhinidae
genus = "Platyrhinoidis"
species = "P. triseriata"
binomial = "Platyrhinoidis triseriata"
binomial_authority = (Jordan & Gilbert, 1880)
synonyms = The banjo shark, california thornback, guitarfish, round skate, shovelnose, thornback ray, or thornback ("Platyrhinoidis triseriata") is a species of ray in the Platyrhinidae family. It is the only member of the genus "Platyrhinoidis". It is found in Mexico and the United States. Its natural habitats are shallow seas, subtidal aquatic beds, estuarine waters, and intertidal marshes.

tree of life

Along with the Banjo Shark in the class chondrichthyes are sharks, sawfish and chimaeras. Upon a phylogenetic tree the chondrichthyes are shown to be most closely related to Placoderms, (an extinct class of vertebrates who were among the first to evolve jaws) and their most related living class are the Osteichthyes (bony fish).
The "Osteichthyes", "Chondrichthyes", "Placoderms", "Sacropterygii" and "Acanthodii" are all members of the superclass "Gnathostomata". This group is believed to have evolved around 365 million years ago during the Paleozoic period (542-257 million years ago).
More specifically, the Banjo Shark is believed to be closely related to the species "Plotoplayrhina" and "Cyclarhus", extinct relatives that are similarly structured and fossils show they look similar. It is also closely related to the existing genus, "Rhinobatidae".
It is believed that the rays (which the Banjo Shark is one of) evolved from sharks around 200mya.The link which all the chondrichthyes share is that they are cartilaginous, meaning that rather than being boney, they contain cartilage.It is believed this group of chondrichthyes must have evolved from a common ancestor, but the common ancestor has "yet to be discovered".

Homologous structure

The banjo Shark (thornback guitarfish) has a homologous feature to its close relative the shark in that both organisms have pectoral fins located along either side of their body, although the sharks fins do appear smaller in bodily proportion then that of the thornback.
This similar feature is used uniquely by the two creatures. The thornback uses its pectoral fins by flapping and moving them in a wave like motion in order to glide through the water, where as the shark uses them to steer and for lift.
By using the pectoral fins (common structural feature) in separate ways, a homology is shown between the shark and thornback.

Analogous structure

The flattened body of the Banjo shark is analogous to that of the flounder. Both organisms feature a similar but not completely the same flat body which allows them to sit on the bottom of the sea bed under sand and earthy elements unnoticed by prey and predators. The eyes on top of their heads also are an analogous feature which adaptively allows each organism to subtly spot prey. The Banjo shark and Flounder are not related but have both evolved these features along separate evolutionary lines.

References

* Carlisle, A.B. & Garayzar, C.V. 2005. [http://www.iucnredlist.org/search/details.php/60112/all Platyrhinoidis triseriata] . [http://www.iucnredlist.org 2006 IUCN Red List of Threatened Species. ] Downloaded on 03 August 2007.
http://www.tolweb.org/Gnathostomata/14843
http://www.reference.com/browse/Gnathostomata
http://www.discoverlife.org/mp/20q?search=Platyrhinoidis+triseriata
http://www.tolweb.org/Gnathostomata/14843
http://www.helsinki.fi/~mhaaramo/metazoa/deuterostoma/chordata/chondrichthyes/elasmobranchii/squalea/rajoidei/rhinobatidae.html#Platyrhinidae]
http://en.wikipedia.org/wiki/Winter_flounder
http://www.montereybayaquarium.org/efc/living_species/default.asp?hOri=1&inhab=134
http://www.fcps.edu/westspringfieldhs/academic/science/bjewell/ocean/adapt/flound/flounder.shtml