Taxonomy and Evolution Taxonomy and Evolution


 
Agnatha and Gnathostomato are the two super-classes of vertebrates, despite the evolution of these species being closely linked they both have very different characteristics.
 
Agnatha
 
There is a small amount of early remains to suggest that Agnatha species evolved during the Cambrian era however more substantial evidence arose during the Ordovician era. The super-class demonstrated their strongest development and abundance during the Silurian and lower Devonian era (Nelson, 1984).
 
Scientifically there is much debate suggesting whether or not Agnatha species are actually vertebrates as they do not obtain a true vertebral column as known in the Gnathostomato species. There is also no presence of jaws or paired fins (Nelson, 1984).
 
Currently there are three extant (living) class of Agnatha including Myxini, Cephalaspidomorphi and Petromyzontidae of which include around 100 species in total (Ubio,2009). Some of these species include lampreys and hagfish.
                       

 
 
Gnathostomata
 
(Note this section refers to fish (Pisces) and not tetrapods)
 
Unlike Agnatha, species of Gnathostomata fish have jaws and vertebral columns and the majority have pair fins. The first evidence of Gnathostomata fish can be found during the Silurian era. This class of species have evolved successfully, so much so, it is estimated that there is over 21 650 extant species of jawed fishes.
 
 
Teleost Fishes
 
Teleost fishes are a sub division from the Osteichthyes subclass which from apart of the Bony fish class (Nelson, 1984). Teleost dominate the majority of freshwater and marine species as well as accounting for almost half of all the vertebrates. They are the most abundant of all freshwater fishes however only form around 40 % of this group (Wootton,1990).
 
Teleost evolved around 70 millions ago over a period of around 115 millions years within the Mesozoic era. There adaptive capability is vast, allowing them to inhabit a variety of different environments from high altitudes to deep oceans and a whole spectrum of varying temperatures (Wootton,1990). Adaptability to changing environments can be a good indicator of success. A prime example of this is there use of homeostatic mechanisms (ability to regulate a constant internal temperature) to maintain temperature consistency. Being Ectothermic means that they are the temperature of the surrounding environment, however they are able to use thermoregulatory behaviour to detected changes in thermal gradient so when moving through it they stay in a relatively narrow temperature range (Wootton,1990).

 

References
 
Nelson, J.R (1984) Fishes of the World, 2nd Edition, USA: John Wiley & Sons
 
Wootton, J.R (1900) Ecology of Teleost Fishes, Fish and fisheries series 16, London: Chapman & Hall