Fossil History And Evolutionary Features

Being a gastropod, T. niloticus originally evolved from a monoplacophoran ancestor (Ruppert, Fox and Barnes, 2004). This occured when the monoplacophoran underwent a 180 degree twist of the visceral mass known as torsion, which is a synapomorphy of all gastropods. The fossil record for the gastropods begins in the early Cambrian period on continues unbroken until the present (Ruppert, Fox and Barnes, 2004). During this initial period they underwent drastic adaptive radiation, with species adjusting to life on land and in freshwater, as well as the marine environment the originated in. Scientists estimate that there are around 15,000 known fossilized gastropod species (Ruppert, Fox and Barnes, 2004). The coiled, recognizable shell is a result of a series of processes: the elongation of the shell to make a retreat for the animal, planispiral coiling, torsion, flexure of the gut to make it U- shaped to accommodate torsion, conispiral coiling and asymmetry (Ruppet, Fox and Barnes, 2004). 

 T. niloticus is of family Vesitgastropoda, also known as Archaeogastropoda (Ruppert, Fox and Barnes, 2004). This is a primitive family, which have a simple ridge osphradium and two nephridia. Oshpradium are thought to filter water of silt before it enters the gills, although scientists are still not entirely sure. Nephridia function like kidneys, removing metabolic waste from the animals circulatory system. The family Trochidae also have two atria only a left bipectinate gill (Ruppet, Fox and Barnes, 2004). Bipectinate gills are also called the ctenidium and is a respiratory structure made up of comblike filaments connected to a central axis. An oblique ventilating current circulates through this bipectinate gill. They also have an operculum, which is attached to the top of the foot at the rear, which is a primitive feature lost in terrestrial snails (Ruppet, Fox and Barnes, 2004).