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Evolution of irregular echinoids

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wastedshadows's version from 2016-01-24 11:59

Section

In the jurassic all echinoids were epifaunal , but since then some have evolved to live infaunally.
Since the jurassic
The test have flattened and elongated
The size of the tubercles and spines have decreased
but the number of spines and tubercles has increased
The periprocot (anus) has shifted posteriorly
The peristome (mouth) has shifted anteriorly
The size of the peristome has decreased

 

sea urchins --------------------------------------> sand dollars
Jurassic --------------------------------------> Present