Perischoechinoidea
Encyclopedia
Perischoechinoidea is a subclass of primitive sea urchin
s that were abundant in the Palaeozoic seas. However, the great majority of species died out during the Mesozoic
, as the more advanced euechinoid
sea urchins became common. Today, only a single order, the Cidaroida
, survives.
Most fossil forms had multiple columns of ambulacral plates, rather than the two rows found in all living species of sea urchin. They also lacked a perignathic girdle around the mouth.
.
Subclass Perischoechinoidea
Sea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...
s that were abundant in the Palaeozoic seas. However, the great majority of species died out during the Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
, as the more advanced euechinoid
Euechinoidea
The subclass Euechinoidea includes almost all living species of sea urchin, and fossil forms going back as far as the Triassic....
sea urchins became common. Today, only a single order, the Cidaroida
Cidaroida
Cidaroida is an order of primitive sea urchins, the only living order of the subclass Perischoechinoidea. All other orders of this subclass became extinct during the Mesozoic, and were even more primitive than the living forms....
, survives.
Most fossil forms had multiple columns of ambulacral plates, rather than the two rows found in all living species of sea urchin. They also lacked a perignathic girdle around the mouth.
Taxonomy
The group is probably a paraphyletic assemblage of stem forms, united only by their lack of more advanced features, rather than a true taxonomic cladeClade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
.
Subclass Perischoechinoidea
- Order CidaroidaCidaroidaCidaroida is an order of primitive sea urchins, the only living order of the subclass Perischoechinoidea. All other orders of this subclass became extinct during the Mesozoic, and were even more primitive than the living forms....
- Order Bothriocidaroida †
- Order Echinocystitoida †
- Order Megalopoda †
- Order Palaechinoida †