Heteractinida is an extinct grade of Paleozoic (Cambrian–Permian) sponges, sometimes used as a class or order. They are most commonly considered paraphyletic with respect to Calcarea (calcareous sponges),[1][2][3] though some studies instead argue that they are paraphyletic relative to Hexactinellida (glass sponges).[4] Heteractinids can be distinguished by their six-pronged (snowflake-shaped) spicules, whose symmetry historically suggested a relationship with the triradial calcarean sponges.[4]
The sponge-like Cambrian Hetairacyathidae may be related to heteractinids,[5] though most instead consider hetairacyathids to be closer to archaeocyaths and radiocyaths.[6]
From Treatise on Invertebrate Paleontology:[5]
Heteractinida is an extinct grade of Paleozoic (Cambrian–Permian) sponges, sometimes used as a class or order. They are most commonly considered paraphyletic with respect to Calcarea (calcareous sponges), though some studies instead argue that they are paraphyletic relative to Hexactinellida (glass sponges). Heteractinids can be distinguished by their six-pronged (snowflake-shaped) spicules, whose symmetry historically suggested a relationship with the triradial calcarean sponges.
The sponge-like Cambrian Hetairacyathidae may be related to heteractinids, though most instead consider hetairacyathids to be closer to archaeocyaths and radiocyaths.
