Middle Jurassic (Bajocian) to Upper Cretaceous (Cenomanian).
|Classification and/or Status||Irregularia, Microstomata, Neognathostomata stem group
Possibly a paraphyletic grade
The Nucleolitidae Agassiz & Desor, 1847 as used by Kier (1962) is a plesiomorphic grade in the stem group Neognathstomata, and is here very much restricted in concept. Petalobrissus is identical in most respects, but differs by having single pores beneath the petals. Apatopygidae are more derived in having both single pores beneath the petals and pyrinoid plates in their phyllodes. The most primitive representative of this group is Pronucleolites.