Middle Cambrian – Lower Permian, ?Upper Permian
Modiomorphida is an extinct order of stationary semi-infaunal suspension feeders.
Characteristics of the Order
- Shell microstructure unknown; generally unornamented.
- Otherwise possessing characters of subclass, as this heterogeneous, poorly understood assemblage is grouped together for convenience.
Treatise on Invertebrate Paleontology, Part N, Vol. 1 of 3 (1969):
- Extinct Paleozoic marine ovoid bivalves; shell microstructure unknown; generally unornamented, otherwise possessing characters of subclass. This is somewhat heterogeneous, poorly understood assemblage is grouped together for convenience. Clearly, it includes ancestral radicals of many major lines.
- (Subclass – Palaeoheterodonta)Equivalve, with closed margins and, where preserved, prismatonacreous shells; ligament amphidetic or poisthodetic, external and parivincular; entition generally consisting of few teeth more or less radial and divergent from umbonal region, striated in some genera, rarely becoming taxodont in few genera (certain Unionoida); where lateral teeth occur, they originate below beaks and are not separated from cardinals by edentulous interval, as in Heterodonta. These forms include the actinodonts (earliest known Bivalvia), Trigonioida and Unionoida. They lead in various lines to the Pteriomorphia, Heterodonta, and possibly to the Mytiloida early in the Paleozoic.