Lower Ordovician – Lower Carboniferous
Petrocrania is an extinct genus of stationary, epifaunal suspension feeders.
Identification in Hand Sample:
- Encrusting brachiopod commonly found on surface of other shells
- Circular, small and very thin
- Ornamentation of underlying shell visible through Petrocrania
- Sometimes, concentric growth lines visiblie
- Composed of calcareous shell
Map point data provided by iDigBio.
- Dorsal valve convex to subconical; beak subcentral to posterocentral;
ornament only of concentric growth laminae or simulating morphology of the host;
dorsal posterior adductor scars typically larger than anterior scars, but not
exclusively; vascular lateralia commonly prominent, sigmoidal; valve margin not
thickened, limbus narrow when developed ; encrusting; ventral valve not known.
Fossils of Ohio (1996):
- A small to medium sized- acrotretid brachiopod. It has a circular outline and
a calcareous shell. This brachiopod commonly is found attached to other types of
shells, such as that of the articulate brachiopod Rafinesquina. Because the shell of
Petrocrania is very thin, the ornament of the underlying surface to which this
brachiopod has attached is commonly visible. Petrocrania is recorded from the
entire Cincinnatian Series and also is found in Silurian and Devonian rocks of Ohio
- Exterior of brachial valve slightly irregular, subquadrate and suboval with
flattened posterior margin, 82% as long as wide (mean for 4 valves). And 26% as
deep as long (mean for 3 valves) with maximum depth at about mid-point; surface
with sporadic concentric growth lines, otherwise smooth; shell with thickened
slightly averted margin corresponding to limbus about 1 mm wide.
- Interior of brachial valve with strongly impressed suboval postero-lateral
adductor scars smaller than suboval elevated anterior adductor scars situated
submedially up to 60% forward of the posterior margin and ornamented by fine
parallel wavy ridges; mantle canal system pinnate with variably impressed
sigmoidal vascular lateralia.
Caster, Dalve, & Pope (1961):
- The inarticulate brachiopods are distinguished from the articulate forms by the absence of a hinge, a different musculature, and shells of calcium phosphate-chitin rather than calcium carbonate. In the Cincinnatian these may be frequently seen as thin, oval shells adhering to larger brachiopod shells (Petrocrania, Schizocrania), as chalky shells with minute punctae (pores) in the valves (Trematis) or as glossy, triangular shells (“Lingula”). Since the inarticulates appear in the Cambrian before the articulates, they are considered the evolutionary precursor of the articulates.