“Many brachiopods of the superfamily Strophomenoidea,including R. alternata, lacked a functional pedicle as adults and so lived unattached to the substrate.Free-lying brachiopods may have been subject to greater risk than attached brachiopods from transport by currents or from burial in soft sediments. Many workers have interpreted the external morphology of these brachiopods as an adaptive response to the free-lying condition. Rather than possessing the biconvex shape common to most other brachiopods, strophomenoids have one convex and one concave valve. In the case of R. alternata, the pedicle valve is convex and the brachial valve concave.”
Small, compressed geometries of R. alternata dominate the Eden and “Lower Waynesville” samples and are correlated with deeper, quiet water substrates. Large, globose, convex forms which dominate the “Bellevue” sample inhabited shoaled, agitated environments. Geniculate forms which prevailed among the “Arnheim” populations were associated with high sedimentation rate habitats. Alation increased progressively through the stratigraphic sequence of samples and indicated morphologic adaptation to either muddy or agitated substrates. Flume experiments demonstrated the greater physical stability of alate-geniculate forms in contrast to plano-convex, ovate shapes under increased current velocities.
Size of R. alternata varied directly with the energy of the environment. Consequently, deeper water inhabitants were characteristically
small whereas shoaled-water dwellers attained greater size.Convexity varied directly with sedimentation rates and geniculate forms of R. alternata lived in habitats periodically subjected to intensified sedimentation rates.
A common and variable species commonly around 40 mm. in width, length, 0.8 width. Cardinal angles rectangular to acute. Convexity variable. Striae alternating in size commonly one coarse to several fine ones.
Common throughout the Trenton-Richmond formations.
Under this name it is customary to group a great variety of shells rather large for brachiopods; both valves curved in the same direction leaving little space between them; pedicle (ventral) valve convex even at the beak; brachial (dorsal) valve concave; surface marked by numerous radiating striae, alternating in size, often with groups of three or four finer striae between the larger coarser ones, except toward anterior margin, where alternation in size is more common. Interior of pedicle (ventral) valve exhibits a large muscle scar, slightly but distinctly impressed in inner surface of shell. This impression somewhat radiately striated, but much larger in size than in Strophomena, often exceeding half the length of the shell. Contrasted with Strophomena, ventral valve distinctly convex along its anterior border, whereas in Strophomena it would appear concave here. In a corresponding manner, brachial (dorsal) valve of Rafinesquina distinctly concave anteriorly, instead of somewhat strongly convex.
Rafinesquinidae vary considerably in size, outline, convexity, degree of geniculation anteriorly, coarseness of striation, and other features. Eventually, the genus will be divided into a number of species and varieties.
In upper Ordovician rocks of Ontario and Quebec the flat forms are most common, the more strongly convex forms rare; for instance, in Proetus zone at Chambly Canton.
Locality and Horizon. Nicolet River section where it ranges from the top of the Proetus zone of the Lorraine to the Waynesville; in the Leptaena or Cryptolithus zone at Petite Caroline; in the Proetus zone in Chambly Canton, the mouth of Huron river, and at Vars; in Pholadomorpha zone in the Nicolet RIver section, and at St. Hilaire, in the so-called Lorraine at Weston, and 3 miles south of Little Current; in the Waynesville on Snake island, in the Nicolet River section, on Huron river at St. Hilaire, in loose blocks near St. Hugues, at Vars, STreetsville, Oakville, near Meaford, at Clay cliffs, 3 miles south of Little Current, and near Kagawong. It is comparatively rare at the Cryptolithus and Leptaena horizons in Ontario, and not known in the Kagawong member on Manitoulin island.