Case & Cappetta (1997, pp. 154-5, text figs. 10a-f, pl. 8, figs. 3a-e) erected Ewingia problematica based on some bizzare placoid scales (broadest definition) from the Kemp Clay (Maastrichtian), Texas. They considered them oral teeth and based on the highly unusual 'root'-design were conflicted as to whether to include it to a raji- or myliobatiform.
In viewing this design, it is difficult to understand their confidence in considering it an (oral) tooth. The cusp lacks an occlusal orientation and the thickness that might be expected for a crushing tooth; as an oral tooth, it would likely be associated with a filter-feeding dentition (they suggested a possible mobulid). Hartstein et all (1999:18) report an Ewingia problematica rostral spine from the Severn Fm, (Maastrichtian) of Maryland and Becker et al (2004:788, fig. 5q-u) included the design (with lateral cusplets) as an oral tooth from the Fox Hills Fm. (Maastrichtian) of South Dakota. These specimens strike me as equally likely to represent dermal/rostral denticles.
A single specimen has been recovered from reworked Late Cretaceous (probably Peedee Fm. - Maastrichtian) North Carolina sediments; this better helps to complete a range that extends from the Chesapeake to Texas and far into the 'Interior Sea'.
Becker, M., Chamberlain, J. and Terry, D., 2004. Chondrichthyans from the Fairpoint Member of the Fox Hills Formation (Maastrichtian), Meade County, South Dakota. Journal of Vertebrate Paleontology, 24(4):780-793,
Case, G. and Cappetta, H.. 1997. A new selachian fauna from the late Maastrichtian of Texas. Münchener Geowissenschaften Abhandungen 34:131-189.
Hartstein, E., Decina, L. and Keil, R., 1999. A Late Cretaceous (Severn Formation) Vertebrate Assemblage from Bowie, Maryland. The Mosasaur, VI:17-23.