It is impossible for minimal case systems to include, in addition to the basic general case, only a single further case exclusively marking noun phrases in construction with (adverbial/grammatical) adpositions.
Standardized
IF there is a case exclusively marking noun phrases in construction with adpositions, THEN there will be still further cases in addition to a basic general case.
It follows that a case governed by a postposition cannot be the first non-basic case in the rise of a case system in any language, nor the last non-basic case in the demise of case systems. The case discussed by Plank is English genitival s: on the assumption that it is a postposition (rather than a case suffix), the question arises why genitival s unlike plural s ceased to govern a segmental voicing alternation of certain nouns (wife / wive-s). Plank’s suggestion is that such voicing would have had the status of a case marking governed by a possessive postposition s.
It follows that a case governed by a postposition cannot be the first non-basic case in the rise of a case system in any language, nor the last non-basic case in the demise of case systems. The case discussed by Plank is English genitival s: on the assumption that it is a postposition (rather than a case suffix), the question arises why genitival s unlike plural s ceased to govern a segmental voicing alternation of certain nouns (wife / wive-s). Plank’s suggestion is that such voicing would have had the status of a case marking governed by a possessive postposition s.