In Syntax of Plautus, W. M. Lindsay writes:
But the earliest form of the Future Infinitive Active, which still survives in some lines of Plautus and has probably been removed by scribes from more, shows merely -urum (indeclinable) without esse, e.g. Cas. 693 “altero te occisurum ait” ‘Casina says she will kill you with one of the two swords.’ This points to an Impersonal Future Infinitive Active, just as we have an Impersonal Future Infinitive Passive, and just as the Gerund (e.g. agitandum est vigilias) was superseded by the Gerundive (e.g. agitandae sunt vigiliae).
I am curious about this older indeclinable form of the future active infinitive. Are there other scholarly works that describe it in greater detail? Have we learnt anything more about it in the century that has passed since Lindsay so briefly described it? What are its origins? Can it be used in ways that the usual form (future active pariticple + esse) cannot? Are there enough citations to even entertain answering that question?
This is a rather general question; I'm really just looking for whatever further information there may be out there on the indeclinable future active infinitive ending in -urum.
Yesterday I found the following quote in the same text, which seems to confirm that speakers of Old Latin did not regard the future active participle to be a 'true' participle:
The independent use of the Fut. Partic., e.g. moriturus te saluto, is unknown to the early Latin writers. The first certain example appears in a fragment of a speech of C. Gracchus (ap. Gell. 11, 10, 4) qui prodeunt dissuasuri (See Sjogren 'Futurum im altlateinischen' pp. 225 sqq.).
This seems to indicate a progression from an indeclinable future active infinitive (occisurum), to a declinable periphrastic future active infinitive (occisurus, -a, -um + esse), and finally to an independent declinable future active participle (occisurus, -a, -um). Perhaps there was an intermediary stage between the first and the second where the indeclinable form became declinable to the accusative subject, followed by the analogical insertion of esse; if this was the case, then it would have briefly fit into the construction of accusativus cum praedicativo before being 'ported over' to the accusativus cum infinitivo.