Laudavēre is an (apparently older) alternative to laudaverunt. What is the origin of this ending? Is it connected with any other known endings or affixes?
Clackson & Horrocks say it is from an alternative 3rd-person plural ending -er plus a "primary marker" -i. I don't know this marker: I only know the primary paradigma as something resembling -m/s/t/mus/tis/nt. What is this marker about?
Palmer seems to be saying it contains the "impersonal" -r- also found in the infinitive (laudare) and in passive forms (laudatur). He, too, speaks of a primary ending -ri.
Both sources agree that -ĕrunt is from the perfective suffix -is-, found in laudav-is-se and laudav-is-ti(s) (and related to the -s- found in the Greek sigmatic aorist). The regular 3rd-person ending -ont was stuck on as expected. The suffix -is- (was) then rhotacized into -ir-, which in turn became -er-.
This would make me suspect that -ere also contains this rhotacized suffix -is-, with some mysterious suffix -e at the end. Neither of the sources mentioned seems to allow for this interpretation, but they don't fully explain -ere either: is this at all possible? Or are there any other theories?