Much to students' annoyance, nouns ending in -us can belong to either the second (servus), third (tempus), or fourth (circus) declensions. I understand the origin of the second and fourth: Proto-Indo-European stems in -o- vs -u-. But the third-declension -us nouns are less predictable:
- tempus → temporis (also corpus, decus, nemus)
- latus → lateris (also genus, funus, scelus)
- jūs → jūris (also crūs, pūs, rūs)
The last of these makes sense to me: the root was originally jūs, and the intervocalic s became r in pre-classical Latin (cf flōs, flōris).
But for the other two patterns, why sometimes e and sometimes o? The two patterns seem very distinct: even nouns of uncertain declension, like penus, -ī/-ūs/-oris, never interchange -eris and -oris forms. What led to this distinction?