The Saturnian was (probably) stress-based, not weight-based.
To borrow from another answer of mine:
In a question about Old Latin meters, an anonymous user brought
up Mercado's convincing argument that the Saturnian was based on
accent. The idea isn't new, but Mercado backs it up with some nice
information-theoretical analysis: basically, the accent theory passes
some statistical tests that all the quantity theories I've seen fail
His idea is that a Saturnian line generally resembles one of the
'⏑'⏑|⏑'⏑ || ⏑'⏑|⏑'⏑
'⏑'⏑|⏑'⏑ || '⏑⏑|⏑'⏑
' marks a stressed syllable and
⏑ an unstressed one. For
example, the Epitaph of Naevius:
Ímmortálēs mortálēs / si fóret fas flére,
flérent dévae Caménae / Náevium poétam:
ítaque póstqu' est Órchi / tráditus thesáuro
oblítī sunt Rómae / lóquier línguā Latínā.
If gods could weep for mortals, the divine Muses would mourn the poet Naevius: after he was delivered to the vault of Hades, Rome
forgot how to speak Latin.
In other words, according to Mercado, the Saturnian meter didn't depend on vowel length or syllable weight at all, except insofar as they change the position of the stress within each word. The pattern involved stressed versus unstressed syllables, somewhat like English poetic meters.
In the Greek-derived system, on the other hand, the pattern was built out of heavy and light syllables, with a syllable's "weight" depending on vowel quantity and coda consonants. This was much better suited for the Ancient Greek language, which had quite a lot of single-syllable particles that could be inserted anywhere into a phrase; Latin didn't have these, so the system didn't fit it nearly as well.