Was the final "-m" a "full-featured" consonant?

Question

Is there any solid evidence supporting or denying the hypothesis that in Classical Latin the syllable-final vowel -m (especially at the end of the word) was only an orthographic convention, but in spoken language it was used to make preceding syllable nasalized?

Some researchers suggest this was true, due to indirect evidence. For example, here someone bases their suggestion on measuring metrical properties of the words in poetry.

This is especially interesting for me because there are similar effects in Sanskrit (like the Dative case ending).

To me, this sounds like a nice start, but still, is there solid, documented evidence supporting or denying the idea of nasalization in this case?

Answer 1

The pronunciation of the letter m at the end of words isn't completely uniform in Classical Latin. W. Sydney Allen, in Vox Latina 30–31, lays out the evidence for several different ways the letter impacted the pronunciation of words.

"In general," Allen says, m at the end of words marked "a mere nasalization of the preceding vowel." He cites the imprecise descriptions of Velius Longus ("almost a foreign letter") and Priscian ("obscurum in extremitate dictionum sonat") as evidence for this, as well as inscriptions in which the final m is omitted:

honc oino ploirume cosentiont...
    [hunc unum plurimi consentiunt]
duonoro optumo fuise uiro
    [bonorum optimum fuisse uirum]

The final vowel preceding an m may have also been lengthened, as suggested by Cato the Elder's spelling of diem as diee.

Another piece of evidence for m not being a true consonant in this position is that syllables ending in m are elided in poetry as if they ended in a vowel, as Joonas's answer explains. Similarly, in cases of aphaeresis, the final m does not protect the syllable as other consonants would, so scriptum est becomes scriptust in inscriptions.

Interestingly, however, inscriptions and grammarians indicate that this treatment of the final m was not universal:

Where, however, a final m was followed by a closely connected word beginning with a stop (plosive or nasal) consonant, it seems to have been treated rather as in the interior of a word, being assimilated to the following consonant

So tam durum becomes tan durum in inscriptions, thanks to the d following the final m in tam. And in etiam nunc, "plenius per n quam per m enuntiatur," according to Velius Longus.

Summary

In general, the final m indicates nasalization, and possibly lengthening, of the final vowel, and it is not truly a consonant. However, preceding a stop consonant in the next word, it sometimes retained its consonantal properties.

Answer 2

The argument raised in the document you cite is good one. In classical Latin poetry elision is used frequently. As a rule, when a word ends in a vowel and the next word begins with a vowel, the vowel in the first word is not pronounced. In addition to this main rule, there are two additional things to note: Word-initial h and word-final m are ignored.

Exceptions to these rules are rare. The poetic meters used in ancient poetry were quite strict, so the elision rules are easy to verify by reading poetry (out loud if you will).

For the purposes of scansion, ending a word in -um (or with any other vowel) behaves exactly as ending a word in a long vowel. This is a good reason to believe that at least word-final -um was pronounced more like a long vowel than a vowel and a consonant.

Poetic considerations give no insight as to whether the long vowel was nasalized or not.