A while ago I posited the idea that the paradigm for 'foot' in Indo-European might have a phonotactic schwa insertion. Glen Gordon spook out against this, saying it was paradigmatic levelling rather than a phonotactic constraint. I didn't want to believe him, simply because phonotactic constraits are so much nicer than paradigmatic levelling, but I found some examples which shows that he's probably right.
So let us look at the paradigm for 'foot'. We have the root *ped- With a paradigm like this:
The ō in the nominative is due to mono-syllabicity of the word. The vocalism of this word is then perfectly acceptable.
The genitive though is slightly more problematic. From the idea of syncope, we'd rather see a word like **pd-ós, which we clearly don't. My idea was then that a *e was inserted (which was previously a schwa), due to phonotactic constraints, after all /pd/ is a rather difficult cluster to pronounce initially, even Greek, known for it's sometimes rather strange initial clusters doesn't allow such a cluster. Glen Gordon argued that this is not so much a phonotactic constraint, but rather the tendency of Indo-European to avoid asyllabic roots alternating with syllabic roots in one paradigm.
At first I did not see why one would assume this, and then I suddenly remembered the root *peth₁- 'to fly'. This root is well attested in many languages, including English (feather). But the Greek word πτερόν 'wing' is the one I'd like to talk about. As we can see, Greek has a perfect zero grade in the root for this word, a Indo-European reconstruction of this word would be:
By the phonotactic constraint I first proposed such a cluster would be forbidden. I would expect a form **peth₁ró-m, so I have to re-evaluate my theory. I'm not quite feeling into giving into Glen's theory yet, and would like to change my theory a bit;
When there's a initial cluster of two consonants of which one of the two elements is voiced, a schwa is insterted between the two.
I believe Greek agrees with me on this one, since there's no clusters like bd, gd, pd, bt etc. to be found in the Greek language initially. Nevertheless I'm going to have to search for words to disprove me again; If you're feeling in a particularly productive mood, I encourage my readers to do the work for me!
Just a little side not to finish this post of, it might be interesting to know that Sanskrit in fact does have the phonotactic constraint that I posited the first time, the word for 'wing' in Sanskrit is patram पत्रम्. Which is a problematic word as well since it brings up some of Sanskrit's sporadic laryngeal vocalisation, but we'll leave that for some other time.
Sanskrit does allow such clusters medially though. The third person plural perfect in Vedic Sanskrit of patati पतति 'to fall/fly' is paptuḥ पप्तुः While in Classical Sanskrit by a bizarre analogy from sad- 'to sit' it became petuḥ पेतुः. To sit in 3ppl perf. is seduḥ < *sazduḥ < *sasduḥ. This e-vocalism rather than a medial CC cluster was generalised to all reduplicating perfects that have identical initial reduplication (Labials, dentals and fricatives). Seems like Sanskrit wasn't particularly fond of the medial zero-grade roots after a while either.