Latin tardus

From: Andrew Jarrette
Message: 45142
Date: 2006-06-27

My Webster's New World College Dictionary of English, which contains IE etymologies of English words, says that Latin <tardus> (> English <tardy>) is "probably" from the IE base *ter- "delicate, weak" which > Greek <tere:n>, "tender".  My question is how Latin /a/ would develop from a root with *e in this position.  If /a/ in <tardus> is not derived from an original *e, then what is the origin of /a/ in this Latin word?  This dictionary similarly derives Latin <mari:tus> "probably" from  the IE base *meri "young wife", akin to *meryo-, "young man", in Sanskrit <marya-> "man, young man, suitor".  If this is the basis of <mari:tus>, how would Latin /a/ derive from IE *e?  I believe there are a significant number of other Latin words in this dictionary with /a/ held to be from *e.  Under what conditions would IE *e become /a/ in Latin?  Or is this dictionary wrong?  Do words such as these suggest that there actually was an *a in IE, not only due to *h2e or *e after *k, *g, *gh?  Could this supposed change of *e > /a/ account for the /a/ in <maneo>? (And could this account for cases of /a/ in Celtic as well (ref. my earlier question on the origin of Celtic /a/)?) 
I mention this issue in connection with the discussion about Latin /a/ from *o after labials.
 
Andrew