Tesauro, tesouro, thesaurus

El Tesauro, que se publicará este otoño, según informó hoy a EFE una portavoz de la editorial, abarca casi todo el vocabulario de la lengua inglesa, desde el Old English (Inglés Antiguo) hasta la actualidad.

I had never seen tesauro in Spanish and thought it was Spanglish, but it is indeed in the dictionary, which calls it dated and refers you to tesoro (the usual word for treasure). One of the definitions of tesoro is indeed that of a thesaurus. Both English thesaurus and Spanish tesoro/tesauro come from Latin thesaurus. I was then curious to know if in Portuguese there was the word tesauro, but I didn't find anything, and among the definitions of tesouro (the usual Portuguese word for treasure), there's nothing related to thesauruses or thesauri (I don't like the latter plural in English). Anyway, in Brazil we normally say dicionário, enciclopédia or sometimes léxico, but a thesaurus is a different kind of dictionary, which I've never seen on the Brazilian market, for which I can't think of an adequate translation in Portuguese. I think the closest we have to a thesaurus are dicionários de sinônimos e antônimos.

I've just checked Portuguese Wikipedia, which has the entry tesauro (but that word is not yet in the dictionary, as I wrote above). It says it's also known as dicionário de ideias afins, which kind of convinces me, but I'm still doubtful whether a dicionário de ideias afins is as comprehensive as a dictionary.

I think what accounts for the emergence of tesauro in Portuguese and Spanish is the English word (and English-language, particularly American, editorial practices). If we were to let Latin evolve naturally into Portuguese and Spanish, we'd have tesoro (Spanish), tesouro (Portuguese) for the thesaurus sense, since Latin words with au turned that diphthong into o and ou in Spanish and Portuguese, respectively. A couple of examples are Latin aurum - Spanish oro, Portuguese ouro; Latin taurus - Spanish toro, Portuguese touro; Latin laurus - Spanish loro, Portuguese louro.