Initial pl-,fl- in Basque

As it's well known, Basque abhors "muta cum liquida" clusters and thus we've got boronde, boronte < Latin fronte- and p(h)ereka, fereka 'stroke' from Romance *freca < Latin fricāre 'to rub'. 

There's however a number of words where Latin pl-, fl- have given Basque l-: 

Latin plācet 'it pleases' > Basque laket 'pleasure'

Latin plānu- 'flat' > Basque lau(n), leu(n), legun 'flat, smooth'
Latin plūma- 'feather' > Basque luma id.

Latin flamma- 'flame, fire' > Basque lama 'flame, heat'
Latin flore- 'flower' > Basque lore id.

The evolution fl- > l-¹ can also be found in Spanish words such as lacio < *flaccidu- 'flacid' and the personal name Laín < Flavinus. IMHO this could be explained if these labials were realized as a voiceless bilabial fricative [φ] like in Proto-Celtic², where IE *pl- became *φl- and later l- in Celtic languages. Of course, we should expect the same evolution in genuine Celtic loanwords in Basque³:

Celtic *φlāro- 'floor' > Basque larre 'meadow'

Celtic *φletro- 'hide, leather' > Basque larru, narru 'skin'

IMHO, the realization of /f/ as [φ] (which in fact is the usual pronounciation in most of Northern Spain (also including Basque-speaking areas) would explain the aspiration of Latin f- as h- in Gascon and Spanish⁴.  

In Basque, this consonant disappears at word-initial, mostly before back vowels, where also /p/was realized as [φ]:

Romance *fīco 'fig' [φiko] > Basque iko id.

Latin fīlu- 'thread' [φilu] > Basque iru id.
Romance *fondo 'bottom' [φondo] > Basque ondo 'side, bottom' 
Romance *fongo 'fungus' [φongo] > Basque onddo 'mushroom'
Latin fōrma- 'mould' [φorma] > Basque horma, orma 'wall'
Latin frāga- 'strawberry' [φ(a)raga] > Basque arraga, araga id.⁵
Romance pollo 'chicken' [φoλo] > Basque oilo 'hen'
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¹ However, in most words the cluster fl- became assimilated to pl-, then palatalized in Spanish and Portuguese, as in flamma- > Spanish llama, Portuguese chama. 
² In Proto-Celtic, PIE *p became a voiceless bilabial fricative *φ (then lost in the daughter Celtic languages) at word-initial and intervocally, but *b in other contexts.
³ In fact, there's ample evidence of Celtic toponymy in the Westernmost part of the Basque country, suggesting the area was previously inhabited by Celtic speakers. 

⁴ Unlike Gascon, Spanish never aspirates clusters fl-, fr- (see note 1). Also Castilian alone doesn't aspirate f- when followed by /w/ (resulting from diphtongation of Latin short /o/), hence fonte- > fuente. However, this doesn't happen in Cantabrian and Eastern Asturian varieties, where we find /xwénte/ instead.

⁵ In the variant arraga, [φr] evolved to a trill [rr], which then adquired a prothetic vowel. Compare Gascon ahraga. 

Assibilation of initial stops

The lamino-alveolar sibilant [s] <z> isn't a native consonant in Basque at word-initial, its possible sources being these ones:

1) The retention of an etymological sibilant in non-standard varieties vs. its loss in standard Basque, as in zapo 'toad' (a loanword from Semitic *ɬ’abb- 'a kind of lizard') vs. standard apo.

2) The result of Iberian *t- in loanwords. We've already seen that in a previous post.

3) The assibilation of an initial stop, either velar (most frequent) or labial (very rare), which often undergone expressive palatalization¹ into [ʃ] <x> or [tʃ] <tx>. For example, non-standard kakur 'dog' has given the assibilated forms zakur, txakur, xakur, and Romance *capel 'hat' has given txapel, xapel, zapel.

An example of assibilation of a labial stop is zegi 'milky cow' vs. behi 'cow' < EPBsq. *pek:i.

The coalescence of assibilation and expressive palatization in the same  word has made Vascologists² incapable of identifying the former. In Basque only coronal consonants [s, ś, ts, tś, d, t, n, l] (and in some dialects also [r, ŕ]) can undergone palatalization, so in order for a velar to be palatalized, it must be first converted to a sibilant.

The chronology of assibilation is ancient, as it's already found in Iberian. For example, zaldi 'horse' (Iberian saldu) is related to Germanic *kulta- 'colt'³, and zamar 'sheepskin jacket'⁴ is related to Germanic *xamísa- 'clothes, skirt'⁵ (PNC *χ^ʕa:mV ‘skin, cloth’).

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¹ A common linguistic device in Basque used to denote diminutive or affective meanings.
² Bengtson among them.
³ Nikolayev reconstructs IE *g^(w)Ald- 'foal, young of an ass', related to NEC *gwalV 'horse'.
⁴ A non-standard word borrowed into Romance (Spanish zamarra, Catalan samarra).
⁵ Hence Spanish camisa 'shirt'.