How does the Icelandic word “finna” come from Proto-Germanic “finþanan”?

Question

finna

From Old Norse finna, from Proto-Germanic *finþanan.

finþanan

From Proto-Indo-European *pent-, *penth- (“to go, pass; path, bridge”). Cognate with Latin pons (“bridge”), Old Indian pánthā (“way, path”).

Related also to Old High German fendo, fendeo (“pedestrian, footsoldier”), Old Saxon fāþi (“walking”), Old English fēþe (“locomotion, walking, gait, pace”)

Descendants Old English: findan Scots: find, fynd English: find Old Saxon: findan Old Dutch: *findan Dutch: vinden Afrikaans: vind Old High German: findan Middle High German: vinden German: finden Old Norse: finna Icelandic: finna Faroese: finna Swedish: finna Danish: finde Norwegian: finne Gothic: (finþan)

I guess there must have existed some common changes in the Old Norse branch, for the daughter languages of that branch altogether lost the "d".

Does anyone know what the name of these phenomena?

Answer 1

Both of these look like regular sound changes between Proto-Germanic and Old Norse. However, I have found no eponyms for these sound changes, if that was what you were asking.

Loss of word-final /nan/

Wikipedia gives this as an example of an innovation in North Germanic:

General loss of word-final /n/, following the loss of word-final short vowels (which are still present in the earliest runic inscriptions).

Proto-Germanic *bindanaN > *bindan > Old Norse binda, but > Old English bindan

Applying this rule to *finþanan gives *finþa.

Assimilation of /nþ/ to /nn/

This is also a regular sound change in Old Norse. According to Marisa Lohr, one of its characteristic innovations is:

Development of /lth/ and /nth/ to /ll nn/ eg. ON finna, past pl. fundinn "to find"; ON gull, OE gold

This same phenomenon appears in many other ON words, such as tǫnn 'tooth', from Proto-Germanic *tanþs, gunnr 'war' from *gunþiz, munnr 'mouth' from *munþaz. (Note that English instead lost the /n/.)