Oh, and I have decided on what the Norse will call North America, when they get around to conceptualizing the continent as a whole; given that pretty much every nation they interact with in any meaningful way refers to it as Turtle Island, they will also call it Turtle Island. Google Translate claims that that's "Skjaldböku Eyju" in Icelandic, but I'd prefer a translation by an actual person into actual Old Norse. Any takers?
This
English-Old Norse dictionary might come in handy, though I have no ideas myself what to call the continent other than the traditional Vinland.
It has no listing for "turtle". :-(
So, "Skjaldböku Eyland" is what I've got so far...
Umm....
Let's back up a bit, shall we.
a really good source for early (mostly Germanic) dictionaries is
http://lexicon.ff.cuni.cz
which has dictionaries of Anglo-Saxon, Old Norse, etc.
(the Old Norse ones are INTO English, not from English)
I searched the whole site for "turtle" and the only hits were for "turtledoves" in Anglo-Saxon. I went ?Hmmm???
So I looked up "List of reptiles of Europe" on Wiki, and ALL the turtles there are from Southern Europe or Russia. None in Germany or Scandinavia.
Therefore, the Norse DIDN'T have a word for turtle, in all likelihood.
So... They need to invent a word. Do they construct something like modern Icelandic "Shieldback" (which is what you have) or German "Shieldtoad" (!?!). Or do they borrow the word from the first group of people they meet? What's Miqmaq for "turtle"?
Or do they give it some totally arbitrary and wrong name - like Columbia, America (after Europeans), Canada (=village in Iroquoian), Quebec (=narrows in Algonkian), etc., etc.
When I do "turtle" in Google Translate, it gives me "skjaldbaka" which is the same root as what you've got, but a different form, possibly genitive or accusative. Edit: Aha! dative.
Note that combining forms with animals do NOT take the genitive "from cattle, birds, beasts, Fær-eyjar, Lamb-ey, Sauð-ey, Hrút-ey, Yxn-ey, Hafr-ey, Svín-ey, Kið-ey, Fugl-ey, Arn-ey, Æð-ey, Má-ey, Þern-ey, Úlf-ey, Bjarn-ey: from vegetation, Eng-ey, Akr-ey, Við-ey, Brok-ey, Mos-ey" (Cleasby and Vigfusson)
No 's' or 'ar' genitives at all. In other words, it wouldn't be "Turtle's Island" but the equivalent of Turtleisland.
Note, too, that while "eyland" exists, meaning island, it is redundant and far rarer than "ey" which means exactly the same thing. DON'T use eyland is my advice.
So.... Possibly something on the close order of "Skjaldbakey", IF you assume they independently come up with the same word as modern Icelandic.
Edit: Note, for instance, that the various words for "turtle" in English are all reasonably late and come from other languages. There is no native English word. Turtle and Tortoise are manglings of French words and Terrapin comes from a native language in the New World.