Have shrunken

A construction I encountered in the great read The language instinct, by Steven Pinker, struck me as odd. Here it is:

The various parts can be grotesquely distorted or stunted across animals: a bat's wing is a hand, a hortse trots on its middle toes, a whales' forelimbs have become flippers and this hindlimbs have shrunken to invisible nubs...

Here is what the Merriam-Webster's Dictionary of Contemporary English Usage says about shrunken:

Shrunk and shrunken are both used as the past participle of shrink. Shrunk is the usual choice when the participle is functioning as a verb:

... the lake has shrunk to a mirage of shimmering blue - William Kittredge, Fiction, vol. 1, no. 3, 1973

Or had they only shrunk? - Wilfrid Sheed, The Good Word and Other Words, 19787

Shrunken is the usual choice when the participle is functioning as a an adjective:

... a somewhat shrunken functionary, barely worth a book - Wilfrid Sheed, The Good Word and Other Words, 1978

... a frail and shrunken fragment of the old dream - Mavis Gallant, N.Y. Times Book Rev., 11 May 1983

So according to this book, using shrunk as the author did is unusual, to which I agree. I'd never seen it used like that before, not that that means anything.