It would seem my previous post was on the right track. The husks were
eaten by large herbivores that are now extinct. From this page:

http://www.redorbit.com/news/science/261045/ice_age_survivors/

Two unusual survivors aren't animals at all but plants, one of which
is so common that you may have eaten one in the past week or so. The
avocado (Persea Americana) is a South American fruit that has survived
until modem times thanks to the intervention of humans and fanning.
The seed inside the avocado fruit is gigantic and needs to be
swallowed and excreted for it to germinate in the wild. However, no
modern herbivore in South America can swallow such a seed without
choking to death.

This may be the case today, but 10,000 years ago South America had
some of the largest mammalian herbivores ever, including the enormous
ground sloth (Megatherium americanutn), which could grow as tall as
two stories. Coprolites (fossilised poo) reveal that these sloths ate
everything in their region, including the humble avocado.

Here is a classic case of parallel evolution, where two organisms
evolved alongside each other in a symbiotic relationship. The loss of
one almost caused the extinction of the other.

Likewise the osage orange tree (Madum pomtfera), which is found on the
North American plains, also yields gigantic fruits that were once
eaten (and similarly distributed) by the American sloth and mammoth -
some of the only herbivores at the time that could manage the tough-
skinned fruit

Luckily both the avocado and the osage live for a long time, and both
managed to survive just long enough for humans to unwittingly save
them. In the case of the avocado it was farmers cultivating them for
their fruit, while the osage was saved by the reintroduction of the
horse by the conquistadors.