The infamous "puncture vine" _Tribulus terrestris_ is a weed in Arizona.
The local bike shop has a "trophy board" with numerous clawed seeds of this
plant glued to it. Truthfully, I've never had a puncture caused by one of
these, but there is a big one growing at the corner of 44th and Washington
which I eye cautiously every morning I ride by it...
(This plant is in the Zygophyllaceae, listed in my book as the "Caltrop
family", but better known to desert denizens as the family of the Creosote
Bush).
A caltrop, as I recall, is a metal device somewhat like toy "jaxs" but with
four spines rather than ball-tipped projections. Used to cripple cattle, or
horses (cavalry, jousting knights, etc..). The puncture vine seed looks a
lot like a thumbtack.
-Michael