Ask Mr. Smarty Plants

The most likely possibility I can think of is that you were struck by small leaflets with spiny stipules. Stipules are the small paired appendages at the base of the petiole (stem) of a leaf.  Several species of Central American acacias have impressively large spiny stipules.  Robinia pseudoacacia (Black locust) is a species native to North America with a bit more modest stipular spines.  It has been reported in California in counties adjacent to Fresno County.  Click "California" on the USDA Plants Database distribution map to see counties where it has been reported in California.  You can also see the spiny stipule on the photograph on that page.  Here is a drawing showing the leaves and the spines and here is a photo.  The tree can grow from 40 to 100 feet tall and is considered invasive in many areas, including California.  One acacia relative, Prosopis pubescens (Screwbean mesquite), with spiny stipules grows in Fresno County and can reach a maximum of about 30 feet.  Here are photos showing the spines.  An insect or small mammal feeding on the tree, could possibly chew on the newly formed leaflets at their base and cause them to fall from the tree with the stipular spines attached.  One species of ant in Africa prunes new leaf shoots (presumably along with the spiny stipules) from acacia trees there.  (Young, T. P., M. L. Stanton & C. E. Christian.  2003.  Effects of Natural and Simulated Herbivory on Spine Lengths of Acacia drepanolobium in Kenya.  Oikos Vol. 101, no. 1: 171-179). 

Whatever the prickly plant-like objects are, it seems likely they fell from the tall trees you said you were walking under.  Your best bet, then, is to go back and try to determine what the trees are.  If you have photos of the trees and/or the prickly object, you should visit our Plant Identification page to find links to plant identification forums that accept photos for identification.