I don't think they are caltrops...
Main Entry: cal·trop
Pronunciation: 'kal-tr&p, 'kol-
Variant(s): also cal·throp /-thr&p/
Function: noun
Etymology: Middle English caltrappe, alteration of calketrappe star thistle, from Old English calcatrippe, from Medieval Latin calcatrippa
1 a plural but singular or plural in construction : STAR THISTLE 1 b : PUNCTURE VINE; also : any of various related herbs (genera Tribulus and Kallstroemia) of the same family (Zygophyllaceae, the caltrop family)
2 : a device with four metal points so arranged that when any three are on the ground the fourth projects upward as a hazard to the hooves of horses or to pneumatic tires