Does fumaric acid form maleic anhydride upon heating? Maleic acid (the cis isomer of the dicarboxylic acid $\ce{C4H4O4}$) readily forms its anhydride on heating. Clayden's organic chemistry textbook mentions this as a point of distinction between it and fumaric acid (the trans isomer), which fails to dehydrate to its anhydride upon heating. However, I have discovered that at temperatures above 200 $^{\circ}\mathrm{C}$, fumaric acid forms maleic anhydride as well. 

Which is the correct answer? I don't think it is likely that fumaric acid can form the anhydride - even at higher temperatures - given the trans configuration and restricted rotation.
Fumaric acid does form maleic anhydride due to partial combustion. From wikipedia article of fumaric acid:


  Fumaric acid converts to the irritant maleic anhydride, upon partial
  combustion.


Also, dehydrating fumaric acid with a  dehydrating agent also forms maleic anhydride. This paper describe how to convert $\ce{C^{14}}$ labelled fumaric acid to maleic anhydride.


  Maleic anhydride-2-$\ce{C^{14}}$ is readily obtained from fumaric
  acid-2-$\ce{C^{14}}$ through dehydration using phosphorus pentoxide
  getting 96% yield under controlled condition.