That is, iguanodontids represent successive steps in the acquisition of advanced hadrosaurian characteristics, and in this view cannot be defined as a single distinct clade. Nevertheless, some researchers have found support for a monophyletic Iguanodontidae consisting of a handful of genera. Some other studies, however, fail to recover the group. The left cladogram was recovered in a 2015 analysis that supports a monophyletic Iguanodontidae, whereas the right cladogram from 2012 study finds the group to be paraphyletic:   Fossilized footprints provide evidence for both quadrupedality and bipedality within iguanodontids. It is thought that iguanodontids were primarily quadrupedal but could optionally walk on two limbs. The ossification of tendons along the neural arches may have played a role in facilitating the dynamic pedality of iguanodontids, as the ossified tendons could help withstand the additional stress incurred on the backbone by standing upright. Some research suggests that organism size plays a role in the determination of pedality, where larger organisms are more likely to choose to walk on all fours than their smaller counterparts. Iguanodontids are low-browsing herbivores that fed extensively on gymnosperms like ferns and horsetails, especially during the early Cretaceous period. These dinosaurs were very effective as herbivores due in part to their combination of bilateral dental occlusion with the transverse power stroke of their chewing mechanism. Additionally, iguanodontids lack a rigid secondary palate, which helps to mitigate torsional stresses during occlusion, a feature that enhanced their ability to break down plant matter. Additionally, the iguanodontids co-evolved with the radiation of angiosperms in the Cretaceous period. Angiosperms typically develop more rapidly and lower to the ground than gymnosperms; their proliferation provided a wealth of easily accessible food for the members of Iguanodontidae. A Question of Attribution  A Question of Attribution is a 1988 one-act stage play, written by Alan Bennett. It was premièred at the National Theatre, London, in December 1988, along with the stage version of "An Englishman Abroad". The two plays are collectively called "Single Spies". The one-act play formed the basis of a 1991 television film of the same name broadcast as part of the BBC's "Screen One" series.