Phrynosoma orbiculare bears one pair of horns on the midline parietal bone (P2), two pairs on each squamosal bone (S1 and S2), and one horn on each frontal bone (F0). Reduced major axis (RMA) regression statistics for the relationship of each horn to head length, and of head length to body length, were estimated and compared for significant differences in slopes, size at smallest head length, and size at greatest head length. F0 and P2 increase at different but low rates, but S1 and S2 grow at a significantly faster rate and approach P2 length toward the upper end of the head length range. This suggests an adaptive role for the cranial horn array in adult lizards. No sexual differences were found in these relationships, but there were significant subspecific differences in the relationships of head length to body length and of S1 and S2 to head length. This allometry of the cranial horn array is hypothesized to represent a plesiomorphic condition among the horned phrynosomatids.
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