To solve this problem, we need to identify which characteristic is not shared by both birds and mammals. Let's analyze each option:$\newline \bullet$Option 1: Ossified endoskeleton$\newline$ - Both birds and mammals have an ossified (bony) endoskeleton.$\newline$ - This is a common feature shared by both groups.$\newline \bullet$Option 2: Breathing using lungs$\newline$ - Both birds and mammals use lungs for breathing.$\newline$ - This is another shared characteristic.$\newline \bullet$Option 3: Viviparity$\newline$ - Viviparity refers to giving birth to live young rather than laying eggs.$\newline$ - Most mammals are viviparous, but birds are oviparous (they lay eggs).$\newline$ - Therefore, viviparity is not a characteristic shared by both birds and mammals.$\newline \bullet$Option 4: Warm blooded nature$\newline$ - Both birds and mammals are warm-blooded (endothermic), meaning they can regulate their body temperature internally.$\newline$ - This is a shared characteristic.$\newline$Based on the analysis, the characteristic that is not shared by birds and mammals is viviparity.$\newline$Therefore, the correct option is Option 3.