Common Ancestry Practice Test

A developmental biologist compared early embryo stages of a chicken and a lizard. In both lineages, embryos form a transient pharyngeal-arch pattern and a segmented tail bud, and the same set of regulatory genes is expressed in similar spatial regions during these stages. Later development diverges, producing distinct adult morphologies. The shared embryonic structures and gene-expression pattern appear before major lineage-specific traits form. Which inference is best supported by these observations?

A paleontologist compared skulls of a modern horse and a fossil mammal found in older rock layers. Both skulls share the same distinctive arrangement of molars: three cusps in a specific triangular pattern and an identical groove between the second and third cusp. Several other mammals in the same deposits lack this cusp pattern. The fossil skull has smaller teeth and a shorter face, but the cusp pattern and groove occur in the same relative positions. Which inference is best supported by these observations?