Paleontologists have reconstructed the evolutionary history of modern sea otters (Enhydra) using fossil remains of four extinct marine mustelid species, designated W, X, Y, and Z. The diagram below illustrates the geographic distribution, geological age, and proposed phylogenetic relationships of these species.

Based on this phylogenetic reconstruction, which extinct species is expected to share the highest percentage of sequence homology in their mitochondrial DNA with modern sea otters?