Myxini and which of the following are phylogenetically sister taxa?

Myxini, commonly known as hagfish, and Petromyzontida, which includes lampreys, are sister taxa because they both belong to the clade Cyclostomata, which is characterized by jawless vertebrates. This evolutionary relationship indicates that Myxini and Petromyzontida share a more recent common ancestor with each other than with any other group of vertebrates.

The Cyclostomata group is crucial for understanding the early evolution of vertebrates and the transition from jawless to jawed forms. This relationship highlights the importance of studying these primitive vertebrates to gain insight into vertebrate evolution and the development of key physiological traits.

In contrast, the other groups mentioned—Cephalochordata (lancelets), Chondrichthyes (sharks and rays), and Urochordata (tunicates)—do not share this specific phylogenetic closeness with Myxini, as they fall under different classifications and exhibit distinct evolutionary paths. This distinction underscores the significance of Myxini and Petromyzontida as a more closely related pair within the vertebrate lineage.