When the species that is most convenient for analysis is a frog like Xenopus, we make the unspoken assumption that Xenopus is a typical amphibian and that its development, including genetic control over development, can be extrapolated to the many tens of thousands of other vertebrate species. But Xenopus is a more bizarre amphibian.

-Xenopus is not a basal amphibian. Xenopus is in the family Pipidae. Once thought to be among the most primitive frogs, pipids have now been shown through rigorous phylogenetic analyses to be among the most highly derived frogs.

-Xenopus is pseudotetraploid and speciation is based on polyploidy, which is unique among frogs.

-Xenopus displays many novel features associated with its aquatic mode of life and evolutionary origins from terrestrial saltatorial ancestor.

-Xenopus lacks a tongue and so has a feeding mechanism (ram feeding) that is highly unusual amog frogs.

-Xenopus has highly specialized keratinous claws.

-Xenopus has a skull that develops precociously in relation to other frogs and in which many of the bones surrounding the eye develop by intramembranous rather than endochondrial ossification.

Brian K. Hall - Evolutionary Developmental Biology