Chordate evolution and the three-phylum system.

Area of Science:

• Evolutionary Biology
• Phylogenetics
• Developmental Biology

Background:

• Traditional metazoan phylogeny places Vertebrata within Chordata, alongside Urochordata and Cephalochordata.
• Chordata, Echinodermata, and Hemichordata form the Deuterostomia group.
• Previous studies have not deeply explored the taxonomic ranking of the three chordate subphyla.

Purpose of the Study:

• To re-evaluate the taxonomic ranks of major animal groups based on evolutionary and developmental data.
• To propose a revised classification reflecting new understanding of chordate and related phyla origins.
• To highlight the significance of larval morphology in understanding deep evolutionary divergences.

Main Methods:

• Comparative analysis of larval development (tadpole-type vs. dipleurula-type).
• Review of accumulating phylogenetic evidence on relationships within Deuterostomia.
• Reclassification of Protostomes into Ecdysozoa and Lophotrochozoa as a precedent.

Main Results:

• Cephalochordates diverged earliest within Chordata; tunicates and vertebrates form a sister group.
• Echinoderms and hemichordates form the clade Ambulacraria.
• Significant larval differences between Ambulacraria and Chordata warrant a higher taxonomic category.

Conclusions:

• Recommend elevating Ecdysozoa, Lophotrochozoa, Ambulacraria, and Chordata to superphylum rank.
• Propose subdividing Chordata into three distinct phyla based on unique characteristics.
• Emphasize the evolutionary occurrence of tadpole-type larvae as key to understanding chordate origins.