The phylogeny, evolutionary developmental biology, and paleobiology of the Deuterostomia: 25 years of new techniques, new discoveries, and new ideas

Over the past 25 years, new techniques, new discoveries, and new ideas have profoundly impacted our understanding of deuterostome interrelationships and, ultimately, deuterostome evolution. During the late 1980s and early 1990s morphological cladistic analyses made predictions about both taxonomic history and homology, predictions that would be tested independent of the morphological characters themselves with the advent of molecular systematics, the rise of evolutionary developmental biology, and continued exploration of the fossil record. Thanks to these three areas of inquiry, we have gone from scenarios where animals like mobile enteropneust hemichordates and chordates were derived from sessile filter-feeding animals like modern lophophorates, echinoderms, and pterobranch hemichordates, to a new perspective where hemichordates are recognized as the nearest living relative of the echinoderms, and that vagile gill-bearing animals like Cambrian vetulicolians are seen-at least by some-as close to the deuterostome last common ancestor, with both sessility and filter-feeding convergent features of deuterostomes (e.g., echinoderm) and non-deuterostomes (e.g., lophophorates) alike. Although much of the backbone of the new deuterostome phylogeny is supported by multiple independent data sets, as are statements of homology of several different morphological characters, in particular the homology of gill slits across Deuterostomia, nonetheless, the next quarter century of study on this remarkable group of animals promises to be as equally illuminating and exciting as the past quarter century.