Fossils unearthed in southwestern Canada in 2012 are providing new insights into one of the world’s oldest known vertebrates, as well as the evolution of their jaws. The creature, dubbed Metaspriggina walcotti (artist’s reconstruction above), was previously known only from two very fragmentary fossils discovered almost a century ago and first described in 1993. Of the hundred or so new specimens taken from several sites boasting rocks between 500 million and 515 million years old, the largest are about 6 centimeters long (thumb-sized) but the body proportions of some incomplete fossils hint that the creatures might have grown somewhat larger. The most complete and best preserved fossils include impressions of eyes, muscle groups, and the supports for gills, the researchers report online today in Nature. Analyses haven’t yet noted any remnants of fins, but the eyes of this active swimmer were large and prominent. A small but sharp-edged circular area in each eye hints that the eyes of these ancient fish included a lens, a relatively modern feature for its era. More significant, though, are details of the curved structures supporting the creature’s gills. The arrangement of these paired structures foreshadows that seen in fish that evolved much later, and the slightly thicker dimensions of the foremost pair of gill supports may reveal the first steps in the evolution of jaws. Detailed analyses place Metaspriggina near the base of the vertebrate family tree and certainly among the earliest fish, the team suggests.