Exceptionally well-preserved fossils of one of the most primitive fishes have been discovered in the Canadian Rockies and the eastern U.S. They belong to Metaspriggina walcotti, and they showcase the origin of jaws -- the earliest this feature has been seen in the vertebrate fossil record.
First discovered in 1993, Metaspriggina lived in the Cambrian period around 505 million years ago. Previously, only two fragmentary fossils have been found. This new discovery includes a whopping 100 new Metaspriggina specimens from the Burgess Shale site near Marble Canyon in British Columbia, along with three other Cambrian deposits from Laurentia (present-day North America and Greenland).
The primitive fish had a narrow, tapering body, with the most complete specimens reaching lengths of just 6 centimeters. Analysis by Simon Conway Morris from University of Cambridge and Jean-Bernard Caron from University of Toronto reveals that the ancient fish had several unambiguous vertebrate features including: a notochord (a skeletal rod supporting the body), paired nasal sacs for sensing, a post-anal tail, and a pair of prominent camera-type eyes like ours (and squid), which is a pretty modern feature for its time. Arrangement of its muscles suggests it was an active swimmer. Here’s a photo:
But the most striking feature of all is a gill structure that foreshadows that of modern vertebrates. They have an array of bipartite bars, or pairs of curved structures for supporting gills (also called branchial arches). Most of the bars are each associated with externally located gills, and their configuration suggests that a bipartite (“two parts”) arrangement is primitive. But there’s more!
The foremost pair of arches is slightly thicker, suggesting that the pair of gill supports closest to the head may be the first step in the evolution of the vertebrate jaw. “Obviously jawed fish came later, but this is like a starting post,” Caron says, “everything is there and ready to go.”
Here’s a diagram. The full key to the abbreviations is included below, though the main features to notice are: Brv, branchial bars (ventral element); Brd, branchial bars (dorsal element); Brp, branchial bar processes.
Modern jawless vertebrates (like lampreys with their sucking mouths) have cartilaginous gill baskets for support. These findings suggest that the gill basket is probably derived -- that is, it came after the primitive traits, isn’t shared with other groups, and thus likely can’t tell us much about the origin of our jaws.
A phylogenetic analysis places Metaspriggina as a basal vertebrate, close to primitive fishes Haikouichthys and Myllokunmingia of the Lower-Middle Cambrian. (These guys didn't appear to have jaws.) “Once the jaws have developed, the whole world opens,” Conway Morris says in a news release. “Having a hypothetical model swim into the fossil record like this is incredibly gratifying.” Speaking of which, watch a video of it swim!
The work was published in Nature this week.
Images: Marianne Collins © Conway Morris and Caron (reconstruction & diagram), Jean-Bernard Caron © ROM (photo)
Key to anatomical features: Mo?, possible position of mouth; Ph, pharyngeal area; An, anus; Brv, branchial bars (ventral element); Brd, branchial bars (dorsal element); Brp, branchial bar processes; Es, oesophagus; Ey, eyes; Gi, gill filaments; Gu, gut; He?, possible heart; Ke, keel; Le, lens; Li, liver; My, myomere; Na, nasal sacs; No, notochord.