LETTER doi:10.1038/nature13414 A primitive fish from the Cambrian of North America Simon Conway Morris1 & Jean-Bernard Caron2,3 Knowledge of the early evolution of fish largely depends on soft- (Extended Data Fig. 4f). Incompleteness precludes a precise estimate of bodied material from the Lower (Series 2) Cambrian period of South size range, but themostcomplete specimens (Fig.1a,b) areabout 60 mm China1,2. Owing to the rarity of some of these forms and a general in length and 8–13 mm in height. Laterally the body is fusiform, widest lack of comparative material from other deposits, interpretations of near the middle, tapering to a fine point posteriorly (Fig. 1a, b and Ex- various features remain controversial3,4, as do their wider relation- tended Data Fig. 4a), whereas in dorsal view the anterior termination is ships amongst post-Cambrian early un-skeletonized jawless verte- rounded (Fig. 1d and Extended Data Fig. 4c–e). The animal was com- brates. Here we redescribe Metaspriggina5 on the basis of new material pressed laterally, as is evident from occasional folding of the body as well from the Burgess Shale and exceptionally preserved material collected as specimensindorso-ventral orientation being conspicuously narrower near Marble Canyon, British Columbia6, and three other Cambrian (Fig. 1a and Extended Data Fig. 5a). Along the anterior ventral margin Burgess Shale-type deposits from Laurentia. This primitive fish dis- there was a keel-like structure (Fig. 1b, g, i, k, l), but no fins have been plays unambiguous vertebrate features: a notochord, a pair of prom- recognized. In the much more abundant specimens of Haikouichthys1,3,4 inent camera-type eyes, paired nasal sacs, possible cranium and arcualia, fins are seldom obvious, suggesting that their absence in Metaspriggina W-shaped myomeres, and a post-anal tail. A striking feature is the might be taphonomic. branchial area with an array of bipartite bars. Apart from the anterior- The diagnostic myomeres are invariably present, but are sometimes most bar, which appears to be slightly thicker, each is associated with highly disorganized especially towards the anterior (Fig. 1b, d, i and externally located gills, possibly housed in pouches. Phylogenetic Extended Data Fig. 1g), suggesting variable levels of decay11. Undisrupted analysis places Metaspriggina as a basal vertebrate, apparently close specimens show open chevrons with the main apices directly anteriorly to the Chengjiang taxa Haikouichthys1–4 and Myllokunmingia1, dem- (Fig. 1h, k and Extended Data Fig. 5c). Dorsally, however, some specimens onstrating also that this primitive group of fish was cosmopolitan show a smaller additional fold with the blunt apex directed posteriorly during Lower–Middle Cambrian times (Series 2–3). However, the (Fig. 1k), while in one specimen (Fig. 1h) another posterior inflection arrangement of the branchial region in Metaspriggina has wider lies towards the ventral side. Overall, therefore, the myomeres have a implications for reconstructing the morphology of the primitive ver- W-shaped configuration. As the body narrows posteriorly the prin- tebrate. Each bipartite bar is identified as being respectively equi- cipal myotomal apices become much more acute (approximately 60u), valent to an epibranchial and ceratobranchial. This configuration and the subsidiary dorsal inflection is probably lost (Extended Data suggests that a bipartite arrangement is primitive and reinforces Figs 1g, i, 4a). The myomeres, totalling at least 40, are considerably more the view that the branchial basket of lampreys7 is probably derived. acute than in Pikaia9 and, in contrast to this chordate, Metaspriggina Other features of Metaspriggina, including the external position of was evidently an effective swimmer15. Parker Slate specimens differ in the gills and possible absence of a gill opposite the more robust anterior- being less slender and having myomeres with a more angular closure most bar, are characteristic of gnathostomes8 and so may be prim- (Extended Data Fig. 6a–d). In dorso-ventral specimens an elongate itive within vertebrates. strand (0.25 mm thick) is identified as the notochord (Fig. 1a–d and Cambrian chordates1–5 are effectively restricted to the Burgess Shale5,9 Extended Data Fig. 1a–d, f). In laterally orientated material the notochord and Chengjiang1–4 Lagersta¨tten, and despite soft-part preservation key lies on the midline, opposite the zone of myotomal closure (Fig. 1h). Occa- structures may be difficult to resolve3,9,10. In addition, differential decay sional narrower strands in the anterior region probably represent parts of various parts of the body may skew interpretation of character states11 of the vascular system (Fig. 1d). and potentially compromise phylogenetic analysis. The head is small and slightly bilobed, with smooth margins and Here we redescribe the poorly known Burgess Shale chordate Meta- possibly a central notch (Fig. 1d and Extended Data Fig. 4c–e). It bears spriggina walcotti5 (Extended Data Fig. 1), on the basis of approximately two prominent eyes (Fig. 1a–d, f, g, i, j, l, m and Extended Data Figs 3, 100 new specimens collected in the Canadian Rockies from several 4b–e, 5a–d, 6d). These appear to have been originally circular (maximum Cambrian Burgess Shale-type deposits. Localities include the Burgess diameter approximately 1.3 mm), but elliptical shapes (Fig. 1d) may re- Shale (Walcott Quarry) in Yoho National Park (Fig. 1g and Extended flect slightly oblique burial. Typically the eye is preserved as a reflective Data Fig. 2), and three sites in Kootenay National Park: Haiduk Peak film, but a well-defined circular area (Fig. 1f) (approximately 0.4 mm) (Extended Data Fig. 3)12 and, most importantly, near Marble Canyon6 appears to be the lens, suggesting a camera-like arrangement. In several (Fig. 1a–f, h–m and Extended Data Figs 4, 5). In addition, Emmonsaspis specimens (Fig. 1c, i, j, l, m and Extended Data Figs 1c, d, 5b), paired (partim) from the Parker Slate Formation13 in Vermont, hitherto ten- circular areas located between the eyes are interpreted as the nasal sacs. tatively identified as a frond-like fossil14, and the Kinzers Formation in The proximity of these structures suggests a single median duct might Pennsylvania (R. Thomas, personal communication), are reinterpreted have connected them. The paired eyes sometimes lie at a steep angle to as Metaspriggina spp. (Extended Data Fig. 6). Despite variations in age the body axis (Extended Data Fig. 5b), suggesting that the head formed (Extended Data Fig. 7 and Supplementary Table 1) and palaeoenviron- a discrete lobe capable of rotation. A median triangular area may repre- mental settings between these occurrences (and by implication tapho- sent cranial cartilage (Fig. 1d and Extended Data Fig. 4c–e), while darker nomic histories) the similarities of anatomy allow reliable reconstructions. dorsal elements might indicate possible arcualia (Fig. 1g). Descriptions are largely based on the more complete ‘Marble Canyon’6 Posterior to the head and ventrally positioned is a large branchial area. fossils (see also Fig. 2). Like other Burgess Shale-type material, fossils are This region is identified on the basis of two sets of bars, seven in total, on preserved as carbonaceous compressions and aluminosilicate minerals each side of the body (Fig. 1d and Extended Data Fig. 4c). In dorso-ventral 1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK. 2Department of Natural History (Palaeobiology), Royal Ontario Museum, 100 Queen’s Park, Toronto, Ontario M5S 2C6, Canada. 3University of Toronto, Department of Ecology and Evolutionary Biology, 25 Willcocks Street, Toronto, Ontario M5S 3B2, Canada. 00 MONTH 2014 | VOL 000 | NATURE | 1 ©2014 Macmillan Publishers Limited. All rights reserved RESEARCH LETTER a c b e f d g j h i m k l Figure 1 | Metaspriggina walcotti (Simonetta and Insom, 1993) specimens eyes (framed area in j with the image flipped vertically). k, ROM62954, lateral from Marble Canyon and Walcott’s Quarry collected by the Royal view showing internal organs. l, m, ROM62928, oblique view, and close up Ontario Museum. a–f, h–m, Specimens collected from Marble Canyon. showing eyes in m. a, b, e, h, j–m, Backscatter scanning electron microscopy g, Specimens collected from Walcott’s Quarry. a–m, Complete (a–c) and partial images (e, j), composite images of both parts and counterparts (f–i, k–m) and (d–m) specimens, anterior to the left (except i). a, ROM62935, oblique view. stitched images at white lines (a, b). An, anus; Ar?, possible arcualia; Bc?, b, c, ROM62938, lateral view and close up of the anterior section (framed area possible extra-branchial cartilage; Brv, branchial bars (ventral element); Brd, in c). d, e, ROM62933, oblique view, and close up of gill bars (framed area branchial bars (dorsal element); Brp, branchial bar processes; Bv, blood vessel; in e). Only the position of some gill bars (Brd1 and Brv1 in e) are highlighted by Cc?, possible cranial cartilage; Es, oesophagus; Ey, eyes; Gi, gill filaments; arrows (see also Extended Data Fig. 4c). f, ROM62946, oblique view, pair of Gu, gut; He?, possible heart; Ke, keel; Le, lens; Li, liver; My, myomere; Na, nasal eyes showing presumed lens (see Extended Data Fig. 5d). g, ROM62964, lateral sacs; No, notochord; Not, notch. Scale bars: 5 mm (a, b, g–i, k, l); 2 mm (d); view. h, ROM62924, lateral view, showing W-shaped myomeres and main 1mm(c, e, f, j, m). inflections (thin arrows). i, j, ROM62932, lateral view, and close up showing 2 | NATURE | VOL 000 | 00 MONTH 2014 ©2014 Macmillan Publishers Limited. All rights reserved LETTER RESEARCH Na Figure 2 | Metaspriggina walcotti (Simonetta and Insom, 1993). Diagrammatic anatomical drawing, dorsal and lateral views showing main Ey morphological features (gills and possible blood vessels; arcualia and extra Mo? branchial and cranial cartilages have not been reconstructed). Mo?, possible position of mouth; Ph, pharyngeal area.