Extended Data Fig. 6: Various growth stages of the Mazon Creek stem lampreys P. zangerli and M. pieckoensis.

a–v, Pipiscius (a–h) and Mayomyzon (i–v) show suites of characters that are compatible with the Priscomyzon series, including prominent eyes, an oral funnel, and short branchial baskets. a, b, ROM 56679, a hatchling carrying a yolk sac. Photograph overlain with outlines of the anatomical structures identified in this study (a) and photograph of the counterpart (b). c, d, FMNH PF16082, a hatchling carrying a yolk sac. Photograph overlain with outlines of the anatomical structures identified in this study (c) and photograph of the counterpart (d). e–h, Comparison of the specimens of P. zangerli at the same scale reveals nearly identical suites of morphological characters between the hatchling and adult, except size and presence or absence of a yolk sac. e, f, Holotype FMNH PF8346 shown as a photograph (e) and interpretive drawing (f), representing the general adult morphology of the taxon and preserved in comparable orientations to the two hatchling specimens. g, The largest specimen known for Pipiscius (FMNH PF8344), showing the circumoral feeding apparatus in dorsal view. h, The smaller hatchling (ROM 56679), shown in interpretive drawing at the same scale as e–g (two adult specimens). i–p, A specimen referred to M. pieckoensis (FMNH PF8167), representing a larva. i–l, Main part. Photograph (i), photograph overlain with outlines of the anatomical structures identified in this study (j), interpretive drawing (k), and scanned photograph taken in the 1960s by D. Bardack to show the original state of tissue preservation in this specimen (l). The photograph in l was discovered in the slide collections of D. Bardack. A full view of the snout can be seen in figure 2 of ref. ²⁸. The specimen was damaged in the snout, and the thin film of organic tissues has deteriorated across the entire specimen. m–p, Detailed photographs and illustration of the head region. Photograph in low-angle lighting to reveal surface textures (m), interpretive drawing (n), photograph in high-angle lighting to reveal the film of preserved tissues (o) and photograph of counterpart in high-angle lighting for comparison (p). q, Scan of photograph taken in the 1960s by D. Bardack, to show original state of tissue preservation in the head region of FMNH PF5687, a juvenile of M. pieckoensis. The photograph in q was discovered in the slide collection of D. Bardack. Further details can be seen in figures 2–4 of ref. ²⁸. r–v, Comparison of the specimens of M. pieckoensis at the same scale reveals character transitions across ontogeny, including the decreasing relative proportions of branchial region. r, s, Holotype FMNH PF5687 shown as a photograph (r) and interpretive drawing (s), representing the juvenile stage. t, Interpretive drawing of FMNH PF8167 at the same scale as r, s (juvenile), u, v (adult). u, v, The largest known specimen (ROM 56787) shown as a photograph (u) and interpretive drawing (v), representing the adult stage. Scale bars, 2 mm (a–d, i–l, m–p), 5 mm (e–h, r–v). Definitions of abbreviations are provided in the legends of Figs. 1–3 and ‘Anatomical abbreviations’ in the Methods.