Presence of the functional head joint mapped onto the latest molecular analysis [15] of the family Stomiidae.

(A, B) Functional head joint absent in basal stomiids (blue box). The notochord does not overlap the basioccipital ventrally (small black box), e.g., in the cleared and double stained (c&s) specimen of (A) Flagellostomias boureei, BMNH 2002.8.5.786–788 and the histological sagittal section (hss) of (B) Idiacanthus antrostomus, SIO 97–85. (C–F) Intermediate stage of the functional head joint (red box), where the chordal sheath slightly overlaps the basioccipital ventrally (best seen in the histological sections (D, F). (C) Grammatostomias dentatus, USNM 234036 (c&s). (D) Grammatostomias circularis, NSMT-P 99317 (hss) and (E) Bathophilus vaillanti, USNM 234150 (c&s). (F) Bathophilus filifer, SIO 03–189 (hss). (G–P) Presence of the functional head joint (green box). The notochord overlaps the basioccipital ventrally in a resting position (G, I, K, M, O). Unfolding of the ventral sheath of the notochord during manipulative elevation of the head in c&s specimens (H, J, L, N, P). Green arrow heads indicate the two genera (Aristostomias and Photostomias) that have the occipital condyle composed of the exoccipitals and the basioccipital. (G) Pachystomias microdon, USNM 297922. (H) Malacosteus australis, USNM 296675. (I) Aristostomias xenostoma, USNM 296715. (J) Photostomias sp., USNM 296650. (K) Eustomias obscurus, USNM 206711. boc, basioccipital; c1, first centrum; exo, exoccipital; na1, first neural arch; nc, notochord. Scale bars, 1 mm.