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![The anterior end of Pikaia, oriented with the mouth facing down, modified from Fig. 8C of [5]. The anterior appendages (ap) are generally interpreted as gills or gill-related structures, and repeat in register with the anterior segments (pink). The pharynx, dorsal organ and ventral blood vessel are shown in outline. The point of the diagram is to show that there is no evidence for appendages positioned forward of the putative somite series, so if the latter indicates the beginning of the trunk, the region constituting what is effectively the head of Pikaia is exceedingly small](publication/377893162/figure/fig2/AS:11431281221704081@1706844229809/The-anterior-end-of-Pikaia-oriented-with-the-mouth-facing-down-modified-from-Fig8C-of.png)
The anterior end of Pikaia, oriented with the mouth facing down, modified from Fig. 8C of [5]. The anterior appendages (ap) are generally interpreted as gills or gill-related structures, and repeat in register with the anterior segments (pink). The pharynx, dorsal organ and ventral blood vessel are shown in outline. The point of the diagram is to show that there is no evidence for appendages positioned forward of the putative somite series, so if the latter indicates the beginning of the trunk, the region constituting what is effectively the head of Pikaia is exceedingly small
Source publication
The Middle Cambrian fossil Pikaia has a regular series of vertical bands that, assuming chordate affinities, can be interpreted as septa positioned between serial myotomes. Whether Pikaia has a notochord and nerve cord is less certain, as the dorsal organ, which has no obvious counterpart in living chordates, is the only clearly defined axial struc...
