Haikouichthys: a tiny Early Cambrian transitional taxon between armored Osteostraci and sturgeons, not a basal lamprey

Updated August 17, 2022
with a new nesting for Haikouichthys between the armored osteostracan, Hemicyclaspis, and sturgeons.

Haikouichthys is supposed to be
and was originally identified as a lamprey ancestor.

Either way,
Shu et al. reported on the importance of this fossil, “These finds imply that the first agnathans may have evolved in the earliest Cambrian, with the chordates arising from more primitive deuterostomes in Ediacaran times (latest Neoproterozoic, (555 Myr BP), if not earlier.”

If Haikouichthys is indeed a basal sturgeon,
ancestral, soft, worm-like chordates, like the more primitive lamprey, emerged in the Ediacaran, “if not earlier.”

Figure 1. Haikouichthys in situ and skull closeup, colors added.

Figure 1. Haikouichthys in situ and skull closeup, colors added. The oral  cavity on both lampreys and galeaspids is ventral.

Haikouichthys ercaicunensis
(Luo, Hu & Shu 1997; Shu et al.1999; HZf-12-127; 2.5cm) is an Early Cambrian basal sturgeon in the LRT. It is a phylogenetically miniaturized taxon at the genesis of extant fish. The pectoral fin (green) is not separate from the body. The gill openings are invisible here, likely behind the skull. A prominent dorsal fin is present. So is a heterocercal tail in other specimens. The series of posterior green diamonds are likely armored scales… like those in sturgeons and osteostracans and like those in early thelodonts, like Thelodus (Fig. 3). They are likely not gonads, but overlaid the area where the gonads are in related taxa.

Figure 2. Subset of the LRT focusing on basal vertebrates as of August 18, 2022 nesting Haikouichthys basal to sturgeons in the Early Cambrian.

Figure 2. Subset of the LRT focusing on basal vertebrates as of August 18, 2022 nesting Haikouichthys basal to sturgeons in the Early Cambrian.

No prior authors
have attempted to put tetrapod homologs on the skull of Haikouichthys or any other Cambrian fish.

Shu et al. 1999 wrote,
The theory of lateral fin folds has had considerable signiificance, but of the fossil agnathans only the anaspids and Jamoytius have such paired fn-folds. The possible occurrence of this condition in these Lower Cambrian agnathans indicates, however, that fin-folds may be a primitive feature within the vertebrates. The occurrence of close-set dorsal fin-radials in Haikouichthys, on the other hand, may be a relatively advanced feature.”


References
Luo H. et al. 1997. New occurrence of the early Cambrian Chengjiang fauna from Haikou, Kunming, Yunnan province. Acta. Geol. Sin. 71, 97-104.
Shu D-G et al. (8 co-authors) 1999. Lower Cambrian vertebrates from China. Nature 402:42-46.

wiki/Haikouichthys

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