Haikouichthys: an Early Cambrian galeaspid in the LRT, not a basal lamprey

Haikouichthys is supposed to be
a lamprey ancestor, but after testing in the LRT nests as an Early Cambrian galeaspid with a small head.

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 a galeaspid,
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 galeaspid in the LRT. It has an appropriately much smaller skull than later galeaspids. The pectoral fin (green) is not separate from the body. The gill openings are invisible here, likely beneath the skull, as in galeaspids, not on the sides, as in lampreys. A prominent dorsal fin is present. So is a heterocercal tail. The series of posterior green diamonds are likely armored scales… like those in later sturgeons and osteostracans and 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. Skull of Dunyu with tetrapod homolog colors applied here.

Figure 2. Skull of Dunyu with tetrapod homolog colors applied here.

The only other tested galeaspid in the LRT,
Dunyu (Fig. 2), is from the Late Silurian andhas a larger skull. We looked at Dunyu earlier here. It is close to the Early Devonian osteostracan, Hemicyclaspis (Fig. 3), which is basal to the extant sturgeon in the LRT. Note that galeaspids arise from soft-bodied thelodonts like Thelodus (Fig. 3), in the LRT.

Figure 3. Subset of the LRT focusing on basal chordates, including Dunyu.

Figure 3. Subset of the LRT focusing on basal chordates, including Dunyu.

Some thelodonts,
like Furcacauda, have a relatively small head, a ventral oral cavity, a dorsal fin and no lateral fins.

Figure 1. Galeaspids from Halstead 1985.

Figure 4. Galeaspids from Halstead 1985.

No prior authors
have attempted to put tetrapod homologs on the skull of Haikouichthys or any other galeaspid (Fig. 4).

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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