Metaspriggina walcotti, a 500 million year old finless, jawless fish

Not a reptile, but we’ll make room for this one:
The biggest news in paleontology in the last six months (IMHO) is not the discovery, but the redescription of Metaspriggina as a VERY basal Burgess Shale (Cambrian) fish (chordate) without fins, without jaws, but with two anterodorsal eyes (Fig.1). Metaspriggina is considered to represent a primitive chordate, transitional between cephalochordates and the earliest vertebrates.

Figure 1. Diagram of Metaspriggina, the basalmost chordate. Lectotype –USNM198612 and former holotype 198611 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.

Figure 1. Diagram of Metaspriggina, the basalmost chordate. Lectotype –USNM198612 and former holotype 198611 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA. This free-swimming “worm” is close to the ancestry of vertebrates, including humans, ultimately.

From the abstract:
“This primitive fish displays unambiguous vertebrate features: a notochord, a pair of prominent camera-type eyes, paired nasal sacs, possible cranium and arcualia, W-shaped myomeres, and a post-anal tail. A striking feature is the branchial area with an array of bipartite bars. Apart from the anterior-most bar, which appears to be slightly thicker, each is associated with externally located gills, possibly housed in pouches.”

Figure 2. A hypothetical basal fish based on a lamprey larva and the cephalochordate, Amphioxus. Image lifted from "From the Beginning, the Story of Human Evolution".

Figure 2. A hypothetical basal fish based on a lamprey larva and the cephalochordate, Amphioxus. Image lifted from “From the Beginning, the Story of Human Evolution”. Such a guess is pretty easy based on phylogenetic bracketing. The tail is off. It should have come much later.

 

Set aside by Walcott for further study, the two known specimens of this species were briefly examined by Conway Morris (1979). Simonetta and Insom (1993) described one of the two specimens (the original holotype specimen) as a potential relative of the Ediacaran organism Spriggina (Eidacaran segmented worm), whereas the second specimen (now the lectotype) was interpreted as a potential chordate. A chordate interpretation for both specimens was proposed (Janvier, 1998; Smithet al., 2001) and a detailed redescription was eventually instigated by Conway Morris (2008) with both specimens being included in the same genus and species.

Figure 2. A variety of basal fish from the Ordovician, Silurian, Devonian and the present day (for the lamprey larva).

Figure 2. A variety of very basal fish from the Ordovician, Silurian, Devonian and the present day (for the lamprey larva). The fish head is hypothetical.

I was not aware of similar specimens coming out of early Cambrian China. Apparently Metaspriggina was close to Haikouichthys and Myllokunmingia (Fig. 3).

Figure 3. A clade of finless chordates that became armored.

Figure 3. A clade of finless chordates that became armored.

And of course, Pikaia, the Cambrian cephalochordate, is also from the Burgess Shale.

Let’s remind ourselves at this point that urochordates and other sessile organisms have no place in the lineage of vertebrates other than as renegade cousins of lancelets that stopped chasing after food and let food come to them.

References
Conway Morris S and Caron, J-B 2014. A primitive fish from the Cambrian of North America”. Nature (London: Nature Publishing Group). doi:10.1038/nature13414. ISSN 0028-0836.
Conway Morris, S 2008. A Redescription of a Rare Chordate, Metaspriggina walcotti Simonetta and Insom, from the Burgess Shale (Middle Cambrian), British Columbia, Canada”. Journal of Paleontology (Boulder, CO: The Paleontological Society) 82 (2): 424–430. doi:10.1666/06-130.1. ISSN 0022-3360. Retrieved 2014-06-13.

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