Pteromyzon, Pharyngolepis, Jamoytius and Metaspriggina

Metaspriggina walcotti (Simonetta and Insom 1993; Morris and Caron 2014; Cambrian, 500 mya; up to 10cm) is an early chordate, naked, jawless and finless, but with two anterodorsal eyes and nares. The orbits comprise the proto-skull. The swimming muscles are larger, so this was probably a mobile feeder, using its eyes to seek prey and avoid predators. Gill slits opened directly. Descendants, like Arandaspis (below), were armored.

Pteromyzon marinus (Linneaus 1758) is the extant lamprey, a large lancelet in which some metamorphiosized adults attach themselves to fish to suck their blood. Others do not feed as adults, but live off reserves obtained by filter feeding while young. The gill openings open to the outside, not into an atrium.

The origin goes back to the early Ordovician. 38 extant and 5 extinct species are known.

Jamoytius kerwoodi (White 1946, Sansom et al. 2010; Early Silurian; 10+cm in length) was once considered the basalmost vertebrate, then a sister to lampreys, then a sister to Euphanerops. Here this eel-like jawless fish nests between armored lancelets and jawless fish with fins, like Thelodus. Jamoytius shares a tiny circular mouth and a naris at the tip of its short snout with these taxa. No earlier studies traced the skull bones.

Pharyngolepis oblongus (Kiaer 1911; Late Silurian; up to 15cm) is a jawless fish with a single median naris starting to split in two. Most skull bones not associated with jaws are present here for the first time. This clade has an anal fin before gnathostomes developed one.

Gabbot SE, Aldridge RJ and Theron JN 1995. A giant conondont with preserved muscle tissue from the Upper Ordovician of South Africa. Nature 374:800–803.

Kiaer JA 1911. Skr. Vidensk. Christiania, Mat.-Naturv. Kl., 1911(7):1–19.

Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.

Morris SC and Caron J-B 2014. A primitive fish from the Cambrian of North America. Nature 512(7515):419–422.

Sansom RS, Freedman K, Gabbott SE, Aldridge RJ and Purnell MA 2010. Taphonomy and affinity of an enigmatic Silurian vertebrate, Jamoytius kerwoodi White. Palaentology 53(6):1393–1409.

Simonetta AM and Insom E 1993. New animals from the Burgess Shale (Middle Cambrian)and the possible significance for the understanding of the Bilateria. Bolletino Di Zoologia 60:97–107.

White EI 1946. Jamoytius kerwoodi, a new chordatefrom the Silurian of Lanarkshire. Geological Magazine, 83, 89–97.

wiki/Metaspriggina
wiki/Lamprey_Pteromyzon
wiki/Jamoytius
wiki/Pharyngolepis

Chordata

Branchiostoma

Craniata

Mxyine

Metaspriggina

Arandaspis

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