According to Wikipedia
“Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times (430 to 370 million years ago) in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, there being currently no evidence that the galeaspids had paired fins. However, Galeaspida are in fact regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase.”
“The defining characteristic of all galeaspids was a large opening on the dorsal surface of the head shield, which was connected to the pharynx and gill chamber, and a scalloped pattern of the sensory-lines. The opening appears to have served both the olfaction and the intake of the respiratory water similar to the nasopharyngeal duct of hagfishes.”
(Fig. 2) from the Late Silurian was described by Zhu et al. 2012.
(Fig. 3) in the large reptile tree (LRT, 1803+ taxa) Dunyu nests between the thelodont, Thelodus, and the osteostracan, Hemicyclaspis (Fig. 3). The resemblance between the three is readily observed. Phylogenetic bracketing (Fig. 3) provides galeaspids with pectoral fins. Closest living relatives are hagfish and sturgeons.
Halstead LB 1985. The vertebrate invasion of fresh water. Philosophical Transactions of the Royal Society London B 309:243–258.
Janvier P 1984. The relationships of the Osteostraci and Galeaspia. Journal of Vertebrate Paleontology 4(3):344–358.
Liu YH 1965. New Devonian aganathans from Yunnan. Vertebrata PalAsiatica 9(2):125–134.
Zhu M and Gai Z-K 2006. Phylogenetic relationships of Galeaspids (Agnatha). Vertebrate PalAsiatica 44:1–27.
Zhu M, Liu Y-H, Jia L-T and Gai Z-K 2012. A new genus of eugaleaspidiforms (Agnatha: Galeaspida) from the Ludlow, Silurian of Qujing, Yunnan, Southwestern China. Vertebrata PalAsiatica. 50 (1): 1–7.