Lasanius is a genus of basal agnathan (jawless) anapsid fish from the Early Silurian. Its fossils are around 10 mm to 150 mm in length.
Fossils have been found in Early Silurian-aged strata in Ayrshire, Muirkirk, Scotland and Seggholm.
A comprehensive redescription published in 2023 found that it was a stem-group cyclostome, related to hagfish and lampreys.
References
R.H. Traquair M.D. LL.D. F.R.S. (1898) IX.—Notes on Palœozoic fishes.—No. II, Annals and Magazine of Natural History, 2:7, 67–70, DOI: 10.1080/00222939808678013
Traquair, R. (1899). XXXII.—Report on Fossil Fishes collected by the Geological Survey of Scotland in the Silurian Rocks of the South of Scotland. Transactions of the Royal Society of Edinburgh, 39(3), 827–864. doi:10.1017/S0080456800035237