Thrissops were fast predatory fish about 60 centimetres (24 in) long, that fed on other bony fish.[2] They had a streamlined body with a deeply cleft tail and only very small pelvic fins. Thrissops was related to the giant Xiphactinus and may have been an ancestor of the modern Osteoglossiformes, the most primitive group of living teleosts, which includes the arapaima.