Gillicus
Gillicus was a relatively small, 2-metre long ichthyodectiform fish that lived in the Western Interior Seaway, in what is now central North America, during the Late Cretaceous. Description. Like its larger relative, "Ichthyodectes ctenodon", "Gillicus" had numerous small teeth lining its jaws, and ate smaller fish by sucking them into its mouth, but the teeth of "Gillicus" are so small that the jaws appear almost toothless at first, which has led to the suggestion that "Gillicus" was also a filter-feeder. "Gillicus" was also eaten by its own relative, "Xiphactinus". One particular long fossil specimen contains a nearly  perfectly preserved long "Gillicus arcuatus" inside its ribcage. The "Gillicus" bones have not been digested so the larger fish must have died soon after eating its prey. The cause of death may have been due to injuries, such as a ruptured major blood vessel, caused by the fin of the smaller fish as it struggled while being swallowed. This fossil specimen, FHSM VP-333 (the "Xiphactinus") and FHSM VP-333 (the "Gillicus" inside) is known as the “fish-in-a-fish” specimen and was collected by George F. Sternberg in 1952. The specimen can be seen at the Sternberg Museum of Natural  History in Hays, Kansas. "Gillicus" remains are also present in at least two other "Xiphactinus" specimens. Nearly all of the "Gillicus" specimens collected from the Smoky Hill Chalk are adults or subadults, which suggests that the early stages of their life were spent elsewhere. In 2009 the first specimen of "Gillicus" from Mexico was described. In 2021, the children's book "The Plesiosaur's Neck" coined the common name "pug fish" for the "Gillicus".