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''Gillicus'' was a relatively small, 2-metre long
ichthyodectiform Ichthyodectiformes is an extinct order of marine stem- teleost ray-finned fish. The order is named after the genus ''Ichthyodectes'', established by Edward Drinker Cope in 1870. Ichthyodectiforms are usually considered to be some of the closes ...
fish that lived in the
Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, and the Western Interior Sea) was a large inland sea that split the continent of North America into two landmasses. The ancient sea ...
, in what is now central
North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and th ...
, during the
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...
.


Description

Like its larger relative, ''
Ichthyodectes ctenodon ''Ichthyodectes'' is an extinct genus of ichthyodectid fish which lived during the Late Cretaceous. Fossils of the species included have been found from Canada to Texas. Description ''Ichthyodectes ctenodon'' was a ichthyodectid around in ...
'', ''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 George Fryer Sternberg (1883–1969) was a paleontologist best known for his discovery in Gove County, Kansas of the "fish-within-a-fish" of ''Xiphactinus audax'' with a recently eaten ''Gillicus arcuatus'' within its stomach. Sternberg was bo ...
in 1952. The specimen can be seen at the Sternberg Museum of Natural History in
Hays, Kansas Hays is a city in and the county seat of Ellis County, Kansas, United States. The largest city in northwestern Kansas, it is the economic and cultural center of the region. As of the 2020 census, the population of the city was 21,116. It is ...
. ''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''.


References

Ichthyodectiformes Prehistoric ray-finned fish genera Coniacian genus first appearances Campanian genus extinctions Cretaceous fish of North America {{paleo-rayfinned-fish-stub