Megacephalosaurus (/ˌmɛɡəˈsɛfəloʊˈsɔːrəs/; "great-headed lizard") is an extinct genus of short-necked pliosaur that inhabited the Western Interior Seaway of North America about 94 to 93 million years ago during the Turonian stage of the Late Cretaceous, containing the single species M. eulerti.

The first specimen of Megacephalosaurus was discovered by two teenage brothers named Frank and Robert Jennrich while collecting fossil shark teeth near Fairport, Kansas.

After conferring with Samuel Paul Welles of UC Berkeley and other paleontologists during the 1950 Society of Vertebrate Paleontology meeting in Albuquerque, New Mexico, Sternberg identified FHSM VP-321 as the Late Cretaceous pliosaur Brachauchenius lucasi.

Although additional associated fossils of the specimen were recovered during its excavation, Sternberg had the skull and left lower jaw embedded on a mount of plaster and displayed it in the museum's public exhibits by 1951.

[6] In November 2003, Robert Jennrich led staff members of the Sternberg Museum to the original locality where he and his brother discovered the specimen.

To confirm the perceived differences, FHSM VP-321 was arranged to be removed from its mount and Everhart collaborated with paleontologists Bruce A. Schumacher of the USDA Forest Service and Kenneth Carpenter of the USU Eastern Prehistoric Museum in a study to examine the underside of the skull.

When it was recovered from the collections of the University of Nebraska State Museum for study, there was strangely no contextual information or records pertaining to the large fossil.

[3] No stratigraphic data was recorded for USNM 50136, but tests on a piece of matrix extracted from the fossil identified nannofossil assemblages that are most associated with deposits of the Greenhorn Limestone likely dated around 93.9 mya at earliest.

Unlike in pliosaurs such as Liopleurodon or Pliosaurus, the maxillary teeth in Megacephalosaurus do not reduce in size towards the base of the skull and are all consistently large.

It is believed that pits represent vestigial dental alveoli that once held an additional pair of teeth in ancestral species but devolved in Megacephalosaurus.

Some characteristics that are shared by most brachauchenines like Megacephalosaurus includes skull features (such as an elongated snout, gracile rostrum, and consistently sized teeth) that are better adapted for a general evolutionary shift towards smaller prey.

[9] A 2018 study by Daniel Madzia of the Polish Academy of Sciences, Sven Sachs of the Natural History Museum, Bielefeld, and Johan Lindgren of Lund University hypothesized that the presence of these inconsistencies indicates that the trait for consistently-sized teeth evolved independently within the Brachaucheninae three times; these occurrences being independently in Luskhan, Stenorhynchosaurus, and in a clade that includes Megacephalosaurus and Brachauchenius.

[4] An early phylogenetic attempt including a Megacephalosaurus specimen was performed in a 2012 study led by Roger Benson of the University of Oxford.

[10] In 2013, Benson led another study that attempted another phylogenetic analysis using a better description of FHSM VP-321 made by Schumacher et al. (2013), being named as Brachauchenius eulerti.

This time, the study recovered FHSM VP-321 from a polytomy clade shared by Brachauchenius and a pliosaur specimen cataloged as DORK/G/1-2 but doubtfully labeled as 'Polyptychodon interruptus', with one outgroup consisting of Kronosaurus, under a strict reduced consensus.

In an alternate strict consensus analysis, the study recovered the FHSM VP-321 from a polytomy clade that includes Pliosaurus and Gallardosaurus.

[11] Another study published in 2015 by Andrea Caua and Federico Fanti of the University of Bologna yielded similar phylogenetic results in a strict consensus method.

[4] It inhabited the Western Interior Seaway that spanned the middle of North America and cut it in two during the Turonian stage of the Late Cretaceous.

[2] The pliosaur was present during the Cenomanian-Turonian boundary event, a period marked with significant worldwide faunal turnovers and extinctions caused by an abnormally intense increase in underwater volcanism, which ushered a global anoxic event that acidified the oceans, increased global temperatures, and caused a mass extinction that led to the disappearance of 26% of the entire marine fauna.

[14] The fossil assemblages of the Turonian-aged portions of the Fairport Chalk and Greenhorn Shale are considered small in terms of the number of present species but nevertheless encompassed a wide ecological diversity.

Many of their fossils are too fragmentary to be properly identified, but known Turonian taxa include but not limited to Enchodus, Pachyrhizodus, and the ichthyodectids Ichthyodectes and Xiphactinus.