The type specimen[a] of Goronyosaurus, BMNH R 5674 consisting of two vertebrae, was found within a section of the Dukamaje Formation called the "Mosasaurus shales" in northwestern Nigeria[5] and was noted by Franz von Nopcsa in 1925, but he gave no species or genus name,[6] and these remains were described more completely by Swinton et al. in 1930.

[1] The holotype material included only isolated vertebrae, a single femur, mandible fragments and teeth which Swinton et al. described and named as the new species Mosasaurus nigeriensis.

Because of the material being found in the same horizon of the same region of Nigeria, and the similarities in size and anatomy, these new specimens were referred to M. nigeriensis by Augusto Azzaroli and his colleagues.

[4] Discussions of the jugal bone were presented significantly in Soliar (1988), due to the supposed morphology that was completely unlike any in known mosasaurs.

Azzaroli et al. (1972) proposed that Goronyosaurus has an extremely long and tall jugal, which directly contrasts with the thin slender bones of other lepidosaurs.

The two main processes of the pterygoid are broken, because of their long and slender shape, but it probable looked similar to related taxa.

The sides of the basisphenoid are unusually steep, and the vidian canal is uncovered, probably due to breakage of the thin sheet of bone that normally covers it.

[10] Due to its unique characteristics and features, Goronyosaurus is notoriously difficult to classify and is left out of most phylogenetic analyses.

[4] A placement within the Tylosaurinae is not necessarily correct however, as the genus was found as forming a clade with the genera Prognathodon, Plesiotylosaurus and Ectenosaurus, which would place it within the Mosasaurinae, in a comprehensive 2010 analysis of the Anguimorpha.

[12] Halisaurinae Plioplatecarpus primaevus Platecarpus tympaniticus Lakumasaurus antarcticus Taniwhasaurus oweni Tylosaurus proriger Tylosaurus nepaeolicus Hainosaurus bernardi Ectenosaurus clidastoides Goronyosaurus nigeriensis Plesiotylosaurus crassidens Prognathodon solvayi Prognathodon overtoni Clidastes liodontus Globidens alabamaensis Mosasaurus hoffmanni Mosasaurus lemonnieri Moanasaurus mangahouangae Plotosaurus bennisoni Plotosaurus tuckeri The clade of Goronyosaurus and Prognathodon, and the other branch with Mosasaurini, were found to be grouped by two unique (unambiguous) features, the lack of frontal bone bordering the nasal opening, and a humerus with a hooked process behind the glenoid cavity.

[13][11] A clade of Goronyosaurus and Plotosaurus was diagnosed by the presence of teeth to the very front of the premaxilla, extension of the tooth row below and behind the orbit, frontally contacting the maxillae, the unforked shape of the contact between skull roof and supratemporal arch bones, location of the vidian canal opening moved posteriorly, absence of zygapophyseal articulations in vertebrae, and complete separation of the deltoid anc pectoral muscle crests on the humerus.

[17] All known Goronyosaurus fossils come from the Dukamaje and Farin Doutchi Formations of Niger and Nigeria, both of which lie within the Iullemmeden Basin.

Precise dating both of deposits have proven difficult due to the absence of reliable index fossils, but geologists generally agree that the formations are at least Maastrichtian in age based on the occurrence of the ammonite Libycoceras.

Some studies suggest the formations can be further constrained to the mid-late Maastrichtian (~70-66 million years ago) based on nannofossil assemblages.

[19] The region of the basin that Goronyosaurus inhabited represented an insular flat coastal and estuarine environment characterized by lagoons and salt marshes.

The Dukamaje Formation may have in particular represented a sabkha with alternating cycles of flooding and evaporation and influence from brackish water.

The specialization towards a crocodile-like skull through an elongated and rigid tube-like build, straight conical interlocking teeth, and highly developed snout sensitivity and sense of smell was well-suited for pursuing prey in the environment's unique murky setting.

Lingham-Soliar (1999) opined that Goronyosaurus represented an incipient stage towards a freshwater invasion of rivers, which would have been achieved had it not been for the Cretaceous–Paleogene extinction.