Centrosaurus apertus, a large bodied ceratopsid from the Late Cretaceous of North America, is one of the most common fossils recovered from the Belly River Group. This fossil record shows a wide diversity in morphology and size, with specimens ranging from putative juveniles to fully-grown individuals. The goal of this study was to reconstruct the ontogenetic changes that occur in the craniofacial skeleton of C. apertus through a quantitative cladistic analysis. Forty-seven cranial specimens were independently coded in separate data matrices for 80 hypothetical multistate growth characters and 130 hypothetical binary growth characters. Both analyses yielded the max-limit of 100,000 most parsimonious saved trees and the strict consensus collapsed into large polytomies. In order to reduce conflict resulting from missing data, fragmentary individuals were removed and the analyses were rerun. Among both the complete and the reduced data sets the multistate analyses recovered a shorter tree with a higher consistency index (CI) than the additive binary data sets. The arrangement within the trees shows a progression of specimens with a recurved nasal horn in the least mature individuals, followed by specimens with straight nasal horns in relatively more mature individuals, and finally specimens with procurved nasal horns in the most mature individuals. The most mature individuals are further characterized by the reduction of the cranial horn ornamentations in late growth stages, a trait that similarly occurs in the growth of other dinosaurs. Bone textural changes were found to be sufficient proxies for relative maturity in individuals that have not reached adult size. Additionally, frill length is congruent with relative maturity status and makes an acceptable proxy for ontogenetic status, especially in smaller individuals. In adult-sized individuals, the fusion of the epiparietals and episquamosals and the orientation of the nasal horn are the best indicators of relative maturity. This study recovers no clear evidence for sexually specific display structures or size dimorphism in C. apertus.

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Ceratodontid lungfishes are generally rare, poorly represented elements of North America’s Mesozoic ecosystems, with previously known maximum diversity in the Late Jurassic. Herein we describe four new species of the form genusCeratodus, from the Cretaceous of the Western Interior, considerably expanding fossil representation of post-Triassic dipnoans in North America. To model taxonomic and morphologic diversity, we adopt a four-fold system of phenetically based species groups, named for exemplars from the Morrison Formation.Ceratodus kirklandin. sp. (Potamoceratodus guentherigroup) andC.kempaen. sp. (C.frazierigroup) represent a hitherto unsampled time interval, the Valanginian.Ceratodus nirumbeen. sp. andC.molossusn. sp. extend the temporal ranges of theC.fossanovumandC.robustusgroups upward to the Albian and Cenomanian, respectively. These new occurrences show that ceratodontids maintained their highest diversity from the Late Jurassic through the mid-Cretaceous (Albian–Cenomanian), an interval of ~60 Myr. The existing record suggests that some of the later (mid-Cretaceous) ceratodontids may have been tolerant of salt water; to date, there is no evidence that they aestivated. Only a few occurrences are known from horizons younger than Cenomanian. Demise of ceratodontids appears to be part of a broader pattern of turnover that occurred at the Cenomanian-Turonian boundary in North America.