Description of a Juvenile Triceratops Skull and Lower Jaw and Comparison of Juvenile Ceratopsian Characteristics

Abstract

A juvenile Triceratops partial skull and lower jaw from the Upper Cretaceous (66 million years) Hell Creek Formation near Jordan, Montana, was collected in 2014. Having been collected near the top of the upper third of the Hell Creek, the fossil material likely belonged to Triceratops prorsus. Main elements from the skull (Emporia State University Paleontology Collection specimen number ESU 2014-1) include the parietal, the right postorbital horn, partial left postorbital horn, the right maxilla, a partial left maxilla, a left nasal, and a rostral. Both left and right dentaries and the predentary were collected. Growth stages of Triceratops are: baby, juvenile, subadult, and adult. The size of the skull material and the posteriorly-curving brow horn indicate that this was an older, medium-to-large sized juvenile specimen, with a snout-to-occipital condyle length of about 65 cm. Erin Roberts Master of Science Physical Sciences, Earth Science Concentration May 9, 2019 Description of a Juvenile Triceratops Skull and Lower Jaw and Comparison of Juvenile Ceratopsian Characteristics

Comparisons among Triceratops juveniles and between juveniles of other ceratopsian species focused on ontogenetic features. Characteristics of juvenile Triceratops are posteriorly curving brow horns, large orbits in comparison to skull size, delta shaped epoccipitals that are not fused to the frill, and unfused nasals. In babies, the brow horn was straight, but in subadults it angled forward. ESU 2014-1 shows individual variation from other Triceratops of the same growth stage in the shorter length of the postorbital horns, the larger size of the bony prominence along the parietal midline, and the shallow scalloping of the frill. A survey of ceratopsian literature revealed that besides the size of the individual, common characteristics for juvenile ceratopsians of several genera include proportionally larger orbits, unfanned and/or scalloped frills, amount of striated bone texture, unfused nasals, lack of horn core sinuses, and unfused skull sutures. Identification of the juvenile growth stage of ceratopsians must be based on the observation of multiple juvenile traits in the individual specimen.