Abstract
Ontogeny—the growth and development of an organism—is among the more poorly understood aspects of the life history of mosasaurs, largely owing to a dearth of fossil material from young individuals. We describe the partial and complete skulls of two subadult individuals of the mosasaurid Tylosaurus proriger from the upper Smoky Hills Chalk Member of the Niobrara Formation in Kansas. We include the more complete of the two specimens in an allometric analysis to better understand proportional changes of the skull through growth. Although our small sample size produces several instances of ‘soft isometry’, we recover the length of the edentulous rostrum as significantly negatively allometric, and quadrate height as significantly positively allometric. In light of our findings, we go on to consider the question of whether T. kansasensis represents an immature ontogimorph of T. nepaeolicus, but find no convincing evidence that this is the case.
Highlights
Mosasauridae is a clade of carnivorous, mostly marine reptiles known from Upper Cretaceous deposits worldwide (Russell, 1967)
We examined allometry in the skull of T. proriger using the linear morphometric dataset of Everhart (2002), itself modified from Russell (1967)
CMN 8162 can be positively attributed to Tylosaurus based on the presence of a prefrontal that does not contribute to the margin of the external naris, a frontal that is excluded from the margin of the orbit by the prefrontal and postorbitofrontal, an edentulous rostral ‘prow’, and 13 maxillary and dentary teeth each (Jiménez-Huidobro et al 2018)
Summary
Mosasauridae is a clade of carnivorous, mostly marine reptiles known from Upper Cretaceous deposits worldwide (Russell, 1967). Aspects of the appearance and life history of its members are well-documented, owing to the excellent fossil record of the family, with some remarkable specimens even retaining traces of soft tissue These have illuminated issues of mosasaur locomotion (Lindgren et al 2009), colouration (Lindgren et al 2010), and physiology (Lindgren et al, 2013). Despite these advances, the ontogeny of mosasaurs remains poorly understood, owing to a lack of reported growth series. Caldwell (1996, 2007), Pellegrini (2007), and Field et al (2015) have discussed various aspects of mosasaur ontogeny, but not the suite of gross morphological changes that attends it. We describe a partial and a complete skull attributable to sub-adults of T. proriger, a large (>12 m long) species known exclusively from the Western Interior
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