Use of continuous cranial shape variation in the identification of divergent crocodile species of the genus Mecistops

Abstract

The discovery of cryptic biodiversity has blossomed under the advancements of genetic techniques, but species identification via morphology remains crucial to effective conservation efforts. In this study, we tested the use of continuous cranial shape variation in distinguishing the two living species of Mecistops: the West African slender-snouted crocodile (M. cataphractus) and the Central African slender-snouted crocodile (M. leptorhynchus). Using a combination of geometric morphometric characters and ratios of linear measurements, we identified statistically significant variation in cranial bone and overall skull shape of mature individuals that corroborates existing molecular and discrete morphological evidence for two distinct, extant species within Mecistops. Specifically, variation in the shape of the nasal appears particularly diagnostic, while ratios involving metrics of snout length to snout width at the premaxillary notch offer distinguishing features easily measured in the field. Because of the complementary results and applications of the morphometric and cranial ratio analyses, we argue that both methodologies remain relevant to species identification. Moreover, we recommend continued cooperation between geneticists and morphologists in diagnosing species of conservation concern.

New frontiers in imaging, anatomy, and mechanics of crocodylian jaw muscles

“…Living crocodylians, and their extinct crocodyliform ancestors, catch the imagination of scientists and the public alike with their unique heads, armored and agile bodies and voracious feeding bouts. Recently, great strides have been made in understanding the functional morphology of the feeding apparatus of crocodylians including studies of anatomy (Bona and Desojo 2011; Bailleul and Holliday, 2017; Sellers et al, 2019; Kuzmin et al 2021; Morris et al, 2021; Sellers, 2021), function (McHenry et al, 2006; Porro et al, 2013; Sellers et al, 2017; Sellers et al, 2022), performance (Erickson et al, 2003; Kerfoot et al, 2014) and evolution (Melstrom and Irmis, 2019; Drumheller and Wilberg, 2019; Carr et al, 2021). Despite a reasonable understanding of the anatomy of the jaw muscles and their related soft tissues within the adductor chamber (e.g., Iordansky 1964; Schumacher 1973; Holliday and Witmer 2007, Porro et al, 2011; Tsai and Holliday 2011; Lessner and Holliday 2020), challenges interpreting their ontogeny, function, and evolution remain.…”