Ontogenetic insights into the significance of mandibular corpus shape variation in hominoids: Developmental covariation between M₂ crypt formation and corpus shape

Abstract

ObjectivesHere, we quantify and compare the cross‐sectional shape of the mandibular corpus between M1 and M2 during growth in Pan paniscus, Pan troglodytes, and Pongo pygmaeus. The goal is to assess the hypothesis that the shape of the corpus is influenced by the development of permanent molars in their crypts, by examining ontogenetic changes in corpus shape and investigating covariation between corpus shape and M2 and M3 molar crypt forms.Materials and MethodsOntogenetic changes in mandibular corpus shape were assessed using landmarks and semilandmarks, and measurements of length, width, and height were used to quantify molar crypts (M2 and M3). Ontogenetic changes in corpus growth from the eruption of M1 to the eruption of M3 were evaluated for each species through generalized Procrustes analysis and principal components analysis in shape–space and form–space. The relationship between corpus shape and molar crypt form was investigated at three different developmental stages using two‐block partial least squares (2B‐PLS) analysis.ResultsThe results show clear differences in growth patterns among all three species and provide evidence that species‐level differences in mandibular corpus growth occur prior to the emergence of M1. The results of the 2B‐PLS analysis reveal that significant covariance between corpus shape and molar crypt form is limited to the developmental stage marked by the emergence of M1, with covariance between corpus shape and M2 crypt width. Corpora that are relatively narrower in the inferior portion of the cross section covary with relatively narrower M2 crypts.ConclusionsThese results have important implications for understanding the taxonomic and phylogenetic significance of mandibular corpus shape variation in the hominoid fossil record.

Anatomy of the mandibular corpus of extant cercopithecids : taxonomy and variation

“…The shape of the extant catarrhines corpus is a useful taxonomic discriminator and has been implicated in taxonomic discrimination among extant cercopithecoids (Takahashi and Pan, 1994; Jablonski et al, 1998; Daegling and McGraw, 2001; Pan et al, 2002, 2008; Wright et al, 2008) and hominoids (Daegling, 1989; Humphrey et al, 1999; Taylor, 2002; Taylor and Groves, 2003; Taylor, 2006a; Pitirri and Begun, 2019; Pitirri et al, 2020). The sources of variation in corpus shape, notably sexual dimorphism and ontogeny, were clarified in extant hominoids (Wood et al, 1991; Daegling, 1996; Brown, 1997; Taylor, 2006b; Singh, 2014; Pitirri and Begun, 2020). Such data are of paramount value to interpret the mandibular hominoid fossil record (White and Johanson, 1982; Chamberlain and Wood, 1985; Lockwood et al, 1996; White et al, 2000; Fabbri, 2006; Skinner et al, 2006; Lague et al, 2008; Haile-Selassie et al, 2015, 2022; Ioannidou et al, 2022).…”