Accessory cusp expression at the enamel-dentine junction of hominin mandibular molars

Abstract

Studies of hominin dental morphology frequently consider accessory cusps on the lower molars, in particular those on the distal margin of the tooth (C6 or distal accessory cusp) and the lingual margin of the tooth (C7 or lingual accessory cusp). They are often utilized in studies of hominin systematics, where their presence or absence is assessed at the outer enamel surface (OES). However, studies of the enamel-dentine junction (EDJ) suggest these traits may be more variable in development, morphology and position than previously thought. Building on these studies, we outline a scoring procedure for the EDJ expression of these accessory cusps that considers the relationship between these accessory cusps and the surrounding primary cusps. We apply this scoring system to a sample of Plio-Pleistocene hominin mandibular molars of Paranthropus robustus, Paranthropus boisei, Australopithecus afarensis, Australopithecus africanus, Homo sp., Homo habilis and Homo erectus from Africa and Asia (n = 132). We find that there are taxon-specific patterns in accessory cusp expression at the EDJ that are consistent with previous findings at the OES. For example, P. robustus M1s and M2s very often have a distal accessory cusp but no lingual accessory cusp, while H. habilis M1s and M2s show the opposite pattern. The EDJ also reveals a number of complicating factors; some apparent accessory cusps at the enamel surface are represented at the EDJ only by shouldering on the ridges associated with the main cusps, while other accessory cusps appear to have little or no EDJ expression at all. We also discuss the presence of double and triple accessory cusps, including the presence of a double lingual accessory cusp on the distal ridge of the metaconid in the type specimen of H. habilis (OH 7–M1) that is not clear at the OES due to occlusal wear. Overall, our observations, as well as our understanding of the developmental underpinnings of cusp patterning, suggest that we should be cautious in our comparisons of accessory cusps for taxonomic interpretations.

Further analyses of the structural organization of Homo luzonensis teeth: Evolutionary implications

“…Indeed, tooth crown outline, as well as EDJ morphology and pulp chamber shape have been previously demonstrated to exhibit a reliable taxonomic signal (Skinner et al, 2008(Skinner et al, , 2009(Skinner et al, , 2016Macchiarelli et al, 2013;Kaifu et al, 2015a;Zanolli et al, 2018Zanolli et al, , 2019aPan et al, 2020). Our results indicate that compared with the dentition of H. habilis-that retains Australopithecus-like features and often bears multiple accessory cusps (Davies et al, 2021)-the external crown morphology of the postcanine teeth of H. luzonensis and H. floresiensis appears more derived and simplified, similar to that of Indonesian late Early to early Middle Pleistocene H. erectus (Fig. 2; SOM Fig.…”

Dental data challenge the ubiquitous presence of Homo in the Cradle of Humankind

“…However, the diversity of Australopithecus -like and Paranthropus -like signals that we have found in the southern African purported Homo hypodigm suggests that it is unlikely that all of these specimens belong to Homo . We propose that, in addition to the four that display a Homo signal, a number of other individuals that show australopith affinities (e.g., SE 1508, StW 80 and 81, and StW 669) should be investigated further and compared with the hominin specimens from Olduvai Gorge, Tanzania, currently under study for internal tooth structure ( 74 ). The presence of another hominin taxon not included in the reference sample and represented by one or more of these intermediate specimens cannot be discarded either.…”

Primate tooth crown nomenclature revisited

“…19D ). Skinner et al (2008) and Davies et al (2021) have previously identified different forms of C6 and C7 expression and positioning along the lingual and distal margins of hominoid molars, and differentiated them accordingly in their schematics and discussion. Potential complications regarding cusp expression types and positioning are further exaggerated with the acknowledgement and inclusion of cusp shouldering features.…”

“…We then report on crown feature variation at the EDJ present in the study sample and propose a ‘new’ expected variation schematic for each group. Davies et al (2021) recently proposed the adoption of conservative terms for accessory cusps on hominin lower molars due to difficulties in defining variations in dentine horn presence on the lingual and distal marginal ridge of the EDJ. Specifically, they adopted the terms distal accessory cusp(s) and lingual accessory cusp(s) instead of cusp 6 and cusp 7, respectively.…”

“…Imaging the EDJ has been shown to not only to record the presence and size of dental crown features with increased accuracy, but also allows greater insights into the developmental origin and individual morphology of dental traits ( Skinner et al, 2008 ; Skinner et al, 2010 ). For example, De Pinillos et al (2014) and Martin et al (2017) addressed the potential taxonomic and phylogenetic utility of dental non-metric traits at the EDJ in Late Pleistocene hominins and modern humans, while others have addressed concerns regarding the homology and identity of certain crown features previously described at the outer enamel surface in fossil hominins ( Anemone, Skinner & Dirks, 2012 ; Ortiz et al, 2017 ; Davies et al, 2021 ).…”