What lies beneath? An evaluation of lower molar trigonid crest patterns based on both dentine and enamel expression

Abstract

The nearly ubiquitous presence of a continuous crest connecting the protoconid and metaconid of the lower molars (often referred to as the middle trigonid crest), is one of several dental traits that distinguish Homo neanderthalensis from Homo sapiens. This study examined variation in trigonid crest patterns on the enamel and dentine surfaces to (1) evaluate the concordance between the morphology of trigonid crests at the inner dentine and the outer enamel surfaces; (2) examine their developmental origin(s); and (3) examine trait polarity through comparison with Australopithecus africanus and Pan. The sample included 73 H. neanderthalensis, 67 contemporary H. sapiens, 5 A. africanus, and 24 Pan lower molars. Results indicate general agreement in the morphology observed on the dentine and enamel surfaces. All but one H. neanderthalensis molar shows some trigonid crest development, whereas trigonid crests occur in low frequency in contemporary humans. Pan and A. africanus both also show high frequencies of a continuous trigonid crest. However, the origin of the trigonid crest differs among groups. H. neanderthalensis uniquely possesses a 'middle' trigonid crest that originates from the mesial accessory ridge of one or both cusps. Based on our results we suggest that presence of a continuous middle trigonid crest at the dentine surface is primitive and the lack of any trigonid crest is derived. Genetic drift may explain the high frequency of trigonid crests in H.neanderthalensis. However, H. neanderthalensis still appears to be derived relative to Pan and A. africanus in its high frequency of the mesial-mesial trigonid crestconfiguration.

Morphological description and comparison of the dental remains from Atapuerca-Sima de los Huesos site (Spain)

“…In particular, their dental morphology has been widely analyzed (e.g., Boule, 1923;Keith, 1924Keith, , 1925Krogman, 1927;Weidenreich, 1937;Boule and Vallois, 1957;Patte, 1959), and, beyond the high fr equencies of taurodontism (e.g., Keith, 1924;Hillson, 1986;Klein, 1999) or pronounced shovel shape (e.g., Mizoguchi, 1985;Crummett, 1995;Bailey, 2002a), other dental traits have been identified as "unique" to this species (e.g., Zubov, 1992a,b;Bailey, 2002a,b;Martinon-Torres et al, 2007a,b;Gomez-Robles et al, 2007, 2011Bailey et al, 2011 ). However, a true assessment of Neanderthal "uniqueness" and its evolutionary significance requires comparison to European Middle Pleistocene populations, considered by many researchers as the direct ancestors of the classic Neanderthals (e.g., Hublin, 1982Hublin, , 1984Hublin, , 1996Hublin, , 2009Stringer, 1985Stringer, , 1993Arsuaga et al, 1993Arsuaga et al, , 1997c.…”

“…Paradoxically, the Mauer mandible has Neanderthal dental proportions (Rosas and Bermudez de Castro, 1998) but it lacks some distinctive morphological traits of the European Middle Pleistocene population, such as the typical combination of the middle trigonid crest and a deep, pit-like anterior fovea (Martinon-Torres, 2006;Bailey et al, 2011 ), or the typical Neanderthal lower second premolars with a reduced exocentric occlusal polygon (Martinon Torres et al, 2006). In this context, we suggest that a revision of the H. heidelbergensis taxon would be necessary, due to the lack of consensus among scholars about its taxonomic distinctiveness and its phylogenetic position (see Stringer, 1985;Arsuaga et al, 1997c;Ta ttersall and Schwartz, 1999;Hublin, 2009).…”

“…In our sample, H. neanderthalensis expresses a continuous crest in 53.3% of cases and it is concomitant with a pit -like anterior fovea in 46.4% of the specimens. Mountmaurin and 3 out of 4 Arago individuals present this Neanderthal-like combination, whereas in Mauer, despite being the holotype, it is absent (Martin6n-To rres, 2006; Bailey et al, 2011 ). In the SH sample, the middle trigonid crest is continuous in 95% of the specimens, and its expression is concomitant with a grade 2 anterior fovea in 85% of the sample.…”

“…The concomitant expression of both fe atures has been considered typical of Neanderthals (Zubov, 1992a,b;Bailey, 2002a,b;Bailey et al, 2011 ) and European Middle Pleistocene populations (Martin6n-To rres et al, 2007a,b), although it can also be found in Asian H. erectus from Java in lower percentages (Martin6n-Torres, 2006;Martin6n-Torres et al, 2007a,b). In our sample, H. neanderthalensis expresses a continuous crest in 53.3% of cases and it is concomitant with a pit -like anterior fovea in 46.4% of the specimens.…”

“…denecting wrinkle in M 2 , C6 in M 2 , C7 in M3. and multiple lingual cusps in P3, instead of in the most stable tooth of the dental field, as their frequencies in those dental classes are more informative to differentiate between H. sapiens and H. neanderthalensis (see Bailey, 2002a,b: Martin6n-Torres, 2006: Bailey et al, 2011. Finally, in order to enlarge the sample size, we have recorded the mesial marginal accessory tubercles and the transverse crest in M 3 and the CS in M 2 , since these traits are easily worn out in the M 2 and the M l , respectively.…”