Description of dental roots of Sahelanthropus tchadensis
The root morphology of the mandibles TM266-02-154-1 and TM292-02-01 are displayed in Figures 1 and 2, respectively. Measurements of maximal root length are presented in Table 3.
Figure 1. Radicular morphology of Sahelanthropus tchadensis TM266-02-154-1. (A) Lingual view, (B) buccal view, (C) Occlusal view, (D) ventral view. The arrow points to a small secondary root growth.
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Figure 2. Radicular morphology of Sahelanthropus tchadensis TM292-02-01. (A) Occlusal view, (B) ventral view, (C) buccal view of left side, (D) lingual view of left side, (E) lingual view of right side, (F) lingual view of right side.
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Table 3. Maximal root length of Sahelanthropus tchadensis specimens
| ||TM266-02-154-1||TM292-02-01 left||TM292-02-01 right|
|I1|| || ||>11.5 mm|
|I2|| ||>14.0 mm||>10.5 mm|
|C1|| ||27.97 mm||>24.0 mm|
|P3||18.11 mm||>12.5 mm||16.53 mm|
|P4||16.44 mm||16.13 mm||>11.5 mm|
|M1||19.75 mm||20.87 mm||20.16 mm|
|M2||18.82 mm||>12.5 mm||>14.5 mm|
|M3||14.71 mm|| || |
Roots of incisors are only partially preserved in TM292-02-01. They are ventrally as deep as the roots of premolars and molars and mesio-distally broad. Their transversal section is rounder than that of African apes. Canine root on TM292-02-01 is robust, especially when compared to the reduced crown, ventrally deep but only slightly curved. Its transversal section is bucco-lingually compressed.
In both TM266-02-154-1 and TM292-02-01, P3 displays two roots: one mesial root with a single pulp canal, and a distal root with two pulp canals. This distal root is bucco-lingually narrow in comparison to two-canal roots of other hominoids, especially African apes (Table 4). The mesial root is more buccal than the distal one; this character is clearer in TM266-02-154-1 than in TM292-02-01. Roots of P3 are subparallel, although the distal root is slightly more curved apically than the mesial one in TM266-02-154-1.
Table 4. Ratio between maximum mesio-distal breadth and maximum bucco-lingual breadth of the distal root of P3
| ||Sample||Mean value||Standard deviation|
|Tomes' roots||Two modern Homo sapiens; two Pan troglodytes; Australopithecus africanus STS52b||0.893||0.031|
|Comparative sample (roots with two canals)||One modern Homo sapiens; five Gorilla gorilla; six Pan sp.||0.524||0.065|
|Sahelanthropus tchadensis||TM266-02-154-1, TM292-02-01||TM266: 0.591, TM292: 0.687|| |
P4 of both specimens display two rectilinear, vertical, subparallel, and robust roots. The bifurcation (the point where horizontal diaphragmatic processes converge, as defined by Kovacs, 1971) of the roots is very apical, and even after the bifurcation the roots are in contact along the two thirds of their length.
M1 and M2 in both specimens display three robust roots, two mesial ones and a distal root. The mesial roots are connected through a very thin blade (as defined by Kupczik, 2003) for half of their length. In TM266-02-154-1, the mesio-buccal root of M2 divides itself at half of its length, leading to the formation of a small secondary root in central position (Fig. 1). This rare configuration is observed in extant specimens [in our comparative sample, in a M1 of Pan troglodytes and a M2 of Gorilla gorilla; Kovacs (1971), also reports its appearance in modern Homo sapiens, Gorilla gorilla, and several Carnivora] and linked to the folding of the epithelial diaphragm during the penetrative phase of root development (Kovacs, 1971). Its single occurrence is likely to have an epigenetic origin (Kovacs, 1971; Kupczik, 2003). Mesial roots of M1 and M2 are slightly curved, mesio-distally thin and bucco-lingually narrow. Distal roots of M1 and M2 are very deep, rectilinear and bucco-lingually narrow. They display two pulp canals, which converge toward the apex. In TM266-02-154-1, the separation between the penetrative and eruptive phase (Kovacs, 1971) of the distal root of M1 is marked by a deep groove.
In TM266-02-154-1, the mesial roots of M3 are similar to those of M1. The distal root is a short, rectilinear, robust root with a triangular section. This morphology of the distal root of M3 is observed in all extant and fossil hominoid species (Emonet et al., 2012).
Comparison with other hominoids
A count of roots and pulp canals, collected from previous studies (Wood et al., 1988; Shields, 2005) and new data, is proposed in Table 1. TM266-02-154-1 and TM292-02-01 display the same number of roots and pulp canals for each tooth, which suggests that this morphology was at least not uncommon in Sahelanthropus tchadensis. The morphology of roots of premolars (two roots and three canals for P3, two roots and four canals for P4) corresponds to the most common morphology in Gorilla, Pan and Australopithecus (Wood et al., 1988; Table 1), which is also found, though uncommonly, in fossil and extant Homo species (Wood et al., 1988; Shields, 2005; Table 1). Ardipithecus ramidus and A. kaddaba display a slightly different morphology, with only one root in P4 (Suwa et al., 2009). In Sahelanthropus specimens, the morphology of roots of M1 and M2 (three roots and four canals) is also the most common morphology in African apes (Table 1) but not in modern Homo sapiens. In the latter species, the large majority of specimens display two roots and three or four canals (Shields, 2005).
The reduction of the size of the crown of C1 in Sahelanthropus tchadensis and Ardipithecus ramidus when compared to extant and fossil apes (Brunet et al., 2002, 2005; Suwa et al., 2009) is not reflected in the root morphology. The length of root of TM292-02-01 (Table 3) is comparable to that of Pan troglodytes (26.8–30.7 mm, Kupczik, 2008), much larger than the length of modern Homo sapiens (13.2–19.2 mm, Kupczik et al., 2008; Le Cabec et al., 2012) or Homo neanderthalensis (17.2–25.6 mm, Le Cabec et al., 2012). The roots of C1 are robust and ventrally deep in Sahelanthropus tchadensis and Ardipithecus ramidus, even more so in S. tchadensis than in A. ramidus (Suwa et al., 2009). This morphology for early hominins is much like that of Pan or Gorilla, while the roots of C1 are thinner and ventrally shallower in australopithecins and Homo (Wood et al., 1988; Kupczik, 2003; Shields, 2005).
The morphology of P3 roots in Sahelanthropus, including the buccal position of the mesial root and the bucco-lingual compression of the distal root, differs from the morphologies observed in African apes. This peculiar morphology is also commonly reported in australopithecins (Wood et al., 1988). The distal root has two pulp canals, but its bucco-lingual breadth is low for a two-canals root (especially in TM292-02-01, Table 4), closer to the values observed in Tomes' roots than other distal roots with two canals (Table 4). Tomes' roots are frequent in early Homo (Wood et al., 1988) and observed also in extant humans (Shields, 2005). The morphology observed in Sahelanthropus could thus represent in a morphological series an intermediary state between African ape morphology and the hominin morphology.
The morphology of P4 of TM266-02-154-1 and TM292-02-01 also differs from that of African apes. The ventrally deep, very straight roots are in contact for at least the two thirds of their length (TM266-02-154-1) up to the three quarters (TM292-02-01); only the most apical parts of the roots are clearly separated. As a result, the transversal section of the root appears like a rounded square (Figs. 1 and 2). This rare morphology is not reported in African apes, but is sometimes observed in Australopithecus (e.g., KNM-ER 3734, 1483, 1501; OH16, 23, 37; Wood et al., 1998). Like for P3, the morphology of roots of P4 in Sahelanthropus evokes that of later hominins, notably Tomes' root (Ardipithecus kaddaba, Australopithecus) and single-rooted P4 (Ardipithecus ramidus, Australopithecus, Homo), and could be interpreted in a morphological series as an intermediary state between African apes and human morphologies.
The roots of M1 and M2 are rectilinear in African apes, while they are curved in humans, concave to the distal (respectively mesial) side for the mesial (distal) roots. The roots of molars of Sahelanthropus are curved like those of humans, even if the distal root of the first molar is only slightly curved (Figs. 1 and 2). They are also absolutely longer (Table 3) than those of Homo sapiens (14.6–15.3 mm for M1, 14.7–15.8 mm for M2, Kupczik et al., 2008, 2010) and Pan troglodytes (12.1–15.4 mm for M1, 11.1–14.3 mm for M2, Kupczik et al., 2008).