Analysis of Megadontia in Hominins

Transcription

1 Analysis of Megadontia in Hominins Introduction Hominin teeth are important for telling us about the dietary strategy of these extinct species. In particular, the degree of megadontia allows inferences into how mechanically demanding a diet was. Extremely large molars with molarized premolars are an indication of an extremely demanding diet, consisting of hard objects. In contrast, small molars in comparison to the incisors indicate a softer diet (Wood and Collard, 1999). An abundance of teeth in the fossil record enables a comprehensive analysis of the dentition in nearly all hominin species. Materials and Methods The dentition of twelve cast specimens was measured using sliding calipers. The species Australopithecus afarensis, A. africanus, Paranthropus boisei, P. robustus, Homo habilis, H. ergaster, H, erectus, H. floresiensis, amd H. neanderthalensis were represented by one cast each. A. afarensis, H. erectus and H. neanderthalensis were represented by composite specimens while the remaining specimens were casts of specific finds. H. sapiens was represented by two casts, one of a female and one of a male. Measurements were done on maxillary teeth where possible, and on right side teeth. The results of these measurements are show in Figure 1. Measurements were taken on the medial incisor, canine, premolars and molars. Teeth measured from the left side or from the mandible are indicated by color coding, yellow and blue respectively. Green represents both mandibular and left. All measurements are given in centimeters. All teeth were measured at the widest point. For incisors, premolars and molars this located at the border of the occlusal surface, while on

2 canines this is at the base of the enamel. Mesial to distal measurements were done as close to the midline of the tooth as possible. Mandibular and maxillary teeth are not strictly comparable as mandibular incisors tend to be much smaller, however each skull did reveal potentially useful information. In the two specimens, H. floresiensis and P. boisei, for which no maxillary I 1 were present, the premolars can still be compared to the molars to assess degree of premolarization. When possible, measurements from the literature were used as a stand in for measurements that were not present in lab specimens. After measurements were taken, a spreadsheet was used to determine multiple ratios in order to assess relative sizes of teeth. These ratios were calculated by simply dividing the more mesial tooth s measurement by the more distal tooth s measurements. I 1 s mesial-distal width was compared to the width of the canine, and to both the mesial-distal and buccal-lingual dimensions of the premolars and molars. Both premolars were compared with molars using both mesial-distal and buccal-lingual dimensions. In an attempt to reveal dentition trends, and compare them to modern apes, data was obtained on Ardipithecus ramidus and Pan troglodytes. The Pan troglodytes data was obtained from a female specimen in the lab and A. ramidus was from one of the Science articles describing the A. ramidus find (Suwa et al., 2009). Results In order to determine the ancestral dental condition, A. ramidus is used as the earliest known hominin. This species is sufficiently old enough at 4.4 million years old that it likely represents a similar condition to the last common ancestor of humans and the Great Apes. A.

3 ramidus shows a reduced level of megadontia compared to australopithecines, and had small incisors indicating that while it did not have a mechanically demanding diet, it probably did not rely as much on fruit as modern day chimpanzees (Suwa et al., 2009). A. ramidus also shows reduced canine size in relation to other teeth, which is one trait that is not shared with earlier apes (Suwa et al., 2009). The next hominin chronologically is A. afarensis. In the cast measured we find that the trend of reduced canine is continued, with the canine being approximately the same size as the medial incisor. The premolars, M 1 and M 2 are all similar size mesially-distally to the medial incisor. M 3 is larger than I 1, but not significantly so (ratio of.8). The premolars do not show extensive molarization, and are significantly smaller than the molars. This is especially obvious in the mesial-distal dimensions, in which the molars are nearly twice the size of the premolars. Buccally-lingually, the premolars are closer to the molars in size but are still smaller. A. africanus shows a much larger amount of megadontia than afarensis. The medial incisor is much smaller than the molars, showing ratios of.7 or less. This is especially apparent in M 3, which is nearly twice as large as the medial incisor. The premolars still do not show molarization; they are smaller relatively than in afarensis and are much smaller than the molars. The canine is significantly smaller than the medial incisor (1.17 ratio) although is larger relatively than the afarensis canine. P. boisei was the first of the robust australopithecines measured. This species shows remarkable megadontia, with the medial incisor measuring less than half the size of the molars. The medial incisor was also significantly smaller than the premolars. The canine is reduced from africanus dimensions and is more similar to afarensis. The molars are not only relatively large,

4 but also absolutely large. Despite being a small bodied species, with males weighing less than 50kg (McHenry, 1992), this specimen exhibited molars much larger than any other hominin, nearly twice as large as H. sapiens. While the premolars are still relatively small compared to the molars, their relative size compared to the incisors indicates that there is molarization present. This was the first hominin measured that showed premolars that were extremely large in comparison to the incisors. The other robust australopithecine measured was P. robustus. Maxillary measurements for the medial incisor were not available from the cast. As a result, measurements from from existing literature were used. These measurements were from a more recent find, DNH 77 (Moggi-Cecchi et al., 2010). P. robustus continues to show a small canine, similar in size to the medial incisor. The premolars are not considerably larger than the medial incisor, a feature different from P. boisei. The premolars exhibit a similar proportion to the molars found in P. boisei although are slightly larger. The molars are much larger than the incisors. They are not as absolutely large as found in boisei, and not as large relative to incisors. A cast of KNM-ER 1813, representing H. habilis was measured. This specimen was very similar in proportions to A. africanus. The extremely large molars found in Paranthropus are no longer present. The premolars are similar in size to the molars, showing the reduced size of the molars in comparison to the rest of the maxilla. The cast of KNM-WT 15000, representing H. ergaster was measured. This specimen exhibits extremely large incisors, much more so than any other species measured. The medial incisor was the largest tooth, even more so than the molars. This species shows the first dramatic shift from the proportions found in Australopithecus. The molars and premolars are no longer

5 extremely large, instead the anterior teeth are much larger. This could be representative of a dietary shift away from needing large molars for crushing hard foods. H. erectus was represented by a composite cast from Asian specimens. This specimen no longer shows the extremely large incisors found in H. ergaster. It shows a return to large molars and premolars found in Paranthropus. The measurements from this cast are somewhat suspect as Asian specimens of H. erectus are very fragmentary and so this cast may not be the most accurate. H. floresiensis is represented by a cast of LB1, the type specimen. Maxillary measurements for the incisors were not available, but the mandibular incisors can be compared to the mandibular M 3. I 1 is much smaller than M 3, which may be a result of mandibular incisors being smaller in general. The premolars show similar proportions to the molars similar to more modern Homo species. The species does show large teeth for its size, with absolute size being similar to H. sapiens. H. neanderthalensis and H. sapiens show very similar tooth proportions. The incisors are similar in size to the molars and premolars. The megadontia shown in Paranthropus and Australopithecus is completely absent. As a comparison to other living apes, a female chimpanzee maxilla was also used. Other than an extremely large canine, the dentition was remarkably similar to H. ergaster. It exhibited extremely large incisors, a frugivorous adaptation. Molars and premolars were relatively small.

6 Discussion Analyzing the trends of molar size through time, a distinct trend emerges. Australopithecus starts out with somewhat large molars, then as Paranthropus emerges we see extremely large molars and premolars. With the emergence of Homo, we see a reduction in molar size, with an emphasis on larger anterior teeth. This can be seen in Figure 2. Earlier Australopithecus and Paranthropus have check teeth times larger in relation to body size than modern humans (McHenry, 1984). The findings in this study are consistent with other studies of hominin teeth (Wood and Collard, 1999; Suwa et al., 2009; Molnar and Gantt, 1977; Moggi-Cecchi et al., 2010; McHenry, 1992, 1984; Lacruz et al., 2008) which show a trend of dietary shifts in hominin evolution. Hominins began with relatively larger molars, designed for mechanically demanding diet. This lead to extreme megadontia in the specialized Paranthropus species. This extreme specialization was later selected against, leading to the trend of reduced dentition seen in more modern hominins and Homo sapiens.

7 Figure 1

8 I1-M2 (mesial-distal) ratio I1-M2 (mesial- 0 Figure 2

9 Literature Cited Lacruz RS, Dean MC, Ramirez-Rozzi F, and Bromage TG Megadontia, striae periodicity and patterns of enamel secretion in Plio-Pleistocene fossil hominins. Journal of Anatomy 213: McHenry HM Relative Cheek-Tooth Size in Australopithecus. American Journal of Physical Anthropology 64: McHenry HM Body size and proportions in early hominids. American Journal of Physical Anthropology 87: Moggi-Cecchi J, Menter C, Boccone S, and Keyser A Early hominin dental remains from the Plio-Pleistocene site of Drimolen, South Africa. Journal of Human Evolution 58: Molnar S, and Gantt DG Functional implications of primate enamel thickness. American Journal of Physical Anthropology 46: Suwa G, Kono RT, Simpson SW, Asfaw B, Lovejoy CO, and White TD Paleobiological Implications of the Ardipithecus ramidus Dentition. Science 326:69-69, Wood B, and Collard M The Human Genus. Science 284:65-71.