on the relative frequencies of hominid maxillary and mandibular teeth and jaws as taphonomic...

HUMAN EVOLUTION Vol. 2 - N. 4 (297-309) - 1987

P. V. Tobias Palaeo-anthropological Research Unit, Department o/ Anatomy University of the Witwatersrand, Johannesburg

Key words: Hominid teeth, Plio- Pleistocene; Taphonomy, East Africa, Southern Africa.

On the Relative Frequencies of Hominid Maxillary and Mandibular Teeth and Jaws as Taphonomic Indicators

In southern African samples of early hominid remains, maxillary and mandibular teeth (deciduous-plus-permanent) have a virtually equal chance of accumulating in the dolomitic limestone cave deposits, of being preserved therein and recovered therefrom. Thus, of 1066 fossil teeth of Australopithecus spp. plus Homo habilis, 51.9 per cent are maxillary and 48.1 per cent mandibular. On the other hand, the East African sample of 847 early hominid, deciduous-plus-permanent teeth, departs more strikingly from a 1:1 ratio: it comprises 41.0 per cent maxillary and 59.0 per cent mandibular teeth. It is inferred that mandibular teeth have a somewhat better chance of accumulating and being preserved in, and being recovered from, the open, fluvial, lacustrine and deltaic sedimentary environments of the East African sites. The dental proportions are approximately matched by the proportions of jaws. For example, the maxilla: mandible proportions at Koobi Fora in northern Kenya are 33.0:67.0 for teeth and 21.6:78.4 for jaws. In other words, the preponderance in favour of mandibular remains is somewhat more marked in the case of jaws than of teeth, this distinction doubtless reflecting the more fragile bony structure of the maxilla and the sturdier construction of the mandible. This first study known to the author of the differential distribution of maxillary and mandibular teeth of the Plio-Pleistocene hominids leads the author to hypothesize that, where environ- mental conditions at the place and time of the death of the hominids have been non-destructive, non-dispersive, relatively mild and protective, maxillae and mandibles may be expected to have been conserved and recovered in approximately equal proportions - - and likewise of maxillary and mandibular teeth. On the other hand, the more brutal and destructive the sedimentary environment and other taphonomic influences have been, at the place and time when the hominid individuals died, the more likely it is that the maxillary and mandibular remains of jaws and teeth will deviate from equality of proportions, generally at the expense of the maxillae and upper teeth. Hence, it is proposed that the upper jaw/lower jaw ratio (Mx/Mn jaw ratio) and the maxillary teeth/mandibular teeth ratio (Mx/Mn dental ratio) may serve as two useful new gauges of the rigour of palaeo-ecological and taphonomic conditions.

Introduction

Ecologists familiar with the many difficulties surrounding the study of modern ecosystems may tend to be sceptical about the val idi ty of palaeo-ecological in terpreta t ions (BEHRENSMEYER & HILt , 1980). Their scepticism may seem justified if the a t tempt is confined to the reconstruct ing of the configurat ion of a palaeocommunity as a neoecolo- gist would see it, namely at a moment in time. The problems may seem to be compounded

298 TOBIAS

when one appreciates that the accuracy of the palaeo-ecological reconstruction depends not only on the nature of the palaeo-community before death, but upon the system of taphonomic agents which selectively collected the remains after death and preserved the fossil sample (HANSON, 1980). Thus taphonomic analysis of how bones are transferred from the biosphere to the lithosphere would seem to be a sine qua non for a comprehen- sive, systematic palaeo-ecological investigation.

While these doubts and difficulties would seem cogent, it should on the other hand be recognised, as BEHRENSMEYER & HILL (1980) have pointed out, that:

<<.. paleoecology can tap one component of natural systems which is essentially unavailable to the recent ecologist: time depth. Ideally, paleoecologists should be able to show which aspects of vertebrate communities are persistent through long periods of time, and such information could be of great importance to modern ecology>>.

(op. cit., p. 301)

It is therefore, in the author's opinion, entirely appropriate that the 1st International Conference on Human Ecology should import this time depth into its proceedings, by including an assay of early hominid taphonomy and palaeo-ecology. Accordingly, the author presents here the first known study on the taphonomic interpretation of the differential distribution of maxillary and mandibular teeth and jaws of the Plio-Pleisto- cene hominids.

Previous studies of comparative faunal skeletal parts

In the Conclusion to their important work on Fossils in the Making, BEHRENSMEYER & HILL (1980) list six sets of 'taphonomic indicators' to be sought and if possible expressed quantitatively in vertebrate fossil deposits. First in their list, they cite:

~d. Skeletal part representation, particularly the ratio of teeth to vertebrae, the number of limb elements, and the presence of easily destroyed elements, as evidence for biological and/or geological processes responsible for primary biases in the bone sample~>.

(op. cit., p. 303)

Previous studies (e.g. DART, 1957a, 1957b,1958; BRAIN, 1974, 1975a, 1975b, 1976, 1978, 1981) have analysed the accumulations of individual components of the vertebrate skeleton in such early hominid cave deposits as Makapansgat, Swartkrans, Kromdraai and Sterkfontein in the Transvaal. From these observations, as well as the experiments of BRAIN (1981), it has become clear that, at least in bovid skeletons, the bones may be sorted into groups according to their ability to resist destruction, d n limb bones, ability to withstand carnivore action is closely related to the compactness of the bony part and may be reflected in the bone's specific gravity; it is also affected by the time at which the epiphysis of a particular extremity fuses to its shaft...~> (BRAIN, 1981: p. 139). Moreover BRAIN has been able to allocate each part of a bovid skeleton a potential survival rating which indicates how likely it is to survive a particular destructive regime. Each part of an antelope skeleton is allocated to a category, having high, medium or low potential survival rating. In his accompanying diagram of a bovid skeleton (loc. cir., fig. 148), both the maxillary and mandibular dentitions are shown to have high ratings.

In studies on a porcupine lair adjacent to the Nossob river in the Kalahari Gemsbok National Park (HUGHES, 1954a,b; BRAIN, 1980), BRAIN reported 24 half-mandibular pieces

ON THE RELATIVE FREQUENCIES OF HOMINID MAXILLARY AND MANDIBULAR TEETH 299

to 15 maxillary pieces (op. cit., Table 7.5, p. 115) of bovids. When these remains were expressed as proportions of the estimated total of bovid individuals, the maxillae comprised 15 out of 81 or 18.5 per cent and the half-mandibles 24 out of 162 or 14.8 per cent. Thus the percentage survival under these conditions of bone accumulation was slightly higher for maxillae than for mandibles.

On the other hand, in a heavily scavenged assemblage of goat bones from a Topnaar Khoikhoi (Hottentot) village in the Central Namib Desert, Namibia, BRAIN (1967a, 1969, 1980) found that the percentage survival was slightly higher for mandibles (30.7 per cent) than for maxillae (26.3 per cent).

In the study by BEHRENSMEYER & BOAZ (1980) of recent bones in Amboseli National Park, Kenya, tooth totals were given and, as in the present study, the totals included those found in maxillae and mandibles as well as isolated teeth. From all habitats, they found 5090 teeth out of 19,526 skeletal parts or 26 per cent. Unfortunately, the dental sample was not sorted into maxillary and mandibular teeth. There were 504 hemimandibles but the figure for maxillae was not given, though crania comprised 381 items. However, it was not stated how many of these crania retained their maxillae. In a hyena den, teeth comprised 20 per cent of skeletal items and hemimandibles and crania 2 per cent each (op. cit., p. 85).

From these studies it seems that maxillae in some recent assemblages survive more frequently, whilst in others mandibles are more frequent. The apparently greater fragility of maxillary structure might have led one to predict that mandibles would almost always be in the ascendant. However, although the relative density of the bone is an important correlated variable (BRAIN, 1981), it is clear that a variety of taphonomic agencies may operate to overcome this anatomical effect. For instance, RICKLAN (1986) has recently demonstrated that the frequencies of survival and recovery of wrist-, hand- and finger- bones of fossil hominids at Sterkfontein are not directly correlated with the density of the individual bones.

In none of these studies, nor in others such as those of GIFFORI) (1980) and HILt (1980), has the author found any distinction being drawn between the relative survival rating of maxillary and mandibular teeth, either in hominids or in comparative fauna.

Relative numbers of hominid maxillary and mandibular teeth

Deciduous teeth

In 1972 the author listed a total of 110 hominid deciduous teeth from 5 southern African and 5 East African sites (TOBIAS, 1972).

Currently, some 207 deciduous teeth, partial or complete, are available from the same 10 African sites. The numbers and percentages, site totals and regional totals, are listed in Table 1.

Some 59.4 per cent (123 teeth) of these are mandibular and 40.6 per cent (84 teeth) are maxillary. Thus, there is a slightly greater likelihood of deciduous mandibular teeth being preserved at and recovered from those 10 sites, than of deciduous maxillary teeth.

In striking contrast to the position at the nine other African sites (Table 1), the deciduous teeth from Olduvai are represented virtually exclusively by maxillary specimens (H. habilis: 0 mandibular to 7 maxillary; A. boisei: 1 mandibular to 8 maxillary). Almost the exact opposite obtains at Swartkrans where specimens (complete and incomplete) of deciduous mandibular teeth outnumber the maxillary teeth with 33 to 15, at Kromdraai

300 TOBIAS

TABLE 1 - Numbers and percentages o] hominid maxillary and mandibular deciduous teeth from Southern and East African sites *.

Maxillary teeth Mandibular teeth

No. % No. % No.

Sterkfontein 17 35.4 31 64.6 48 Makapansgat 0 0 2 100.0 2 Taung 10 50.0 10 50.0 20 Swartkrans 15 31.25 33 68.75 48 Kromdraai 0 0 8 100.0 8

S. African Cave Deposits 42 33.3 84 66.7 126

Omo 4 44.4 5 55.6 9 Koobi Fora 5 29.4 12 70.6 17 Hadar 10 50.0 10 50.0 20 Olduvai 15 93.75 1 6.25 16 Laetoli 8 42.1 11 57.9 19

E. African Open Sites 42 51.9 39 48.1 81

Grand Total S. Af. and E. Af. 84 40.6 123 59.4 207

* Since we are concerned here with the likelihood of preservation of individual teeth, the numbers given in this Table and the next are numbers of actual teeth, not of specimens or individuals.

with 8 to 0 and at Sterkfontein with 31 to 17. The figures for Makapansgat are 2 to 0 (deciduous mandibular to deciduous maxillary), whilst from Taung we have 10 to 10. The Omo list furnished by HOWELL & COPI'ENS (1973) features 5 mandibular to 4 maxillary deciduous teeth, those of Laetoli are 11 to 8 (WHITE, 1977, 1980), of Hadar 10 to 10 (JOHANSON, WHITE & COVVENS, 1982) and of Koobi Fora 12 to 5 (R. E. LEAKEY et al., 1978).

Three patterns of preservation of deciduous teeth are detectable:

a) approximate equality of numbers of deciduous maxillary and mandibular teeth: this characterises the samples from the South African A. africanus sites of Taung and Makapansgat (the latter sample is minuscule), Laetoli, Hadar and Omo;

b) preponderance of maxillary teeth: the sample of 16 deciduous teeth from Olduvai Gorge;

c) preponderance of mandibular teeth: as in the samples from Swartkrans, Krom- draai, Sterkfontein and Koobi Fora.

The pooled data, irrespective of taxa, for the a site-samples total 38 mandibular to 32 maxillary; for the b site-sample, 1 mandibular (see above) to 15 maxillary; and for the c site-samples 84 mandibular to 37 maxillary!

These interesting differences in the hominid deciduous dental elements may be the result of sampling biases, or they may bespeak differences in taphonomic circumstances governing the accumulation and/or the preservation of bones at the various sites.


Permanent Teeth

In an earlier analysis, the author found that, from 14 African sites, the hominid permanent teeth comprised 425 mandibular and 469 maxillary teeth (TOBIAS, 1972, p. 71). These figures refer to numbers of individual teeth, not of specimens or individuals. Overall, there was a slightly lesser chance of mandibular teeth being preserved and recovered than of maxillary teeth: of the total of 894 teeth, 47.5 per cent were mandibular and 52.5 per cent maxillary. To these totals must now be added hundreds of additional specimens subsequently discovered or described. We have at least 140 more from Omo (based on HOWELL & COPPENS, 1974); 178 more from Koobi Fora (after R. E. LEAKEY et al., 1978); 170 from Hadar (after JOHANSON, WHITE & COPPENS, 1982); 84 from Laetoli (from WHITE, 1977, 1980); 273 more from Sterkfontein (based on TOBIAS etaL, 1977, and on the author's subsequent annual reports of the Palaeo-anthropology Research Group of the University of the Witwatersrand up to September 1985); 17 from Nariokotome III (BROWN et al., 1985); 13 more from Swartkrans (GRINE, 1981); 4 more from Kromdraai (based on check-list, Table 1, of Vrba, 1981; GRINE, 1982); and a few additional items from Makapansgat, Sterkfontein, Swartkrans and East African sites. This gives a total of at least 879 additional, African, early hominid, permanent teeth, since I completed my last tally early in 1971 (TOBIAS, 1972). In other words, the stockpile of early hominid permanent teeth from the African continent doubled in the 15 years from 1971 to 1985!

The overall total of early hominid permanent teeth currently available is thus 1773, of which 964 have stemmed from the 5 southern African sites and 809 from 10 east African sites (Table 2).

When we examine the distribution of hominid maxillary and mandibular permanent teeth from all 15 South and East African site-samples, we see first that 845 or 47.7 per cent are maxillary and 928 or 52.3 per cent mandibular. These figures compare with the values for hominid deciduous teeth of 40.6 per cent maxillary and 59.4 per cent mandibular.

Thus, for both deciduous and permanent teeth of African early hominids, there is a slightly greater chance of mandibular teeth being preserved and recovered, than of maxillary teeth.

When the samples of hominid permanent teeth from the five South African cave deposits are examined, then irrespective of taxa involved, there is a small majority (54.25 per cent) of maxillary teeth. Although the departure from a 50-50 distribution is small and probably not significant, under the conditions of bone accumulations and preservation prevailing in these caves, there is a suggestion of a slightly greater likelihood of maxillary permanent teeth being accumulated in these cave deposits, within which bone preservation appears to have been excellent (C.K. BRAIN, pers. comm). This contrasts sharply with the relevant frequencies of hominid deciduous teeth in the cave deposits, where a large majority (66.7 per cent) of mandibular deciduous teeth has come to light.

The relative frequencies of hominid permanent teeth in the East African open sites are quite different from those in the southern African cave deposits: approximately 60 per cent are mandibular and about 40 per cent maxillary. In four of the five East African sites which have yielded appreciable numbers of hominid permanent teeth -- Omo, Koobi Fora, Hadar and Laetoli -- the frequencies of mandibular teeth vary from 56 to 67 per cent. Again, the Olduvai sample of permanent teeth is exceptional among the East African site samples in showing a majority -- albeit a small one -- of maxillary teeth. Thus the conditions of tooth accumulation and survival at Olduvai have strongly favoured deciduous maxillary teeth and slightly favoured permanent maxillary teeth.

302 TOBIAS

TABLE 2 - Numbers and percentages o/hominid maxillary and mandibular permanent teeth/rom Southern and East African sites *

Maxillary teeth Mandibular teeth

No. % No. % No.

Sterkfontein 263 56.0 207 44.0 470 Makapansgat 26 44.1 33 55.9 59 Taung 2 50.0 2 50.0 4 Swartkrans 219 54.75 181 45.25 400 Kromdraai 13 41.9 18 58.1 31


Omo 88 44.0 112 56.0 200 Koobi Fora 62 33.3 124 66.7 186 Hadar 56 32.9 114 67.1 170 Olduvai 65 51.6 61 48.4 126 Laetoli 34 40.5 50 59.5 84 Nariokotome III 8 9 17 Peninj 0 16 16 Chesowanja 6 0 6 Garusi 3 0 3 Lothagam 0 1 1

E. Af. Open Sites 322 39.8 487 60.2 809


* Since we are concerned here with the likelihood of preservation of individual teeth, the numbers given in this Table and the next are numbers of actual teeth, not of specimens or individuals.

Deciduous and permanent teeth combined

Table 3 summarises the total numbers of hominid maxillary and of mandibular teeth for the nine dentally richest African sites, irrespective of whether the teeth are deciduous or permanent and irrespective of taxon. As in the previous tables, the totals include teeth in jaws and isolated teeth.

It seems clear that, under conditions prevailing at the fluvial, deltaic and lake margin deposits of Laetoli, Omo and Koobi Fora, hominid mandibular teeth - - both deciduous and permanent - - seem to have stood a better chance of accumulating, being preserved and recovered than maxillary teeth. From these sites have come 201 maxillary teeth (deciduous plus permanent), but 314 mandibular teeth (combined) that is, 30.9 and 61.0 per cent respectively. Hadar is in similar case in respect of permanent teeth; its value of 67.1 per cent mandibular permanent teeth is the highest of all those for which percentages are cited in Table 2; on the other hand, its modest sample of hominid deciduous teeth (n = 20) is equally divided between maxillary and mandibular dentitions. The overall totals for Hadar are thus 66 maxillary teeth (deciduous plus permanent) and 124 mandibular teeth (combined), that is, 34.7 and 65.3 per cent respectively. With these figures, the Olduvai combined samples contrast sharply: there are 80 maxillary teeth (deciduous plus permanent) and 62 mandibular teeth (combined), the corresponding percentages of the


TABLE 3 - Numbers and percentages of hominid maxillary and mandibular teeth, deciduous and permanent combined, [rum nine Southern and East A/rican site-samples *

Maxillary teeth Mandibular teeth No.

No. % No. %

Sterkfontein 280 54.1 238 45.9 518 Makapansgat 26 42.6 35 57.4 61 Swartkrans 234 52.2 214 47.8 448 Kromdraai 13 33.3 26 66.7 39


Omo 92 44.0 117 56.0 209 Koobi Fora 67 33.0 136 67.0 203 Hadar 66 34.7 124 65.3 190 Olduvai 80 5&3 62 43.7 142 Laetoli 42 40.8 61 59.2 103

E~ African Open Sites 347 41.0 500 59.0 847


* In contrast with the preceding tables, this table does not include the teeth from Taung, Nariokotome III, Peninj, Chesowanja, Garusi and Lothagam, comprising 29 maxillary and 38 mandibular teeth in all.

total Olduvai sample being 56.3 and 43.7. At Olduvai the collection and preservation of mandibular teeth is slightly less favoured, it would seem, than that of maxillary teeth, as is true also of the total samples (deciduous plus permanent) from Sterkfontein (54.1 and 45.9 per cent), Swartkrans (52.2 and 47.8 per cent) and combined southern African sites (51.9 and 48.1 per cent).

The overall southern African sample of 1066 deciduous plus permanent teeth, of which 51.9 per cent are maxillary and 48.1 per cent mandibular, departs so slightly from a 50-50 distribution that it would be reasonable to infer that hominid maxillary and mandibular teeth have a virtually equal chance of accumulating in the dolomitic limestone cave deposits, of being preserved therein and recovered therefrom. On the other hand, the East African sample of 847 deciduous plus permanent teeth, comprising 41.0 per cent maxillary and 59.0 per cent mandibular teeth, departs more strikingly from a 50-50 distribution. It would be a fair inference that hominid mandibular teeth have a somewhat better chance of being preserved in and being recovered from the open, fluvial, lacustrine and deltaic sedimentary environments of these sites.

It is interesting in this connection to see that the 33-67 per cent of maxillary to mandibular teeth from Koobi Fora finds a parallel in the large predominance of mandibles over maxillae among the hominid fossils from Koobi Fora (BEHRENSMEYER, 1978). This aspect will now be considered.

Distribution of hominid skeletal parts

BEHRENSMEYER (1978) examined the distribution of hominid skeletal parts among the Koobi Fora sample, in relation to taxon (Australopithecus or Homo), and to sedimenta-

304 TomAs

ry environment (fluvial or lake margin). In her Table 4 (op. cit., p. 173), she listed 34 isolated mandibular specimens and only 4 isolated maxillae. However, two other mutually exclusive categories, crania (including parietals) and associated parts (including skull and mandible), undoubtedly included some cranial specimens with maxillae. Accordingly, in order to test whether the ratio of maxillary and mandibular teeth paralleled the ratio of hominid maxillae to mandibles at Koobi Fora, I resorted to the illustrated Hominid Catalogue of R. E. LEAI(EY et al. (1978). The Catalogue features the total Koobi Fora hominid assemblage up to 1978. Thus it includes some later-discovered specimens that had not been available to A. K. BEHRENSMEYER at the time when she presented her analysis at the Wenner-Gren Workshop in New York City in January-February, 1974.

Irrespective of taxon, sedimentary environment or whether the specimen was isolated or associated, it transpires that the Koobi Fora sample includes 40 mandibular specimens and only 11 maxillary. Of the total gnathic sample of 51 specimens, 21.6 per cent are maxillae and 78.4 per cent are mandibles. In comparison, maxillary teeth from Koobi Fora comprise 33.0 per cent of the total dental sample and mandibular teeth, 67.0 per cent. In broad terms, these figures agree in showing a heavy preponderance of mandibular teeth and jaws. The predominance is more marked in the case of the jaws themselves.

As might be expected from the more fragile bony structure of the maxilla and the much sturdier construction of the mandible (which was well-known, it seems, to long- haired Samson), the survival of the maxilla under destructive or unprotected conditions is relatively infrequent. The frequency of hominid maxillary teeth accumulated seems to be related to this, since they are found in most of these open East African sites far less frequently than are the mandibular teeth, especially at Koobi Fora, Hadar and Laetoli. Though Olduvai Beds I and II represent a lake margin situation similar in some respects to that of Koobi Fora (BEHRENSMEYER, 1978), hominid maxillary teeth, curiously enough, have had as good a chance of being accumulated, surviving and being found at Olduvai as at Sterkfontein. It is not immediately clear what taphonomic and palaeo-ecological factors at Olduvai have seemingly operated to protect the maxillary teeth and, presumably, the maxillae from destruction and loss. Of course it remains possible that the 56.3-43.7 figures for Olduvai maxillary and mandibular teeth are an artefact of biased sampling and that more Olduvai teeth may tip the scales into line with the ratios in the larger samples from Omo, Koobi Fora and Hadar. Moreover, as SHIPMAN (1981) has pointed out, ~Comparing the representation of different body parts in different assemblages is a major problem. Equally problematical is assessing the significance of observed similarities or differences in skeletal representatiom>. (op. cit., p. 137). As far as the author has been able to ascertain, BEHRENSMEYER'S (1978) study and the earlier one by TOBIAS (1972) are the only previous investigations to have analysed the occurrences of the skeletal parts of the very ancient hominids themselves, while the present study is the first known to the author which examines the differential distribution of maxillary and mandibular teeth of the Plio- Pleistocene hominids.

Fragility of the maxillae as a cardinal factor

In this study it has been shown that the circumstances of accumulation in the southern African cave sites and at Olduvai favoured the collection and preservation of hominid maxillary teeth somewhat more than that of mandibular, whereas the other East African open sites preferred hominid mandibular teeth.


It is not suggested here that a difference in structure, enamel or dentine density or thickness, might account for the varying frequencies of upper and lower teeth encountered in the dentally rich hominid assemblages. However, the differences in frequencies of maxillary and mandibular teeth may be related to the differential accumulation and/or survival of the maxillae and mandibles themselves, as is suggested by the moderatly good concordance of the relative frequencies of upper and lower teeth and jaws at Koobi Fora. If this correlation can be confirmed from other sites of fossil hominid assemblages, then it may be possible to infer that in a particular site, whatever factors favour the accumulation and/or preservation of one set of jaws over the other are likely indirectly to influence the dental composition of the site-sample.

In the case of teeth, then, it would not be any differential survival rating of the dental structures themselves, but of their socketing host organs, the maxillae and mandibles, which would account for the differential accumulation and recovery rates of maxillary and mandibular teeth. The jaws themselves, on the other hand, seem to vary, as between maxillae and mandibles, in their structural rigidity and strength, a common anatomical and orthodontic observation.

A new taphonomic hypothesis and two new taphonomic indicators

It may be predicted that, where taphonomic conditions have been non-destructive, non-dispersive, relatively mild and protective, maxillae and mandibles may be expected to have been conserved and recovered in approximately equal proportions; moreover, if the hypothesized interrelatedness of teeth and their respective socketing host organs is correct (TOBIAS, 1987, 1988), one would expect the maxillary and mandibular teeth also to be available in nearly equal proportions. On other hand, the more brutal and destructive the sedimentary environment and other taphonomic influences have been, the more likely it is that the maxillary and mandibular remains of jaws and teeth will deviate from equality of proportions, generally at the expense of the maxillae and upper teeth and in favour of the mandibles and lower teeth.

Hence, it is suggested that the upper jaw/lower jaw ratio (Mx/Mn jaw ratio) and the maxillary teeth/mandibular teeth ratio (Mx/Mn dental ratio) may serve as useful gauges of the rigour of the taphonomic environment, that is the rigour of all processes which can differentially affect the accumulation, preservation and recovery of maxillae and mandibles. In much the same way, BEHRENSMEYEg (1975) and BEHRENSMEYER & BOAZ (1980) have used the relative numbers of teeth and vertebrae (the T/V ratio) in fossil assemblages, to assess whether or not a bone assemblage had been selectively sorted prior to burial.

The Mx/Mn dental ratio proposed here suggests that the modest hominid assemblages from Kromdraai (dental ratio 0.50) and Makapansgat (0.74) had been more rigorously sorted prior to inhumation than the large ones of Sterkfontein (1.18) and Swartkrans (1.09). Similarly, harsher sorting agencies would seem to have pertained in the hominid assemblages of Koobi Fora (0.49), Hadar (0.53), Laetoli (0.69) and Omo (0.79), whereas a blander taphonomic regimen would appear to have been the lot of the Olduvai hominids (1.29). The Mx/Mn dental ratio of the Olduvai assemblage suggests slightly less destructive taphonomic sorting and milder, more protective conditions than prevailed even within the dolomitic cave repositories of the Transvaal.

These hypotheses and the proposed new taphonomic indicators (Mx/Mn dental ratio and Mx/Mn jaw ratio) require to be tested rigorously on the fossil hominid caches, the associated comparative faunae and analogous recent bone assemblages.

306 TOBIAS

Are there species differences in Mx/Mn ratios?

Several investigators have addressed the problem of the relative survival0f skeletal parts of different species and higher systematic groups (e.g. BRAIN, 1967b, 1969, 1974, 1980, 1981; BEHRENSMEYER, 1975, 1976, 1978; BEHRENSMEYER & BOAZ, 1980; HILL, 1980; SHIPMAN, 1981).

BRAIN (1981) has drawn the conclusion that <~skeletons of primates, and probably also of carnivores, are far more susceptible to destruction than are antelope skeletons derived from animals of equivalent live body weight~ (op. cit., p. 139). It is possible, therefore, that the greater vulnerability of primate skeletons including those of hominids may, under destructive or unprotected conditions, lead to the uncovering of more subtle differences in the potential for survival, accumulation and preservation, between upper and lower jaws and teeth.

For example, a moderately rigorous palaeo-ecological and taphonomic regime might be expected to affect the more vulnerable Primate skeletal parts to a greater extent than it influences the bovid remains in the same assemblage. On the hypotheses enunciated above, the Mx/Mn ratios may under these circumstances be expected to depart from unity to a moderate extent among the Bovidae and to a more marked extent among the Hominidae and Cercopithecidae. These deductions from the primary hypothesis are readily amenable to testing.

Many related questions could be followed up with profit. For example, within the Hominidae, are the proportions of maxillary and mandibular teeth and jaws related to the systematic composition of each sample? In an open deposit reflecting a harsh sedimentary environment, would an accumulation of robust or hyper-robust australopithecine fossils include more of their sturdily-constructed maxillae with dentitions, than would be found in an assemblage of the more lightly-constructed or 'gracile' hominids (e.g.A. africanus, H. habilis)? BEHRENSMEYER (1978) reports a significantly greater number of Australopithecus (? A. boisei) remains from East Turkana fluvial deposits than of Homo (?H. habilis and/or H. erectus), while the <~two taxa~> occur in similar numbers in lake margin deposits. From these and other data recorded by her, she concludes that the differing patterns of abundance of the different hominids are to be explained as ecological. <<More Australo- pithecines lived (or died) in fluvial habitats, probably gallery forest and bush near rivers, than Homo. Both inhabited lake margins~). (op. cit., p. 187).

An examination of the data tabulated by BEHRENSMEYER (op cit.), including taxo- nomic assignments by R.E.F. LEAKEY, F.C. HOWELL and B.A. WooD, reveals that 39 identified hominid jaw specimens from Koobi Fora are distributed thus:

Maxillae Mandibles Mx/Mn Jaw Ratio

Australopithecus 5 20 0.25 Homo 5 9 0.56 Combined 10 29 0.34

If the identifications are correct, the data show (a) that the ratios have low values, departing appreciably from unity, and (b) that the Australopitbecus ratio departs from unity to a greater degree than the Homo ratio. This is the opposite of what one might have expected on the assumption that the very robust australopithecines have a less vulnerable


maxilla than have species of Homo. It appears at first blush to have a more vulnerable maxilla than Homo! However, it should be noted, as BEHRENSMEYER points out, that the two hominids are differentially distributed according to environment of burial (op. cit., her Table 1). Of the identified Homo specimens, 28 per cent are buried in fluvial and 72 per cent in lake margin deposits, whereas the figures for the Australopithecus assemblage are 62.5 and 37.5 per cent respectively. The data suggest that the fluvial sedimentary environment is more destructive of maxillae than is the lake margin environment, irrespective of the taxon. In this instance, the robusticity of the A. boisei maxillae is apparently not equal to the challenge of a putatively harsh sedimentary environment. If we break the data down further, both by taxon and by environment of burial, as well as by upper or lower jaw, the sub-sets become too small for meaningful analysis at this stage.

It may be noted that the Olduvai hominids were largely in lacustrine deposits and this presumably blander sedimentary environment may be a major factor (though not necessarily the only one) accounting for the seemingly favoured accumulation of maxillae at Olduvai.

Accumulation versus conservation

The present study has uncovered apparent differences in the accumulation of, and the potential for survival of hominid dentitions, between intracavernous fossil deposits and open site deposits. If we assume that taphonomic conditions are more potentially dispersive and destructive at open sites and more protective and conservational in cave deposits, we might expect that, in the more protected cavernous environment, in which the preservation of bone appears to be excellent, the seemingly more fragile hominid maxillae would have been conserved virtually as readily as the mandibles, once the upper and lower jaws gained access to the cave deposit. It follows that within these intracavernous deposits, any statistically significant difference in the frequency of maxillae and mandibles would then be attributed, not to intracavernous preservation, but rather to the differential impact of the collecting agency on the upper and lower jaws and their contained teeth. On BRAIN'S carnivore hypothesis, the impact on maxillae and on mandibles of the feeding habits of the carnivores, as the prime collecting agents, might be expected to account for any real differences in the incidence of upper and lower jaws and teeth. On the other hand, the open environment, with all its hazards such as stream dispersal and flash-flood destruction, would pose a greater threat to the accumulation of the apparently more fragile maxillae and their contained teeth, than to that of mandibles and mandibular teeth. Such a taphonomic difference probably accounts for the relative abundance of hominid maxillary teeth in cave deposits and their relative dearth in the open site deposits, lacustrine, deltaic and fluviatile, of East Africa.

The foregoing constitutes no more than a pilot study of the tapho-data on hominid teeth preserved and discovered at ten to fifteen African early hominid sites.

Hypotheses have been proposed on the relative survival potential of maxillae and mandibles, and on the interrelatedness between the proportions of maxillary and mandibular teeth and the proportions of the respective jaws, even when most teeth are isolated specimens. Some preliminary tests of the hypotheses have failed to refute them. Two new indices have been introduced: Mx/Mn dental ratio and Mx/Mn jaw ratio, and their potential usefulness has been illustrated.

BEHRENSMEYER'S and the author's earlier start in the analysis of hominid skeletal parts and the present small assay at the hominid dental and gnathic tapho-data indicate

308 TOBIAS

that it would be worthwhile for such studies to be extended. The immense assemblages of hominid bones from 9 or 10 Afr ican sites provide invaluable material enabling investigators to under take for the Homin idae the kinds of analysis that R. A. DART and C. K. BRAIN carried out decades ago on Bovidae and Cercopi thecoidea.

ACKNOWLEDGEMENTS - - My appreciation is extended to Professor Cristina Bernis and the Organiz- ing Committee of the 1st International Conference on Human Ecology. Thanks are due to Dr. C. K. Brain, Mrs. V. E. Strong and Mrs H. White. Generous financial assistance was received from the Foundation for Research Development.

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Received: 26 June 1987. Accepted: 20 July 1987.

on the relative frequencies of hominid maxillary and mandibular teeth and jaws as taphonomic...

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