The enigmatic molar from Gondolin, South Africa: Implications for Paranthropus paleobiology

Journal of Human Evolution 63 (2012) 597e609

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The enigmatic molar from Gondolin, South Africa: Implications for Paranthropus paleobiology Frederick E. Grine a, b, *, Rachel L. Jacobs c, Kaye E. Reed d, J. Michael Plavcan e a

Department of Anthropology, Stony Brook University, Stony Brook, NY 11794-4364, USA Department of Anatomical Sciences, Stony Brook University, Stony Brook, NY 11794-8081, USA c Interdepartmental Doctoral Program in Anthropological Sciences, Stony Brook University, Stony Brook, NY 11794-4364, USA d School of Human Evolution and Social Change, Institute of Human Origins, Arizona State University, Tempe, AZ 85827-4101, USA e Department of Anthropology, University of Arkansas, Fayetteville, AR 72701, USA b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 October 2011 Accepted 28 June 2012 Available online 16 August 2012

The specific attribution of the large hominin M2 (GDA-2) from Gondolin has significant implications for the paleobiology of Paranthropus. If it is a specimen of Paranthropus robustus it impacts that species’ size range, and if it belongs to Paranthropus boisei it has important biogeographic implications. We evaluate crown size, cusp proportions and the likelihood of encountering a large-bodied mammal species in both East and South Africa in the Early Pleistocene. The tooth falls well outside the P. robustus sample range, and comfortably within that for penecontemporaneous P. boisei. Analyses of sample range, distribution and variability suggest that it is possible, albeit unlikely to find a M2 of this size in the current P. robustus sample. However, taphonomic agents - carnivore (particularly leopard) feeding behaviors - have likely skewed the size distribution of the Swartkrans and Drimolen P. robustus assemblage. In particular, assemblages of large-bodied mammals accumulated by leopards typically display high proportions of juveniles and smaller adults. The skew in the P. robustus sample is consistent with this type of assemblage. Morphological evidence in the form of cusp proportions is congruent with GDA-2 representing P. robustus rather than P. boisei. The comparatively small number of large-bodied mammal species common to both South and East Africa in the Early Pleistocene suggests a low probability of encountering an herbivorous australopith in both. Our results are most consistent with the interpretation of the Gondolin molar as a very large specimen of P. robustus. This, in turn, suggests that large, presumptive male, specimens are rare, and that the levels of size variation (sexual dimorphism) previously ascribed to this species are likely to be gross underestimates. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Gondolin Taxonomy South Africa East Africa Paranthropus robustus Paranthropus boisei Molar size Variation Cusp proportions Species biogeography Taphonomy Swartkrans Drimolen

Introduction Fossils of Paranthropus robustus are known from five sites situated within a 5 km radius of one another in the Bloubank Valley of South Africa (Fig. 1). All represent clastic sediment infillings of karst caves that formed in the Precambrian Malmani dolomitic limestones of the Monte Cristo Formation. A small, albeit significant collection, including the type specimen, is known from Kromdraai (Broom, 1938; Grine, 1982), but the species is best represented at Swartkrans (e.g., Broom and Robinson, 1952; Grine, 1989) and Drimolen (Keyser et al., 2000; Moggi-Cecchi et al., 2010). Several molars attributed to P. robustus have also been recovered from Sterkfontein Member 5B (Kuman and Clarke, 2000), and a badly crushed facial skeleton and a few teeth have been found at Cooper’s (Berger et.al., 2003; Steininger et al., 2008; de Ruiter et al., 2009).

* Corresponding author. E-mail address: [email protected] (F.E. Grine). 0047-2484/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jhevol.2012.06.005

The geochronological ages of these Paranthropus-bearing deposits have been the subject of prolonged investigation. Ignoring the spectacularly bizarre range - 4.38 Mae0.36 Ma - derived from ESR dating of tooth enamel (Blackwell, 1994; Curnoe et al., 2001, 2002), most faunal estimates indicate accumulation between about 1.9 Ma and 1.5 Ma (Vrba, 1985; Delson, 1988; McKee et al., 1995; Keyser et al., 2000; Kuman and Clarke, 2000). Palaeomagnetic determinations (Thackeray et al., 2002; Herries et al., 2009) are, of course, concordant because they are grounded by these biochronological estimates. The UePb determinations from speleothems at Cooper’s and Swartkrans (de Ruiter et al., 2009; Pickering et al., 2011) do not contradict the faunal estimates.

Gondolin In 1997, two hominin teeth were discovered in a “breccia” dump at Gondolin, some 20 km northwest of the other Paranthropusbearing sites (Menter et al., 1999) (Fig. 1). The Gondolin cave

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Figure 1. Location of Early Pleistocene Paranthropus-bearing sites in South Africa.

system, which developed in Eccles Formation dolomites, is surrounded by greater topographic relief than those in the Bloubank Valley (Herries et al., 2006). Gondolin presents a number of “breccia” dumps created by lime-mining activity, and two in-situ fossiliferous deposits. In 1979, E.S. Vrba conducted a brief excavation of one of the in-situ deposits, designated GD 2, which yielded a number of vertebrate fossils, but no primates (Watson, 1993). The fossils from Vrba’s excavations include “stage III” Metridiochoerus andrewsi, which suggests an age of between 1.9 and 1.5 Ma (Watson, 1993). Using this species as a basis for interpolation, Herries et al. (2006) argued that the normal geomagnetic polarity of GD 2 indicates deposition during the Olduvai (C2n) subchron between 1.95 and 1.78 Ma (Cande and Kent, 1995). Subsequent excavation of the in-situ GD 1 deposit produced a vertebrate fauna equivalent to that from GD 2, but these sediments preserve a wholly reversed polarity signature. This has been interpreted as suggesting deposition either just prior to or immediately following the Olduvai subchron (Adams et al., 2007). Adams et al. (2007) argued that GD 1 is the likely source for the “breccia” blocks in Dump GDA that yielded two hominin teeth (Menter et al., 1999). Thus, the Gondolin (GD 1 ¼ GDA) fossils might be ca. 2.0 Ma (or older), or perhaps 1.7e1.5 Ma. Neither estimate places them outside the probable range of the Bloubank Valley Paranthropus-bearing deposits. These dates also fall comfortably within the geochronological range of Paranthropus boisei in East Africa, which extends from about 2.3 Ma in lower Member G of the Shungura Formation (Suwa, 1988) to some 1.4 Ma at Konso (Suwa et al., 1997; Katoh et al., 2000). The Paranthropus molar from Gondolin The first hominin recovered from Gondolin (GDA-1) consists of the distolingual third of a lower molar. Menter et al. (1999) concluded that although it was not possible to attribute this fragment to any taxon, it was unlikely to belong to Paranthropus. The second hominin specimen (GDA-2) is a very large mandibular left second molar crown lacking roots (Fig. 2). Its size and the presence of a large tuberculum sextum (C6) led Menter et al. (1999) to attribute it to Paranthropus sp. indet. Although its mesiodistal (MD) and buccolingual (BL) diameters were observed to be substantially larger than those of known P. robustus homologues, because of the geographic proximity of Gondolin to the Bloubank Valley sites, Menter et al. (1999: 305) were “content to conclude only that this tooth is a surprisingly large-sized specimen

representing a population of South African robust hominids”, and that it “would probably be acceptable to attribute this tooth to P. cf. robustus”. Tobias (2000) quickly enumerated the three possibilities entailed by this molar: 1) it is indeed a very large specimen of P. robustus, 2) it is the first indication of P. boisei in South Africa and, his least favorite, 3) it attests to the presence of a novel species of “robust australopithecine”. Each of these possible interpretations has significant implications for our appreciation of Paranthropus paleobiology. Some workers have likened P. robustus size variation to a chimpanzee-like level of dimorphism, whereas others have inferred higher (e.g., gorilla-like) levels for it (e.g., Steudel, 1980; McHenry, 1991; Lockwood et al., 2007). If GDA-2 is attributable to P. robustus, the degree of size variation (possibly sexual dimorphism) ascribed to this species is likely to be notably underestimated. The resultant substantial increase in its size range would have significant biological consequences (Calder, 1984). On the other hand, if GDA-2 is attributable to P. boisei, it would have major implications for Early Pleistocene hominin biogeography (Strait and Wood, 1999). Fossils attributable to P. boisei, or the presumptive P. aethiopicus - P. boisei lineage are known from sites that extend from southern Ethiopia to northern Malawi (Suwa et al., 1997; Kullmer et al., 1999) (Fig. 3). The discovery of the Paranthropus maxilla at Malema more than doubled the previously known NortheSouth range of P. aethiopicus - P. boisei. Malema is nearly 2000 km from Konso, Ethiopia, and another 2000 km separates Malema from the South African Paranthropus-bearing localities. If the GDA-2 molar attests to the presence of either P. boisei or a novel species of Paranthropus in South Africa, this might have paleoecological implications (Giacominia et al., 2009). The notion that the Paranthropus specimens from Kromdraai and Swartkrans represent two species, namely P. robustus and Paranthropus crassidens, as proposed by Broom (1938, 1949), gained some support from cranial and especially deciduous dental comparisons (Howell, 1978; Grine, 1982, 1985), but subsequent discoveries at Drimolen (Keyser et al., 2000; Moggi-Cecchi et al., 2010) have blurred these apparent differences. Even though there is scant evidence for the recognition of two species of Paranthropus in the Bloubank Valley deposits, this should neither cloud nor preclude such interpretations for the Gondolin fossils. Because of the significant implications that follow from the specific attribution of the Gondolin Paranthropus molar, we

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Figure 2. The Gondolin GDA-2 Paranthropus M2 reconstructed from stacked micro-CT images. A) occlusal view, mesial to top and buccal to left; B) occlusal view showing delineation of the cusps and reconstruction of the mesial and distal crown contours; C) distobuccal view; D) mesiobuccal view. Images courtesy of M. Skinner.

undertake its taxonomic assessment using two lines of evidence: 1) overall crown size, and 2) the proportional sizes of the cusps. While the taxonomic assessment of any fossil should be morphologically driven, contextual information is not wholly irrelevant. In

particular, the level of biogeographic correspondence between East and South Africa in the ranges of penecontemporaneous largebodied mammal species may inform on the probability of encountering a given species of Paranthropus in both regions. Thus,

Figure 3. Location of the Paranthropus-bearing localities in East Africa. Malema is situated some 2000 km from Omo Shungura and Konso; a further, 2000 km separates Malema from the Paranthropus-bearing sites of South Africa.

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following our assessment of the morphological characteristics of the Gondolin molar, we review the biogeographic context of the South African Paranthropus fossils to provide a probabilistic framework within which to interpret these results. Crown size Menter et al. (1999) observed that the GDA-2 crown diameters fall above the observed ranges and more than three SDs above the corresponding P. robustus sample means, but within the observed ranges of the P. aethiopicus/boisei sample. However, their P. robustus sample did not include specimens from Drimolen, and there is some discrepancy between the GDA-2 diameters recorded by Menter et al. (1999) and Kuykendall and Conroy (1999). The discrepancies in these values (Table 1) may reflect simple interobserver error, or differences in the definitions that were employed. The MD diameter recorded here (by FEG) is much closer to that obtained by Menter et al. (1999); it was measured according to the definition used by Tobias (1967), where the maximum distance between the mesial and distal surfaces is determined by the longitudinal axis of the crown. The pristine MD diameter estimated here is somewhat less than that estimated by Menter et al. (1999). The maximum BL diameter, which pertains to the talonid, was here measured according to the definition employed by Tobias (1967), where the maximum distance between the buccal and lingual surfaces is determined at a right angle to the longitudinal axis of the crown. Our determination of the maximum BL diameter falls between the values recorded by Menter et al. (1999) and Kuykendall and Conroy (1999), albeit slightly closer to the latter. The BL and reconstructed MD diameters recorded here for GDA2 fall above the corresponding P. robustus sample ranges, and some 3.5 and 3.0 SDs from the respective means (Table 2). At the same time, however, the GDA-2 dimensions fall within the observed P. boisei sample ranges, and within one SD of the means. Given the similarities of the GDA-2 values to those for P. boisei homologues, the questions to be addressed pertain to its relationship to the P. robustus sample (Fig. 4). Four such questions can be phrased. 1) What is the probability of encountering a tooth that is some 3.0 to 3.5 SD from the mean in a large sample of a highly dimorphic, largebodied hominid species? 2) Does the inclusion of GDA-2 in the P. robustus sample increase its degree of relative variation beyond that seen in a living species? 3) What does the inclusion of GDA-2 in the P. robustus sample do to its range structure vis-à-vis other species samples? 4) What is the likelihood of drawing an outlier such as GDA-2 and at the same time drawing 24 other specimens that display variation comparable to that of the P. robustus sample when sampling the distributions of highly dimorphic, large-bodied extant hominids? We address these sequentially below.

Table 2 M2 crown dimensions for Paranthropus robustus and P. boisei samples.

MD diameter P. robustus P. boisei BL diameter P. robustus P. boisei

n

Mean

SD

SE

Obs. Range

24 15

16.21 18.37

1.02 1.30

0.208 0.335

13.8e17.7 16.4e20.3

28 14

14.70 17.01

0.92 1.07

0.174 0.285

13.0e16.5 14.7e18.5

Paranthropus robustus specimens derive from Kromdraai, Swartkrans and Drimolen. Specimens attributed to P. aethiopicus are excluded from the P. boisei sample. The majority of P. boisei crowns were measured by one of us (FEG); values for Omo 471968-46 are from Coppens (1971), KNM-ER 404, KNM-ER 818 and Peninj 1 from Wood (1991), KNM-ER 25520 from Brown et al. (2001), and KGA 10-525 and KGA 10-2705 from Suwa et al. (1997).

basis of molar size distributions in the most dimorphic largebodied hominids, Gorilla gorilla and Pongo pygmaeus. Reference samples may be chosen based on the degree of expected variation in the fossil sample or, more conservatively, by selecting extant species that exhibit high degrees of variation (Plavcan and Cope, 2001). Because Paranthropus (P. robustus and P. boisei) has been argued by some to exhibit degrees of craniodental dimorphism similar to that of Gorilla (Lockwood et al., 2007), we employ G. gorilla as a conservative reference. Orangutans exhibit higher degrees of postcanine dental variation than gorillas (Kelley and Plavcan, 1998) and are likewise also included here. The MD and BL measurements for balanced-sex samples of G. gorilla and Po. pygmaeus M2s were compiled from Mahler (1973), supplemented with data provided by S. King and M.H. Wolpoff (Table 3). The gorilla sample consists primarily of specimens from West-Central Africa. In light of current debate over orangutan alpha taxonomy (e.g., Muir et al., 2000; Brandon-Jones et al., 2004; Steiper, 2006), that sample was restricted to specimens from Borneo. As such, our comparative samples minimized geographic variation. The bootstrap analyses were performed in MATLAB using a script written by Anne Su. Sample sizes of 25 and 29 (which represent the P. robustus þ GDA-2 sample sizes for the MD and BL diameters respectively) were selected from the two comparative samples. The re-sampling procedure was conducted 1000 times (with replacement) for each dimension to determine the probability of recovering a sample with a specimen as large as GDA-2 for each extant species. The bootstrap results for the P. robustus samples reveal the probabilities of recovering a gorilla sample with

Assessment of probability Bootstrapping has been used extensively in assessing the taxonomic homogeneity of fossil hominin assemblages (e.g., Lockwood et al., 1996; Miller, 2000), and is utilized here to assess the probability of recovering a P. robustus molar as large as GDA-2 on the

Table 1 Maximum mesiodistal (MD) and buccolingual (BL) crown diameters recorded for the GDA-2 Paranthropus M2 from Gondolin. MD meas.

MD est.

BL meas.

Source

18.8 18.4

19.6 e

18.1 17.8

18.7

19.3

17.9

Menter et al. (1999) Kuykendall and Conroy (1999) This study

Figure 4. Mesiodistal (MD) and buccolingual (BL) diameters of the GDA-2 M2 compared with those of the Paranthropus robustus and P. boisei samples. Vertical line ¼ sample mean; horizontal line ¼ observed sample range; shaded rectangle ¼ 1 SD of mean; open rectangle ¼ 1 SE of mean.

F.E. Grine et al. / Journal of Human Evolution 63 (2012) 597e609 Table 3 M2 Crown dimensions for Gorilla gorilla and Pongo pygmaeus samples. Species MD diameter Gorilla gorilla

Pongo pygmaeus

BL diameter Gorilla gorilla

Pongo pygmaeus

Sex

n

Mean

SD

Obs. Range

Male Female Combined Male Female Combined

147 147 294 40 40 80

17.62 16.46 17.04 14.56 13.23 13.90

1.00 0.93 1.13 1.11 0.93 1.22

15.5e20.4 13.8e18.9 13.8e20.4 12.6e17.5 11.7e16.0 11.7e17.5

Male Female Combined Male Female Combined

147 147 294 40 40 80

15.70 14.49 15.09 13.63 12.31 12.97

0.93 0.82 1.06 0.81 0.71 1.01

13.3e18.7 12.8e16.9 12.8e18.7 12.2e16.0 11.0e14.5 11.0e16.0

Table 4 Univariate statistics and coefficients of variation (CVs) for M2 crown diameters of Plio-Pleistocene hominin and extant hominid samples. Species

a molar as MD and BL aberrant as GDA-2 are 0.5% (p ¼ 0.005) and 0.8% (p ¼ 0.008) respectively. The probabilities of recovering an equivalent orangutan sample are 9.1% (p ¼ 0.091) and 0.1% (p ¼ 0.001) respectively. Thus, in three of the four possible comparisons, it is highly unlikely that one would sample at random a molar as unusually large as GDA-2.

MD diameter Paranthropus robustus Paranthropus robustus Paranthropus boisei Paranthropus boisei Australopithecus afarensis Australopithecus africanus Gorilla gorilla

Pongo pygmaeus

Pan troglodytes Pan paniscus Homo sapiens

Assessment of variability A separate question is whether the inclusion of GDA-2 in the P. robustus sample serves to increase its degree of relative variation beyond that seen in a living species (Pilbeam and Zwell, 1972; Cope and Lacy, 1992, 1995; Plavcan, 1993; Donnelly and Kramer, 1999). Of course, as noted by Plavcan and Cope (2001), it is not possible to falsify a single-species hypothesis for a fossil sample on the basis of relative variation alone since comparatively high levels of variation in fossil samples may be interpreted as suggesting a greater degree of sexual dimorphism in some extinct taxa (e.g., Kelley and Xu, 1991; Richmond and Jungers, 1995). Nevertheless, the coefficient of variation (CV ¼ [SD/Mean]  100) is commonly used to gauge whether the degree of variation exhibited by a fossil sample is excessive in comparison to similarly-sized samples of other, closely related extant and/or extinct species. Table 4 lists CVs of the P. robustus and P. boisei samples with and without the GDA-2 specimen included in each. As expected, there are slight increases in the MD and BL means as well as their associated standard deviations in the expanded P. robustus sample. However, the CVs of the expanded P. robustus are not remarkable by comparison with those of other species. In both diameters, the P. robustus CVs exceed those for Australopithecus africanus and P. boisei, but are lower than those for Australopithecus afarensis. The P. robustus MD CV is also exceeded by those for one or both sexes in samples of Pongo pygmaeus, Pan troglodytes and even some human populations. The BL CV for the P. robustus M2 sample is exceeded by those for a sample of female chimpanzees, and a sample of human males. Moreover, if the same bootstrapping methods described above are employed, the probabilities of recovering similar P. robustus MD and BL CVs are 20.2% and 34.8% respectively for the gorilla sample, and 84.4% and 67.3% respectively for the orangutan sample. Thus, the addition of the Gondolin molar to the P. robustus sample does not appreciably increase its metric variability. Indeed, its level of relative variability is comparable to that of P. boisei and A. africanus, and lower than that of A. afarensis. Of course, a problem with the use of the CV in this exercise is the number of M2s that comprise the P. robustus sample. With n ¼ 24 for the MD diameter and n ¼ 28 for the BL diameter, a single molar

601

Homo sapiens BL diameter Paranthropus robustus Paranthropus robustus Paranthropus boisei Paranthropus boisei Australopithecus afarensis Australopithecus africanus Gorilla gorilla

Pongo pygmaeus

Pan troglodytes Pan paniscus Homo sapiens Homo sapiens

Sex

n

Mean

SD

CV

Reference/Notes

C

24

16.21

1.02

6.29

Without GDA-2

C

25

16.33

1.17

7.16

With GDA-2

C

15

18.37

1.30

7.08

Without GDA-2

C

16

18.43

1.28

6.95

With GDA-2

C

23

13.84

1.15

8.31

This study

C

31

15.97

0.97

6.07

This study

M F C M F C M F M F M F M F

147 147 294 40 40 80 19 47 17 25 69 74 18 29

17.62 16.46 17.04 14.56 13.23 13.90 11.40 11.00 9.80 10.20 11.02 10.61 10.97 10.60

1.00 0.93 1.13 1.11 0.93 1.22 0.89 0.65 0.60 0.60 0.71 0.86 1.09 0.82

5.68 5.65 6.63 7.62 7.03 8.78 7.81 5.91 6.12 5.88 6.44 8.11 9.94 7.74

This study This study This study This study This study This study Swindler (1976) Swindler (1976) Johanson (1974) Johanson (1974) Bailit et al. (1968) Bailit et al. (1968) Taverne (1980) Taverne (1980)

C

28

14.70

0.92

6.25

Without GDA-2

C

29

14.81

1.08

7.29

With GDA-2

C

14

17.01

1.07

6.29

Without GDA-2

C

15

17.07

1.05

6.15

With GDA-2

C

25

13.80

1.43

10.36

This study

C

35

14.56

0.92

6.32

This study

M F C M F C M F M F M F M F

147 147 294 40 40 80 17 46 17 26 70 67 18 29

15.70 14.49 15.09 13.63 12.31 12.97 10.90 10.20 9.20 9.10 11.08 10.63 10.84 10.50

0.93 0.82 1.06 0.81 0.71 1.01 0.75 0.86 0.60 0.60 0.69 0.73 0.85 0.71

5.92 5.66 7.02 5.94 5.77 7.79 6.88 8.43 6.52 6.59 6.23 6.87 7.84 6.76

This study This study This study This study This study This study Swindler (1976) Swindler (1976) Johanson (1974) Johanson (1974) Bailit et al. (1968) Bailit et al. (1968) Taverne (1980) Taverne (1980)

The two human samples included here reflect those with the highest CVs for males and females among a selection of six studies with reasonable sample sizes. The other four studies are those of Moorrees (1957), Van Reenen (1966), Jacobson (1982) and Kieser et al. (1985).

would have to be much larger than GDA-2 to inflate the CVs enough to match those of the A. afarensis sample. Such a tooth would have to measure 21 mm MD and 21.6 mm BL to inflate the P. robustus CVs to 8.31 and 10.36 respectively. Assessment of range Another means by which to address the question of what the inclusion of GDA-2 does to the structure of the P. robustus sample vis-à-vis other species samples is to examine their ranges. The relationship of the minimum and maximum diameters within

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a sample may be expressed as a scale-free ratio, the “coefficient of the range” (CR ¼ [(max e min)/max]  100). The CR is a means by which the range of variation in a fossil assemblage can be gauged in comparison to similarly-sized samples of other, closely related species (Table 5). The CR for the M2 MD diameter of the P. robustus sample without GDA-2 (22.0%) is comparable to those of P. boisei and A. africanus, and even with the inclusion of GDA-2, the relative range of the P. robustus sample (28.5%) is smaller than those of A. afarensis, G. gorilla and Po. pygmaeus. A molar would have to measure 20.5 mm MD in order to increase the P. robustus CR to the level seen in orangs and A. afarensis. The CR for the BL diameter of the P. robustus sample without GDA-2 (21.2%) already matches that of P. boisei, but even with the inclusion of GDA-2, the P. robustus sample CR is smaller than those of A. afarensis and the ape species. A molar would have to measure 19.0 mm BL in order to increase the P. robustus CR to the levels seen in these other species samples. Thus, in neither dimension does GDA-2 match what one would expect for the largest M2 of P. robustus given the relative size ranges of A. afarensis and living great apes. As such, inclusion of GDA-2 in the P. robustus sample does not sample either the potential or expected extremes of the species. Assessment of distribution The foregoing analyses demonstrate that it is possible to draw a specimen as different from the P. robustus sample as GDA-2 in the distributions of highly dimorphic apes such as Gorilla or Pongo, and that relative variation that results from the inclusion of GDA-2 in the P. robustus sample is not excessive in comparisons to extant great ape and other fossil hominin samples. However, skewed sex ratios and unbalanced sample sizes may frustrate the straightforward use of CV and CR analyses. Fig. 5 shows the size distribution of gorilla and orangutan M2s, with the P. robustus and Gondolin specimens superimposed (Fig. 5A). With reference to the notable gap between GDA-2 and the next largest P. robustus molar, the question to be addressed is the likelihood, when sampling a Gorilla- or Pongo-like distribution, of drawing such an outlier and at the same time drawing 24 other specimens that display variation comparable to that of the P. robustus sample. To approach this, a program was written in

Table 5 Observed sample ranges and the “coefficient of the range” (CR) for M2 crown dimensions in fossil hominin and combined-sex extant hominid species samples. Species MD diameter Paranthropus robustus Paranthropus robustus Paranthropus boisei Australopithecus afarensis Australopithecus africanus Gorilla gorilla Pongo pygmaeus Pan troglodytes BL diameter Paranthropus robustus Paranthropus robustus Paranthropus boisei Australopithecus afarensis Australopithecus africanus Gorilla gorilla Pongo pygmaeus Pan troglodytes

n

Obs. range

CR

Reference/Notes

24 25 15 23 31 294 80 66

13.8e17.7 13.8e19.3 16.4e20.3 11.2e16.6 14.5e17.7 13.8e20.4 11.7e17.5 9.5e12.9

22.0 28.5 19.2 32.5 18.1 32.4 33.1 26.4

Without GDA-2 With GDA-2 This study This study This study This study This study Swindler (1976)

28 29 14 25 35 294 80 63

13.0e16.5 13.0e17.9 14.7e18.5 12.0e17.5 12.8e17.0 12.8e18.7 11.0e16.0 8.3e12.5

21.2 27.4 20.5 31.4 24.7 31.6 31.3 33.6

Without GDA-2 With GDA-2 This study This study This study This study This study Swindler (1976)

CR ¼ [(max  min)/max]  100. The Gorilla and Pongo samples comprise equal numbers of males and females; the Pan sample comprises 47 females and 19 males for the MD diameter, and 46 females and 17 males for the BL diameter.

MATLAB by one of us (MJP) where specimens were randomly drawn with replacement from samples of Gorilla and Pongo until the sample size matched that of P. robustus inclusive of GDA-2 for the MD and BL dimensions. Because Pongo, Gorilla and P. robustus differ in tooth size, data were ln-transformed to control for the effect of size on variances (Sokal and Braumann, 1980; Sokal and Rohlf, 1995; Plavcan and Cope, 2001). The program then calculated the standard deviation of the sample exclusive of the largest drawn specimen, and the mean of that same subsample. This procedure was repeated 1,000,000 times for each dimension of each extant taxon, counting 1) the number of times that the largest specimen fell more than the number of SDs from the sample mean that GDA-2 falls from that of the P. robustus mean, 2) the number of times that the sample SD exceeded that of the P. robustus sample, and 3) the number of times that both conditions were met. The results are summarized in Table 6. It is exceedingly unlikely that one would draw a sample like that of P. robustus plus an outlier such as GDA-2 from a sample of Gorilla or Pongo molars. However, examination of Fig. 5B reveals a rather striking comparison between the P. robustus þ GDA-2 distribution and that of Gorilla. The P. robustus sample overlaps almost exactly the female gorilla distribution, while GDA-2 falls in the upper part of the male gorilla distribution. This is not to suggest that all of the P. robustus specimens in the present sample are female and GDA-2 is a male. Rather, the P. robustus sample clearly comprises a number of undoubted male specimens (Broom and Robinson, 1952; Keyser et al., 2000; Lockwood et al., 2007). We feel reasonably confident that SK 1, SK 6/100, SK 12, SK 34, SK 81, SK 858, SK 876, SK 3976, SKX 4446 and DNH 68 can be so attributed on the basis of tooth and/or corpus size. Those for which both diameters of the M2 could be recorded (SK 1, SK 858, SK 3976, SKX 4446 and DNH 68) are highlighted in Fig. 5B. We do not presume these specimens to be exhaustive, but choose to illustrate a selection of probable male specimens for heuristic purposes. Interestingly, they all fall within the lower part of the male gorilla distribution. It is both noteworthy and telling that, when combined with GDA-2, the range of presumptive P. robustus males falls comfortably within that of male gorillas, while the presumptive P. robustus female range is nearly identical to that of female gorillas. Cusp proportions The utility of molar cusp proportions in taxonomic (specieslevel) comparisons has been explored in a number of studies (Lavelle, 1978; Wood et al., 1983; Uchida, 1992; Matsumura et al., 1992; Suwa et al., 1994; Grine et al., 2009). These data appear to be particularly effective in sorting mandibular molars, and have proved useful in taxonomic evaluations of Plio-Pleistocene hominin fossils (Wood et al., 1983; Suwa et al., 1994). Although the GDA-2 crown is worn flat, the occlusal fissures separating the principal and subsidiary cusps are clearly visible, enabling them to be delineated (Fig. 2). The planimetric cusp areas were measured (by FEG) using an occlusal image generated from stacked micro-CT slices courtesy of Matt Skinner. The image was imported into Adobe Photoshop 7.0, where the occlusal fissures were highlighted and the mesial and distal contours corrected for interproximal wear. Areas of the adjusted image were measured using Image J 1.41o (NIH; http://rsb. info.nih.gov/ij). Each cusp area was traced five times; the highest and lowest values were discarded, and the average of the remaining three values was used to compute relative cusp area. The GDA-2 crown has a large tuberculum sextum and a smaller distoconulid wedged between it and the hypoconulid. This configuration has been referred to as a “double”, “bifid” or “duplex” C6 by some workers. The methods employed here to define cusp boundaries followed those of Wood et al. (1983), where the areas of

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603

Figure 5. Scatterplots of the mesiodistal (MD) and buccolingual (BL) diameters of M2s comprising extant, highly dimorphic hominid samples with Paranthropus robustus and GDA-2 crowns superimposed. A) Pongo and Gorilla samples compared. Gold diamonds are Pongo males and females; blue triangles are Gorilla males and females; black circles are Paranthropus robustus molars from Kromdraai, Swartkrans and Drimolen; black star is GDA-2. B) Gorilla males and females compared. Gold diamonds are Gorilla females, blue boxes are Gorilla males; black and red circles are Paranthropus robustus molars from Kromdraai, Swartkrans and Drimolen. Red circles denote a number of the specimens considered to be male by Lockwood et al. (2007) and us (i.e., SK 1, SK 858, SK 3976, SKX 4446 and DNH 68); note that all fall within the lower part of the male Gorilla distribution. The black star is the GDA-2 crown. This highlights the distance between GDA-2 and the next largest P. robustus homologue being equivalent to that separating a large Gorilla male from smaller male and female conspecifics. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

the principal cusps and the tuberculum sextum were recorded separately. In this instance, the tuberculum sextum and distoconulid were treated as a single entity. The relative proportions of the GDA-2 cusps are compared to those recorded by Suwa et al. (1994) for P. robustus and P. boisei M2s in Table 7 and Fig. 6. The proportional sizes of the trigonid cusps (protoconid and metaconid) do not differ significantly between P. robustus and P. boisei, and nor does the relative size of the hypoconulid. On the other hand, P. robustus and P. boisei differ significantly in the relative sizes of the hypoconid and entoconid. The hypoconid is significantly larger in P. robustus (t ¼ 3.7044; df ¼ 23; p < 0.01), whereas the entoconid is significantly larger in P. boisei (t ¼ 3.4298; df ¼ 23; p < 0.01). The relative sizes of these two cusps on GDA-2 are nearly identical to the corresponding P. robustus means. Although P. robustus and P. boisei do not differ significantly from one another in the relative size of the C6, GDA-2 exceeds both means by more than 2 SD (Table 7). The tuberculum sextum on GDA-2 has achieved its size primarily at the expense of the hypoconulid (Fig. 6). A “bifid” C6, such as possessed by GDA-2, is reasonably common among P. robustus homologues, where approximately 42% have two (e.g., DNH 60; SK 37) or even three (e.g., SK 25, SK 3976) cuspulids between the entoconid and hypoconulid. A “bifid” C6 is also not

Table 6 Likelihood, when sampling a Gorilla- or Pongo-like distribution, of drawing an outlier such as GDA-2 and at the same time drawing another 24 specimens that show relative variation comparable to that of the Paranthropus robustus sample. Condition met Pongo MD BL Gorilla MD BL

Range

SD

Both

0.0662 0.0019

0.2053 0.2814

0.0094 0.0011

0.0086 0.0019

0.2858 0.1569

0.0035 0.0001

This procedure was repeated 1,000,000 times for each tooth dimension of each extant taxon, counting 1) the number of times that the largest specimen fell more than the number of standard deviations from the sample mean that GDA-2 falls from the P. robustus mean, 2) the number of times that the sample standard deviation exceeded that of the P. robustus sample, and 3) the number of times that both conditions were met.

uncommon on P. boisei M2s, although at a somewhat lower incidence (c. 33%). Here too, some molars possess two cusps (e.g., KNM-ER 3230), while others have three (e.g., KNM-WT 17396) in this region. Biogeographic context As noted above, if the GDA-2 molar is attributable to P. boisei, it would have major implications for Early Pleistocene hominin biogeography and for South African australopith paleoecology. We address this issue by examining the exchange of other mammals between East and South Africa during this period of time. It is difficult to assess possible biogeographic ranging patterns from the few large-bodied mammals that have been identified to species from GD1 (Adams et al., 2007) or GD 2 (Adams, 2006) at Gondolin (Table 8). However, it is possible to compare mammalian species presence between the two regions using data from other South and East African Paranthropus-bearing sites. The species that have been identified in the literature at 13 East and 8 South African localities are recorded in Table 9. Because we are particularly interested in the species rather than the genera that are shared between regions, only those taxa that have been identified to the species level are included. Thus, taxa that have been identified as “Genus sp.,” compared with a given species (i.e., those prefixed “cf.”), or seen as sharing affinities with a given species (i.e., those prefixed “aff.”) have been excluded. The only exceptions pertain to Homo habilis and Homo erectus. With regard to the former, we ignore the questions that have been raised as to its specific identification in South Africa (Grine et al., 1996), and follow MacLatchy et al. (2010) on its possible presence in Sterkfontein Member 5A. As such, we do not subscribe to arguments to the effect that the Stw 53 cranium represents A. africanus (Kuman and Clarke, 2000; Clarke, 2008; Berger et al., 2010). With regard to H. erectus, we follow numerous workers (Rightmire, 1990, 1998; Antón, 2003; Gilbert et al., 2003; Baab, 2008) in regarding Homo ergaster as a subjective junior synonym, and recognize the existence of H. erectus at the South African sites of Swartkrans (Members 1e3) and Sterkfontein (Member 5B). An important consideration in comparisons such as those in Table 9 is the possibility that species identification, more so than generic identification, depends upon the alpha taxonomy employed

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Table 7 Relative cusp proportions of Paranthropus M2s. n

Protoconid Mean

GDA-2 P. robustus P. boisei

16 9

0.216 0.227 0.221

Metaconid

SD

Mean

0.017 0.017

0.206 0.217 0.219

Hypoconid

SD

Mean

0.017 0.020

0.204 0.203 0.185

Entoconid

SD

Mean

0.012 0.011

0.158 0.160 0.191

Hypoconulid

SD

Mean

0.023 0.019

0.105 0.123 0.120

C6

SD

Mean

SD

0.021 0.025

0.112 0.071 0.064

0.016 0.021

The P. robustus and P. boisei data are taken from Suwa et al. (1994: Table 6). The C6 data represent single as well as multiple cuspulids between the entoconid and hypoconulid.

by different workers who might have first-hand familiarity with fossils from only one of the regions or site areas. However, a number of researchers in the recent past have compared fossils in both regions, with the result that such biases (i.e., misidentification of the same species) have likely been reduced (e. g., Vrba, 1995; Gentry, 2010; Werdelin and Peigné, 2010). The results of this comparison show that while there is considerable generic correspondence, there are comparatively few species that are common to both South and East Africa in the Early Pleistocene. Thus, of the 157 species that have been identified from the Paranthropus-bearing sites, only 20 (12.7%) are common to both geographic regions. Therefore, it is possible but improbable to find the same species in eastern as well as southern Africa. Moreover, seven of the species common to both are faunivorous (six large carnivores and the aardvark), which is interesting in light of their rarity in the fossil record. Given the abundance of artiodactyl remains in these deposits, there are surprisingly few bovid species represented in both regions, and this does not appear to be attributable to sampling error given the occurrence of the faunivorous species. With regard to the shared herbivorous species, four are grazers, two are browsers, and two are mixed feeders. While many of the herbivores are large (species of Elephas and Ceratotherium), others are fairly small (e.g., Antidorcas recki). The number of primate species common to both regions is, in some measure, dependent upon the alpha taxonomy subscribed to by individual researchers. Theropithecus oswaldi is recognized here as occurring in three East African and three South African localities, and there is also only one colobine (Cercopithecoides williamsi) common to both regions. Like. T. oswaldi, it was a large terrestrial folivore (Delson, 1992; Benefit, 2000). With regard to the Homo fossils, if both H. habilis and H. erectus are recognized in South African sites, then there are possibly two Early Pleistocene hominin species whose ranges extended from East to South Africa. However, while the existence of H. erectus in South Africa is universally accepted for fossils that derive from

Swartkrans Member 2 (Grine, 2005), other of the South African Homo fossils have been argued to possibly attest to a species lineage unknown in East Africa (Grine et al., 1996). Thus, if H. habilis does not exist in South Africa, the similarity between the two biogeographic provinces is further reduced. Large and medium-sized carnivores are found across subSaharan Africa today and, for the most part, they are not limited by habitat. The Early Pleistocene large-bodied carnivores are presumed to have the same eurytopic ecological requirements as those living today. If faunivores are removed from the comparison, then only 8.9% of the remaining species are common to both regions (reducing the number of shared species to 14). Of the herbivores, four are large to very large species and today these types of animals (rhinos, giraffes, zebra) are also present in both regions. This, then, leaves eight shared medium-sized mammals: two hominins (both of which are species of Homo), two cercopithecids, two grazing pigs, A. recki (a mixed feeder that appears to have preferred grass), Kobus leche (a specialized grazer) and Tragelaphus strepsiceros (a browser). Although Tragelaphus (cf. or aff.) scriptus has been identified at many of the sites in both regions, there is no site in which a definitive attribution has been made. Recent DNA analyses suggest that living populations of Tragelaphus scriptus have been separated Table 8 The large-bodied mammalian fauna recorded from Gondolin. GD 1

GD 2

Artiodactyla Bovidae Oreotragus oreotragus Antidorcas cf. A. recki Redunca sp.

Antidorcas recki Redunca sefulathabeng Damaliscus (?niro) Connochaetes sp. Taurotragus oryx Tragelaphus angasi Tragelaphus strepsiceros

Suidae Metridiochoerus andrewsi Carnivora Canidae Canis mesomelas Hyaenidae Crocuta crocuta Perissodactyla Equidae Equus sp.

Equus sp.

Rhinocerotidae Ceratotherium simum Hyracoidea Procavidae Procavia antiqua

Figure 6. Histogram of relative cusp areas of the M2 recorded for Paranthropus robustus, P. boisei and GDA-2. The proportional contributions of the hypoconid and entoconid differ significantly between P. robustus and P. boisei; in each instance the GDA-2 value accords with the P. robustus sample mean.

Rodentia Hystricidae Hystrix makapenensis Data are from Adams (2006) and Adams et al. (2007).

F.E. Grine et al. / Journal of Human Evolution 63 (2012) 597e609 Table 9 Species shared between Early Pleistocene East and South African fossil sites. East

South

Antidorcas recki Kobus leche Tragelaphus strepsiceros

7 3 9

6 2 3

Metridiochoerus andrewsi Metridiochoerus modestus

7 6

5 2

10

2

Canis mesomelas

1

6

Acinonys jubatus Caracal caracal Panthera leo Panthera pardus

1 1 2 5

2 3 4 5

Crocuta ultra

3

4

Equus burchelli

3

3

13

1

Cercopithecoides williamsi Theropithecus oswaldi

1 3

3 4

Homo habilis Homo erectus

3 3

4 4

Elephas recki

11

1

1

3

Artiodactyla Bovidae

Suidae

Giraffidae Sivatherium marusium Carnivora Canidae Felidae

Hyaenidae

605

previous species comparison, and serves to further estimate how much species interchange is evident between the two regions. Species indices between each pair of sites were calculated using the Jaccard measure of similarity (Cj ¼ j/a þ b  j), which is based on the presence/absence of individual species (not their abundance) and creates an index between 0 and 1 (Cheetham and Hazel, 1969). We placed these pairwise indices in a matrix and calculated a minimum spanning tree that connects sites based on their taxonomic similarity such that the sites that share the most species have a much shorter distance or span between them. Plotting these results on a graph scaled to latitude and longitude for purposes of visualization (Fig. 7) reveals that the Late Pliocene Early Pleistocene fossil localities display clear regional groupings that correspond to major differences between East and South Africa despite temporal and/or habitat differences among them. The South African sites are tightly clustered with one another. It is perhaps significant that while they show low affiliation to West Turkana, Kenya, they exhibit virtually none with the much closer Chiwondo Beds in Malawi. Although the Omo, West Turkana and Koobi Fora localities are tightly clustered, they are also distinct from

Perissodactyla Equidae Rhinocerotidae Ceratotherium simum Primates Cercopithecidae

Hominidae

Proboscidea Elephantidae Tubulidentata Orycteropodidae Orycteropus afer

Numbers refer to the number of localities/strata in East and South Africa respectively from which the species were recovered (e.g., Shungura Member C ¼ 1 Locality; Swartkrans Member 2 ¼ 1 Locality). East African sites are: Shungura Formation, Ethiopia, Members C - G; Koobi Fora Formation, Kenya, Upper Burgi, KBS, Okote Members; Nachukui Formation, Kenya, Lokalalei, Kaitio Members; Olduvai Gorge, Tanzania, Beds IeII; Chiwondo Beds, Malawi. Data for East African sites from Leakey (1967), Harris (1991), Reed (1997), Turner et al. (1999), Sandrock et al. (2007), Bishop (2010), Werdelin and Peigné (2010), Gentry (2010), MacLatchy et al. (2010). South African sites are: Swartkrans Members 1e3; Sterkfontein Member 5; Drimolin; Kromdraai B; Gondolin; Cooper’s D. Data for South African sites from Reed (1997), Turner et al. (1999), de Ruiter (2003), Adams (2006), Adams et al. (2007), de Ruiter et al. (2009); Bishop (2010), Werdelin and Peigné (2010); Gentry (2010), MacLatchy et al. (2010).

between northwestern and southeastern Africa for an estimated 3.0 Myr, with only small amounts of gene flow between adjacent populations (Moodley and Bruford, 2007). Indeed, because the bushbuck exhibits a distinct mtDNA haplotype from the Cape to KwaZulu-Natal, it may warrant separate specific status as Tragelaphus sylvaticus, as originally described by Sparrman in 1780 (Moodley et al., 2009). To further examine the distribution of species between East and South African biogeographic provinces, we collected data from the literature on species recovered from 25 Late Pliocene - Early Pleistocene sites dated to between ca. 2.5e1.2 Ma. In this instance, the rationale for collection was the general time period over which Paranthropus fossils are distributed rather than specific Paranthopus-bearing localities. As such, this analysis expands our

Figure 7. A stylized minimum spanning tree based on species similarities among late Pliocene e early Pleistocene (ca. 2.5e1.2 Ma) fossil sites in East and South Africa, adjusted so as to be mapped onto a grid of African latitudes and longitudes. Lower indices of association are indicated by dotted lines. The distances among the South African sites are exaggerated for display purposes. East African sites: Koobi Fora (Upper Burgi, KBS and Okote members), Shungura Formation (members C - G); West Turkana (Lokalalei, Kaitio, Natoo and Kalachoro members plus the Nariokotome, KNMWT 17000 locality); Hadar (Maka’amitalu [A.L. 666 locality]); Olduvai (Beds IeII); Uraha (Chiwondo Beds). South African Sites: Drimolin, Sterkfontein (Members 4 and 5); Swartkrans (Members 1e3), Kromdrai (A and B). Data for East African sites from Leakey (1967); Harris (1991); Reed (1997); Turner et al. (1999); Sandrock et al. (2007). Data for South African sites from Reed (1997), Turner et al. (1999), de Ruiter (2003), de Ruiter et al. (2008).

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one another. Given that regional fossil sites group tightly together, these data are consistent with South Africa having been relatively isolated in the Early Pleistocene. In sum, there are very few species of large-bodied mammals common to both East and South Africa in the Early Pleistocene. Consideration of their ecology and body sizes suggests that some barrier prevented wholesale movement between the two regions. Although there are several notable exceptions (mostly relating to faunivores), it would seem possible, but not probable to encounter a large-bodied mammal species, and especially one that is herbivorous, in the Early Pleistocene of both East and South Africa. Discussion The specific attribution of the very large, hominin M2 crown (GDA-2) that was discovered in 1997 at the site of Gondolin has important implications for the paleobiology of Paranthropus. If this very large molar is, indeed, attributed to P. robustus, this has significance for the size range of that taxon, and if it is attributable to P. boisei, it holds significant biogeographic implications. As such, we examined the likelihood that GDA-2 represents a specimen of either P. robustus or P. boisei. Using the sample distributions of highly dimorphic apes such as Gorilla and Pongo as comparators, we find that it is possible, albeit unlikely to draw a specimen as different from the conventional P. robustus assemblage as GDA-2, but that the level of variation that results from the inclusion of GDA-2 in the P. robustus sample does not differ substantially from that of ape and other australopith samples. Thus, inclusion of GDA-2 in the P. robustus hypodigm does not sample its potential, expected extremes. At the same time, it is exceedingly unlikely that one would draw a sub-sample like that represented by the P. robustus molars with an outlier such as GDA-2 from orangutan or gorilla samples. However, it is important to note that the analyses relating to overall crown size employed extant hominid samples that were not selected by the same taphonomic agent(s) that were likely responsible for the structure of the Swartkrans and Drimolen P. robustus assemblages. This is especially relevant when considering the results of our resampling experiment on sample distributions. Size, taphonomy, and the structure of the P. robustus assemblage Brain (1981, 1993, 1994) has presented convincing evidence that the large-bodied mammals, including the australopiths, found in the South African cave sites, and especially at Swartkrans, were very likely accumulated through a taphonomic filter governed primarily by large carnivore feeding behavior. The relative abundance of baboon and hominin fossils, which together comprise over 40% of all individuals at Swartkrans, is most reasonably explained in terms of carnivore food debris, with the leopard (Panthera pardus) having been implicated as a primary agent of accumulation. There is solid evidence of leopard canine puncture marks on the calotte of at least one Paranthropus individual (SK 54) from Swartkrans (Brain, 1969, 1970, 1974). While the possible involvement of other largebodied carnivores (e.g., Dinofelis and Hyaena) at Swartkrans has been raised, leopards are seen as the major contributor to the site, having likely used the cave as a feeding lair throughout the deposition of the Paranthropus-rich Member 1 deposit (Pickering et al., 2008). Although puncture pits that correspond to hyaena tooth sizes appear on bones in Members 2 and 3, leopards cannot be eliminated as having contributed to these accumulations as well (Pickering et al., 2004). Modern African leopards will attack a fairly broad range of prey sizes, including small (ca. 165 kg) buffalo. However, they prefer game no larger than themselves (i.e., ca. 37e61 kg for females and

males respectively (Bailey, 1993)), and the majority of successful attacks are on animals that weigh less than half their mean body weights. Pivotal success rates for kills entail prey species between 30 and 100 kg (Pienaar, 1969; Radloff and Du Toit, 2004; OwenSmith and Mills, 2008), and the average prey body masses for female and male leopards is 25.2 kg and 34.2 kg respectively (Radloff and Du Toit, 2004). Thus, average prey body mass is some 60e70% that of an adult leopard. Reliable body weights for P. robustus are difficult to determine because of the dearth of postcranial bones that can be confidently attributed to this taxon. This is because of the presence of Homo in the same deposits (Grine, 2005; Moggi-Cecchi et al., 2010). Nevertheless, the clear numerical superiority of craniodental fossils of Paranthropus vis-à-vis Homo at both Swartkrans and Drimolen (Moggi-Cecchi et al., 2010), and the occasional morphological distinction between homologous elements (e.g., the proximal radius (Grine and Susman, 1991)) enable the attribution of fossils to the former with at least a reasonable likelihood. McHenry (1976) obtained estimates for several postcranial bones from Swartkrans that he regarded as belonging to P. robustus. The SK 3981 vertebrae yielded a value of 36.1 kg, while the SK 82 and SK 97 femoral head diameters suggested 49.8 kg and 52.7 kg respectively. The SK 82 and SK 97 femoral shafts yielded somewhat higher weights of 55.1 kg and 56.5 kg respectively for McHenry (1988), whereas Steudel (1980) obtained estimates of ca. 69 kg (the average weight of a male orangutan) from their diaphyseal circumferences. Jungers (1988) reported values from Swartkrans hip joints between 37.1 kg and 57.5 kg. Expanding the sample to include all of the postcranial elements from Swartkrans led McHenry (1991) to note that the Member 1 specimens yielded estimates of less than 54 kg, while those from Member 3 suggested weights around 45 kg. Subsequent estimates reported by McHenry (1992) for Swartkrans and Kromdraai postcrania range from 30.0 kg (the SK 3981b lumbar vertebra) to 59.8 kg (the SK 97 femoral shaft). He perceived the smaller and larger fossils as likely representing females and males, with respective averages of 31.9 kg and 40.2 kg for the sexes. Several studies have explored the potential of cranial measurements to predict body mass in fossil hominins (e.g., Steudel, 1980; Aiello and Wood, 1994; Kappelman, 1996), and with a few notable exceptions (Braga and Thackeray, 2003; Schwartz and Tattersall, 2003) distinguishing between the skulls and teeth of Paranthropus and Homo is a fairly straightforward task. Steudel (1980) found orbital width, palate breadth and bizygomatic breadth to be particularly powerful predictors among extant hominoids, but was cautious about employing the latter two variables to estimate body size in P. robustus because of their obvious relationship to its hypertrophied masticatory apparatus. Accordingly, she obtained estimates of around 37 kg (the average weight of a female orangutan) for the SK 46, SK 48 and SK 79 crania. Kappelman (1996) predicted a weight of 46.8 kg for SK 48 using orbital area, while Spocter and Manger (2007) obtained values of between 48 kg and 52 kg for it from a variety of upper facial and orbital dimensions. Regardless of their derivation (i.e., specific measurement, element or regression model), the body weight estimates based on the postcranial remains are quite consistent, ranging between about 30 and 60 kg, and the cranial variables have yielded estimates that are consistent between 37 and 52 kg. These values fall within the range for pivotal prey kill success for modern leopards, and the higher estimates for some P. robustus specimens from Swartkrans and Kromdraai begin to push that envelope somewhat. In this context, it is significant that the Paranthropus assemblages from Swartkrans and Drimolen (and even Kromdraai, despite its comparative paucity of fossils) are notable for their high

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proportion of juvenile individuals (McKinley, 1971; Mann, 1975; Brain, 1981; Keyser et al., 2000; Shaaban, 2002; Moggi-Cecchi et al., 2010). Assemblages that have been taphonomically skewed by predation are expected to contain inordinately high proportions of juvenile individuals and limited body weight variation among adults. It is precisely in such assemblages that very large (presumptive male) specimens are expected to be rare. Lockwood et al. (2007) have argued that the P. robustus adults comprising the Swartkrans assemblage are dominated by younger, smaller (non-dominant) males. Although the sex assignment of some of the specimens they considered may be open to question, their observation that large (“Rank 9”) individuals are comparatively rare in the collection is a matter of record. McHenry (1992) observed that, in comparison to other Plio-Pleistocene hominin taxa, the estimates for P. robustus body weights attest to a comparatively low level of intraspecific variation. In this context, it is perhaps revealing that the observed P. robustus þ GDA-2 molar size distribution overlaps that of G. gorilla. More specifically, most of the Swartkrans, Drimolen and Kromdraai specimens overlap almost exactly the distribution of female gorillas. While a number of these specimens, which almost certainly include some male individuals, also overlap the lower part of the male Gorilla distribution, the GDA-2 molar sits comfortably within the upper reaches of that distribution. By comparison with highly dimorphic gorillas, then, one might reasonably conclude that GDA-2 likely represents a large male individual, and that the P. robustus assemblage from Swartkrans, Drimolen and Kromdraai is dominated by females and small males. This suggests that the degree of sexual size dimorphism inferred for P. robustus by Lockwood et al. (2007) and others before has likely been grossly underestimated. Other evidence bearing on alpha taxonomy Is there any other evidence to suggest that GDA-2 represents an expectedly rare, very large (presumptive male) individual of P. robustus rather than another larger-toothed species such as P. boisei? Paranthropus robustus and P. boisei differ significantly in the relative sizes of the M2 hypoconid and entoconid, and the proportional sizes of these cusps on GDA-2 are nearly identical to the corresponding P. robustus means. Moreover, the “bifid” C6 displayed by GDA-2 is relatively common among P. robustus homologues (42%), and somewhat rarer among P. boisei specimens. Among other large-bodied mammals, and especially noncarnivorous forms, there are very few species common to both South and East Africa in the Early Pleistocene. Of some 151 identified species, less than 15% are common to both regions, and a good proportion of these are faunivores. Thus, it would seem possible, but not probable to encounter a large-bodied herbivorous mammal in both East and South Africa during the temporal range of Paranthropus. With regard to the Plio-Pleistocene hominin fossils, we are unaware of any authority who currently maintains that any australopith species is known from both East and South Africa. Thus, despite the initial attribution of fragmentary East African fossils to A. africanus (e.g., Howell, 1969, 1978; Tobias, 1980), this view has not prevailed under the weight of evidence. Similarly, neither A. afarensis nor A. garhi are known in South Africa, despite the presence of like-aged deposits there. On the other hand, if the Homo fossils from Sterkfontein, Swartkrans and Drimolen are attributable to H. habilis and H. erectus, then there is at least one, if not two penecontemporaneous hominin species whose range(s) extended from South Africa to Ethiopia. Regardless of their ultimate specific attributions, however, early Homo taxa almost certainly possessed broader ranges of dietary behaviors than either P. robustus or P. boisei (Ungar et al., 2006).

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Taken together, various lines of evidence and reasoning indicate that Menter et al. (1999) were correct in their initial assessment of GDA-2. The most likely interpretation of the Gondolin molar is that is a very large (and expectedly rare) specimen of P. robustus. As such, the levels of size variation (sexual dimorphism) that have been ascribed to this species are likely to be gross underestimates. The resultant substantial increase in its constituent size range has significant consequences for our understanding of the paleobiology of P. robustus. Conclusions The hominin M2 (GDA-2) from Gondolin is substantially larger than any other P. robustus homologue known; its MD and BL diameters fall some 3.0 to 3.5 SD above the corresponding P. robustus sample means, but within the ranges for penecontemporaneous P. boisei samples from East Africa. We have explored the likelihood that it represents a specimen of either taxon by evaluating 1) overall crown size using various metrics of assessment, 2) crown morphology through cusp proportions, and 3) the biogeographic likelihood of encountering a large-bodied mammal species in the Early Pleistocene in both East and South Africa. Observations on overall crown size are mixed. Although analyses of sample range, distribution and variability suggest that it is possible, albeit unlikely to find a M2 of this size in the current sample representing P. robustus, the extant hominid samples employed in these comparisons were not selected (skewed) by the same taphonomic agent(s) responsible for the structure of the Swartkrans and Drimolen P. robustus assemblages. These were very likely accumulated as a result of carnivore (especially leopard) feeding behaviors. Given the prey body mass preferences of leopards, assemblages accumulated by them are expected to be taphonomically skewed, displaying inordinately high proportions of juvenile individuals and limited body weight variation among adults. This describes the Swartkrans assemblage, and it is precisely in such an assemblage that very large (presumptive male) specimens are expected to be rare. Evidence from cusp proportions is congruent with the interpretation that GDA-2 represents a rare, large (presumptive male) individual of P. robustus rather than another larger-toothed species such as P. boisei. The two species of Paranthropus differ significantly in the relative sizes of the hypoconid and entoconid, and those on GDA-2 are nearly identical to the corresponding P. robustus means. Finally, there are comparatively very few large-bodied (nonhominin) mammal species common to Early Pleistocene sites in both South and East Africa. Less than 15% of identified species are common to both, and a good proportion of these are carnivores. Thus, it is possible, albeit not probable to encounter a large-bodied mammal, and especially one that is herbivorous, in East and South Africa in the Early Pleistocene. Taken together, these various lines of evidence indicate that Menter et al. (1999) were correct in their initial assessment of GDA2. The most likely interpretation of the Gondolin molar is that is a very large (and expectedly rare) specimen of P. robustus. As such, the levels of size variation (sexual dimorphism) ascribed to this species have been grossly underestimated. Acknowledgments We are grateful to M. Dagosto and M.H. Wolpoff for providing Paul Mahler’s data and to S. King and M.H. Wolpoff for sharing additional data on extant hominid molar sizes. We thank A. Su for the MATLAB bootstrapping script; E. Delson, S. Frost, O. Kullmer and F. Schrenk graciously shared their data on species composition of

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Early Pleistocene sub-Saharan African localities. For access to specimens we thank M. Raath and B. Zipfel (University of the Witwatersrand), C. Menter (University of Johannesburg), S. Potze (Ditsong National Museum of Natural History), and E. Mbua, F. Manthi and S. Muteti (National Museums of Kenya). Thanks to M. Skinner for generously permitting us to use his reconstructions of the GDA-2 molar crown from micro-CT scans. The late Charles Lockwood generated Fig. 7. We thank the anonymous reviewers for their comments, and the editor, M.F. Teaford, for his careful scrutiny and cogent suggestions for improvements to our manuscript.

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