Results

There are two main questions that need to be addressed in this attempt to identify morphological correlates of a protracted humeral posture for the purpose of tracing its evolutionary history: (1) are any of the morphological features examined functionally related to humeral protraction, and (2) do these features distinguish primates from other mammalian groups? In answer to the first question, correlations between each of the examined variables and humeral protraction angles from Larson et al., (2000)2 were analyzed. As summarized in Table 4, protraction angles were significantly correlated with roundness of the proxi-

2 Only a subset of the comparative sample could be used since not all taxa examined here were included in Larson et al. (2000).

Placental Mammals Groups
Figure 3. Fossil proximal humeri from Miocene localities in Uganda. Scale bar equals 5 mm. Upper row: MUZM 173 from Moroto 1 locality, (A) medial view, (B) superior view, and (C) lateral view. Lower row: BUMP 101 from Napak CC locality. (D) medial view, (E) superior view, and (F) lateral view.

mal humeral epiphysis, relative humeral head size, relative greater tubercle width, bitubercle angle, and anterior projection of the greater tubercle.

The second question regarding the distinctiveness of these features in primates was addressed by comparing group means for each of the variables (Table 5). Compared to the other mammalian groups, primates possess more obtuse spinoglenoid angles, relatively larger humeral heads, and less anteriorly projecting greater tubercles (not significant for rodents) (Table 5). Primates as a group were most different from carnivores displaying, in addition to those features listed above, larger bitubercle angles, more rounded proximal

Table 4. Correlations between humeral protraction angles at touchdown* during quadrupedal locomotion and study variables (statistically significant correlations in boldface)

Variable name

nt

r

Prob r = 0

Spinoglenoid angle

22

0.39

NS

Bitubercle angle

22

0.62

0.002

Rel Grt Tub Proj

22

-0.72

<0.001

Rel Grt Tub Wd

22

-0.44

0.04

Grt Tub Hti

22

-0.06

NS

Roundness Hum Epi

22

0.55

0.008

Hum Hd shape

22

0.19

NS

Rel Hum Hd Sz

22

0.46

0.03

Suprasp facet shape

22

0.30

NS

Suprasp facet angle

22

-0.42

0.05

Rel supra facet Sz

22

0.05

NS

Rel Intertub Wd

22

-0.05

NS

*Humeral protraction angles taken from Larson et al. (2000)

^Humeral protraction angles were available for only 22 out of the 52 species examined here ^Greater tubercle height was not correlated with body size

*Humeral protraction angles taken from Larson et al. (2000)

^Humeral protraction angles were available for only 22 out of the 52 species examined here ^Greater tubercle height was not correlated with body size humeral epiphyses, less long and narrow supraspinatus insertion facets, and shorter and more narrow greater tubercles. However, these latter features did not distinguish primates from either rodents or marsupials. Additional features distinguishing primates from marsupials were humeral head shape and intertubercular groove width, and from rodents was angle of the supraspinatus insertion facet. Figure 4 presents drawings of representative humeri illustrating some of these differences.

Within the set of features examined, therefore, is a subset that is both correlated to humeral protraction angles, and distinguishes primates from (all or at least some) other mammals. Those features are relative humeral head size, relative greater tubercle projection and width, bitubercle angle, and roundness of the proximal humeral epiphysis. In addition, spinoglenoid angle distinguished primates from all three other mammalian groups, although it is not significantly correlated to protraction angle. These six variables were used in a canonical discriminant analysis to explore whether the combination could be used to identify a primate pattern of glenohumeral morphology. Figure 5 displays a bivariate plot of the sampled taxa for canonical discriminant functions 1 and 2, which together explain 91.1% of the variation in the analysis. Function 1 clearly separates primates from rodents and marsupials, and the variable having the highest correlation with this function is spinoglenoid angle. Primates are separated from carnivores along function 2, and variables

Table 5. Group means and comparisons of means (boldface values indicate statistically significant differences between primates and other mammalian groups at p < 0.05)

Variable name

Primates

Rodents

Carnivores

Marsupials

Smilodectes

BUMP 101

MUZM 173

Spinoglenoid angle

81.34+10.70*

63.61+2.70

73.08+4.58

66.59+2.94

76.00

-

-

Bitubercle angle

67.50+5.50

65.88+10.24

47.44+9.98

73.99+10.38

78.67

53.33

60.67

Rel Grt Tub projection

12.73+7.61

26.67+14.14

46.66+13.71

32.51+7.22

18.48

33.55

27.29

Rel Grt Tub Wd

126.26+8.62

120.25+11.25

137.67+6.35

123.15+3.16

131.14

146.66

119.51

Grt Tub Ht

-0.56+1.04

-0.47+0.87

1.19+2.25

-0.19+0.72

-1.81

-0.46

-0.58

Roundness Hum epiphysis

101.26+3.96

102.42+8.17

85.27+6.87

103.53+7.67

102.24

90.87

90.05

Hum Hd shape

85.15+4.23

85.08+4.66

83.57+4.76

91.43+4.27

83.07

82.31

88.17

Rel Hum Hd size

141.49+8.60

131.20+8.17

125.56+7.34

130.05+6.99

141.49

147.89

135.77

Suprasp facet shape

40.08+7.78

41.70+8.42

27.87+5.07

40.31+8.12

37.61

29.57

27.48

Suprasp facet angle

32.53+7.36

41.07+6.89

38.31+7.28

36.51+6.39

37.33

34.33

38.50

Rel supra facet size

56.91+2.69

58.92+3.57

58.68+4.44

54.80+1.92

50.19

46.43

55.72

Rel intertub Wd

45.08+8.76

49.94+12.39

47.23+5.66

55.00+4.69

47.69

41.17

54.98

*One standard deviation.

*One standard deviation.

Figure 4. Lateral and superior views of representative humeri from the different mammalian groups examined here. Scale bar represents 1 cm. Primate humeri are characterized by a head that sits more on top of the humeral shaft than those of the other groups.

correlated with this axis are shape of the proximal humeral epiphysis, bitu-bercle angle, anterior projection of the greater tubercle, and relative greater tubercle width.

Inclusion of the specimen of Smilodectesgracilis as a separate group places it within the primate sample between Eulemur fulvus and Otolemur crassi-caudatus. However, Smilodectes displayed a higher loading on function 3 (which accounts for an additional 8.7% of the variance) than any of the other primate species, and in this dimension was somewhat more like the Celebes or spotted cuscus (Ailurops ursinus or Spilocuscus maculatus) (Figure 6).

In order to be able to include the two proximal humeral fragments from the Miocene of Uganda, it was necessary to remove spinoglenoid angle from the analysis. The resulting distribution of taxa along functions 1 and 2, which together explain 89.3% of the variance (Figure 7), shows more overlap between

Figure 5. Bivariate plot of functions 1 and 2 of the canonical discriminant analysis based on the six variables that were correlated to humeral protraction angles and/or distinguished primates from some or all other mammalian groups. U: Primates; A : Carnivores; o : Marsupials; d: Rodents; Smihdectes; "h Group Centroids.

Figure 5. Bivariate plot of functions 1 and 2 of the canonical discriminant analysis based on the six variables that were correlated to humeral protraction angles and/or distinguished primates from some or all other mammalian groups. U: Primates; A : Carnivores; o : Marsupials; d: Rodents; Smihdectes; "h Group Centroids.

Figure 6. Three dimensional plot of functions 1, 2, and 3 of six variable canonical discriminant analysis. Symbols as in Figure 5. While Smilodectes falls among primates along functions 1 and 2, it is nearer to marsupials on function 3.

Figure 7. Bivariate plot of functions 1 and 2 of the canonical discriminant analysis with spinoglenoid angle removed from the analysis BUMP 101; MUZM 173; other symbols as in Figure 5. More overlap between groups is evident than in six variable analysis. Smilodectes coincides with rodent group centroid, BUMP 101 falls among the carnivores, but MUZM 173 appears to be a primate.

Figure 7. Bivariate plot of functions 1 and 2 of the canonical discriminant analysis with spinoglenoid angle removed from the analysis BUMP 101; MUZM 173; other symbols as in Figure 5. More overlap between groups is evident than in six variable analysis. Smilodectes coincides with rodent group centroid, BUMP 101 falls among the carnivores, but MUZM 173 appears to be a primate.

groups, and predicted group membership places Saimiri and Cebus albifrons in the rodent group, and the rodent Uromys sp. in primates. Smilodectes falls within rodents along functions 1 and 2, but is more similar to primates and marsupials on function 3 (accounting for an additional 7.3% of the variance) (Figure 8). BUMP 101 falls within the carnivore group, and MUZM 173 is just within primates, falling nearest to Lemur catta on all three discriminate functions.

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