Methods

Beginning a step with a more protracted forelimb posture is likely to entail a glenohumeral joint configuration in which the humerus is more aligned with the scapula, and the cranial margin of the glenoid fossa encroaches upon the anterior edge of the humeral head articular surface and the greater and lesser tubercles. Therefore, the logical places to explore for features related to this distinctive shoulder posture are the proximal humerus and glenoid fossa. Fortunately, the functional morphology of the proximal humerus has been well studied (e.g., Fleagle and Simons, 1982; Godfrey, et al. 1991; Harrison, 1989; Rose, 1989; Larson, 1995, 1996; Ruff and Runestad, 1992; Rafferty and Ruff, 1994), and some distinctive primate characters have already been suggested. In particular, Baba (1988) compared the proximal humeri in five primate species (including humans) to those of three nonprimate mammalian species and described differences in features related to range of motion, stability of the shoulder joint, and power for glenohumeral retraction. Six of his measurements describing the size and shape of the humeral head and proximal humeral epiphysis were included in this study (Figure 1, a-f; Table 1). Additional linear and angular measurements were constructed to attempt to further describe the configuration of the proximal humerus and the orientation of the glenoid fossa (Figure 1, h-i; Figure 2; Table 1).

Linear measurements were taken with a digital caliper, and all angular measurements (except spinoglenoid angle) were taken with the aid of a torsiometer (Krahl, 1944; Larson, 1996). Spinoglenoid angle was measured using a clear plastic goniometer. Holding a scapula up to a light, the base of the scapular spine could be aligned with one limb of the goniometer while the other was rotated until it was in line with the superior and inferior margins of the glenoid.

Figure 1. Linear measurements taken on the proximal humeri of the comparative sample (described in Table 1) (redrawn from Baba, 1988). Measurements a-f taken from Baba (1988).
Herpestes Scapula
Figure 2. Angular measurements taken on the proximal humeri and scapulae of the comparative sample (described in Table 1).

Table 1. Measurements taken

Sagittal diameter* Transverse diameter* Humeral head length* Humeral head width* Intertubercular width* Greater tubercle height*

Greater tubercle width Supraspinatus facet width Supraspinatus facet length Supraspinatus facet angle

Bitubercle angle

Spinoglenoid angle

Maximum anterio-posterior length of the proximal humeral epiphysis (Figure 1-a) Maximum medio-lateral width of the proximal humeral epiphysis (Figure 1-b) Maximum anterior-posterior length of the humeral head (Figure 1-c)

Maximum medio-lateral width of the humeral head (Figure 1-d)

Width between medial and lateral margins of the intertubercular groove (Figure 1-e) Difference between the most superior point of the greater tubercle and the most superior point on the humeral head (Figure 1-f). If tubercle projects above the head, value is positive, if tubercle is below the level of the head, value is negative

Distance between the anteriormost margin and the most posterior point of the greater tubercle (Figure 1-g) Maximum width of supraspinatus insertion facet on the greater tubercle (Figure 1-h) maximum length of supraspinatus insertion facet on the greater tubercle (Figure 1-i) Angle described by a line passing through the long axis of the supraspinatus insertion facet relative to the axis of the humeral head (Figure 2) Angle described by a line across the anteriormost points of the greater and lesser tubercles relative to the axis of the humeral head (Figure 2) Angle between a line connecting the superior and inferior margins of the glenoid fossa and the base of the scapular spine (Figure 2)

The comparative sample consisted of scapulae and humeri of 52 different small- to medium-sized mammalian taxa (derived from the collections of the American Museum of Natural History, and the Stony Brook University Anatomical Sciences Museum), including 9 prosimians, 10 New World monkeys,1 11 rodents, 13 carnivores, and 9 marsupials (Table 2). All individuals were adult, as judged from epiphyseal fusion, and each scapula and humerus pair were from the same individual.

Three shape variables and five ratios (using the geometric mean of eight linear measurements on the proximal humerus as a size surrogate) were constructed from the linear measurements to facilitate comparison of species differing in overall size (Table 3). Sample means were derived for each species. Canonical discriminate analysis was used to determine whether taxa would be sorted into their appropriate groups according to the measurements explored here.

Table 2. Comparative sample

Prosimians

n

Rodents

n

Carnivores

n

Lemur catta

3

Uromys anak

1

Vulpes vulpes

3

Eulemur fulvus

3

Uromys caudimaculatus

1

Genetta genetta

3

Varecia variegata

3

Tamiasciurus hudsonius

2

Nandinia binotata

3

Cheirogaleus major

1

Protoxerus strangeri

2

Ailurus fulgens

3

Loris tardigradus

3

Sciurus abertmimus

3

Herpestes sanguineus

3

Arctocebus calabarensis

3

Aplodontia rufa

3

Martes americana

3

Nycticebus coucang

3

Cratogeomys castanops

3

Martes flavigula

3

Otolemur crassicaudatus

3

Spermophilus sp.

1

Bassariscus astutus

3

Galago senegalensis

3

Marmota monax

1

Potos flavus

3

Cavia porcellus

1

Nasua nasua

3

Anthropoids

Rattus sp.

1

Leopardis pardalis

3

Cacajao calvus

2

Canis latrans

3

Pithecia sp.

4

Marsupials

Procyon lotor

1

Chiropotes satanas

3

Didelphis virginiana

3

Cebus apella

3

Didelphis albiventis

3

Cebus albifrons

3

Caluromys philander

3

Aotus trivirgatus

2

Philander sp.

3

Saimiri sciureus

2

Trichosurus vulpecula

2

Callicebus moloch

3

Pseudocherius herbertensis 3

Callithrix sp.

3

Phalanger orientalis

2

Saguinus fuscicollis

3

Ailurops ursinus Spilocuscus maculatus

2 1

1 Most estimates suggest that early euprimates were small in body size (e.g., Cartmill, 1974; Dagosto and Terranova, 1992; Martin, 1972; Rose, 1995) and therefore only small taxa were selected for the comparative sample. Most anthropoids of small body size are platyrrhines.

1 Most estimates suggest that early euprimates were small in body size (e.g., Cartmill, 1974; Dagosto and Terranova, 1992; Martin, 1972; Rose, 1995) and therefore only small taxa were selected for the comparative sample. Most anthropoids of small body size are platyrrhines.

Table 3. Shape variables and ratios

Roundness of proximal humeral epiphysis Humeral head shape Humeral head size Relative humeral head size Supraspinatus facet shape Supraspinatus facet size Relative supraspinatus facet size Relative greater tubercle projection Relative greater tubercle width Relative intertubercular width SIZE (Geometric mean of 8 linear measurements)

(Transverse Diam / Sagittal Diam) x 100

(Humeral Head Wd / Humeral Head Lgth) x 100 (Humeral Head Wd x Humeral Head Lgth)1/2 (Humeral Head Size / SIZE) x 100

(Supraspinatus Facet Wd / Supraspinatus Facet Lgth) x 100 (Supraspinatus Facet Wd x Supraspinatus Facet Lgth)1/2 (Supraspinatus Facet Size / SIZE) x 100 ((Sagittal Diam - Humeral Head Lgth) / Size) x 100

(Greater Tubercle Wd / SIZE) x 100 (Intertubercular Wd / SIZE) x 100 (Transverse Diam x Sagittal Diam x Humeral Head Wd x Humeral Head Lgth x Supraspinatus Facet Wd x Supraspinatus Facet Lgth x Greater Tubercle Wd x Intertubercular Wd)1/8

To investigate the applicability of the resulting analysis to fossil material, three test cases were included. The first was casts of a proximal humerus (USNM 17994#2) and partial scapula (USNM 21815#7) from the Eocene form, Smilodectesgracilis. The others were two undescribed possible primate fossil proximal humeri (Figure 3) from Miocene localities in Uganda (MUZM 173 from Moroto I locality dated > 20.6 MYA (Gebo et al., 1997), and BUMP 101 from Napak CC locality thought to be approximately 19 MYA (MacLatchy, pers.com)) loaned to the author by Dr. Laura MacLatchy of the University of Michigan.

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