Indicators of Locomotor Habits in Xenarthrans: Evidence for Locomotor Heterogeneity among Fossil Sloths

Indicators of Locomotor Habits in Xenarthrans: Evidence for Locomotor Heterogeneity amongFossil SlothsAuthor(s): Jennifer L. WhiteSource: Journal of Vertebrate Paleontology, Vol. 13, No. 2 (Jun. 8, 1993), pp. 230-242Published by: Taylor & Francis, Ltd. on behalf of The Society of Vertebrate PaleontologyStable URL: http://www.jstor.org/stable/4523502 .

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WHITE-LOCOMOTOR HABITS IN FOSSIL SLOTHS 231

cluded are from the Pleistocene radiation of the An- tillean Islands; again, none approach the size of the mainland Pleistocene "giants." Previous suggestions that certain Antillean genera were arboreal (e.g., An- thony, 1916; Matthew and de Paula Couto, 1959; de Paula Couto, 1967; Hirschfeld and Webb, 1968; Webb, 1985) have been based largely on their relatively small size. Appendix 1 presents estimates of body weight in fossil taxa included in this study.

MATERIALS AND METHODS

A suite of features was examined on three major joints of the appendicular skeleton: the elbow, hip, and knee joints. Three specific indices were chosen to represent aspects of the morphology of these regions (depicted in figures for each index). Dorsal Olecranon Projection is an index of the dorsal extent of the olecranon process (perpendicular to the ulnar shaft) from the center of the trochlear notch, divided by the length of the trochlear notch from the most proximal, anterior point to the junction of the trochlear and radial notches (see Fig. 1). The long axis of the ulnar shaft, which in many taxa exhibits a marked curvature, was deter- mined by vertically aligning the proximal and distal ends of the ulna. The drawings that appear skewed in Figure 1 appear so because the distal end is not depicted. It may also be noted in Figure 1 that the shape of the trochlear notch varies somewhat among taxa. This variation occurs primarily in mediolateral width, depth of the concavity, and projection of the coronoid process; length as measured here is subject to relatively less variation. Similar olecranon indices have been employed to reveal locomotor differences in primates (e.g., Jolly, 1972; Walker, 1974) and carnivores (e.g., Van Valkenburgh, 1987). Femoral Neck Angle (used by workers such as Jenkins and Camazine, 1977; Fleagle and Meldrum, 1988; Kappelman, 1988) is the angle formed by a line passing through the center of the neck (or articular surface of the head if there is no neck) and the long axis of the shaft of the femur (see Fig. 3). Distal Femur Shape is an index of the anteroposterior depth of the distal femur divided by the mediolateral width of the bicondylar surface (see Fig. 6), and has been employed by many workers including Taylor (1976), Fleagle and Meldrum (1988), and Kappelman (1988).

Extant primate and xenarthran taxa used in this study are listed in Table 1, along with a summary of the primary locomotor habits of each taxon and average body weights. Within both primates and anteaters, a spectrum of locomotor behaviors from suspension to fully terrestriality is exhibited. For the purpose of this study, taxonomic units are meant to be functionally uniform entities. Among primates, species are used as

FIGURE 1. Lateral view of the proximal ends of left ulnae of extant primates (A-D), anteaters (E-G), sloths (H-I), and

A B C D

I -

E F G

- J I! I

H I J K L

1

I I

,I - / - 7

~iYB

~':a~i

fossil sloths (J-O). The Dorsal Olecranon Projection index is depicted in A, D, E, G, H, K, L, M, and 0; horizontal dotted lines are the X values and vertical dotted lines across the trochlear notch are the Y values. A, Pongo pygmaeus; B, Alouatta seniculus; C, Cercopithecus aethiops; D, Papio anubis; E, Cyclopes didactylus; F, Tamandua tetradactyla; G, Myrmecophaga tridactyla; H, Bradypus tridactylus; I, Choloepus didactylus; J, Neocnus gliriformis; K, Acratocnus odontrigonus; L, Megalocnus rodens; M, Hapalops longiceps; N, Hapalops elongatus; 0, Prepotherium potens. Scale bars = 1 cm.



232 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 13, NO. 2, 1993

TABLE 1. Extant primate and xenarthran taxa measured in this study, with corresponding locomotor categories and ap- proximate body weights. Primate descriptions are taken from Fleagle (1988), xenarthran descriptions from Wetzel (1985b).

Mass Taxon Locomotor behavior (kg)

Primates Hylobates lar 75% suspensory and quadrumanous climbing; 25% leaping and bi- 5-6

pedal behaviors Pongo pygmaeus highly arboreal and suspensory; quadrumanous climbing 40-80 Ateles paniscus 70% suspension, brachiation, and climbing; 25% arboreal quadrupedal- 9

ism; 5% leaping and bipedality Alouatta seniculus 80% arboreal quadrupedalism; 15% suspension; 5% leaping 6-7 Presbytis melalophos 20% arboreal quadrupedalism; 10% suspension; 70% leaping 6-7 Saimiri sciureus 55% arboreal quadrupedalism; 5% suspension; 40% leaping <1 Cercopithecus aethiops 90% quadrupedal (20% terrestrial); 10% leaping 4-5 Papio anubis primarily terrestrial quadrupedalism 15-25 Gorilla gorilla highly terrestrial; infrequent climbing 170

Xenarthra Choloepus hoffmanni; fully arboreal and suspensory 5-7

C. didactylus Bradypus tridactylus; fully arboreal and suspensory 4-5

B. variegatus Cyclopes didactylus fully arboreal 0.24 Tamandua tetradactyla; both arboreal and terrestrial quadrupedalism; digging behaviors 4-5

T. mexicana Myrmecophaga tridactyla fully terrestrial; digging behaviors; infrequent bipedality 25-40

taxonomic units because there are known behavioral differences between many primate species within a single genus. There are only three genera of extant anteaters, and two are represented by only one species each (Myrmecophaga tridactyla and Cyclopes didactylus). Tamandua includes two species, T. tetradactyla and T. mexicana, which are combined for analyses presented here because there are no significant locomotor differences between them. Similarly, taxonomic units used for the tree sloths (Bradypus and Choloepus) are a mixture of B. variegatus and B. tridactylus, and C. hoffmanni and C. didactylus, respectively. Although there are subtle differences in size, geographical range, and activity patterns between species of tree sloths, for the purposes of this study species fall into identical locomotor categories within a genus.

Fossil sloth taxa measured for this study are listed in Table 2. The identification of species and genera are of course always arbitrary to some extent in fossils, especially when many species are based upon single bones. I therefore represent many fossil species by single specimens, except in cases where I was confident that a group of specimens were all members of the same species.

Tables of paired comparisons among means for extant primates and extant xenarthrans, for each of the three indices, are presented in Table 3. Indices were first tested for normality and homogeneity of vari- ances. The parametric GT2 method of unplanned comparisons among means was used for normal and ho- mogeneous data. Heterogeneous data were tested for equality of means using the Games and Howell meth-

od. Kruskal-Wallis and Mann-Whitney U-tests for differences in locations of means were employed for non- normal data. These tests were performed with the BIOM package of statistical programs (Rohlf, 1988). Means and standard deviations for femoral neck angle data were obtained by following methodologies outlined in Zar (1984) for circularly distributed data. Paired comparisons of means were then performed with non-parametric Kruskal-Wallis and Mann-Whitney U-tests.

RESULTS AND DISCUSSION

Dorsal Olecranon Projection

The correlation between orientation, shape, and size of the olecranon process of the ulna and mode of locomotion has been well documented in primates (Jolly, 1972; Walker, 1974; Bown et al., 1982; Fleagle and Meldrum, 1988; Ford, 1990) and carnivores (Van Val- kenburgh, 1987; Taylor, 1989). This process serves as the insertion point for m. triceps brachii, which extends the forearm at the elbow. Suspensory animals have extremely small olecrana, because of a minimal need for forearm extension, which is achieved by gravity (Fig. 1A, H, I). Climbers and diggers often have rather long, proximally extended olecrana (in line with the shaft of the ulna), with little dorsal extent; there may in fact be a slight ventral orientation (Fig. 1B, C, F). The forearm is habitually bent at the elbow in climbers and diggers, and such an olecranon orientation max- imizes triceps leverage during forearm extension. In contrast, the olecranon tends to project dorsally in terrestrial animals, for maximum mechanical advantage




TABLE 2. Fossil specimens used in this study and indices (with sample sizes) for which they were measured. AMNH = American Museum of Natural History; MCZ = Museum of Comparative Zoology; YPMPU = Princeton collection at Yale Peabody Museum; KMNH = University of Kansas Museum of Natural History; MPUH = Museo Poey de la Universidad de La Habana, Cuba; MNCN = Museo Nacional de Ciencias Naturales, La Habana, Cuba; OARR = private collection of Oscar Arredondo, Cuba. DO refers to Dorsal Olecranon Projection index; FA refers to Femoral Neck Angle; DF refers to Distal Femur Shape index; "*" refers to the author's identification numbers for individuals lacking specimen numbers.

Taxon Specimens Indices

Antillean Neocnus gliriformis AMNH 49950 DO, N = 1 Neocnus bairiensis MNCN 425/1 DF, N = 1 Neocnus minor MNCN 417/2; OARR 499, 522, DO, N = 3; FA, N = 1;

1007 DF, N = 1 Neocnus major MNCN 417/121; OARR 1005, DO, N = 2; FA, N = 3;

1006, 512, 1023* DF, N = 2 Mesocnus herrerai MPUH 1014* DO, N = 1 Mesocnus torrei MNCN 420/3; MPUH 93, 98, 74 DF, N = 3; DO, N = 1; FA, N = 1 Mesocnus browni OARR 730, 708, 707, 731, 727, DO, N = 7; DF, N = 4; FA, N = 1

711, 713, 940, 944, 945, 941, 950, 681

Megalocnus rodens MCZ 4443, 10197, 10203, 8430, DO, N = 9; FA, N = 10; DF, N = 10189, 17616, 10185, 10170; 15 AMNH 49971, 49977, 49978, 49972,49995; MNCN 420/6, 424/2; MPUH 275, 272; OARR 1031*, 690, 1032*, 1033*

Miocnus antillensis AMNH 49944; OARR 1025* DO, N = 2; FA, N = 1 Acratocnus odontrigonus MCZ 10136, 4442, 16931, 9769, DO, N = 10; FA, N = 18; DF, N =

16917, 16758, 16918, 16757; 20 AMNH 14170, 17362A, 17362C, 17362D, 17363C, 17363E, 94722, 17711B, 17711C, 94719, 94721, 94722, 177166, 17716L, 17716G, 177161D, 177161A, 177166C, 177166A, 17711FA, 17711FB, 177161E, 177161C

Santacrucian

Eucholoeops fronto AMNH 9241 FA, N = 1; DF, N = 1 Pelecyodon arcuatus AMNH 9240 DO, N = 1; DF, N = 1 Prepotherium potens YPMPU 15345 DO, N = 1; FA, N = 1; DF, N = 1 Planops sp. KMNH 5559 FA, N = 1; DF, N = 1 Hapalops longiceps YPMPU 15523 DO, N = 1; FA, N = 1; DF, N = 1 Hapalops indifferens YPMPU 15110 FA, N = 1; DF, N = 1 Hapalops platycephalus YPMPU 15536 DO, N = 1; FA, N = 1 Hapalops elongatus YPMPU 15160 DO, N = 1

of m. triceps brachii while the forelimb is extended (Fig. lD, G).

The Dorsal Olecranon Projection index is graphically presented in Figure 2. Among primates, a rough gradient exists from the highly suspensory Hylobates through the primarily terrestrial Papio. Papio, with a great dorsal projection, is statistically distinguishable from all other taxa (Table 3). Primates practicing varying degrees of arboreal quadrupedalism (see Table 1) are distributed between the extremes.

The three genera of anteaters are clearly distin- guished by this morphological index; therefore, the dorsal olecranon projection is an effective discrimi- nator of locomotor habits among these xenarthrans (Table 3, Fig. 2). Although the numerical range of the

measurement varies between primates and anteaters, the measurement succeeded in separating locomotor modes within both primates and anteaters, and the magnitude of the ranges are comparable for the two groups.

Extant and fossil sloths (Figs. 1H-O, 2) exhibit a range comparable to that of anteaters and primates for the dorsal olecranon projection index. Sample sizes for each fossil taxon are presented in Table 2. The extant suspensory tree sloths, Bradypus and Choloepus, man- ifest the smallest olecrana. If all fossil sloths were really terrestrial, one would expect most to fall on the right side of the graph with dorsally projecting olecrana as do the three species of Mesocnus and Megalocnus rodens from Cuba, and the Santacrucian Prepotherium



234 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 13, NO. 2. 1993

ARBOREALITY ---> TERRESTRIALITY

Hylobates - Ateles -

Saimiri - I

Alouatta -

Pongo - , Presbytis

Gorilla - -

Cercopithecus -

Papio

Cyclopes Tamandua -

Myrmecophaga -

Choloepus Bradypus -

Neocnus gliriformis* -

Pelecyodon arcuatus. -

Hapalops longiceps. - H. platycephalus*

Miocnus antillensis* - Acratocnus odont.* -

Neocnus minor* - Hapalops elongatus* -

Neocnus major* -

Prepotherium potens -. Mesocnus torrei* -

Mesocnus browni* - Megalocnus rodens - Mesocnus herrerai -_

0.3 0.6 0.9 1.2 1.5 1.8 2.1

Index of Dorsal Olecranon Projection

FIGURE 2. Plot of means (vertical bars) and standard deviations (horizontal bars) for Dorsal Olecranon Projection for primates, anteaters, and sloths. Squares represent single values; "*" refers to Antillean fossil sloths; "+" refers to Santacrucian fossil sloths.

(Fig. 1L, O). Although no fossil exhibits the extreme olecranon reduction seen in tree sloths, several taxa fall on the left side of the graph, one even within the range of the extant tree sloths, because they exhibit relatively small (Fig. IJ), or merely proximally projecting (Fig. 1K, M) olecrana. This morphology is associated with suspensory or climbing activities, while a substantial dorsal projection is seen in more terrestrial animals that habitually maintain an extended forelimb.

Femoral Neck Angle

Several features were examined on the proximal femur, including shape and orientation of the femoral head and its articular surface, and size of the greater trochanter. The measurement reported on here is the angle of the neck relative to the shaft, where zero degrees is a completely proximally oriented head. Sus- pensory and highly arboreal animals tend to have highly curved, globular heads, and small greater trochanters, to maximize the range of mobility within the acetab- ulum (Fig. 3A, E, H, I). In addition, the angle of the neck is small because the head projects superiorly (or proximally) from the shaft which allows the femur to

move in a multitude of directions (Walker, 1974; Flea- gle and Meldrum, 1988). In contrast, highly cursorial animals and terrestrial and leaping primates practice more limited femoral movements, and characteristically have flatter femoral heads, more perpendicularly oriented necks, and larger, proximally projecting greater trochanters (Fig. 3C, D, G). This arrangement seems to facilitate adduction of the femur while the hind limbs are moved in a primarily parasagittal plane as seen in primates, bovids, and some carnivores (although this generalization does not apply to all car-

A B C D

\

SI

E F G

H I J K L

M N 0

I r 1

FIGURE 3. Anterior view of proximal ends of left femora for extant primates (A-D), anteaters (E-G), sloths (H-I), and for fossil sloths (J-O). Femoral Neck Angle is depicted on A, D, E, G, H, L, and 0; the angle is formed at the junction of the dotted lines. A, Pongo pygmaeus; B, Alouatta seniculus; C, Cercopithecus aethiops; D, Papio anubis; E, Cyclo- pes didactylus; F, Tamandua tetradactyla; G, Myrmecophaga tridactyla; H, Bradypus tridactylus; I, Choloepus didactylus; J, Acratocnus odontrigonus; K, Miocnus antillensis; L, Me- galocnus torrei; M, Eucholoeops fronto; N, Hapalops indifferens; 0, Prepotherium potens. Scale bars = 1 cm.




TABLE 3. Paired comparisons among means for extant primates, sloths, and anteaters for each of the three indices. N: sample size; NS: not significant. Primates: AL = Alouatta, AT = Ateles, CE = Cercopithecus, GO = Gorilla, HY = IHylobates, PA = Papio, PO = Pongo, PR = Presbytis, SA = Saimiri. Xenarthrans: BR = Bradypus, CH = Choloepus, CY = Cyclopes, MY = Myrmecophaga, TA = Tamandua.

A. Dorsal olecranon projection Primates

Kruskal-Wallis: * P = 0.05, Xenarthrans ** P = 0.025, *** P = 0.005; Kruskal-Wallis: *** P = 0.001;

Games and Howell: + P = 0.05, ++ P = 0.01 Games and Howell: + P = 0.05

N Genus N Genus

7 HY 21 CH 3 AT NS 25 BR NS 7 SA *** ** 11 CY *** +

10 AL NS NS NS 41 TA *** + + 6 PO + NS * NS 18 MY *** + + + 5 PR NS NS NS NS NS CH BR CY TA 3 GO + NS ** NS NS NS

11 CE ++ + *** NS NS NS NS 7 PA ++ ++ *** ++ ++ + ++ +

HY AT SA AL PO PR GO CE

B. Femoral neck angle Primates

Kruskal-Wallis: * P = 0.05, Xenarthrans ** P = 0.025, *** P = 0.01, **** P = 0.001;

Mann-Whitney U: + P = 0.025, ++ P = 0.01, Kruskal-Wallis: +++ P = 0.001 *** P = 0.001

N Genus N Genus

6 PO 26 BR 3 AT NS 23 CH NS 7 HY *** ++ 13 CY NS NS 7 SA *** ++ NS 39 TA *** *** *** 5 PR *** ** NS + 17 MY *** *** *** ***

10 AL *** ** * + NS BR CH CY TA 3 GO ** * ** ** NS *

11 CE **** *** **** **** *** +++ NS 6 PA **** *** **** **** *** +++ NS NS

PO AT HY SA PR AL GO CE

C. Distal femur shape Primates Xenarthrans

Parametric GT2: Games and Howell: * P = 0.05, ** P = 0.01 + P = 0.05, ++ P = 0.01

N Genus N Genus

6 PO 13 CY 3 AT NS 23 BR + 7 HY NS NS 23 CH ++ NS 3 GO NS NS NS 40 TA ++ ++ ++ 9 AL ** * * NS 18 MY ++ ++ ++ ++

11 CE ** ** ** NS NS CY BR CH TA 7 SA ** ** ** NS NS NS 5 PR ** ** ** NS NS NS NS 6 PA ** ** ** NS NS NS NS NS

PO AT HY GO AL CE SA PR

nivores) (Jenkins and Camazine, 1977; Fleagle and Meldrum, 1988; Kappelman, 1988).

Femoral Neck Angle clearly separates locomotor types among primates both graphically and statistically (Fig. 4, Table 3). Pongo and Ateles are statistically

distinguishable from all other primate taxa; these species practice the most suspension using both the fore- limbs and hind limbs. On the other extreme, Gorilla, Cercopithecus, and Papio form a cluster of terrestrial quadrupeds. The four genera in the middle of the range




ARBOREALITY ---> TERRESTRIALITY

Pongo - .... Ateles -

Hylobates - Saimiri •-

Presbytis -

Alouatta - ---- Gorilla -

Cercopithecus Papio

Cyclopes - Tamandua -

Myrmecophaga -

Bradypus - Choloepus -

Hapalops platyceph.+ -

Eucholoeops fronto+ - H. indifferens -

Miocnus antillensis - Acratocnus odont.-

Megalocnus rodens* -

Hapalops longiceps Planops sp.-

Prepotherlum potens - Neocnus major* Neocnus minor*-

Mesocnus brownl* - Mesocnus torreli -

15 25 35 45 55 65

Index of Femoral Neck Angle

FIGURE 4. Plot of means (vertical bars) and standard deviations (horizontal bars) for Femoral Neck Angle for primates, anteaters, and sloths. Squares represent single values; "*" refers to Antillean fossil sloths; "+" refers to Santacru- cian fossil sloths.

perform varying degrees of arboreal quadrupedalism (see Table 1). Whereas Hylobates occupies the most suspensory position on the graph of the dorsal olecranon projection, its value for angle of the femoral neck probably reflects its occasional leaping and bipedal behaviors.

Xenarthran femora are shaped quite differently from those of primates, and again, the numerical range of values for this measurement is shifted relative to that of primates. However, the three behaviorally distinct genera of anteaters are clearly discriminated (Fig. 4, Table 3).

The fossil sloths reveal a range comparable in magnitude to that of the anteaters for this index (Fig. 4). Although quite variable within each tree sloth genus, the indices for both fall on the left side of the graph in accordance with their suspensory habits. Fossil sloth taxa represented by many individuals also show a substantial amount of variability, yet a diversity of femoral neck orientation is evident (Fig. 3J-O). The range exhibited in Figure 4 is actually a conservative represen- tation of the variation in Femoral Neck Angle for fossil sloths because this one measurement cannot take into account other morphological variations of the femur among taxa. Fossil sloth femora are remarkably di-

verse in shape, robustness, and placement of features. It is of course necessary to position every femur in the same manner, to obtain comparable Femoral Neck Angle measurements among different taxa. This is a simple matter in animals with relatively straight, narrow femora, such as primates, anteaters, and tree sloths. Several fossil sloths (e.g., Eucholoeops and Hapalops (Santacrucian), and Miocnus and Acratocnus (Antil- lean)) also have this type of femur. However, Prepothe- rium (Santacrucian), Megalocnus (Antillean), and Me- socnus (Antillean), in particular, have very differently shaped femora.

A comparison between the Antillean species Mega- locnus rodens and Acratocnus odontrigonus illustrates the above point (Fig. 5). In Figure 4, these taxa have very similar means and standard deviations for femoral neck angle. Other features of the femur, however, suggest that these sloths probably used their femora in distinct manners. Acratocnus (Fig. 5B) has a slender, straight femur with a cylindrical shaft. The distal end is shallow anteroposteriorly and wide mediolaterally and exhibits a slight bicondylar angle. The proximal end is characterized by very distinct, narrow trochanters, a short neck, and a globular head that projects superior to the greater trochanter. The greater trochanter is the site of insertion for the gluteal muscles, which abduct and extend the femur. The femoral head of highly arboreal animals projects more superiorly than the greater trochanter, which allows for a great range of movement of the thigh by these muscles (Flea- gle and Meldrum, 1988).

In contrast, the femoral shaft of Megalocnus (Fig. 5A) is flattened anteroposteriorly, and the lateral side of the shaft is expanded into a bladelike ridge that extends both along the distal half of the shaft, and distally from the greater trochanter. The distal end is quite deep anteroposteriorly, asymmetrical, and angled medially; these features give an almost curved ap- pearance to the bone. The medial condyle is extremely enlarged both mediolaterally and anteroposteriorly relative to the lateral condyle; this suggests that most weight was borne on the medial side of the knee joint in a habitually abducted posture of the knee (Jungers, 1976; Fleagle and Meldrum, 1988). The proximal end is very wide and flat, and the huge, robust greater trochanter is continuous with the lateral ridge while projecting superior to the head. A greater trochanter projecting superior to the head limits abduction of the femur in terrestrial primates and cursorial bovids that habitually only protract and retract the hind limb (Fleagle and Meldrum, 1988; Kappelman, 1988). The lesser trochanter is less conspicuous and is located more distally relative to the head than in Acratocnus. The head sits directly on the shaft with essentially no neck, and is flatter, with proportionally less articular surface area, relative to the femoral head of Acratocnus.

Thus, whereas the femur of Acratocnus appears to be suited for a high degree of mobility at the hip joint, the morphology of the femur of Megalocnus seems to suggest reduced mobility at the hip, along with an ab-



WHITE - LOCOMOTOR HABITS IN FOSSIL SLOTHS 237

ducted posture of the knee. Having a characteristically abducted knee indicates that the distal end of the femur would be held lateral to the hip. Consequently, the proximal end would actually be angled medially, caus- ing the greater trochanter to project even more superiorly, and the head to project more perpendicularly.

Distal Femur Shape

The shape of the distal femur is highly correlated with locomotor mode in primates, carnivores, and bovids (Taylor, 1976, 1989; Fleagle and Meldrum, 1988; Kappelman, 1988). Suspensory and highly arboreal animals that continually flex the hind limb tend to have mediolaterally broad, anteroposteriorly shallow, sym- metrical distal femora and patellar grooves (Fig. 6A, E, H). Terrestrial and cursorial animals have anteroposteriorly deep distal femora, often with high medial patellar lips and asymmetrical femoral condyles (Fig. 6C, D, G). A deep distal femur increases the leverage arm for extension of the leg at the knee by m. quad- riceps femoris, and a high patellar lip may serve to prevent displacement of the patella, over which the extensors insert.

A substantial bicondylar angle and a greatly enlarged medial condyle are suggestive of an abducted posture of the knee (Jungers, 1976). This morphology may be an adaptation for walking on the outer side of the pes during terrestrial locomotion, or for climbing because

A B)

FIGURE 5. Anterior view of the femora of A, Megalocnus rodens and B, Acratocnus odontrigonus. Scale bars = 1 cm.

A B C D

A3

-I

E F G H

I J K

L M N

,-

-

FIGURE 6. Distal view of the distal ends of left femora for extant primates (A-D), anteaters (E-G), sloths (H-I), and for fossil sloths (J-N). The Distal Femur Shape index is depicted on A, D, E, G, J, L, and N; vertical dotted lines are X values and horizontal dotted lines are the Y values. A, Pongo pygmaeus; B, Alouatta seniculus; C, Cercopithecus aethiops; D, Papio anubis; E, Cyclopes didactylus; F, Ta- mandua tetradactyla; G, Myrmecophaga tridactyla; H, Bra- dypus tridactylus; I, Choloepus didactylus; J, Pelecyodon arcuatus; K, Hapalops longiceps; L, Prepotherium potens; M, Acratocnus odontrigonus; N, Mesocnus torrei. Scale bars = 1 cm.

it would allow powerful adduction of the two feet (Flea- gle and Meldrum, 1988; Taylor, 1989).

Primates show a clear separation between statistically indistinguishable highly arboreal, climbing species (Pongo, Ateles, and Hylobates), and the equally coherent group of species that practice more leaping and terrestrial activities (Cercopithecus, Saimiri, Pres- bytis, and Papio) (Fig. 7, Table 3). Gorilla and Alouatta fall in the middle of the range. Alouatta is primarily an arboreal quadruped (Table 1), and Gorilla appears to be an exception in that it is a primarily terrestrial animal, yet has a relatively shallow distal femur. Go- rillas are known to climb, and perhaps the shallowness is related to this activity.

The separation between the three anteater genera is remarkable, demonstrating that this index is very use- ful for discriminating locomotor modes among members of this family (Fig. 7, Table 3).






pability, based on observed correlations between that trait and that function, the animal did not necessarily include that inferred behavior in its natural repertoire; conversely, it is possible in some cases to perform an activity without exhibiting the commonly associated morphology (Bock and von Wahlert, 1965; Coombs, 1983). Nevertheless, investigators are generally limited to studies of comparative functional morphology and biomechanical analyses when inferring the locomotor behavior of extinct animals.

Clearly, there is a striking diversity of form within the two radiations of fossil sloths included in this study, as well as in other groups. This type of morphological variability certainly is associated with locomotor diversity in other mammalian orders and in xenarthrans; therefore it is not surprising that fossil sloths also would have practiced a variety of locomotor habits. The three indices (and other related features) presented here reveal ranges of morphological variation comparable to those seen in primates and anteaters, in both the San- tacrucian and Antillean radiations. Certain taxa do fall consistently on either the "arboreal" side of each graph (e.g., Hapalops longiceps, Acratocnus odontrigonus) or the "terrestrial" side (e.g., Mesocnus torrei, M. browni). However, it is also evident that a taxon may exhibit a mosaic of features; that is, a taxon may fall on the extreme "arboreal" side of a graph for one index, yet occupy a more "terrestrial" position based on a different index (e.g., Neocnus minor). This reflects differ- ential use of the forelimb, hind limb, and various regions of the body, and a complex and variable behavioral repertoire, all of which likely characterized the group of fossil xenarthrans traditionally thought of as ground sloths.

Evidence presented herein for a diversity of posture and locomotor modes among fossil sloths serves two purposes: 1) the inference of arboreality in fossil sloths through functional analysis discredits any remnants of the traditional "ground" versus "tree" sloth dichoto- my, while lending support to previous claims of arboreality, and 2) a more complete understanding of morphological variation and its implications concern- ing locomotor modes in fossil sloths may shed light on the evolution of the suspensory posture of the extant sloths.

ACKNOWLEDGMENTS

I thank Ross MacPhee for the opportunity to travel to Cuba to examine collections; Oscar Arredondo for access to his private collections; Guy Musser (AMNH), John Alexander (AMNH), and Mary Ann Turner (YPM) for the loan of material; Wolfgang Fuchs (AMNH), Linda Gordon (USNM), Farish Jenkins (MCZ), Mary Ann Turner (YPM), Desui Miao (KMNH), and Manuel Rivero de la Calle (MPUH) for facilitating access to specimens; and John Fleagle, Da- vid Krause, Robert Emry, Margery Coombs, H. Greg- ory McDonald, and Laurie Godfrey for reviewing and improving drafts of the manuscript. Financial support

for this project was provided by National Science Foundation Grant BNS-9012154 to John Fleagle, and by the American Museum of Natural History (Theo- dore Roosevelt Memorial Fund).

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Received 21 November 1991; accepted 28 May 1992.



WHITE - LOCOMOTOR HABITS IN FOSSIL SLOTHS 241

APPENDIX

Estimated body weights for fossil sloth taxa included in this study. Anteroposterior femoral head diameter (FHD) was chosen as the dependent variable because it is easily measured, the proximal femur is relatively abundant in the fossil record, and FHD has been shown to be highly correlated with body weight (BW) in primates (Jungers, 1987, 1990). Regressions were performed on both raw and logged values of FHD. A log transformation is often advocated when the scales of measurement vary (e.g., linear vs. volumetric), and it may decrease prediction errors (Jungers, 1987). For the extant data, the correlation (r) between raw FHD and BW is 0.879; between log FHD and BW, r = 0.967. For these reasons, predicted values from logged data may be more appropriate and accurate.

Least squares (LS) regression is most commonly used to predict body mass (e.g., Martin, 1990; Van Valkenburgh, 1990). However, when the values to be predicted are far outside the range of the known values, as in this case, reduced major axis (RMA) is likely to be a better predictive technique (Jungers, 1988), and least squares probably underestimates the Y values in the range of the greatest X values. In this study, the values of FHD for the 10 extant xenarthran species used to compute regression equations range from 5.5 mm to 28.9 mm (Table 1), while the values for fossil individuals range from 21.3 mm to 58.5 mm (Table 2). As FHD increases away from the mean X and Y extant values, the BW estimates between LS and RMA, and between raw and logged data, become more divergent. Because of the limited overlap between the range of FHD for extant and fossil individuals, and because estimated BW for the largest specimens varies substantially depending on the type of regression used, confidence limits are probably meaningless, especially since the fossils are not part of the reference sample (Jungers, 1988).

Taking into consideration all the aforementioned points and the nature of these data, the body weights estimated from the RMA regression against log FHD are preferred here,

although I must emphasize that I do not place much confidence on the accuracy of these estimates, especially for the largest individuals. It should be noted that in Gingerich's (1990) BODYMASS program, 12 limb element lengths and diameters of Hapalops longiceps 15523 yield a weighted geo- metric mean estimated body weight of 26 kg (with very wide confidence limits), and a weight of 55 kg in a multiple regression of all 12 measurements based on 36 mammalian species. BODYMASS seems to be designed to work best, however, for animals that are generally less robust than fossil sloths. Sherman (1984) reports estimated weights for "Ha- palops" of 25.5 kg (against log scapular length) and 48.2 kg (against humeral width), but hind limb dimensions may be more accurate predictors of body mass because body weight is more consistently borne by the hind limb than by the forelimb. The predicted values for H. longiceps 15523 in Table 2 range from 41.3 to 81.4 kg. The higher estimated values in the RMA (log) column for all the taxa do not seem unreasonable, and may provide rough upper limits on body weight; given the absolute size and robustness of the bones, values computed from raw FHD seem to be underestimates.

Regression Equations.

Least Squares (raw):

BW = 1.357946-X - 13.80087

Least Squares (log):

log BW = 2.749356-log X - 2.554307

Reduced Major Axis (raw):

BW = 1.54419-X - 16.66901

Reduced Major Axis (log):

log BW = 2.84306-log X - 2.66262

APPENDIX--TABLE 1. Data from 10 xenarthran species (sloths, vermilinguas, and armadillos) that were used to compute regression equations in which the independent variable is anteroposterior femoral head diameter (FHD) in millimeters, and the dependent variable is average body weight (BW) in kilograms. Means and standard deviations (SD) are presented for FHD and BW, and sample sizes (N) for FHD. Body weight data are from Wetzel (1985a, b).

FHD (mm) BW (kg)

Extant species Mean SD N Mean SD

Bradypus variegatus 14.9 1.24 15 4.34 0.85 B. tridactylus 15.2 0.97 7 4.01 0.28 Choloepus hoffmanni 18.0 1.33 16 5.72 0.69 C. didactylus 18.4 1.98 8 6.07 1.09 Myrmecophaga tridactyla 28.9 2.14 18 32.90 7.10 Tamandua mexicana 14.9 1.18 9 4.32 0.68 T. tetradactyla 14.2 0.67 28 4.83 0.90 Cyclopes didactylus 5.5 0.69 16 0.235 0.64 Euphractus sexcinctus 12.7 0.98 16 5.39 0.94 Dasypus novemcinctus 11.4 0.73 28 3.30 0.56




APPENDIX -TABLE 2. Body weight estimates for a sample of fossil sloth individuals representing most of the taxa included in this study (not necessarily the individuals depicted in Figs. 2, 4, and 7, depending on availability of the measurement). Anteroposterior femoral head diameter (FHD) is the independent variable; body weight is the dependent variable. Both least

squares (LS) and reduced major axis (RMA) regressions were performed on raw and logged values of FHD.

Estimated body weight (kg)

LS RMA

Fossil specimens FHD (mm) Raw Log Raw Log

Neocnus minor 21.3 15.1 12.5 16.2 13.0

Neocnus major 25.8 21.2 21.2 23.2 22.4 25.3 20.6 20.1 22.4 21.2

Mesocnus torrei 27.2 23.1 24.5 25.3 26.1 32.0 29.7 38.4 32.7 41.4

Megalocnus rodens 42.5 43.9 83.7 49.0 92.7 44.6 46.8 95.6 52.2 106.3 43.8 45.7 90.9 51.0 101.0 35.9 34.9 52.6 38.8 57.4 52.9 58.0 152.8 65.0 172.7 41.8 43.0 80.0 47.9 88.4 48.6 52.2 [21.0 58.4 135.7 36.0 35.1 53.0 38.9 57.8

Miocnus antillensis 28.9 25.4 29.0 28.0 31.0 33.9 32.2 45.0 35.7 48.7

Acratocnus odontrigonu. 24.7 19.7 18.8 21.5 19.8 24.3 19.2 18.8 20.9 18.9 31.2 28.6 35.8 31.5 38.5 26.3 21.9 22.4 23.9 23.7 27.7 23.8 25.8 26.1 27.4 26.6 22.3 23.1 24.4 24.5 24.1 18.9 17.6 20.5 18.5 25.9 21.4 21.5 23.3 22.7 23.1 17.6 15.7 19.0 16.4 26.9 22.7 23.8 24.9 25.3 22.7 17.0 14.9 18.4 15.6 28.3 24.6 27.4 27.0 29.2

Prepotherium potens 55.5 61.6 174.3 69.0 197.9 58.5 65.6 201.5 73.7 229.9

Planops sp. 51.2 55.7 139.7 62.4 157.4

Hapalops longiceps 40.6 41.3 73.8 46.0 81.4

Hapalops indifferens 36.1 35.2 53.4 39.1 58.3

Hapalops platycephalus 29.0 25.6 29.3 28.1 31.3 31.3 28.7 36.1 31.7 38.8

Hapalops elongatus 25.3 20.6 20.1 22.4 21.2

Eucholoeopsfronto 32.8 30.7 41.1 34.0 44.4



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Indicators of Locomotor Habits in Xenarthrans: Evidence for Locomotor Heterogeneity among Fossil Sloths