Derived Features Of The Mrca

There are a number of characteristics of the postcranium that are hypothesized to distinguish the MRCA of primates from other mammals (even though most have been modified in some groups of primates), and most importantly from the most likely sister taxon of Primates, whether that be some member of the Plesiadapiformes, the Scandentia, or the Dermoptera. A partial list of these characters, their presumed mechanical implications and probable biological roles is given in Table 1.

Features Related to Grasping

As noted by many others, a significant number of features in the list are attributed to the presence of grasping extremities, particularly the foot. Among these are—an opposable hallux (which is a shorthand way of referring to numerous morphological modifications, including changes in the shape of the entocuneiform-first metatarsal joint (Szalay and Dagosto, 1988)), replacement of the hallucal claw with a nail, and changes in the shapes and orientations of tarsal bones and joint facets on crural and tarsal bones to accommodate an inverted foot, most of which serve to increase the range of inversion and eversion of the lamina pedis. Inversion is necessary to advantageously position the foot for use of an opposable hallux.

The modifications to the primate tibiotalar articulation have been described and discussed by several authors (Dagosto, 1985; Hafferl, 1932; Lewis, 1980a). The rotation of the joint surface of the medial malleolus, the convexity of the malleolus and the coordinated concavity of the malleolar cup on the talus, and the medial curve of the tibial facet on the talus dictate that as the talus moves from a plantarflexed to a dorsiflexed position, it also abducts relative to the tibia. The resulting close-packed dorsiflexed-abducted-pronated position of the talus allows for full range of motion of the lamina pedis at the subtalar and transverse tarsal joints with a stable talus that is functionally part of the leg. Lewis (1980a) proposed that this suite of features is related to grasping, and there is no doubt that the lamina pedis would be advantageously positioned to invert and grasp. However, there may be a more general relationship to the requirements of an arboreal milieu. The freer motion at the subtalar joint (STJ) and transverse tarsal joint (TTJ) that the upper ankle joint (UAJ) morphology allows simply permits the foot to attain variable positions.

The subtalar joint of primates is not greatly modified relative to any archontan ancestor. The posterior talocalcaneal facets are slightly more elongated (reflected in the posterior trochlear shelf). The anterior talocalcaneal facet is longer than in plesiadapids, but not much longer than in Ptilocercus, most likely as the result of its longer talar neck. Elongation of the talar neck may be related to the general lengthening of the tarsus typical of leapers. However, other arboreal nonleaping mammals (Potos, Felis wiedii, Ptilocercus) exhibit long talar necks, and it may simply permit greater subtalar motion for inversion-eversion (Jenkins and McClearn, 1984).

The transverse tarsal joint of primates exhibits several derived characters. The talonavicular joint is more spherical (mediolateral and dorsoplantar dimensions of the talar head approximately equal) compared to the condition seen in plesiadapids, tupaiids, or dermopterans, and mammals generally, where the mediolateral dimension exceeds the dorsoplantar (Dagosto, 1986, 1988; Hooker, 2001). Probably correlated with this, the dorsal margin of the talar head is not indented as in the other taxa (Hooker, 2001). In other mammals motion at the TTJ is a combination of pronation-supination and mediolateral translation (Jenkins and McClearn, 1984). The unmodified ovoid shape of the primate talonavicular joint dictates that the pronation-supination component is enhanced and the mediolateral translation is reduced, producing a more purely axial rotation. This is possibly related to the enlargement and functional independence of the hallux, and thus to grasping abilities (Dagosto, 1986).

There is some disagreement concerning the morphology, function, and polarity of the calcaneocuboid joint. Decker and Szalay (1974) contrasted the form of this joint in primates where a subconical projection on the proximo-plantar edge of the cuboid fits into a depression on the distoplantar surface of the calcaneus, with that of Plesiadapiformes and other mammals where the more evenly rounded convex cuboid surface articulates with a more evenly concave calcaneocuboid facet. Stressing the shape differences, they emphasized the pivotal nature of the joint. Lewis (1980b) criticized their interpretation, noting that pivotal action at the joint does not require a physical pivot (i.e., protuberance). Both kinds of joints function as "pivots" (i.e., they allow some rotation between the calcaneus and cuboid), but in different ways. In terms of joint geometry, the nonprimate type of joint is a modified ovoid, and the primate joint is an unmodified sellar (nomenclature of joint shape is from MacConaill, 1973). The modified ovoid allows a limited degree of conjunct rotation (inversion-eversion) accompanying medial and lateral translation (adduction-abduction), while the sellar shape of the primate joint, plus the central location of the projection, allows an arcuate slide (inversion-eversion) but limits the accompanying degree of mediolateral translation (Hafferl, 1929).

The form of the primate calcaneocuboid joint, like that of the talonavicular joint, allows increased inversion-eversion at the transverse tarsal joint without appreciable mediolateral sliding of the cuboid and navicular against the talus and calcaneus. This is also likely a mechanism for repositioning the foot into a grasping attitude, while increasing stability at the transverse tarsal joint.

A more serious problem for recognizing this feature as a primate apomor-phy is that similar morphologies occur in other mammals including Cynocephalus, Tupaia, and some plesiadapiforms (Beard, 1993; Lewis, 1980b). Although Cynocephalus clearly has a primatelike protuberance, neither Ptilocercus nor Dendrogale have a protuberance, and Tupaia and Urogale have only the smallest hint of one. The presence of a protuberance in Plesiadapiformes is variable (Beard, 1989). In contrast to Beard, who favorably compared the function and morphology of the dermopteran and primate form of the joint, Hooker (2001) questions the homology between them.

Another feature of the primate foot that has been associated with grasping small supports is the high phalangeal index (digits are long relative to metatarsus) (Lemelin, 1999). Hamrick (this volume) demonstrates that this is accomplished through elongation of the proximal, rather than intermediate phalanges.

Some features of the primate forelimb skeleton have also been thought to be related to grasping. Godinot (Godinot, 1992; Godinot and Beard, 1991) cites the relatively short carpus, long digits, and elongated scaphoid tubercle (see also Hamrick, 1997). The thumb shows some independence and grasping ability (Altner, 1971), and the phalangeal index is high (Lemelin, 1999).

There seems to be relatively little disagreement about either the functional implication of these features (e.g., they enhance the ability of the extremity to grasp), or that the biological role of grasping is to allow/facilitate movement on supports that are small relative to the size of the animal. "Small branches," however, is a rather inexact term that needs more clarification in order to assess selective forces and performance attributes of differing morphologies (Crompton, 1995). The terms "small branch," "relatively small branch," "terminal branches," "fine branches," "canopy," and "shrub layer" are used almost interchangeably, but may have different implications for morphology.

An opposable hallux gives a performance advantage when an animal is balancing or moving on top of a branch small enough so that the narrow triangle of support induces high moments of pitching and rolling (Napier, 1967). There are also performance advantages in other situations, e.g., climbing on or clinging to an oblique or vertical support (Napier, 1967). The ability to use a power grip on a single support is, however, compromised as support size decreases relative to the size of the animal, or more precisely, the size of the grasping organ (Napier, 1980). Studies of human grasping have shown that there is a rather narrow range of cylinder size that provides optimal performance; larger and smaller diameter tool handles require greater muscle activity, and increase the rate of fatigue (Ayoub and LoPresti, 1971; Pheasant and O'Neill, 1975). Bishop (1964) found that on fine branches (<1.2 cm), G. sene-galensis does not use a power grasp; rather it balances with the support across its palm. When challenged with such tiny supports, lemurs, indriids, and even tarsiers either grasp several at once or sit, stand, hang, or cling from larger branches, rather than attempt to support themselves or walk on top of single, very small twigs. For any particular body size, there is an optimal substrate size range on which a power grasp can be applied. This optimum should be deter-minable from hand or foot size, span of grasp (which depends on the opposability of the hallux and the size of the palm or sole), and hand-phalangeal proportions. Bishop (1964) for example, has measured the "effective grasp" of the hand in primates. Her data indicates that Tupaia glis has a smaller effective grasp (both absolutely and relative to body size) than in prosimians. This suggests that, at least at the upper end of the "optimal substrate size range" prosimian hands are designed to grasp branches that are large relative to their body size. Although no similar studies have been done on primate feet, the opposable hallux suggests that they are also designed to allow the use of a power grasp on a range of support sizes (and types, see below), including supports that are larger than most terminal branches. Unless one also proposes that the ancestral euprimate was small enough that the largest support it could reasonably grasp was a few millimeters or less in diameter (Gebo, 2004) there is no reason to suppose that an opposable hallux evolved in an animal restricted to walking along single supports in the terminal branch area. Depending on the size of the animal, an opposable hallux is also useful in areas of the arboreal environment other than "terminal branches" and in situations other than walking slowly quadrupedally on single branches.

Beyond Grasping and "Small Branches"

The grasping foot, and its implication for small branch use, is a key feature of the nocturnal visual predation (NVP) model. It was in fact the only postcranial system considered in the original formation of the model. There are, however, aspects of both the grasping mechanism and other features of the primate postcranial morphotype that differ from other mammalian small branch users. This suggests that, for the primate skeleton, something more than simply grasping small branches is involved.

The most obvious of these differences is the presence of nails on the lateral digits. There is no extant mammalian analog for this, making it difficult to interpret. The suggestions are: (1) Nails are superior to claws when moving within the small branch milieu either because claws interfere with digital flexion when grasping very small supports, or because the expansion of the digital pads, which increases stability on small supports, is correlated with reduction of terminal phalanx length and claw length (Cartmill, 1974a; 1974b; Clark, 1959; Hamrick, 1998; Hershkovitz, 1970; Lemelin and Grafton, 1998; Napier, 1980); and (2) Claws interfere with landings after leaps (Szalay, 1972). In support of the first hypothesis, Hamrick (1998) has demonstrated a morphocline in apical pad size, terminal phalanx breadth, and small branch use in platyrrhines. Cartmill cites the examples of Cercartetus and Burramys, small marsupials that have reduced claws on lateral digits. Burramys, however, does not seem to exploit a small branch environment (see Rasmussen and Sussman, this volume), and Caluromys, a marsupial which does, retains claws, as do most other arboreal didelphids and phalangerids with grasping extremities. The difficulty with Szalay's hypothesis, according to Cartmill (1974a), is that other mammals that jump from trees retain claws. It is argued below, however, that the frequency of leaping in primates distinguishes them from other mammals, and thus may have been a more significant factor in the evolution of their postcranium.

It is also possible that in this case, we have misidentified the target of selection. The developmental relationship between terminal phalanx length and claw size (Hamrick, 1998; 1999) suggests that if short terminal phalanges are biomechanically advantageous for some aspect of behavior (leaping, climbing, or grasping) claw reduction may simply be a passive result.

The anatomy of the primate pedal grasping mechanism itself is quite different from that of tree shrews, plesiadapids, or dermopterans (Szalay and Dagosto, 1988). Features of the ancestral primate that distinguish it from these animals include:

(1) Strong medial shift of the facet for the first metatarsal on the ento-cuneiform which increases the arc of the joint and therefore the range of movement, and also puts the hallux into a permanently abducted position.

(2) Change from a shallow sellar joint to a deeper sellar joint, which increases the stability of the articulation (matched only by Carpolestes (Bloch and Boyer, 2002)).

(3) Enlarged peroneal process on the first metatarsal.

The NVP proposes that the grasping extremities of primates evolved to facilitate "cautious and controlled movements in pursuit of prey" (Cartmill, 1974b, p. 76) or "prolonged and stealthy locomotion on slender terminal branches" (Cartmill, 1974a, p. 442). Therefore, one possible explanation of these traits is that they contribute to better control in slow movement or maintaining stationary postures on small supports. Lorises, however, which fit this behavioral profile, do not have a deeper joint or a larger peroneal process than other primates, in fact the opposite is true (Szalay and Dagosto, 1988). Likewise, arboreal didelphids do not exhibit an appreciably deeper joint nor a larger peroneal process than their more terrestrial relatives.

These differences could also possibly be attributed to more frequent use of small supports or use of differently sized supports. In the absence of good support use data for small marsupials, tree shrews, and even primates, this is hard to evaluate. This is a critical question for determining the reason for the unique foot morphology of primates—is the minimal optimal power grasp substrate size of primates larger, smaller, or the same as in other small branch users? Caluromys, for example, seems to be able to move in "fine branches" without any particular disadvantage without having nails on lateral digits or any of the primate specializations of the hallucal-metatarsal joint (Charles-Dominique et al., 1981; Rasmussen, 1990). If support size characteristics of primates are not different from those of other "small branch users" there needs to be an alternate explanation for the unique features of the primate foot. More frequent or critical use of oblique and/or vertical branches is one possibility; the mechanical consequences of using supports of various orientations needs to be further investigated. The influence of aspects of positional behavior other than quadrupedalism also needs to be considered. Climbing on or clinging to relatively small supports may prove to be an important factor in the evolution of the unique primate grasping mechanism. Alternatively, Szalay and Dagosto (1988) have hypothesized that the distinctive features of the primate entocuneiform-first metatarsal joint are adaptations to buttress the joint during landings after leaps.

Features Related to Leaping

In addition to the features associated with grasping, there is another set of features present in the primate morphotype that seem to be related mechanically to facilitate leaping. The importance of leaping in the behavior of early primates, for the structure of the primate skeleton, and for the origin of primates has been recognized by numerous authors (Clark, 1959; Conroy and Rose, 1983; Crompton, 1995; Dagosto, 1988; Gregory, 1920; Martin, 1972; Morton, 1924; Napier and Walker, 1967a, 1967b; Szalay and Dagosto, 1980; Szalay and Delson, 1979). Demes and colleagues (Demes et al., 1995, 1996, 1999) have demonstrated that the forces limbs are exposed to during leaping are larger than those of other locomotor behaviors and are especially large in small animals, so we should expect this behavior to leave its particular mark on primate limbs.

Like "small branches," leaping is an imprecise term (Oxnard et al., 1990). Virtually all mammals are capable of propelling themselves into the air using their limbs. Proponents of GL are proposing something more—that leaping constitutes a significant means of displacement. "Significant" means that leaping is used frequently, regularly, and contributes appreciably to a meter of travel. It does not, however, require that leaping is the only method employed in moving from one location to another, nor does it imply the inability to walk slowly on small branches. Most primates are capable of performing both these behaviors. The GL argument is that leaping is more consequential to the MRCA than it is to other archontans, plesiadapids, or other arboreal mammals that are described as "agile," "acrobatic," or occasionally leaping (e.g., squirrels, marsupials, carnivores, tree shrews). It is clear that leaping is an important part of the locomotor behavior of most primates (Oxnard et al., 1990; Walker, 1974). For the majority of prosimians and small anthropoids in the GL category leaping makes up at least 20%, and usually significantly more, of locomotor events. For the few nonprimate arboreal mammals on which there is data, leaping is never more than 30% of events, and is usually much less (Garber and Sussman, 1984; Sargis, 2001; Stafford et al., 2003; Youlatos, 1999).

Leaping is also a multifaceted behavior. Factors that need to be considered in any analysis are: (a) frequency of leaps relative to other locomotor behaviors; (b) number of leaps per unit time; (c) length of leaps relative to body size; (d) size, orientation, and compliance of landing and takeoff supports; (e) body size; (f) the specific kinematic properties of leaping performed; and (g) the histochemical properties of muscles (Crompton, 1993; Demes et al.,

1996; Emerson, 1985; Warren and Crompton, 1998). Other than the first, these aspects of leaping are, unfortunately, rarely documented.

There are several features of the primate postcranium that appear to be morphological compromises to permit/facilitate leaping in a hindlimb otherwise adapted for the mobility necessary for grasping. Szalay and Dagosto (1988) interpreted the shape of the primate entocuneiform-first metatarsal joint as a mechanism to resist forces encountered during takeoff and landing while still allowing the rotations necessary for grasping. Similarly, a morphological compromise to preserve the grasping ability of the foot, while remodeling it to facilitate leaping, is the hypothesis proposed by Morton (1924) to explain the lengthening of the tarsal, rather than metatarsal region of the foot in small arboreal primates, and by Preuschoft et al. (1995) to explain the lengthening of the proximal rather than distal limb segments. Likewise, some of the upper ankle joint characters discussed above (e.g., rotation of medial malleolus, etc.) are more exaggerated in taxa that de-emphasize leaping and least expressed in those that leap most frequently (Dagosto, 1985). Both notharctines and omomyids show only moderate expression of the features, suggesting that they were frequent leapers, moderate expression is primitive for primates, and/or that primitive primates were frequent leapers.

In addition to these "compromise" features, the primate morphotype is also characterized by others that are best explained (by both mechanical implications of the features and distribution within both mammals and primates), as functionally related to leaping. Among these features are an elongated ilium and short ischium, elongation of hindlimbs and elements of the hindlimb (femur, tibia, tarsus), a proximally placed third trochanter, an antero-posteriorly deepened knee, and a relatively tall talar body.

Within primates, specialized leapers have especially short ischia and somewhat longer ilia (Anemone, 1993; Anemone and Covert, 2000; McArdle, 1981; Napier and Walker, 1967a; Walker, 1974). The relationship between these features and leaping in mammals as a whole is more complex—other mammalian leapers have a long ischium and long ilia are found among cursors as well as leapers (Howell, 1944; Smith and Savage, 1956). Compared to other archontans, extant prosimian primates are characterized by a relatively long ilium and short ischium (Anemone, 1993; Anemone and Covert, 2000). Notharctus and Omomys, however, have relatively longer ischia and short ilia compared to most extant prosimians, exhibiting values more similar to those of tupaiids (Anemone and Covert, 2000; Gregory, 1920). On the other hand, they do exhibit the dorsal projection of the ischium typical of leaping primates (Fleagle and Anapol, 1992).

Several contributions have demonstrated that primates have longer hindlimbs relative to forelimb length, trunk length, or body mass than other mammals including arboreal members of the Carnivora and Rodentia (Alexander et al., 1979; Anemone, 1990, 1993; Polk et al., 2000). Anemone (1993) provided the most phylogenetically informative comparison demonstrating that tupaiids have a lower hindlimb index (femur + tibia relative to trunk length) than prosimian primates. A potential problem with using trunk length as a body mass surrogate is that leapers often have short trunks as well as long limbs (Emerson, 1985). A comparison of hindlimb length (femur + tibia) with body mass is presented in Figure 2 (statistics are given in Table 3). It illustrates that primates do indeed have hindlimbs that are longer than tree shrews and those arboreal marsupials that have been proposed as models for primitive primates. The exceptions to this generalization are the cheirogaleids, which have the shortest hindlimbs relative to body mass of any

In body mass

Figure 2. Regression (OLS) of ln hindlimb length (femur + tibia) on ln body mass. AP = Adapis parisiensis; CP = Cebuella pygmaea; NC = Nycticebus coucang; PP = Perodicticus potto. See Table 2 for samples and Table 3 for additional information.

In body mass

Figure 2. Regression (OLS) of ln hindlimb length (femur + tibia) on ln body mass. AP = Adapis parisiensis; CP = Cebuella pygmaea; NC = Nycticebus coucang; PP = Perodicticus potto. See Table 2 for samples and Table 3 for additional information.

primate (Jungers, 1985), and Cebuella, which has the shortest hindlmb of any platyrrhine (Davis, 2002). Notharctus, Smilodectes, Shoshonius, and Adapis are the only fossil primates for which this value can be estimated, and with the exception of Adapis (and possibly Europolemur (Franzen, 2000)), they too have relatively long hindlimbs. Dermopterans, however, also have long hindlimbs like primates although presumably for different reasons (Runestad and Ruff, 1995).

Simple ballistic formulas dictate the relationship between limb length and leaping ability (Alexander, 1968, 1995; Emerson, 1985). Other things being equal, a long limb permits the propulsive force to act over a longer distance and therefore for a longer time, enabling a longer (or higher) leap for a given amount of muscle mass. A long hindlimb has the additional advantage of being able to be used as a brake to absorb landing forces (Preuschoft et al., 1995). Longer hindlimbs (relative to body mass) are found not only in primates, but in kangaroos, jumping rodents, and frogs (Emerson, 1985; Marsh, 1994). Experimental work has demonstrated that an increase in hindlimb length increases jumping performance (as measured by leap distance) in interspecific comparisons of Anolis lizards and frogs (Emerson, 1985; Losos, 1990; Marsh, 1994).

Each element of the hindlimb is longer in primates (including notharctines and omomyids) than in tree shrews (or plesiadapids). The femur and tibia show essentially the same pattern as the total hindlimb (Figure 3A and B), with the femur being somewhat more elongated than the tibia. In primates, the femur is often longer than the tibia, while in tree shrews (and most other mammals), the tibia is longer than the femur. Among mammalian leapers, primates appear to be unique in lengthening the femur rather than the metatarsus and tibia. This might be taken as evidence that femoral elongation is not associated with leaping; however Morton (1924) and Preuschoft et al. (1995) have argued that the grasping foot of primates, and its associated musculature, acts as a constraint against elongation of the distal elements of the limb. Among primates, specialized leapers have longer femora than more quadrupedal forms (Anemone, 1990, 1993; Connour, 2000; Jouffroy and Lessertisseur, 1979; Lessertisseur and Jouffroy, 1973).

The elongated tarsus of primates, which results in a "reverse alternating" foot (Lewis, 1980b) has also been thought to be functionally related to leaping. Alexander (1995) has shown with modeling that a three-segment limb produces longer jumps than a two segment limb, and this is the likely reason

o Primates □ Scandentia a Dermoptera

♦ Fossil primates

• Plesiadapidae x Marsupials

+ Cheirogaleidae

In body mass

« Primates □ Scandentia t. Dermoptera

♦ Fossil primates

• Plesiadapidae x Marsupials

+ Cheirogaleidae

In body mass

Figure 3. (A) Regression (OLS) of ln femur length on ln body mass. Conventions as in Figure 2. IG = Ignacius graybullianus; PT = Plesiadapis tricuspidens; TG = Tinimomysgraybulliensis. (B) Regression (OLS) of ln tibia length on ln body mass. NI = Nannodectes intermedius; PS = Phenacolemur simonsi.

Table 2. List of taxa, sample sizes, body weights, and sources of information for variables (Analyzed in Table 3 and illustrated in Figures 2-7)

Taxon N for long bones N for foot bones Source for body weight Additional sources and notes

Prosimians

Table 2. List of taxa, sample sizes, body weights, and sources of information for variables (Analyzed in Table 3 and illustrated in Figures 2-7)

Microcebus m urin us

13-18

14 1

Cheirogaleus medins

6

6 1

Chdrogakus major

8

7 1

Daubentonia madajjascarknsis

5

5 1

Galnjjo senejjaknsis

6

6 1

Gnlnjjo moholi

9

9 1

Galago crassicaudatus

10

7 1

Galnjjo aUeni

5

5 1

Gnlnjjoides demidovii

15

13 1

Euoticus ekjjantulus

12

16 1

Avahi lanijjer

8

11 1

Propithecus rerreauxi

9

11 1

Propithecus diadema

3

11 1

Indri indri

8

12 1

Eukm u rfulvus

15-19

25 1

Eulemur mongoz

5

7 1

Lemur catta

11

13 1

Varccia rariejjata

11-14

14 1

Hapa/cmur jjriseus

9-13

12 1

Lepilemur mustelinus

17

17 1

PerodicLicus potto

10-15

15 1

Arctoccbus calabarensis

10

11 1

Nycticebus coucang

12

13 1

Lor is tardijjradus

8

7 1

Tarsius syrichta

10-15

12 1

Tarsius bancanus

9

6 1

Anthropoids

Aotus azarac

5

5 1

Aotus lemur in us

6

6 1

Table 2. List of taxa,

sample sizes, body weights, and

sources of information for variables (Analyzed in

Table 3 and illustrated in Figures

2-7) —[Continued)

Taxon

N for long bones

N for foot bones

Source for body weight

Additional sources and notes

Saimiri sciureus

10

15

1

Callicebus torquatus

1

2

1

Callicebus donacophihts

5

3

1

Cebus apella

6

6

1

Cebus capucinus

7

7

1

Chiropotes satanas

4

4

1

Pitbecia pithecia

6 3

6 3

1 I

Callim ico jjoeld a Leon topithecus rosalia

4

4

1

Saguinus kucopus

4

3

1

Saguinus mid as

4

4

1

Callithrix jacchus

5

4

1

Cebuella pyjjmaea

5

5

1

Tupaiids

Ptilocercus lowii

Sargis, 2000

1

2

Dendrogak murinus

1

1

3

Tupaia jjlis

3

3

2

T. gracilis

1

1

2

T. minor

1

1

4

T. tana

2

2

2

Urojjak everetti

3

3

3

T. montana

0

2

Sargis, 2000

Dermopterans

Cynoccphalus rariejjatus

5

2

5

Cynocephalus volans

6

3

5

Fossil primates

Notbarctus tcnebrosus

1-2

4

6

(Gebo etal., 1991)

Sm ilodectes gracilis

1-2

7

6

(Gebo etal., 1991)

Cantius mckcnnai

0

0-5

6

(Gebo et al., 1991)

Cantius trigondous

0

4-6

6

(Gebo etal., 1991)

Cantius abditus

0

2-8

6

(Gebo etal., 1991)

Ail apis parisiensis

1-4

2(a), 19(c)

6

(Dagosto, 1983)

Shoshon ius cooperi

1-2

2-3

(Dagosto etal., 1999)

Bridget" B ?Otnomys

0

8 (a), 14 (c)

6

(Dagosto etal., 1999)

Bridjjer C ?Hemiacodon

0

10 (a), 5 (c)

6

(Dagosto etal., 1999)

Teton ius hom un cuius

0

2-3

6

(Dagosto etal., 1999)

Washakius insijjnis

0

1-2

6

(Dagosto etal., 1999)

Plesiadapids

Plesiadapis tricuspidens

1-3

1-2

7

Phenacolemur simonsi

1

0

7

Beard, pers comm.

Na n n od ectes in termed ius

1

0

8

Beard, pers. comm

Ignatius graybulHanus

1

1

7

Beard, pers. comm.

Tin imomys jjraybulliensis

1

0

8

Beard, pers. comm.

Nannodectes jjidleyi

0

1

7

Est from figure in (Simpson, 1940)

Marsupials

Caluromys derbianus

7

0

9

Philander opossum

3

0

10

Marmosa robinsoni

3

0

10

All measurements are from the author's own data unless otherwise noted. In the "N for foot bones" column, a = number of tali; c = number of calcanei. Sources for body weights: 1. (Smith and Jungers, 1997), male and female values averaged; field weights preferred to lab weights; 2. (Emmons, 2000); 3. (Sargis, 2000); 4. (Sargis, 2001); 5. (Runestad and Ruff, 1995); 6. (Dagosto and Terranova, 1992); 7. (Conroy, 1987) prosimian equation; 8. (Fleagle, 1999); 9. (Rasmussen, 1990); and 10. (Lemelin, 1999).

Table 3. Results of tests of slopes and intercepts of limb variables regressed against body mass

Tupaiids Tupaiids Tupaiids versus versus versus primates prosimians Anthropoids

Tupaiids Tupaiids Tupaiids versus versus versus primates prosimians Anthropoids

OLS

II

OLS

II

OLS

II

Hindlimb length

0.001

0.001

0.001

0.005

0.050/0.024

0.009/0.007

Femur length

0.001

0.000

0.000

0.000

0.029/0.026

0.003/0.001

Tibia length

0.004

0.005

0.005

0.005

0.133/0.043

0.056/0.003

Femoral condyle

0.020

*

0.018

0.07

*/*

*/*

depth

Talar height

0.004

0.004

0.001

0.002

0.890/0.830

0.976/0.659

Tarsus length

0.000

*

0.000

*

0.292/0.250

0.043/0.008

Humerus length

0.268

0.096

0.541

0.401

*/*

*/*

Forelimb length

0.097

*

0.140

0.102

*/*

*/*

Results are presented for least squares regression (OLS) and Model II (II). p values in the Table are for a test of elevation differences after slopes were found not to differ; *, the slopes differed. Results for the "Tupaiid versus Anthropoids" comparison are given first including, and then excluding Cebuella, which is an outlier and has a strong influence on the results. ANOVA and ANCOVA were used to test slopes and intercepts for OLS regressions. (S)MATR (Falster et al., 2003) was used to test for slope and intercept differences for reduced major axis regression. See also Figures 2-7 and their legends.

Results are presented for least squares regression (OLS) and Model II (II). p values in the Table are for a test of elevation differences after slopes were found not to differ; *, the slopes differed. Results for the "Tupaiid versus Anthropoids" comparison are given first including, and then excluding Cebuella, which is an outlier and has a strong influence on the results. ANOVA and ANCOVA were used to test slopes and intercepts for OLS regressions. (S)MATR (Falster et al., 2003) was used to test for slope and intercept differences for reduced major axis regression. See also Figures 2-7 and their legends.

for a long tarsus in primates and the extremely long tarsus of specialized leapers like galagos and tarsiers. Analysis of the tarsus (calcaneus + cuboid) is complicated by the extreme elongation of this element in tarsiers and galagos, and the different scaling among subgroups of primates (Figure 4). Tree shrews, dermopterans, and Plesiadapis tricuspidens have a relatively shorter tarsus than similarly sized nonlorisine prosimians, but anthropoids do not differ from tree shrews in this regard.

In addition to elongation of the hindlimb, each joint of the primate hindlimb evidences the role of leaping. Compared to tree shrews, der-mopterans, or plesiadapids, primates have more proximally placed femoral third trochanters that are at approximately the same level as the lesser trochanter, rather than being distal to it (Anemone, 1993; Bloch and Boyer, this volume ; Sargis, 2000; Silcox, 2001). This has the likely effect of decreasing the mechanical advantage of the hip extensors, but increasing their speed of action, and is a morphology typical of leaping primates (Anemone, 1993; McArdle, 1981). The same morphology is seen in all adapids and omomyids for which femora are known.

» Primates □ Scandentia a Dermoptera

♦ Fossil primates

• Plesiadapidae + Cheirogaleidae

» Primates □ Scandentia a Dermoptera

♦ Fossil primates

• Plesiadapidae + Cheirogaleidae

In body mass

Figure 4. Regression (OLS) of ln tarsus length (calcaneus + cuboid) on ln body mass. Conventions as in Figure 2. AC = Arctocebus calabarensis; LT = Loris tardigradus.

Primates (including adapids and omomyids) have femoral condyles that are antero-posteriorly deep relative to their medio-lateral width, the consequence of which is to increase the mechanical advantage of the knee extensors (Anemone, 1993; Napier and Walker, 1967a, 1967b; Tardieu, 1983; Walker, 1974). Among primates, the depth of the condyles is greater in more frequently leaping primates, and shallowest in the slow climbing lorises. Figure 5 illustrates that the anterior-posterior dimension of the lateral condyle is larger in most primates than in tree shrews, dermopterans, or plesiadapids. Neither tree shrews nor other Archonta share the particular development of the ridge of the lateral condyle, a feature which likely prevents patellar dislocation by an enlarged vastus lateralis in leaping primates (Anemone, 1993; Bloch and Boyer, this volume; Sargis, 2000; Silcox, 2001).

The upper ankle joint (UAJ) of primates differs from that of plesiadapids, dermopterans, and tupaiids in several ways. Most strikingly, the body of the talus is very high relative to talar length or body mass (Figure 6). The medial

o Primates □ Scandentia a Dermoptera

♦ Fossil primates

• Plesiadapidae + Cheirogaleidae x Marsupials o PP NC

o PP NC

ln body mass

Figure 5. Regression (OLS) of ln femoral lateral condyle depth on ln body mass. Conventions as in Figure 2.

o Primates □ Scandentia a Dermoptera

♦ Fossil primates

• Plesiadapidae + Cheirogaleidae

Figure 5. Regression (OLS) of ln femoral lateral condyle depth on ln body mass. Conventions as in Figure 2.

Tupaiids y = 0.39x - 1.07 R = 0.99

ln body mass

Figure 6. Regression (OLS) of ln talar height on ln body mass. Conventions as in Figure 2.

and lateral crests are approximately equal in height (the lateral is slightly higher in Plesiadapis, Nannodectes, tupaiids, Ptilocercus, and Cynocephalus). In primates, the medial and lateral edges of the trochlea are sharper and more defined than the more rounded edges typical of plesiadapids and dermopter-ans, although there is less of a difference between primates and tupaiids. The trochlea of primates is deeper than in Dermoptera or Plesiadapiforms, but not so different from tupaiids.

Within lemuriform primates it is the more frequently leaping species that have taller talar bodies, increased radii of curvature, and longer arclengths at the UAJ (Dagosto, 1986; Ward and Sussman, 1979), suggesting a benefit of this morphology to leaping primates. Other things being equal (i.e., the degree of overlap of the tibia on the talus), the greater height of the talar body increases the arc length of the upper ankle joint and therefore the range of plantarflexion-dorsiflexion. The greater depth of the talar trochlea and the sharper talar borders increase medio-lateral stability at this joint. Tarsiers are a notable exception to this characterization, having relatively low talar bodies (Godinot and Dagosto, 1983).

In sum, numerous features of the hindlimb support the hypothesis that leaping was an important factor in the evolution of the primate postcranial skeleton.

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