Introduction

The goal of this chapter is to reconstruct aspects of the postcranial morphotype of the order Primates and to assess their significance for the positional behavior of the ancestor. What derived features of the limb skeleton are likely to have distinguished the last common ancestor of primates from more remote ancestors and what implications does this set of features have for the way of life of the ancestor of Primates? In pursuing this goal, the following questions are addressed:

(1) What are the derived characters of the postcranium that characterize the most recent common ancestor of the primates?

(2) What are the functional and biological role attributes of these characters individually?

(3) Do the functional/biological role attributes of the traits as a whole constitute a cohesive story? Can they be explained by a single selective factor or a set of selective factors arising from a particular way of life?

(4) If primate synapomorphies cannot be attributed to a single way of life, does the evidence suggest an order in which characters were added to the morphotype, and thus a plausible functional/behavioral sequence?

Marian Dagosto • Deptartment of Cell and Molecular Biology, Northwestern University, Chicago IL 60611; Research Associate, Division of Mammals, Field Museum of Natural History, Chicago IL

These questions raise many issues, the primary one being, of course, what does one mean by the phrase "Origin of Primates?" This topic is dealt with further at the end of this chapter, but for immediate clarification, the intention is to explain the behavioral significance of the set of features that characterize the Most Recent Common Ancestor (MRCA; Figure 1) of crown group primates. Crown group primates are the anthropoids, lemurs, tarsiers, adapids, and omomyids. There remain few serious challenges to the hypothesis that this group of mammals shared a common ancestor relative to other extant and fossil mammals. The derived features of the postcranium that distinguish this ancestor from the outgroup are given in Table 1. A formal phylogenetic analysis of the features is not given here since it seems fairly certain from both morphological and molecular evidence that primates are part of the Euarchonta (Springer et al., this volume), and for most of the features listed in Table 1, primates differ from any of the most likely outgroups (Scandentia, Dermoptera, Plesiadapiformes, Rodentia, Lagomorpha), as well as from the majority of other mammals. The few exceptions are noted in the text.

Question 2 entails having a philosophy for formulating and evaluating hypotheses about functional and biological role in fossil organisms. This is discussed in the next section. (See also Szalay, this volume.) Questions 3 and 4 ask if the set of traits can be reasonably considered to be a correlated complex—can a single niche, habitus, or way of life explain all or most of them?

The most comprehensive "single niche" model for the Origin of Primates is the "nocturnal visual predation" model (NVP) developed by Cartmill (1972; 1974a; 1974b). It explains the grasping extremities, loss of claws, and optical convergence of primates as being related to a way of life involving visually directed predation in the small branch niche by nocturnal animals.

Outgroup

MRCA

MRCA

Figure 1. The most recent common ancestor (MRCA), is the common ancestor of the crown group primates, which include the adapids, omomyids, tarsiers, and anthropoids.

Table 1. Derived features that distinguish primates from other Archonta

Trait

Features of the forelimb

Suggested mechanical function

Suggested biological role/s Reference

Forelimb

Relatively long hands and short forearms

Humerus

Trochlea elongate and cylindrically shaped with strong lateral edge and deep groove separating the humeroradial and humerolunar joints

Hand

Paramesaxony; Elongation of third metacarpal

Relatively short carpus/long digits

Elongate scaphoid tubercle

Large pisiform, triquetrum quadrangular in dorsal view; hamate without hamulus; capitate short and narrow Independent thumb Claws replaced by nails

Grasp relatively large branches when landing

Increased support in supinated positions

Deepens radial margin of the carpal tunnel; acts as windlass for the flexor digitorum profundus

Vertical clinging and leaping; seize live prey

Grasping

Grasping small diameter supports

Grasping ? Grasping

(Godinot and Beard, 1993; louffroy et al., 1991)

(Szalay and Dagosto, 1980)

(Godinot, 1992; Godinot and

Beard, 1991; louffroy et al., 1991) (Godinot and Beard, 1991; Hamrick, 1997)

(Godinot and Beard, 1991; 1993)

(Altner, 1971)

Table 1. Derived features that distinguish primates from other Archonta—( Continued)

Trait

Features of the hindlimb

Suggested mechanical function

Suggested biological role/s Reference

Hindlimb

Elongated relative to body size: femur, tibia, tarsus Pelvis

Flattened, expanded ilium Long ilium

Short ischium

Femur

Patellar groove deep and narrow, condyles anteroposteriorly deeper than mediolaterally wide; lateral epicondyle more anteriorly projecting than medial

3rd trochanter at same level as lesser Tibia

Moderate degree of medial rotation of medial malleolus Prominent, inferiorly long, pyramid shaped medial malleolus with a distally convex surface

Increase distance and/or time of Leaping propulsive force

Hypertrophy of gluteus medius Leaping

Increase speed of gluteals for femoral Leaping extension; increased power of femoral flexors Mechanical advantage of hip extensors Leaping

Increase mechanical advantage of knee Leaping extensors

Increased speed of action of hip extensors

Increases abduction of talus relative to tibia during dorsiflexion; allows lamina pedis full range of motion at subtalar and transverse tarsal joints

Leaping

Grasping/climbing/variable orientations of foot

(Martin, 1972; Connour, 2000; Polk et al., 2000; Silcox, 2001 )

(Anemone, 1993; Fleagle and Anapol, 1992; McArdle, 1981)

(Szalay et al, 1987; Silcox, 2001)

(Silcox, 2001)

(Dagosto, 1985; Hafferl, 1932; Lewis, 1980a)

Inferior tibial joint surface as long or longer than wide

(same in Ptiloccrcus) Foot

Elongation of tarsal elements: talar neck, distal part of calcaneus, navicular, cuboid, cuneiforms High phalangeal index; proximal phalanges longer than intermediates or terminals Relatively short terminal phalanges Reverse alternating foot Talus

Talar body tall

Medial and lateral crests of talar trochlea more or less equal in height; sharper edges

Lengthen joint facet for tibia on trochlea posteriorly Posterior trochlear shelf moderately developed

Talar trochlea is deep More spherical talonavicular joint; medial and lateral sides are equivalent in size; distal margin of head not indented

Related to long, narrow talar trochlea

Increase effective length of hindlimb. Elongation of talar neck may be for increased subtalar motion

Developmentally related to loss of claws

Related to tarsal elongation

Increase radius of curvature of upper ankle joint

Reduce concomitant rotations during flexion/extension at upper ankle joint

Increase range of plantarflexion

1. Buttress for plantarflexed foot in pushoff

2. Support for lengthened posterior astragalar calcaneal face

Increases stability of upper ankle joint

Allows axial rotation at talonavicular joint

(Dagosto, 1985; Sargis, 2000)

Leaping Grasping/climbing

(Hall-Craggs, 1965; Jenkins and McClearn, 1984; Martin, 1972; Morton, 1924)

(Lemelin, 1999; Hamrick, 1999; Hamrick, 2001)

(Hamrick, 1998, 1999) (Lewis, 1980b) (Dagosto, 1986) (Dagosto, 1986)

Dagosto, 1986 (Dagosto, 1986)

(Dagosto, 1986; Decker and Szalay,

1974; Szalay and Decker, 1974) (Dagosto, 1986) (Dagosto, 1986; Hooker, 2001)

Table 1. Derived features that distinguish primates from other Archonta—(Continued)

Trait

Features of the hindlimb

Suggested mechanical function

Suggested biological role/s Reference

Calcaneus

Sellar shaped calcaneocuboid joint

Foot

Medial shift of entocuneiform-first metatarsal joint (also in Carpolestes) More restrictive Sellar entocuneiform-first metatarsal joint (also in Carpolestes) Habitual abduction of hallux

(also in Carpolestes) Enlarged peroneal process on first metatarsal Enlarged hallux Nails instead of claws on all pedal digits

Reduces translational component of Grasping movement at the calcaneocuboid joint; increases axial rotation

Opposability of hallux

Stability of hallucal-metatarsal joint

Opposability of hallux

Buttress MTl-entocuneiform joint

Grasping

Grasping

Leaping

Grasping ? Grasping

(Dagosto, 1986; Szalay and Decker, 1974)

(Szalay and Dagosto, 1988) (Szalay and Dagosto, 1988)

(Szalay and Dagosto, 1988) (Szalay and Dagosto, 1988) (Szalay and Dagosto, 1988)

It envisions a slow moving quadrupedal ancestor. In contrast, the "grasp leaping" (GL) hypothesis of Szalay (Szalay and Dagosto, 1980; Szalay and Delson, 1979; this volume) proposes a more agile animal. The term was coined to recognize a unique category of positional behavior that is typical of many primates and thought to be ancestral for Primates. It includes both leaping and grasping as significant elements and thus distinguishes grasp-leapers from other "arboreal quadrupeds." It does not claim that behaviors other than leaping (i.e., quadrupedalism, climbing) are not used. It does not imply the specialized leaping of galagines, tarsiers, or indriids, which are placed in a separate, more derived locomotor category (vertical clinging and leaping). And, although it hypothesizes that primate limb morphology may be a compromise for the demands of grasping and leaping, it does not require tandem coevolution of grasping and leaping. The model presented by Szalay and Dagosto (1988; and later in this chapter) explicitly recognized the presence of a more primitive kind of grasping morphology and behavior in archontans prior to the MRCA.

GL is somewhat more limited than NVP in that it simply seeks to describe a locomotor mode of the ancestor, and is not intended as a complete description of a way of life. It is compatible with, but does not require, the hypothesis of Clark (1959), Collins (1921), and Crompton (1995) that the visual acuity necessary for leaping and landing with the precision characteristic of nocturnal primates may have been an important factor in the development of orbital convergence. GL does not require, nor is it specifically linked to any dietary regime. The positional behavior elements of GL are similar to reconstructions of the ancestral primate offered by Martin (1972; 1990) and Crompton (1995).

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