Encephalization in Extinct Primates

Are the primates of the Paleogene—the period of the Early Tertiary comprising Paleocene, Eocene, and Oligocene epochs—also generally highly encephalized? This question must be addressed in relative fashion as well. Almost all mammalian lineages with substantial fossil records show varying degrees of progressive increase in encephalization throughout the Tertiary (Gould, 1975a; Jerison, 1973); therefore, any determination of relative brain size in extinct early primates must of necessity be rooted in a contemporary mammal-wide context. For example, studies by Gurche (1982), Jerison (1973, 1979), and Radinsky (1970, 1975, 1977, 1979) have revealed that Paleogene primate endocasts yield estimated EQ values which in general are lower than seen in average extant prosimians or strepsirhines (Figures 4 and 5). The absolute values given by Jerison (1979) are summarized in Table 2 and demonstrate levels generally lower than the value of 1.0 for average living mammals (as defined by Jerison's 1973 sample and best-fit). However, Jerison (1973: 373) calculated an average value of only 0.20 (or roughly one-fifth the size of an average living mammal) for archaic mammals of the Early Tertiary, and therefore, the various taxa of fossil Eocene euprimates do indeed exhibit generally high levels of encephalization than their contemporaries. Some

log 10 Body weight(g)

Figure 3. Group averages for brain and body weight in mammals. Note the positive deviation for the primates. Symbols: B = bats; I = insectivores; R = rodents; M = marsupials; L = lagomorphs; E = edentates; P = primates; C = carnivores; U = ungulate; S = pinnipeds; D = cetaceans (modified from Martin, 1983).

Figure 4. Figures of endocranial size and skull morphology in three extinct primates compared to extant Indri and Microcebus. All reconstructions drawn to same approximate size (modified from Radinsky, 1979).

Indri Microcebus

Figure 4. Figures of endocranial size and skull morphology in three extinct primates compared to extant Indri and Microcebus. All reconstructions drawn to same approximate size (modified from Radinsky, 1979).

2 OD

Modern prosimians Fossil prosimians

log Body weight

Figure 5. A plot of brain-body values for extant prosimian primates, with reconstructed estimates for selected taxa of fossil primates. Note the smaller average relative brain size in the fossil taxa. (No = Notharctus; S = Smilodectes; A = Adapis; Ne = Necrolemur; Te = Tetonius) (from Gurche, 1982, with the kind permission of Springer Science and Business Media).

Table 2. Estimates of relative brain size in selected fossil primates

Species

Source of estimate

Mean EQ

Minimum EQ

Maximum EQ

Plesiadapis tricuspidens

HJ

0.50

0.40

0.60

Smilodectes gracilis

JG

0.47

0.35

0.86

JG

0.44

0.36

0.64

LR

0.41

HJ

0.53

Notharctus tenebrosus

JG

0.49

0.36

0.92

Tetonius homunculus

JG

0.43

0.33

0.67

LR

0.42

HJ

0.71

Necrolemur antiquus

JG

0.56

0.35

0.76

LR

0.79

HJ

0.94

Rooneyia viejaensis

JG

0.81

0.60

1.07

LR

0.97

HJ

1.23

Adapted from Conroy, 1990 and Gurche, 1982; as determined by Gurche (JG), Radinsky (LR), and Jerison (HJ). Conroy (1990) provides a range of 0.67-1.89, with a mean of 1.09, for extant prosimians (based on the data of Stephan et al., 1970). Note that Jerison (1979) calculated a mean EQ value of 0.20 for archaic mammals of the Early Tertiary.

Adapted from Conroy, 1990 and Gurche, 1982; as determined by Gurche (JG), Radinsky (LR), and Jerison (HJ). Conroy (1990) provides a range of 0.67-1.89, with a mean of 1.09, for extant prosimians (based on the data of Stephan et al., 1970). Note that Jerison (1979) calculated a mean EQ value of 0.20 for archaic mammals of the Early Tertiary.

estimates of relative brain size from the crushed skull of Plesiadapis tricuspidens also yield values higher than average contemporary mammals (Jerison, 1979). The traditional characterization of early primates as having relatively large brain sizes (e.g., LeGros Clark, 1971; Martin, 1973) appears quite valid, although this pattern requires additional corroboration through the retrieval of reasonably well-preserved fossil skulls from the earliest Tertiary deposits.

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