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The scaling of basicranial flexion and length.

D S Strait1

  • 1Department of Anthropology, The George Washington University, Washington, DC 20052, USA. dstrait@gwu.edu

Journal of Human Evolution
|October 28, 1999
PubMed
Summary

Cranial base flexion in primates is not driven by large brains, but by short cranial bases. This finding challenges previous assumptions about primate brain evolution and cranial morphology.

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Area of Science:

  • Primate Paleontology
  • Comparative Anatomy
  • Evolutionary Biology

Background:

  • Cranial base angle (CBA) and index of relative encephalization (IRE) correlations suggest relative brain size influences cranial base flexion.
  • IRE exhibits positive allometry with body mass, contrasting with brain volume's negative allometry.

Purpose of the Study:

  • To investigate the factors driving the correlation between IRE and CBA in primates.
  • To re-evaluate the role of relative brain size versus cranial base length in basicranial flexion.

Main Methods:

  • Examined scaling relationships of IRE and its components across 87 primate species.
  • Analyzed allometric scaling of basicranial length (BL) and noncortical brain components (diencephalon, mesencephalon, medulla).

Main Results:

  • Positive allometry of IRE is attributed to the strong negative allometry of basicranial length (BL) relative to body mass.
  • BL and noncortical brain components (diencephalon, mesencephalon, medulla) exhibit isometric scaling against each other.
  • Basicranial flexion is linked to a short cranial base, not necessarily a large brain.

Conclusions:

  • Basicranial flexion is an adaptation to accommodate brain size within a limited cranial base length.
  • The evolution of a short cranial base, influenced by noncortical brain component scaling, is the primary driver of cranial base flexion.
  • This challenges the notion that large brains are the main factor in primate cranial base evolution.

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