The primate appendix: a reassessment.

Area of Science:

• Primate Anatomy
• Evolutionary Biology
• Comparative Morphology

Background:

• The vermiform appendix is considered a key trait of Hominoidea (apes and humans).
• Appendix-like structures are reported in various primate species, causing definitional ambiguity.
• Existing criteria for appendix identification lack standardization and quantitative rigor.

Purpose of the Study:

• To critically evaluate current definitions of the vermiform appendix in primates.
• To identify inconsistencies in morphological and histological criteria used for appendix identification.
• To propose a path toward a more accurate and consistent definition of the vermiform appendix.

Main Methods:

• Comprehensive literature review of primate appendix morphology and histology.
• Analysis of commonly used criteria, including gross shape and lymphoid tissue concentration.
• Comparison of features in taxa with and without appendices.

Main Results:

• Current definitions rely on subjective shape descriptions and histological features like lymphoid concentration.
• These features are not consistently quantified and vary with age, complicating comparisons.
• Primitive primate traits may be mistaken for definitive appendix characteristics.

Conclusions:

• Existing criteria for the vermiform appendix are insufficient for reliable use in primate systematics.
• Further research into the morphology and ontogeny of the cecal region is essential.
• A more rigorous definition is required to accurately identify the vermiform appendix across primate taxa.