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Updated: Jan 19, 2026
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Assessing Spatial Learning and Memory in Small Squamate Reptiles
Published on: January 3, 2017
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A developmental synapomorphy of squamate reptiles
James R Stewart1, Daniel G Blackburn2
1Department of Biological Sciences, East Tennessee State University, Johnson City, Tennessee, 37614.
Evolution & Development
|September 24, 2019
Summary
The yolk cleft/isolated yolk mass complex is a unique feature in developing squamate reptile eggs. This developmental trait is a key characteristic defining the Squamata clade.
Area of Science:
- Developmental biology
- Comparative embryology
- Herpetology
Background:
- Squamate reptiles (lizards and snakes) are primarily defined by skeletal features, which can be ambiguous.
- A unique developmental feature in squamate eggs has been identified, not observed in other amniotes.
Purpose of the Study:
- To describe the yolk cleft/isolated yolk mass complex in squamate eggs.
- To establish the yolk cleft/isolated yolk mass complex as a developmental synapomorphy for Squamata.
Main Methods:
- Observation and description of embryonic development in over 65 squamate species across 12 families.
- Comparative analysis of egg development across Squamata and other amniote groups.
Main Results:
- The yolk cleft/isolated yolk mass complex, involving extraembryonic mesoderm penetration and yolk partitioning, is consistently found in developing squamate eggs.
- This complex forms the basis of the omphaloplacenta in viviparous squamates and is absent in mammals, birds, turtles, and crocodilians.
- Loss of the isolated yolk mass is noted in a few highly placentotrophic skink species with very small eggs.
Conclusions:
- The yolk cleft/isolated yolk mass complex is a unique developmental synapomorphy of the Squamata clade.
- This feature likely played a crucial role in the evolution of placentation in extinct and extant viviparous squamates.
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