ZOO 2090 Lecture Notes - Lecture 11: Yolk Sac, Eureptilia, Paleothyris
1 May 2018
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ZOO2090 – Early Amniotes and Life on Land
Early Tetrapods diversified into three lineages:
• Lisamphibia – Temnospondyls
• Lepospondyls
• amniotes
Early Amniotes:
• evolved in early/mid-Carboniferous and show affinities with anthracosaurs
• earliest were often small (~20cm long) and have slight but highly ossified skeleton (lizard
like)
• jaws, teeth and neck are modified for improved predation
• Hylonomus and Paleothyris were found inside fossilized hollow tree stumps in NS
Amniote Tree:
• Early amniotes most related to mammals and “mammal like” reptiles = Synapsids
• Turtles, stem parareptiles, stem eureptiles, lepidosaurs, archosaurs = Sauropsids
o Turtles, Lepidosaurs, Archosaurs = Diapsids
*how living species fit inside amniote tree
Key Features of Amniote
• Cleidoic Egg
o Semi-permeable shell which allows gas exchange but keeps fluid inside
o Extra embryonic membranes:
▪ Protection and gas transfer:
• Chorion surrounds embryo and yolk sac
• Amnion surrounds embryo with water
▪ Respiration and waste storage
• Allantois fills with waste as yolk proteins are used
o Cledoic egg allowed full development of the organism on land and loss of larval
stage
o Through more efficient respiration in the air, it also allowed the embryo to
become larger before hatching compared to amphibians; more energy is invested
in each egg
o It requires internal fertilization
• Low skin permeability
o Greater variety of skin elaborations (keratin)
o Presence of lipid
• Coastal (rib) ventilation of the lungs:
o Allow for long neck (able to draw air through a long tub, buccal pump not
needed) more nerves = fine control
o Space for elaboration of nerves that supply forelimb
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