GEOL 431 Vertebrate Paleobiology

Spring Semester 2015

The Amniotic Egg

Synapomorphies:

• The amniotic egg: An air-breathing egg characterized by a shell and extraembryonic membranes. These enclosed the amniote embryo in a private pond during its development and mediated gas-exchange with the external environment.
  
  
  Amblyrhynchus cristatus the Galapagos marine iguana
• Claws or nails at the end of digits. This novelty allows digging, and burying of eggs. (So are these characters fully independent?)
• An egg-tooth on the snout to facilitate hatching. (Again independent?)
• Complete loss of water-breathing larva.
  
• Complete loss of lateral line system
  
• Internal fertilization by means of a penis.
• Two sacral vertebrae.
  
• Formation of the astragalus (a large tarsal that articulates with the tibia) from the fusion of at least three previous tarsals.

In a way, claiming that these are seven separate characters seems dishonest since all but the last two are associated with one important life-style change: All amniotes reproduce using an air-breathing egg or its derivative.

This is huge. The egg of a lissamphibian is relatively small and simple. The embryo develops inside the perivitelline space enclosed by a Vitelline capsule, outside of which is a thick jelly capsule. Gas exchange is a limiting factor. Gasses enter and leave the egg by simple diffusion. Complicating this, the eggs are often laid in a gelatinous mass where diffusion through the egg mass is slow. Embryos have comparatively low metabolic rates and must hatch at an early developmental stage so as not to asphyxiate.
Major features of the amniotic egg:

• It is an air-breathing egg, in contrast to those of other vertebrates.
• Components of the egg:
  
  • A semipermeable leathery or calcareous shell.
  • Albumen (and other proteins)
  • Three extraembryonic membranes:
    • Chorion: Encloses the embryo, yolk and other membranes. Facilitates gas exchange with exterior.
    • Amnion: Encloses the embryo
    • Allantois: Receives nitrogenous waste products from embryo (including CO₂) and facilitates gas exchange. It develops into the urinary bladder.
    • Yolk sack Encloses the yolk mass and facilitates the transfer of water and nutrients to the embryo.
    Together, these components manage the exchange of gasses, water, and nutrients between the embryo, yolk, albumen, and exterior. NOTE: These membranes are outgrowths (i.e. parts of) of the embryo.
    
  • Large reserve of yolk compared to anamniotes. Lissamphibians divide the yolk mass between the various body cells through complete cleavage of the ovum (see earlier lecture). In amniotes, the yolk mass is too large for this. Thus, while the animal pole of the zygote cleaves, forming a disk of cleaving cells on the surface of the yolk, the yolk, itself, doesn't.
    
• This arrangement has been variously modified. In placental mammals, the yolk and shell are suppressed, and the membranes modified into the placenta and umbilical cord. In birds and crocodylians, the shell membrane is heavily calcified to form a rigid shell.
  

Possible advantages of the amniotic egg:

• Enable embryos to develop more quickly and to a greater extent prior to hatching.
• Free animals from dependency on bodies of water for reproduction and larval stage.
• Reduce competition for pools of water in which larvae can mature
• Avoid predation on larvae by aquatic predators.

The odd reproductive strategies of some frogs reflect that they, too experience these selective pressures. Some colorful examples:

• Foam nests (result)
• Mouth/stomach/vocal sac brooding
• Direct development

But these strategies require very specific environments and/or make heavy demands of parents. The amniotic egg, while metabolically expensive, can be laid and forgotten. Scenarios of it origin are, necessarily, speculative, but the general consensus is that its precursor would have been:

• a relatively large egg, like that of oviparous caecilians (right). It would have:
  • required some kind of membrane to hold it together
  • needed to be internally fertilized
  • required the embryos to employ some kind of gas-exchange structure (like the gills of caecilian embryos.)
• laid by a relatively small animal, capable of finding moist microclimates.

The evolution of the egg, the mechanism of internal fertilization, and the extraembryonic membranes must have proceeded in lockstep. Indeed, oviparous caecilians seem to have convergently (?) evolved something like an amniotic egg precursor.