From Amphibians to Amniotes

The Basics

During the early Carboniferous period amphibians were the most dominant land animals that contained vertebrae(otherwise known as vertebrates). Invertebrates (species with no vertebrae) also inhabited land, and were much larger. This dominance didn’t last very long as amniotes diverged. Unlike amphibian species amniotes were completely adapted to land and had no reliance on aquatic environments even for reproductive purposes. You may have never heard of the term amniotes before, but they are a family which contains birds, mammals and reptiles.

Amphibian eggs are made of a vitalene membrane which made them extremely goo-ey (See fig 1). These eggs were unable to retain water and therefore if placed in a dry environment would desiccate and the embryo would die. These eggs usually produce relatively underdeveloped hatchlings such as tadpoles. In the early stages of development tadpoles remain in an aquatic environment however in late development they undergo metamorphesis, some remaining in water whilst others move to a new terrestrial home (on land).

Fig1: Illustration of amphibian eggs.

Amniote eggs are adapted to survive on terrestrial environments as they have a hard semi permeable shell which protects the embryo from harsh environments. Additional support is given by 3 extra-embryonic membranes: chorion, allantois, and amnion (See fig 2). The hatchlings from amniotic eggs are more developed than that of amphibians, largely due to the vessels within the yolk sac allowing the transferal of nutrients. Oxygen uptake is also more efficient in amniotic eggs allowing for an increased metabolic rate and thus a larger egg. A larger egg is important as it allows the embryo to grow and develop more than would be permitted in an amphibian egg.

Fig 2: This image highlights the amniotic egg structure. Additional support provided by extra-embryonic membranes.

It has been postulated that this adaptation came about as laying eggs on land had a reduced threat of predation in comparison to water. Modern amphibians as described in my previous article are the only surviving non-amniote tetrapods but these were not the ancestor of the earliest amniotes. It is likely that amniotes actually diverged from ancient amphibians.

Differences Between Amphibians and Early Amniotes

Amphibians and early amniotes differ in a variety of ways, early amniotes have :

  • Smaller, deeper, narrower skull
  • A more elaborated tongue
  • A reduction in movable bones in the skull
  • A Strengthened ankle
  • Keritinatious scales
  • Dietary requirements: Have a varying diet which may have been their key to success. The radiation of insect diversity may also have helped with their survival.
  • More complex lungs containing either alveoli or faveoli.
  • A permanent septum to separate ventricles in the heart making the heart more powerful.

The Divergence of Amniotes

The divergence of amniotes can be tracked using skull anatomy and fenestrae – holes in the skull.

  • Anapsids – No fenestrae
  • Synapsids – One fenestrae in the lower skull
  • Eurapsid – One fenestrae in the upper skull
  • Diapsid – Two fenestrae

Fig3:  skeleton on the left shows the structure of ancient synapsids. To the right is a Triceratops an ancientsauropsid.

These skull anatomies have split the amniotes into two groups: Synapsids (mammals) and Sauropsids (birds and reptiles (Diapsid, Eurapsid, and Anapsids ). See the image below.

Synapsids_Sauropsids

How do amniotes differ from each other?

Amniotes differ in a variety of ways, firstly they can contain either a 3 chambered or 4 chambered heart. Birds and mammals contain four chambers, however reptiles contain only 3 chambers. Amniotes also mediate their body temperature differently. Some are ectotherms whilst others are endotherms. Ectotherm means that the body is regulated by the environment surrounding it. Endotherms use their own metabolic processes to generate heat. Endotherms are therefore more flexible with the environment in which they live in comparison to ectotherms.

Synapsids have evolved to more efficiently travel over land by reducing the amount of trunk movement and instead increasing the amount of leg propulsion. They have also evolved to respire more efficiently using their ribs and diaphragm to inhale and exhale air. Sauropsids have evolved other mechanisms. They lack a diaphragm therefore in crocodiles for example they use rib , liver  and pubic bone movement to aid respiration. For other sauropsids like lizards once movement starts respiration almost ceases. Therefore more advanced tetrapods can move faster and farther. Amniotes also differ in how they handle their metabolic wastes, they all convert ammonia to urea as it is less toxic. Some however produce uric acid which is precipitated out as salts. All synapsids produce urea which is managed by highly developed kidneys. These kidneys help to retain H20 more efficiently than in sauropsids.

As food sources were plentiful reptiles thrived and began to grow larger and larger, this was followed by an mass extinction of species in the Permian era which allowed the dominance of these large reptiles known as the dinosaurs. This will be explained in my next article.

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