This figure above shows the circulatory system of frogs. This evolutionary analysis will focus on the frog’s heart and cardiac development.

“Five hundred and fifty million years ago the hearts of the entire animal world were simple tubular hearts, something that still characterizes heart development at the early embryonic stages in animals today. However, as time went by in certain lineages the hearts became increasingly sophisticated. As they became more sophisticated, new switches for the add-ons were put into place.”

In modern amphibians (frogs, toads, salamanders) the atrium is completely separated by a partition into two atria. The right atrium receives venous blood from the body while the left atrium receives oxygenated blood from the lungs. The ventricle is undivided, but venous and arterial blood remain mostly separate by the arrangement of vessels leaving the heart. 

The heart muscle of salamanders are so thoroughly channeled with spaces between muscle fibers that the heart’s own pumping action squeezes through sufficient oxygenated blood. 

"Macrophage populations are largely restored in axolotls, at which point any pro-angiogenic functions are likely impacted by rigid fibrotic ECM associated with early macrophage depletion. New blood vessel growth is likely to impact the speed of regeneration in axolotls however the thin trabeculated (non-compacted) structure of the myocardium likely allows some access of oxygenated blood to the injured tissue. Therefore is is possible that angiogenesis may not be an absolute requirement for allowing the regeneration of an amphibian heart."

➖ Atrial Septum Reduction in Lungless Salamanders

Within an approximate number of 700 species of living salamanders, it has been said that nearly two thirds are lungless. Since then, this resulted to contradictory hypotheses regarding the atrial septum morphology of these lungless salamanders. The results of the study showed that lung loss dramatically impacted the configuration of the circulatory system and the effects of evolutionary lung loss on cardiac morphology, particularly, in atrial septum reduction in lungless salamanders. With this, we can infer that the morphology of lungless salamanders represents an evolutionary loss of a prominent
portion of the atrial septum.

"Although some species of plethodontids retain the pulmonary arch, adult lungless amphibians lack pulmonary veins. Consequently, all blood flowing into the heart originates from the left and right common cardinal veins and the posterior cardinal vein via sinus venosus. Since this blood is from a single mixed source, lungless amphibians do not require separation of blood in the heart. In fact, the presence of an atrial septum might be disadvantageous in this situation, which could lead to extreme pressure differentials between atrial chambers."

• Belongs to convergently lungless species.
• Characterized by partial septation.
• Atrial morphology of the convergently lungless salamander Onychodactylus japonicus is similar to that of plethodontids and evolved independently.

• Species under lungless plethodontid salamanders (Plethodontidae).
• Plethodontid salamanders have partial
  atrial septa and incomplete separation of the atrium into left and right halves.

• Axolotls can regenerate, repair or replace its arms, legs, tail, lower jaw, brain and heart. 
• This species is a popular model organism in the field of regenerative medicine, due to its ability to regenerate for instance functional heart muscle after injury.