Whereas in mammals the heart, which has four chambers, two ventricles and two atria, delivers oxygenated blood to the organs by one channel while the other directs blood to the lungs for re-oxygenation, the reptiles have three-chambered hearts with two ventricles and only one atrium.

True.

Explanation:

The heart of the mammals is divided in half by the right atrium and ventricle (AD, RV) and the atrium and left ventricle (LA, LV). The atria communicate through a small fissure within the ventricle, and the ventricles communicate through the pulmonary trunks. In the atrium there are two vena cavae and a pulmonary artery with two outlets. In the ventricle there is an artery with only one outlet, the aorta and a bicuspid valve. This entire structure supplies oxygenated blood to the organs through one channel, while the other directs blood to the lungs for re-oxygenation.

On the other hand, the heart of reptiles has three cavities: two atria (one right and one left) and one ventricle. The heart of crocodilian reptiles has four cavities: two atria and two ventricles (such as birds and mammals). However, even in crocodilians there is a mixture of blood types (venous and arterial) that pass through the heart, although to a lesser extent than in amphibians. Thus we can consider the circulation of reptiles double and incomplete. Because of this, the animals in this group are pecilodermic, that is, they adapt the body temperature to the ambient temperature.

for a different answer set the answer would be because the oxygen in the water had depleted (apex)