Sea Lion jaw-15-12 million years-miocene

Seal vertebrae-20-12 million years miocene

Seal Humerus-15-5 million years-miocene/pliocene

Sea Lion rooted fang and lateral tooth-15-12 million years-miocene

Sea lion rooted fang and lateral tooth-15-12 million years-miocene

Pinnipeds have had to overcome numerous problems associated with moving efficiently in the dense aquatic medium, and this adaptation has reduced their ability to move about on land. Otariids have hindflippers that can be turned under the body for terrestrial locomotion, whereas phocid seals cannot turn their hindflippers under the body and instead use lunging movements to get around on land.

Perhaps the most complex suite of adaptations required for making a living in the ocean is the physiology associated with breath-hold diving to foraging depths. In addition to adaptations for dealing with great pressures, pinnipeds exhibit physiological adaptation for apnea, increased oxygen storage, brady-chardia. hypoperfusion, hypometabolism, and neuronal and hormonal control of cardiac and spleen function.

The sensory systems of pinnipeds enable them to successfully navigate, forage, and communicate in a variety of environments. Seals hear and see relatively well both in the air and underwater. Because the behavior of sound and light in water is markedly different than that in air, this again requires plasticity in their sensory physiology. Ultimately, sensory physiology must provide the appropriate visual and auditory information to facilitate social interactions on land, while allowing detection and capture of prey and detection and avoidance of predators at sea. Adaptations include well-developed underwater directional hearing and visual sensitivity at low light levels.

Living in salt water poses osmoregulatory problems for pinnipeds. In addition, pinnipeds must stay in water balance during periods on shore during which they may fast completely from food or water. Because animals also lose water for evaporate cooling, osmoregulatory strategies are linked to thermoregulation.

Pinnipeds are exposed to a remarkably variable range of environmental temperatures. They are able to tolerate frigid ocean temperatures at depth as well as high amounts of thermal radiation encountered when hauled out on land. Adaptations that help pinnipeds retain heat in the ocean environment, such as thick blubber or dense fur, may also promote overheating on land. Adaptations that may play a role in thermoregulation include large body size, blubber or dense fur, countercurrent heat exchange systems, and possibly high metabolic rates.

Skeletons of Enaliarctos have been found from as far back as the late Oligocene which spanned from about 30 to 24 million years ago. This particular epoch marked the transition from the Palaeogene to the Neogene period which was characterized by a period of global cooling culminating in the ice ages in the Pleistocene.
Pinnipedia originally consisted of 5 families. As well as Enaliarctidae, there is another extinct family of called the Desmatophocidae. These primitive seals were closely related to the phocid seals. Whilst fossil evidence is poor, skull fossils found show that they would have relied more on sight to hunt as their inner ear was not as developed for underwater hearing as more modern pinnipeds.

 

 

 

 

 

 

 

 

https://en.wikipedia.org/wiki/Pinniped#Evolutionary_history