Iniidae


Iniidae is a family of river dolphins containing one living genus, Inia, and four extinct genera. The living genus lives in the river basins of South America, but the family formerly had a wider presence across the Atlantic Ocean.
Iniidae are highly morphologically different from marine dolphins due to adaptations suited to their freshwater river habitat. They display high amounts of sexual dimorphism through color and size. Seasonal movement between flooded plains and rivers is common, due to the variation of seasonal rain. There has been little research done on the family, in particular the species aside from the Amazon river dolphin.

Evolution

The South American river basins were flooded by marine waters, creating a new brackish habitat that allowed marine mammals to move into them. During the Miocene era, the sea level began to recede, trapping the mammals within the continent.

Morphology

Their necks are flexible, since their cervical vertebra are movable, unlike most whales, dolphins, and porpoises, which have fused neck vertebrae, rigidly aiming their heads forward. The Iniidae have other morphology common to species adapted to freshwater river habitats, including highly reduced or absent dorsal fins, avoiding entanglement in vegetation from the flooded terrestrial plains, and large, wide, paddle-like pectoral fins that allow maneuverability in confined areas cramped by vegetation. Other adaptations including a long rostrum, skull and jaw, and reduced orbits.
Iniidae share other characteristics with their marine toothed whale relatives. Their stomachs include a fore-stomach, a singled single-chambered main stomach, and a pyloric stomach with connecting channels. Iniidae have lost their fur and lack true vocal cords. They share a similar structure of the tympanic bulla and lung shape, the position of their diaphragm, and the position of the blowhole to the back of the head with their marine ancestors.
Iniidae dolphins develop multiple sets of teeth, having conical, small teeth that differ slightly in the front of the mouth. The teeth extend lingually in the back, and in the front, have a small depression on each side. These mammals are carnivorous, finding prey via echolocation.

Speciation

There is scientific debate on the number of species within the genus Inia. The main issue is whether there are two or three species, or whether those can be considered sub-species. According to some researchers, Inia geoffrensis, Inia humboldtiana, Inia boliviensis are three separate species, while many consider I. geoffrensis and I. boliviensis to be the only two. Martin in 2004 found evidence that genetic exchange occurs at multiple sites on the Amazon, even places hundreds of kilometres apart.

Taxonomy

The family was described by John Edward Gray in 1846.
Current classifications include a single living genus, Inia, with one to four species and several subspecies.
The family also includes three extinct genera described from fossils found in South America, Florida, Libya, and Italy.