Iniidae is a family of river dolphins containing one living genus, Inia, and four extinct genera. Extant species are native to the river basins of South America, but the fossil record indicates a wider historical range.

Iniidae are highly morphologically different from marine dolphins due to adaptations to their freshwater river habitat. They display high amounts of sexual dimorphism in color and size. Seasonal migration between flooded plains and rivers is common, due to the variation in rainfall. Apart from the Amazon river dolphin, the family remains fairly under-researched.

Evolution

The South American river basins were flooded by marine waters during the mid-Miocene era, creating a new brackish habitat which allowed marine mammals to move into them. Marine animals that could tolerate the osmotic difference in the salt and freshwater systems permeated these habitats and adapted. Over time, the sea level began to recede, trapping the mammals within the continent in the river basins.

Morphology

Because their cervical vertebra are movable, the necks of Iniidae are flexible; this sets them apart from most whales, dolphins, and porpoises, which have fused neck vertebrae, rigidly aiming their heads forward. The Iniidae have other morphological characters in common with other species adapted to freshwater river habitats, including highly reduced or absent dorsal fins, which help them avoid 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 include 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 single-chambered main stomach, and a pyloric stomach with connecting channels. They also share similarities with their marine ancestors in terms of lung shape, the position of the diaphragm, the position of the blowhole toward the back of the head, and the structure of the tympanic bulla. Iniidae have lost their fur and lack true vocal cords.

Iniidae dolphins develop multiple sets of teeth, including small conical 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.[citation needed]

Speciation

There is scientific debate on the number of species within the genus Inia. The main issue is whether there are between 2-4 species, or whether any of them should be considered sub-species. According to some researchers, Inia geoffrensis, Inia humboldtiana, and Inia boliviensis are three separate species, while many consider I. geoffrensis and I. boliviensis to be the only two. Martin (2004) found evidence that genetic exchange occurs at multiple sites on the Amazon, even in 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.