Biology: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 during the mid-Miocene era, creating a new brackish habitat that 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 makes them 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, which helps 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 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 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.

Speciation

There is scientific debate on the number of species within the genus Inia. The main issue is whether there are two or three (or even four) 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 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.

• Superfamily Inioidea
  • Family Iniidae
    • Genus †Goniodelphis
      • G. hudsoni
    • Genus Inia
      • Inia araguaiaensis - Araguaian river dolphin
      • Inia boliviensis - Bolivian river dolphin
      • Inia geoffrensis - Amazon river dolphin
      • Inia humboldtiana - Orinoco river dolphin
    • Genus †Isthminia
      • †Isthminia panamensis
    • Genus †Meherrinia
    • Genus †Ischyrorhynchus (syn. Anisodelphis)
      • I. vanbenedeni (syn. Anisodelphis brevirostratus)
    • Genus †Saurocetes (syn. Saurodelphis, Pontoplanodes)
      • S. argentinus (syn. Pontoplanodes obliquus)
      • S. gigas

References

Wikidata ☰ Q54873 entry