Late Cretaceous - Present, 95–0 Ma
Sirenidae, the sirens, are a family of aquatic salamanders. Family members have very small fore limbs, and lack hind limbs altogether. In one species, the skeleton in their fore limbs is made of only cartilage. In contrast to most other salamanders, they have external gills bunched together on the neck in both larval and adult states. Sirens are found only in the Southeastern United States and northern Mexico.
Sirens are quite distinct from other salamanders, hence they form their own suborder, Sirenoidea. Sometimes, they are even referred to as a completely distinct order (Meantes or Trachystomata). Genetic analysis confirms the sirens are not closely related to any other salamander group. Many of their unique characteristics seem to be partly primitive and partly derivative.
Sirens are generally eel-like in form, with two tiny, but otherwise fully developed, fore limbs. They range from 25–95 cm (9.8–37.4 in) in length. They are neotenic, although the larval gills are small and functionless at first, and only adults have fully developed gills. Because of this, sirens most likely have evolved from a terrestrial ancestor that still had an aquatic larval stage. Like amphiumas, they are able to cross land on rainy nights.
Except for some patches of small teeth on their palates and on the splenial bone on the inner side of their lower jaws, their mouths have lost all dentition and have been replaced with a horny sheath that resembles a beak. Sirens feed mainly on worms, small snails, shrimps, and filamentous algae.
If the conditions of a water source are unsuitable, a larva will shrink its gills to mere stumps, and may not function at all. They are also able to burrow into mud of drying ponds and encase themselves with a cocoon of mucus to survive periods of drought. During such periods, they breathe with their small but functional lungs.
Unlike other salamanders, an interventricular septum is present in the heart. At least two of the species can produce vocalizations. The structure of sirens' reproductive systems suggests they employ external fertilization.
The siren family (Sirenidae) is subdivided into two genera, with two species each:
- Genus †Habrosaurus Gilmore 1928
- Genus †Kababisha Evans et al. 1996
- Genus †Noterpeton Rage et al. 1993
- †Noterpeton bolivianum Rage et al. 1993
- Genus Pseudobranchus Gray 1825 dwarf sirens
- Genus Siren (Österdam 1766) sirens
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- San Mauro, Diego; Miguel Vences; Marina Alcobendas; Rafael Zardoya; Axel Meyer (May 2005). "Initial diversification of living amphibians predated the breakup of Pangaea" (– Scholar search). American Naturalist 165 (5): 590–599. doi:10.1086/429523. PMID 15795855.[dead link]