Editing Halosaur (section)

{{short description|Family of eel-like deep-sea fishes}}
{{More citations needed|date=November 2024}}
{{Automatic taxobox
| name = Halosaurs
| fossil_range = {{fossil range|78|present|earliest=Coniacian|Late [[Campanian]] to present}}
| image = Halosaur (48976447277).jpg
| image_caption = 
| display_parents = 3
| taxon = Halosauridae
| authority = [[Albert Günther|Günther]] 1868<ref>{{cite web | website=Collection of genus-group names in a systematic arrangement | author= | year= | title=Part 7- Vertebrates | url=http://mave.tweakdsl.nl/tn/genera7.html | volume= | pages= | doi= | display-authors=etal | access-date=30 June 2016 | archive-url=https://web.archive.org/web/20161005114629/http://mave.tweakdsl.nl/tn/genera7.html | archive-date=5 October 2016 | url-status=dead }}</ref>
| subdivision_ranks = Genera
| subdivision = see text
| synonyms = * Halosauropsinae <small>McDowall 1973</small>
}}

'''Halosaurs''' are [[eel]]-shaped [[fish]]es found only at great ocean depths. As the family '''Halosauridae''', halosaurs are one of two families within the order [[Notacanthiformes]]; the other being the deep-sea spiny eels, [[Notacanthidae]]. Halosaurs are thought to have a worldwide distribution, with some 17 species in three genera represented. Only a handful of specimens have been observed alive, all in chance encounters with [[Remotely operated underwater vehicle]]s.

The term "halosaur" refers to the type genus, ''[[Halosaurus]]'', which is a [[Greek language|Greek]] [[compound word]], ''hals'' meaning "sea" and ''sauros'' meaning "[[lizard]]".  Halosaurs have a spotty fossil record, the oldest known genus being ''[[Echidnocephalus]]'' from the [[Late Cretaceous]] ([[Campanian]]) strata of [[Westphalia]], [[Germany]],<ref name=":1" /> and the second-oldest known genus, ''[[Laytonia]]'', from [[Miocene]] strata of [[California]] and [[Oregon]].  The fossil specimens already bear strong resemblance to the modern genera.  The halosaurs' greatly elongated bodies end in whip-like tails; their scales are large. One small dorsal fin is close to the sharply pointed, mostly scaleless head. The tail fin is greatly reduced, with the anal fin being the largest fin. Their pectoral fins are slender and greatly elongated. Their mouths are somewhat large, with the lower jaw shorter than the upper jaw. The [[swim bladder]] is absent in all known species, except for ''Aldrovandia oleosa'', which has a very small bladder.<ref>[https://www.jstor.org/stable/1443498 Aldrovandia oleosa, a New Species of the Halosauridae, with Observations on Several Other Species of the Family]</ref>

The largest species, the 90-cm (3-ft) long [[abyssal halosaur]] (''Halosauropsis macrochir'') is also one of the most deep-living fish, recorded at depths of 3,300 m (11,000&nbsp;ft). Halosaurs have developed certain adaptations to life at these extreme depths, where no light penetrates. Their [[lateral line]] system enabling the detection of vibrations in the water is highly developed; the pores run the length of the fish's body. Some species are also known to hold their elongated pectorals erect and forward, possibly providing a further means of detection.

Halosaurs are [[benthic]] fish, spending their time cruising over or resting on the sea floor, where temperatures may be just 2–4&nbsp;°C. They propel themselves with rhythmic, lateral undulations of their bodies, not unlike [[sea snake]]s. Halosaurs are thought to prey mainly on benthic [[invertebrate]]s, such as [[polychaete]] worms, [[echinoderm]]s, and [[crustacea]]ns such as [[copepod]]s, but they may also consume small [[fish]]es and [[cephalopod]]s.

In life, most halosaurs are grey to bluish-black in colour. Like other notacanthiform fish, halosaurs are able to regenerate their tails easily if lost. This adaptation can be likened to certain terrestrial [[reptile]]s, such as the [[glass lizard]], which sacrifices its tail to evade predators.