''Humboldtichthys'' is an extinct genus of Glass Knifefish from the

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

Yecua Formation The Yecua Formation is a geological Formation in what is now Bolivia. Studies suggest that the Yecua Formation preserves a coastal setting with humid to semiarid floodplains, shorelines and tidal as well as shallow marine environments including ...

of Bolivia. A single species is currently recognized: ''Humboldtichthys kirschbaumi''.

History and naming

The earliest remains of gymnotiform electric fish were discovered in the

Bolivian Bolivian may refer to: * Something of, or related to Bolivia ** Bolivian people ** Demographics of Bolivia ** Culture of Bolivia * SS ''Bolivian'', a British-built standard cargo ship A cargo ship or freighter is a merchant ship that carries ...

95 km west-northwest of Santa Cruz by petroleum geologists and easily recognized to belong to a gymnotiform fish based on several characteristic anatomical features. From material collected, Gayet & Meunier described ''Ellisella kirschbaumi'' in 1991, unaware that the genus name was already occupied by a genus of soft coral (''

Ellisella ''Ellisella'', commonly known as sea whip, is a Genus (biology), genus of soft coral in the Family (biology), family Ellisellidae. ''Ellisella'' constitutes like bushy shrub gorgonians which dominant color is brown to reddish and polyps are whi ...

''). The holotype specimen, RL 1596-4, represents an incomplete fragment of the animal, preserving the posterior margin of the skull and anterior elements of the body. An additional six fossils were also referred to the genus by Gayet & Meunier. As the name was already occupied however, the animal was given the genus name ''Humboldtichthys'' in the place of ''Ellisella'' in 2000 and the referred specimens were re-examined by Albert & Fink (2007). In their examination they compared the holotype specimen with all 6 referred specimen and 3 further gymnotiform fossils from the Yeccua Formation. They concluded that the collected material represents more than one species, with most specimens potentially representing members of extant genera. ''Humboldtichthys'' translates to " Humboldt's fish. Originally the name ''Ellisella kirschbaumi'' was meant to honor both American physiologist

Max Mapes Ellis Max Mapes Ellis, (December 3, 1887 - August 26, 1953) was an American physiologist. He was married to the American ichthyologist Marion Durbin Ellis (1887-1972) in 1909. Early life and career Ellis was the son of Horace and Grace V. Ellis. He w ...

and German ichthyologist Frank Kirschbaum, who pioneered the research of gymnotiform growth and regeneration.

Description

The holotype specimen preserves the right side of ''Humboldtichthys'' with a 5.5 cm long axis, including impressions of the opercle and preopercle as well as the anterior portion of the body. The surface of opercle and preopercle are lamellar with multiple deep radial striations. Although such striations are also present in modern members of ''Sternopygus'', they are noticeably more pronounced in ''Humboldtichthys'' and the only diagnostic feature not called into question by Albert & Fink (2007). The anterior limb of the preopercle is broad and crescent shaped, while its dorsal limb preserves a portion of a relatively broad preopercular-mandibular laterosensory canal. The entire preopercle is oriented oblique to the main axis of the head. The opercle is triangular in shape with its dorsal, posterior and ventral margins slightly convex. However poor preservation renders it difficult to determine the exact shape of these bones. The origin of the anal fin is located immediately behind the posterior margin of the operculum and the holotype specimen preserves 28 pterygiophores and 15 fin rays, all of which unbranched. The pectoral fin possesses a minimum of 10 fin rays with a posterior displacement of approximately 1-2 cm. Compared to the depth of the body the ribs are relatively short.

Phylogeny

In 1994 Gayet ''et al.'' hypothesized that ''Humboldtichthys'' represented a basal gymnotiform and sister taxon to all other gymnotiforms other than the family Apteronotidae. In their 2007 study, Albert & Fink score each knifefish specimen from the Yecua Formation individually due to the lack of overlapping diagnostic material and the fact that knifefish are known to occur alongside other knifefish genera, meaning that sharing a single locality may not be reason enough to assign them to a single specimen. They recovered one referred specimen as Gymnotiformes ''incertae sedis'' and 4 more specimens as members of Sinusoidea. The holotype specimen of ''Humboldtichthys'' was recovered as a sister taxon to the extant '' Sternopygus''. One specimen is placed within Sternopygidae ''incertae sedis'' while the remaining three specimens analysed were all recovered as ''Sternopygus incertae sedis''.

Paleobiology

''Humboldtichthys'' is known from the Bolivian Yecua Formation, which represents a humid to semi-arid environment close to the coast with a variety of bodies of water. Fossil remains, including stomach content from previously referred specimens, confirms the presence of both small bodied characids and

catfish Catfish (or catfishes; order Siluriformes or Nematognathi) are a diverse group of ray-finned fish. Named for their prominent barbels, which resemble a cat's whiskers, catfish range in size and behavior from the three largest species alive ...

in addition to the knifefish fauna. Grass-like aquatic

macrophytes Aquatic plants are plants that have adapted to living in aquatic environments ( saltwater or freshwater). They are also referred to as hydrophytes or macrophytes to distinguish them from algae and other microphytes. A macrophyte is a plant that ...

have also been found. Albert & Fink suggest that the specific locality where ''Humboldtichthys'' and the other knifefish remains have been found may preserve what was once a slow moving terra firma (non floodplain) stream, a common habitat for multi-species gymnotiform assemblages.

References

{{Portal, Palaeontology Prehistoric ray-finned fish genera Fossil taxa described in 1994 Fossils of Bolivia Miocene Gymnotiformes