''Limnofregata'' ("Freshwater frigatebird") is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of primitive

frigatebird Frigatebirds are a family of seabirds called Fregatidae which are found across all tropical and subtropical oceans. The five extant species are classified in a single genus, ''Fregata''. All have predominantly black plumage, long, deeply forked ...

. The two known species were described after fossils from the Early

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

Green River Formation The Green River Formation is an Eocene geologic formation that records the sedimentation in a group of intermountain lakes in three basins along the present-day Green River in Colorado, Wyoming, and Utah. The sediments are deposited in very fine ...

(c.49 million years ago) of

Wyoming Wyoming () is a U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the south ...

. A number of good complete and partial skeletons, some with feather impressions, are known of the type species, ''Limnofregata azygosternon'', and ''L. hasegawai'' is known from two skulls and most of one torso.

Description

Birds of the genus ''Limnofregata'' resembled modern frigatebirds, but had shorter less-hooked bills and longer legs, and longer slitlike nasal openings. They stood tall when on the ground and had a wingspan of about . The

beak The beak, bill, or rostrum is an external anatomical structure found mostly in birds, but also in turtles, non-avian dinosaurs and a few mammals. A beak is used for eating, preening, manipulating objects, killing prey, fighting, probing for food ...

was shorter than that of modern frigatebirds, and lacked the typical hook at the end, resembling a strong

booby A booby is a seabird in the genus ''Sula'', part of the family Sulidae. Boobies are closely related to the gannets (''Morus''), which were formerly included in ''Sula''. Systematics and evolution The genus ''Sula'' was introduced by the Frenc ...

beak more than that of today's frigatebirds. The species differ conspicuously in size, with ''L. hasegawai'' being as much larger when compared to ''L. azygosternon'' as today's largest species of frigatebird (''

Fregata magnificens The magnificent frigatebird (''Fregata magnificens'') is a seabird of the frigatebird family Fregatidae. With a length of and wingspan of it is the largest species of frigatebird. It occurs over tropical and subtropical waters off America, betw ...

'') is compared to the smallest extant one (''

Fregata ariel The lesser frigatebird (''Fregata ariel'') is a seabird of the frigatebird family Fregatidae. At around 75 cm (30 in) in length, it is the smallest species of frigatebird. It occurs over tropical and subtropical waters across the Indian ...

''). The bill of ''L. hasegawai'' was notably larger still than that of its congener, whereas its feet were smaller, echoing a pattern found in the extant '' Fregata'' species. In 2014, a third species, ''Limnofregata hutchisoni'', was described from the

Wasatch Formation The Wasatch Formation (Tw)Shroba & Scott, 2001, p.3 is an extensive highly fossiliferous geologic formation stretching across several basins in Idaho, Montana Wyoming, Utah and western Colorado.

Paleobiology

The Eocene frigatebird genus ''Limnofregata'' comprises birds whose fossil remains were recovered from prehistoric freshwater environments, unlike the marine preferences of their modern-day relatives. They are thought to have lived like gulls of the genus ''Larus''. They inhabited the freshwater or

brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...

lakes that in time formed in today's

Green River Green River may refer to: Rivers Canada * Green River (British Columbia), a tributary of the Lillooet River *Green River, a tributary of the Saint John River, also known by its French name of Rivière Verte *Green River (Ontario), a tributary of ...

valley due to the uplift of the

Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico in ...

, feeding on smaller vertebrates and probably harassing other shorebirds for food and feasting on dead fish (e.g. ''

Knightia ''Knightia'' is an extinct genus of clupeid bony fish that lived in the freshwater lakes and rivers of North America and Asia during the Eocene epoch. The genus was erected by David Starr Jordan in 1907, in honor of the late University of Wyom ...

'') during summer dieoffs due to

oxygen depletion Hypoxia refers to low oxygen conditions. Normally, 20.9% of the gas in the atmosphere is oxygen. The partial pressure of oxygen in the atmosphere is 20.9% of the total barometric pressure. In water, oxygen levels are much lower, approximately 7 p ...

in the

eutrophic Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytoplank ...

lakes. Modern frigatebirds show pronounced

sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...

in size, apparently to avoid competition due to different

soaring Soaring may refer to: * Gliding, in which pilots fly unpowered aircraft known as gliders or sailplanes * Lift (soaring), a meteorological phenomenon used as an energy source by some aircraft and birds * ''Soaring'' (magazine), a magazine produced ...

behavior of male and female wings. As ''Limnofregata'' was most likely not a soaring bird, it can be expected that the sexes were much alike. It is not known if the males had a prominent throat balloon for advertising for mates as in their modern relatives, but ''Limnofregata'' throat bones differ markedly from those of modern frigatebirds.

References

* Olson, Storrs L. & Matsuoka, Hiroshige (2005): New specimens of the early Eocene frigatebird ''Limnofregata'' (Pelecaniformes: Fregatidae), with the description of a new species. ''Zootaxa'' 1046: 1–15
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{{Taxonbar, from=Q961985 Bird genera Fregatidae Eocene birds of North America Fossil taxa described in 1977