#limnofregata

The modern landscape of the American west is but a far cry from its past. Early Eocene Green River formation preserves an incredible snapshot of time 50 million years ago. The locality of these fossils was once a huge lake system bordered by forests of sycamore and palmettos, and teeming with animals of all kinds, including birds, which were in the middle of an enormous radiation of forms.

Calciavis: A palaeognath which was a member of the lithornithids. Like them, it fed on insects from the forest floor and was a powerful flier. Some fossils preserve colour pigments of its feathers, suggesting this bird was a glossy black.

Limnofregata: The earliest known frigatebird that flew over the water just as its aeronaut descendants. Like modern frigates, it plucked prey from the water and the shorelines of the lake system.

Plesiocathartes: A bizarre bird that has had a complicated history. Now we know that it was an early relative of the cuckoo-roller from Madagascar and probably lived a similar lifestyle.

Celericolius: An early member of the mousebird family that had long wings and even longer tail. It may have flown at high speed through the forests.

Foro: This american bird is in fact an early member of turacos, birds which are unique to Africa today. Like them, it mostly fed on soft fruit.

Prefica: A strange bird who'se closest living relative is the oilbird of South America, which is a nocturnal frugivore. It could be that this ancient relative lived in a similar way.

Nahmavis: An early member of the shorebird family known from a spectacular fossil that preserves feathers.

Gallinuloides: The earliest known true galliform, this small fowl was among the early ancestors of guans, quails, turkeys and grouse.

By Ripley Cook

Etymology: Freshwater Frigatebird

First Described By: Olson, 1977

Classification: Dinosauromorpha, Dinosauriformes, Dracohors, Dinosauria, Saurischia, Eusaurischia, Theropoda, Neotheropoda, Averostra, Tetanurae, Orionides, Avetheropoda, Coelurosauria, Tyrannoraptora, Maniraptoromorpha, Maniraptoriformes, Maniraptora, Pennaraptora, Paraves, Eumaniraptora, Averaptora, Avialae, Euavialae, Avebrevicauda, Pygostaylia, Ornithothoraces, Euornithes, Ornithuromorpha, Ornithurae, Neornithes, Neognathae, Neoaves, Aequorlitornithes, Ardeae, Aequornithes, Suliformes, Fregatidae 

Referred Species: L. azygosternon, L. hasegawai, L. hutchisoni

Status: Extinct

Time and Place: Limnofregata is known from between 53.5 and 48.5 million years ago, in the Ypresian age of the Eocene of the Paleogene 

Limnofregata has been found entirely within the Fossil Butte Member of the Green River Formation in Wyoming

Physical Description: Limnofregata is one of the earliest known examples of the Frigatebirds - a group of water-associated birds with a distinctive appearance. These birds have long bills for catching fish in the oceans and huge, distinctive pouches on their throats. They are able to soar for weeks on end, making them highly adapted for their water-journeying lifestyle. Limnofregata, being a very early example of the group, look like what you’d expect a transitional form of the frigatebirds to be - showcasing some of the same distinct shape of the modern group, while still not being quite there yet.

Limnofregata showed significant similarity to living Frigatebirds - like living members of the species it had short, stocky legs, stronger bones in the wings, and wide hips. This indicates that it was already on the path towards high levels of specialization for flight. However, its wings were not quite as long, in terms of proportion, as living Frigatebirds; this indicates that it didn’t utilize soaring as much in its life. Limnofregata also had a shorter bill and long, slitlike nose openings, more like modern boobies; it also lacked the noticeable hook found on modern Frigatebird bills. The size of the nostrils reduced in Frigatebirds as they transitioned to more saltwater habitats (aka, the open ocean), so the large nostril openings in Limnofregata point to its freshwater habitat.  Limnofregata was also not sexually dimorphic in terms of size, an important characteristic of living Frigatebirds.

Limnofregata varied in size amongst the different species, but in general it had a wingspan of between 100 to 120 centimeters - in Imperial units, that’s between 3 and 4 feet long. This is a little more than half the length of living Frigatebird wingspans, but their bodies are roughly the same size as living Frigatebird, adding more evidence to their wings being less adapted for soaring.

By Scott Reid

Diet: Given their distinct difference in beak shape compared to modern Frigatebirds, and their completely different habitat (rather than being associated with the open ocean, Limnofregata lived near a lake; read more in the Ecosystem subsection), it is unlikely that it had a similar diet. Instead, Limnofregata was probably more of an opportunist, like modern gulls - feeding on a wide variety of food, of which fish was only one kind of many. As such, it was much more omnivorous than its living relatives.

Behavior: Limnofregata was heavily associated with a lake ecosystem, and, as something more analogous to living gulls, it would have been the asshole of that lake ecosystem. Rather than soaring about looking for sources of fish, like living Frigatebirds, it would have probably stayed close to the shore and the edge of the freshwater lakes in its environment, feeding on smaller vertebrates and harrassing other birds in the area for their food, attempting to grab the catches of other birds and eating their fish instead of catching the food themselves. As opportunists, they would have congregated especially frequently at moments of extensive food production, such as when large numbers of fish died off in the ecosystem as oxygen depleted in the lakes.

Given that the sexes were alike in size, it’s possible that they didn’t have as extensive systems of sexual display as living Frigatebirds do; though, of course, no throat pouches have been fossilized either way, so we can’t be certain. As large numbers of these birds have been fossilized together, it is likely that they formed large flocks of convenience, like modern gulls today.  

By José Carlos Cortés

Ecosystem: The Green River Formation is another example of a well-documented ecosystem of the global rainforest of the early Eocene; in fact, it is older than the Messel Pit, and showcases unique creatures as a result. Similar to the Messel Pit, the Green River Formation is associated with a fossil lake system scattered amongst the jungle; however, given that the animals of Green River were not quite as small as those of the Messel Pit, the vegetation does not seem to have been quite as dense. The dominant tree type surrounding the lake were sycamore trees, though ferns, palms, and many other plant types were present as well. The entire system was greatly affected by changes in sediment as the Rocky Mountains formed around them. This commonly lead to great influxes of nutrients - like when industrial companies dump phosphorus into lakes today - overwhelming the lake system and suffocating the fish inside. This influx of sediment and minerals allowed for the ecosystem’s unique preservation; it also would have lead to periodic upwelling of dead fish for Limnofregata to munch on. These fish included animals such as rays, catfish, suckers, and relatives of modern herrings and sardines, along with many others. Crocodiles such as Borealosuchus were present, and probably would have fed upon Limnofregata. There were also some of the earliest primates, bats, and an armadillo-like mammal as well.

Of course, birds were extremely common, to the point of having feathers preserved in many cases. Other dinosaurs known from the Fossil Butte Member include the Lithornithids (flighted, tree-dwelling relatives of modern Ostriches and Emu) Pseudocrypturus and Calciavis; the flamingo-like duck Presbyornis; the pheasant relative Gallinuloides; the early swift and hummingbird relative Eocypselus; the frogmouth-like Fluvioviridavis and Prefica; the mousebird relative Anneavis; the early woodpecker relative Neanis; the Parrots of Prey Cyrilavis and Tynskya; and the early passerine relatives with parrot feet Zygodactylus and Eozygodactylus. Thus, the Green River Formation was another example of a great place to see how dinosaurs were getting on after the extinction.

Other: Feather impressions are known from Limnofregata, but they aren’t very exciting in terms of morphology.

Species Differences: Since all three species are from the same place, they are mainly distinguished from one another based on shape and size. L. hasegawai was much larger, about the size of the largest modern Frigatebirds; L. azygosternon was somewhat somewhat smaller and more comparable with the smaller species of the group today. L. hutchisoni differs in terms of how its wing bones are shaped, but was large like L. hasegawai, potentially even larger.

~ By Meig Dickson

Sources under the cut