Palaelodidae

Palaeolodus ambiguus skeleton

Palaelodids were a successful group of bird found across all continents except for Antarctica, with their diversity being the greatest during the Miocene period.

• Eurasia

The most extensive record of the family can be found in Europe, beginning with Adelalopus in the early Oligocene of Hoogbutsel, Belgium.[8] While this genus is currently only found in one locality, the younger Palaelodus appear to have been much more widespread, being found in the form of thousands of bones in Saint-Gérand-le-Puy,[4][13] France, and multiple localities in Germany including the Mainz Basin,[13] Nördlinger Ries[7] and the Steinheimer Basin.[7] Megapaloelodus remains were also found in Saint-Gérand-le-Puy,[4] the Steinheimer Basin and Nördlinger Ries.[7] Other fossils have been discovered in Bohemia in the Czech Republic[7] and Romania.[11] Records in Asia are more sparse, with only a single described species known from the Middle Miocene Ooshin Formation in Mongolia.[10]

• America

America primarily yielded remains of the large genus Megapaloelodus, which is chiefly known from formations of the west coast. This includes fossils from Oregon's Juntura Formation[1] and California's Barstow Formation[14] and the Sharktooth Hill Bonebed.[15] Further inland fossils were found in the Rosebud Formation of South Dakota.[3] These fossils mostly date to the Miocene, with the exception of the material from Oregon which suggests an early Pliocene age. Further south Megapaloelodus was found in the Miocene to Pliocene Almejas Formation of Cedros Island (Baja California).[16] In South America Megapaloelodus fossils were found in the Andalhualá[9] and Ituzaingó Formation[17][18] of Argentina while material assigned to Palaelodus cf. ambiguus have been identified from Brazil's Taubuté Basin.[19]

• Africa

While there are no described species from Africa, remains are still known from multiple localities. Two differently sized types have been recovered from the early Oligocene Jebel Qatrani Formation of Fayum, Egypt.[20] A possible species of Megapaloelodus was found in the early Miocene Northern Sperrgebiet of Namibia.[21]

• Oceania

Three species of Palaelodus are currently accepted from Australasia. Two of these were present in the Oligocene to Miocene Etadunna and Namba Formation within the Lake Eyre Basin.[5] While the third was found in New Zealand's Bannockburn Formation, corresponding to the early Miocene in age.[7] While palaelodid remains are primarily known from these Neogene localities, isolated remains were also discovered in the much younger Kutjitara Formation, indicating that the group may have survived until 0.2 mya.[5]

Many of the rock units that yielded palaelodid remains are thought to have represented shallow lakes or similar environments at the time the bird fossils were deposited. Furthermore, many of these are believed to have been either occasionally or permanently brackish or even saline, indicating that these animals had habitat preferences similar to modern flamingos. The genus Adelalopus for instance appears to have inhabited the shores of a lagoon in what is now Belgium.[8] In a 1989 study Jacques Cheneval analysed the bird species of the type locality of Saint-Gérand-le-Puy, finding that over half of the identified taxa show a preference for brackish conditions while freshwater exclusives appear to have been absent.[26] This was later supported by studies that proposed that the locality underwent cyclical dry and wet seasons, with the salinity of the lake changing through the evaporation of water. This appears to have been an attractive environment for early phoenicopteriforms, given the presence of vast numbers of Palaelodus and more rarely Megapaloelodus and Harrisonavis.[22] Additionally, the discovery of juvenile birds and eggs suggests that palaelodids also bred there.[27] Another similar environment was present in the Lake Eyre Basin, which is thought to have supported salt lakes throughout much of the Neogene and early Quaternary. These localities mirror those of Europe in preserving not just the remains of juvenile palaelodids but also the fossils of more derived flamingos that seemingly coexisted with their more basal relatives.[5] In the case of Australia specifically, this included Phoenicopterus novaehollandiae and Phoeniconotius eyerensis at Lake Palankarinna.

However, palaelodids show some additional range beyond brackish lakes and lagoons. The Jebel Qatrani Formation, where two indetermined palaelodids were discovered, is thought to represent a mix of tropical lowland swamp and marshland supported by a series of river channels that empty into the Tethys ocean.[28] Palaelodus aotearoa from New Zealand inhabited paleo-Lake Manuherikia, which preserves a rich fauna of water fowl and even the remains of crocodiles. Worthy and colleagues describe the environment as a large lake fed by a river delta that was highly affected by wave activity and currents within the lake itself.[7] The Northern Sperrgebiet of Namibia also preserves the presence of freshwater without signs of saline lakes like those inhabited by the European and Australian taxa.[21] The Barstow Formation also likely represented a freshwater rather than brackish environment.[23]

Palaelodids may have fed on caddisfly larvae and other small invertebrates

Little is known about the precise habits of most palaelodids given the lack of substantial fossil material outside of Palaelodus ambiguus. Subsequently, most of what has been proposed for this group is based on said species. Palaelodids lack several of the adaptations found in flamingos that help them in their filter feeding lifestyle, such as the curved jaw, attachment point for keratinous lamellae and the absence of the pronounced kink in the spine prior to the shoulder girdle. The absence of these features all point towards the fact that they had a notably different way of feeding despite their affinity for similar environments. One possibility that has been suggested is that palaelodids fed on various aquatic invertebrates, specifically snails and caddisflies. The later are known to have been especially abundant in the French deposits, where their casings make a substantial contribution to the local limestone. It is possible that the modern filter-feeding apparatus of flamingos evolved from this or a similar feeding method.[13]

Their precise way of life has also been the subject of much debate. Earlier research on P. ambiguus, primarily the works of Cheneval and Escuillié, proposed that they may have been leg-propelled divers.[4][6] More recently this hypothesis has been called into question, partially due to the fact that no diving bird possesses limbs as elongated as what was present in palaelodids. Worthy and colleagues suggest a wading lifestyle, arguing that the compressed and elongated limbs may have aided in wading more easily through deep water.[7] Mayr on the other hand proposes a swimming lifestyle, pointing towards the reduced ability of the toes to flex being unsuited for wading. In his hypothesis the webbed feet and compressed toes would be more suited to work as propulsion while swimming.[25][22] Given its more fragmentary nature, Megapaloelodus has been subject to less speculation regarding its lifestyle. Miller notes that the bones of the lower leg appear to have a mechanism that could "lock" them in place. This could help stabilize the bird while standing in a position without moving.[23]