Palaelodus
Palaelodus is an extinct genus of bird of the Palaelodidae family, distantly related to flamingos. They were slender birds with long, thin legs and a long neck resembling their modern relatives, but likely lived very different livestyles. They had straight, conical beaks not suited for filter feeding and legs showing some similarities to grebes. Their precise lifestyle is disputed, with researchers in the past suggesting they may have been divers, while more recent research suggests they may have used their stiff toes as paddles for swimming while feeding on insect larvae and snails. This behavior may have been key in later phoenicopteriforms developing filterfeeding bills. The genus includes between five and eight species and is found across Europe, Australia, New Zealand, Asia and possibly South America. However some argue that most of the taxa named from Europe simply represent differently sized individuals of one single species. Palaelodus was most abundant during the Late Oligocene to Middle Miocene periods, but isolated remains from Australia indicate that the genus, or at least a relative, survived until the Pleistocene.
History and naming
The genus Palaelodus was first described by French scientist Alphonse Milne-Edwards in 1863 on the basis of fossils discovered in France's early Miocene deposits of the Saint-Gérand-le-Puy area. Milne-Edwards identified and named three distinct species: Palaelodus ambiguus, Palaelodus gracilipes and Palaelodus crassipes. In the years following this initial description, Milne-Edwards named two more species: Palaelodus minutus and Palaelodus goliath. In 1933 P. minutus was sunk into P. gracilipes by Lambrecht, a decision not immediately followed by other paleontologists like Brodkorb or Švec, but later accepted by Jacques Cheneval in 1983 during a major revision of the palaelodids of Saint-Gérand-le-Puy. Besides agreeing with the synonymity between P. gracilipes and P. minutus, Cheneval also placed P. goliath in the genus Megapaloelodus, an assessment followed by Heizmann & Hesse. A more conservative number of species was suggested by Mlíkovský in 2002, who placed M. goliath back in Palaelodus, but in turn sunk all of the remaining European Palaelodus species into P. ambiguus, reasoning that they can only be differentiated by size and thus simply represent variation within the species. Although acknowledging that the species proposed by Milne-Edwards may indeed be simply differently sized members of a single taxon, Worthy and colleagues argue that the synonymity proposed by Mlíkovský is premature until a comprehensive comparison of the European material is conducted. Later publications likewise do not follow Mlíkovskýs proposed species model. Palaelodus remains were first recognized in Australia in 1982, but not described until 1998 when Baird and Vickers-Rich erected two new species, P. wilsoni and P. pledgei, based on fossils from the Lake Eyre Basin. Despite being well known from postcrania remains, unambiguous fossils of the skull were long unknown until largely complete crania were described by Cheneval and Escuillié in 1992. Two distal right tibiotarsi recovered in 2008 and 2009 from the Saint Bathans Fauna of the Bannockburn Formation, New Zealand, were described in 2010 by Worthy and colleagues as another new species, Palaelodus aotearoa, and Palaelodus kurochkini was described from remains dating to the Miocene of Mongolia. The most recently named species is P. haroldocontii from the Late Miocene of Argentina.The name Palaelodus derives from the Ancient Greek "palaios" for "ancient" and "elodes" which means "inhabitant of marshes".
Species
- P. ambiguus
- P. haroldocontii
- P. pledgei
- P. wilsoni
- P. aotearoa
- 'P. kurochkini'''''
Disputed species
- P. gracilipes
- P. crassipes
Description
The neurocranium of Palaelodus shares several ancestral traits with the skulls of modern grebes of the order Podicipediformes. Among its autapomorphic traits is the position and development of the fossae glandulae nasales, two depressions for the nasal glands situated between the orbits of the animal. This trait helps differentiate the skull of Palaelodus from the skulls of any other known birds. The temporal fossae form somewhat of an intermediary between grebes and flamingos, more pronounced than in the later but not as deep as in the former. The premaxilla superficially resembles that of cranes, making the bill of Palaelodus appear straight and highly distinct from the curved bills of modern flamingos.The mandibular ramus is notably deep with an almost straight upper edge and a lower edge that bends and narrows only far behind the symphysis. The rami lack the spongy texture typically associated with flamingos and the upper rim of them is not widened either. Towards the back of the mandible an elongated fossa is found that is also unlike that seen in flamingos, instead resembling the condition observed in grebes. Towards the front of the mandible meanwhile there are distinct foramina, preceding a short but deep mandibular symphysis. The skull of Palaelodus also clearly shows the presence of salt glands.
Specimens from the Mainz Basin as well as Saint-Gérand-le-Puy both show that the notarium, a series of fused vertebrae of the shoulder girdle, consists of five vertebrae rather than the four seen in all extant mirandornithes. The notarium further differs in the orientation of the first vertebra, which in flamingos faces downwards, contributing to a marked kink in the spine of the animal that is not nearly as pronounced in Palaelodus. The first three vertebrae of the notarium all bear a ventral process, while flamingos only show ventral processes on one or two of them which are far less pronounced. In this condition Palaelodus again seems to show an intermediate condition between flamingos and grebes, as the later have well-developed ventral processes on all the vertebrae of the notarium. As the precise number of vertebrae prior to the notarium is unknown, Mayr assumes the same count as in flamingos with a similar division of the neck vertebrae based on bending properties. Based on this, the central and caudal cervicals appear largely similar to those of flamingos, being similarly elongated but lacking the foramina towards the front of the individual vertebrae. Regarding the cranial cervicals, some differences can be identified. The 7th or 8th appear more elongated relative to modern flamingos with a deeper crest formed by the spinous processes, while the known vertebrae thought to be closest to the head, the 4th or 5th, appear less elongated than in flamingos.
The pedal phalanges, the bones that make up the middle toes, of Palaelodus are compressed mediolaterally unlike those of flamingos, deep and with weakly developed convex distal articulation points that lack a furrow. This later characteristic would impact the flexion of the toes and is associated with webbed feet used for locomotion in the water. Although this is also true for grebes to some extent, podicipediforms show dorsoventrally flattened toes, indicating that this is not an ancestral trait and was instead acquired independently in both lineages. The ungual phalanges could not be described by Mayr in his detailed analysis of Palaelodus material, however he notes that older figures seem to indicate that the toe tips were not flattened like in grebes or flamingos and instead show the state typical for other bird groups.
Although still relatively long, the legs of Palaelodus were not nearly as elongated as those of modern flamingos. In particular, the tarsometatarsus was notably shorter than the humerus while the opposite is true for phoenicopterids. The tarsometatarsus further differs from flamingos in that it is laterally compressed, more similar to what is seen in grebes. The pelvis also differs from flamingos, being more narrower than in the extant waders.
Phylogeny
The family Palaelodidae is the sister taxon of modern flamingos, with both being placed in the order Phoenicopteriformes. Palaeolodids such as Palaelodus are considered to be an important link in understanding the relationship between flamingos and their next closest relatives, the diving grebes with which they from the clade Mirandornithes. This relationship is well supported by both molecular and morphological evidence and the Palaelodidae form a link between the two extant groups with cranial anatomy and general proportions similar to flamingos but legs akin to those of grebes. The following phylogenetic tree depicts Mirandornithes as recovered by Torres and colleagues in 2015.Paleobiology
Locomotion
Due to its unique anatomy and intermediary position within mirandornithes, the exact ecology of Palaelodus is not entirely understood. Cheneval and Escuillié both suggest that Palaelodus may have been a diver using its webbed feet for propulsion, however this hypothesis has been questioned by the works of Mayr as well as Worthy and colleagues, both of whom suggest different alternatives. In Worthy et al. it is suggested that palaelodids were wading birds, more akin to flamingos, while publications by Mayr suggest a swimming lifestyle.Some of the traits used to infer a diving lifestyle were noted to be also present in flamingos while a pneumatised humerus, as present in Palaelodus, is entirely unknown in divers. Worthy and colleagues point out that, while having laterally compressed tibiotarsi may be a trait shared with diving birds, no bird with such a lifestyle features the same degree of limb elongation as present in Palaelodus. They counter that the compression, as well as other traits of the hindlimbs, could just as well be adaptions to more easily wade through deeper water. Mayr meanwhile specifically points to several traits that according to him support a swimming lifestyle. The compression of the toes is unlike what is seen in any modern wading bird, while the decreased ability to flex the toes suggests the use of the webbed feet as stiff paddles. The distinctly narrower pelvis also points towards a different way of life than that of flamingos.