Presbyornithidae
Encyclopedia
Presbyornithidae were a family
of waterbirds with an apparently global distribution that lived until the Earliest Oligocene
, but are now extinct. Initially, they were believed to present a mix of characters shown by waterbirds, shorebirds and flamingo
s and were used to argue for an evolutionary relationship between these groups (Feduccia 1976), but they are now generally accepted to be "wading ducks", the sister taxon of the Anatidae
, and thus essentially modern waterbirds. They were generally long-legged, long-necked birds, standing around one meter high, with the body of a duck, feet similar to a wader
but webbed, and a flat duck-like bill adapted for filter feeding. Apparently, at least some species were very social birds that lived in large flocks and nested in colonies.
As the "wading duck" moniker implies, they were waterfowl whose elongated legs enabled them to live a lifestyle similar to the "proto-flamingos" (e.g., Palaelodus
) - which were not really ancestors of the modern flamingos, but a group that evolved in parallel with them and in fact seems to have taken over part of the presbyornithid's ecological niche
after the latter became extinct. Thus, while probably somewhat capable of swimming, they would have preferred to strain the shallow waters of their habitat for food and were also able to snatch up insect
s and small crustacean
s on dry land, just like some species of modern ducks, e.g., the Laysan Duck
, hunt for brine flies.
and early Palaeogene is that shore habitats offered most resources for ancestors of modern birds. The reasons seem to have been that arboreal niches were where the main radiation of the Enantiornithes
had taken place some time earlier, and later on because the C-T mass extinction
affected both aquatic and terrestrial habitats extensively, leading to the almost total collapse of their trophic webs
. In marine habitats, the climatic changes associated with the mass extinction's cause(s) caused a wholesale die-off of oceanic phytoplankton
and thus their food webs were destroyed from the bottom up. In terrestrial habitats on the other hand, apart from the loss of the primary production
capacity, the keystone species
, which were in almost all cases dinosaur
s, disappeared, leading the trophic webs on dry land to collapse also from the inside out.
Specialized taxa of the older bird radiations that were very well adapted to their particular ecological niche and dependent on the intactness of the trophic webs had generally no chance to survive such mass extinctions. It is now apparent that at least the main evolutionary lineages of modern bird families already existed at the end of the Cretaceous
, albeit they were somewhat marginal compared to the dominant, earlier groups of birds such as Enantiornithes
and Confuciornithidae. This serves to show that in evolution
the possession of derived or "modern" characters can actually be a disadvantage when a species needs to compete against well-established but more "primitive" lineages, especially as it must be understood that "primitive" refers only to descendence from a lineage that had been established a longer time ago, not that these species were any more generalist
or less well-adapted than "modern" forms. In fact, that there were "no" (probably rather: very few) arboreal Neornithes by the end of the Cretaceous is today believed to be because the "primitive" Enantiornithes had had more time to develop adaptations to an arboreal lifestyle and were actually able to outcompete the "modern" arboreal forms, leaving vacant only a few possibilities for early Neornithes to evolve an arboreal lifestyle.
At any rate and their evolutionary relationships nonwithstanding, most bird taxa that survived the mass extinction seem to have been living in environments where they could utilize both terrestrial as well as marine or limnic food resources (the ancestors of the Galliformes
probably being the one noteworthy exception). Until the trophic webs had diversified and become complex enough again, such generalist forms were at a competitive advantage. When specialization became a feasible evolutionary strategy again, however, they were outcompeted by more advanced taxa. Note that here, too, "generalist" does not imply that these birds were competitively inferior in their entire ecological niche, only that whenever some form evolves specialization for living in part of this niche, the generalis is at a competitive disadvantage in that particular part of its niche. As time progresses and consequently opportunities for specialization accumulate, it may happen that the generalis forms are either forced to specialize themselves to maintain a competitive edge, or disappear, their niche being in effect divided up by specialist forms.
Principles in evolution as demonstrated by the Presbyornithidae
are unequivocally accepted to belong to the Presbyornithidae:
There is one species generally accepted in Headonornis and Telmabates each. Presbyornis contains 2 or 3 described species. The genus Nautilornis is today considered a synonym of Presbyornis, which is found in a wide range of Late Paleocene
to Early Oligocene deposits in North America
and Europe
. Additionally, most of the bones referred to Headonornis have been found to belong to Presbyornis, and the remaining coracoid may do so too (Dyke, 2001). Telmabates lived in today's Patagonia
during the Eocene
.
Apart from these unequivocal presbyornithids, there are some genera which are tentatively assigned to this family pending the discovery of more complete material. As many fossils from the Early Palaeogene show somewhat ambiguous characters, it is not easy to place these early modern birds unequivocally into one lineage or another. That they were ecologically generalized and are usually known from very few fossil
remains only serves to worsen this situation.
Possible genera of presbyornithids include:
of which the first is known from Late Cretaceous deposits in Mongolia
, the second latter from the Early Eocene of England
, and the third is an enigmatic Late Cretaceous or Paleogene
form from Central Asia
. There are some other, undescribed, presbyornithid or possible presbyornithid remains, such as the partial right scapula
BMNH
PAL 4989, which was considered part of Headonornis hantoniensis, but cannot be positively refererred to a known taxon, or the Early Cretaceous remains from the Mongolian Barun Goyot Formation
at Uday Sayr and the Nemegt Formation
of Tsagaan Kushu.
Vegavis
from the Late Cretaceous
of Antarctica was in all likelihood not a presbyornithid, but rather about equally close to these and the living ducks, geese and swans.
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
of waterbirds with an apparently global distribution that lived until the Earliest Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
, but are now extinct. Initially, they were believed to present a mix of characters shown by waterbirds, shorebirds and flamingo
Flamingo
Flamingos or flamingoes are gregarious wading birds in the genus Phoenicopterus , the only genus in the family Phoenicopteridae...
s and were used to argue for an evolutionary relationship between these groups (Feduccia 1976), but they are now generally accepted to be "wading ducks", the sister taxon of the Anatidae
Anatidae
Anatidae is the biological family of birds that includes ducks, geese and swans. The family has a cosmopolitan distribution, occurring on all the world's continents except Antarctica and on most of the world's islands and island groups...
, and thus essentially modern waterbirds. They were generally long-legged, long-necked birds, standing around one meter high, with the body of a duck, feet similar to a wader
Wader
Waders, called shorebirds in North America , are members of the order Charadriiformes, excluding the more marine web-footed seabird groups. The latter are the skuas , gulls , terns , skimmers , and auks...
but webbed, and a flat duck-like bill adapted for filter feeding. Apparently, at least some species were very social birds that lived in large flocks and nested in colonies.
As the "wading duck" moniker implies, they were waterfowl whose elongated legs enabled them to live a lifestyle similar to the "proto-flamingos" (e.g., Palaelodus
Palaelodus
Palaelodus is an extinct genus of birds distantly related to flamingos. They were slender birds with long, thin legs and a long neck. Little is known about the shape of their skull or beak. Some paleontologists think Palaelodus was able to swim under water, chasing prey, but the morphology of their...
) - which were not really ancestors of the modern flamingos, but a group that evolved in parallel with them and in fact seems to have taken over part of the presbyornithid's ecological niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
after the latter became extinct. Thus, while probably somewhat capable of swimming, they would have preferred to strain the shallow waters of their habitat for food and were also able to snatch up insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s and small crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s on dry land, just like some species of modern ducks, e.g., the Laysan Duck
Laysan Duck
The Laysan Duck , also known as the Laysan Teal because of its small size, is an endangered dabbling duck endemic to the Hawaiian Islands...
, hunt for brine flies.
Significance in avian evolution
The implication of the plethora of this and other, ecologically similar Neornithes (e.g., the wastebasket taxon "Graculavidae") from the Late CretaceousLate Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
and early Palaeogene is that shore habitats offered most resources for ancestors of modern birds. The reasons seem to have been that arboreal niches were where the main radiation of the Enantiornithes
Enantiornithes
Enantiornithes is an extinct group of primitive birds. They were the most abundant and diverse avialans of the Mesozoic. Almost all retained teeth and clawed fingers on each wing, but otherwise looked much like modern birds externally. Over 50 species of Enantiornithines have been named, but some...
had taken place some time earlier, and later on because the C-T mass extinction
Cretaceous-Tertiary extinction event
The Cretaceous–Paleogene extinction event, formerly named and still commonly referred to as the Cretaceous-Tertiary extinction event, occurred approximately 65.5 million years ago at the end of the Maastrichtian age of the Cretaceous period. It was a large-scale mass extinction of animal and plant...
affected both aquatic and terrestrial habitats extensively, leading to the almost total collapse of their trophic webs
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...
. In marine habitats, the climatic changes associated with the mass extinction's cause(s) caused a wholesale die-off of oceanic phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
and thus their food webs were destroyed from the bottom up. In terrestrial habitats on the other hand, apart from the loss of the primary production
Primary production
400px|thumb|Global oceanic and terrestrial photoautotroph abundance, from September [[1997]] to August 2000. As an estimate of autotroph biomass, it is only a rough indicator of primary production potential, and not an actual estimate of it...
capacity, the keystone species
Keystone species
A keystone species is a species that has a disproportionately large effect on its environment relative to its abundance. Such species play a critical role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem and helping to determine the types and...
, which were in almost all cases dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s, disappeared, leading the trophic webs on dry land to collapse also from the inside out.
Specialized taxa of the older bird radiations that were very well adapted to their particular ecological niche and dependent on the intactness of the trophic webs had generally no chance to survive such mass extinctions. It is now apparent that at least the main evolutionary lineages of modern bird families already existed at the end of the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
, albeit they were somewhat marginal compared to the dominant, earlier groups of birds such as Enantiornithes
Enantiornithes
Enantiornithes is an extinct group of primitive birds. They were the most abundant and diverse avialans of the Mesozoic. Almost all retained teeth and clawed fingers on each wing, but otherwise looked much like modern birds externally. Over 50 species of Enantiornithines have been named, but some...
and Confuciornithidae. This serves to show that in evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
the possession of derived or "modern" characters can actually be a disadvantage when a species needs to compete against well-established but more "primitive" lineages, especially as it must be understood that "primitive" refers only to descendence from a lineage that had been established a longer time ago, not that these species were any more generalist
Generalist
Generalist may refer to:* a person with a wide array of knowledge, the opposite of which is a specialist.* a physician who is focused on primary care and is not a specialist in a certain medical field...
or less well-adapted than "modern" forms. In fact, that there were "no" (probably rather: very few) arboreal Neornithes by the end of the Cretaceous is today believed to be because the "primitive" Enantiornithes had had more time to develop adaptations to an arboreal lifestyle and were actually able to outcompete the "modern" arboreal forms, leaving vacant only a few possibilities for early Neornithes to evolve an arboreal lifestyle.
At any rate and their evolutionary relationships nonwithstanding, most bird taxa that survived the mass extinction seem to have been living in environments where they could utilize both terrestrial as well as marine or limnic food resources (the ancestors of the Galliformes
Galliformes
Galliformes are an order of heavy-bodied ground-feeding domestic or game bird, containing turkey, grouse, chicken, New and Old World Quail, ptarmigan, partridge, pheasant, and the Cracidae. Common names are gamefowl or gamebirds, landfowl, gallinaceous birds or galliforms...
probably being the one noteworthy exception). Until the trophic webs had diversified and become complex enough again, such generalist forms were at a competitive advantage. When specialization became a feasible evolutionary strategy again, however, they were outcompeted by more advanced taxa. Note that here, too, "generalist" does not imply that these birds were competitively inferior in their entire ecological niche, only that whenever some form evolves specialization for living in part of this niche, the generalis is at a competitive disadvantage in that particular part of its niche. As time progresses and consequently opportunities for specialization accumulate, it may happen that the generalis forms are either forced to specialize themselves to maintain a competitive edge, or disappear, their niche being in effect divided up by specialist forms.
Principles in evolution as demonstrated by the Presbyornithidae
- Generalist forms which have more "fall-back" potential if part of an ecological niche gets destroyed are better adapted to survive mass extinctions than specialized forms which occupy a narrow ecological niche.
- Primitiveness in descent does not translate into "primitiveness" in morphology or adaptation. The former is an absolute value defined by when the lineage in question separated from relatives which later underwent additional radiation. The latter is dependent on when the last mass extinction created opportunities for the survivors to embark on a new adaptive journey.
- After a lineage has been able to evolve uninterrupted for a considerable amount of time, there is a trend for generalists to be competitively excluded from more and more of their niche(s) by specialist forms, both related and unrelated, that by and by adapt to particular parts of the generalist's niche.
- With continuing uninterrupted evolution of a lineage, remaining plesiomorphies are usually non-adaptive: Presbyornis was a bird which, although primitive by descent and generalist by ecology, was uniquely and highly adapted to its particular mode of life, in a way that is not found anymore in modern waterfowl, because the ecological niche to which it was most well adapted was later partitioned away for the most part, with other opportunities for generalist forms arising in the process.
Systematics
Three generaGenus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
are unequivocally accepted to belong to the Presbyornithidae:
- PresbyornisPresbyornisPresbyornis is an extinct genus of anseriform bird. It contains two unequivocally accepted species; the well-known P. pervetus and the much lesser-known P. isoni. P. pervetus was approximately the size and shape of a goose, but with longer legs; P. isoni, known from a few bones, was much larger,...
(type) - Headonornis (disputed)
- Telmabates
There is one species generally accepted in Headonornis and Telmabates each. Presbyornis contains 2 or 3 described species. The genus Nautilornis is today considered a synonym of Presbyornis, which is found in a wide range of Late Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
to Early Oligocene deposits in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
. Additionally, most of the bones referred to Headonornis have been found to belong to Presbyornis, and the remaining coracoid may do so too (Dyke, 2001). Telmabates lived in today's Patagonia
Patagonia
Patagonia is a region located in Argentina and Chile, integrating the southernmost section of the Andes mountains to the southwest towards the Pacific ocean and from the east of the cordillera to the valleys it follows south through Colorado River towards Carmen de Patagones in the Atlantic Ocean...
during the Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
.
Apart from these unequivocal presbyornithids, there are some genera which are tentatively assigned to this family pending the discovery of more complete material. As many fossils from the Early Palaeogene show somewhat ambiguous characters, it is not easy to place these early modern birds unequivocally into one lineage or another. That they were ecologically generalized and are usually known from very few fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
remains only serves to worsen this situation.
Possible genera of presbyornithids include:
- TeviornisTeviornisTeviornis is a genus of extinct birds. One species has been described, T. gobiensis. It lived in the Maastrichtian stage at the end of the Late Cretaceous period, some 70 million years ago. It is known from fossils collected from the Nemegt Formation of Gobi, south Mongolia.The fossils include only...
- Proherodius
- ZhylgaiaZhylgaiaZhylgaia is a genus of birds known only from fossils. Its remains have been recovered from a Late Cretaceous or Paleogene deposit in Central Asia....
of which the first is known from Late Cretaceous deposits in Mongolia
Mongolia
Mongolia is a landlocked country in East and Central Asia. It is bordered by Russia to the north and China to the south, east and west. Although Mongolia does not share a border with Kazakhstan, its western-most point is only from Kazakhstan's eastern tip. Ulan Bator, the capital and largest...
, the second latter from the Early Eocene of England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
, and the third is an enigmatic Late Cretaceous or Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
form from Central Asia
Central Asia
Central Asia is a core region of the Asian continent from the Caspian Sea in the west, China in the east, Afghanistan in the south, and Russia in the north...
. There are some other, undescribed, presbyornithid or possible presbyornithid remains, such as the partial right scapula
Scapula
In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....
BMNH
Natural History Museum
The Natural History Museum is one of three large museums on Exhibition Road, South Kensington, London, England . Its main frontage is on Cromwell Road...
PAL 4989, which was considered part of Headonornis hantoniensis, but cannot be positively refererred to a known taxon, or the Early Cretaceous remains from the Mongolian Barun Goyot Formation
Barun Goyot Formation
The Barun Goyot Formation , dating from the Late Cretaceous Period, is located within and is widely represented in the Gobi Desert basin, in the Ömnögovi Province of Mongolia....
at Uday Sayr and the Nemegt Formation
Nemegt Formation
The Nemegt Formation is a geological formation dating from the Late Cretaceous sedimentary from the Gobi Desert of Mongolia. It overlies and sometimes forms folds with the Barun Goyot Formation. It consists of river channel sediments and contains fossils of fish, turtles, crocodilians, birds and a...
of Tsagaan Kushu.
Vegavis
Vegavis
Vegavis is a genus of extinct bird that lived during the Late Cretaceous of Antarctica, some 65 mya. It belonged to the clade Anseriformes...
from the Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
of Antarctica was in all likelihood not a presbyornithid, but rather about equally close to these and the living ducks, geese and swans.
See also
- Galloanserae
- HesperornithesHesperornithesHesperornithes is an extinct and highly specialized clade of Cretaceous toothed birds. Hesperornithine birds, apparently limited to former aquatic habitats in the Northern Hemisphere, include genera such as Hesperornis, Parahesperornis, Baptornis, Enaliornis, and probably Potamornis, all...
for a basalBasal (phylogenetics)In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
("ancient") lineage of early modern birds that contained what were probably the most derived ("advanced") birds ever to live.
