Presbyornithidae
Conservation status: Fossil
Fossil range:
Late Cretaceous - Early Oligocene |
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Scientific classification |
Kingdom: |
Animalia
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Phylum: |
Chordata
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Class: |
Aves
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Order: |
Anseriformes
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Family: |
Presbyornithidae
Wetmore, 1926 |
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Genera |
see text |
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 flamingos
and were used to argue for an evolutionary relationship
between these groups, 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 1 meter high, with the body of a duck, feet similar
to a
wader but webbed, and a flat ducklike 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 insects and small
crustaceans on dry land, just like some species of modern
ducks, e.g. the Laysan Duck, hunt for brine flies.
Significance in avian evolution
The implication of the plethora of this and other,
ecologically similar Neornithes (e.g. the wastebin taxon "Graculavidae")
from the Late Cretaceous 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 dinosaurs, 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
- 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 by specialist forms,
both related and unrelated, that one by one adapt to
part 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 birds, 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
Four
genera are unequivocally accepted to belong to the
Presbyornithidae:
- Presbyornis (type)
Headonornis (disputed)
Telmabates
Vegavis
There is one species generally accepted in Headonornis,
Vegavis and Telmabates each. Presbyornis
contains 2 or 3 described species. Vegavis is known
from the
Late Cretaceous of Antarctica, whereas Telmabates lived in
today's Patagonia during the Eocene. 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).
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 former is known from Late Cretaceous
deposits in
Mongolia and the latter from the Early Eocene of England.
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.
References
- Dyke, Gareth J. (2001): The Fossil Waterfowl
(Aves: Anseriformes) from the Eocene of England.
American Museum Novitates 3354: 1-15.
PDF fulltext
- Wetmore, Alexander (1926): Fossil birds from
the Green River Deposits of Easter Utah. Annals of
the Carnegie Museum 16: 391-402.
External links