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VERY DIVERSE ENANTIORNITHINES
Little thread going about enantiornithine birds. Nick Pharris, our
favourist linguist, wrote...
> Careful. There was a time not long ago when almost any Mesozoic
> bird might be referred to as an "enantiornithine". More recently,
> the term has been restricted to a single, natural group of early
> birds, a branch of the Ornithurae ["bird tails", since in members of
> this group, which includes modern birds, the tail vertebrae are
> fused into a stump].
Whoops Nick, enantiornithines are emphatically not ornithurines and
never have been.
I'm not about to mail in a big listing of all enantiornithine taxa -
others on the list could do a more competent job of that than I, but
I would like to say a few things about our changing perception of
what an enantiornithine IS.
When Cyril Walker first created 'subclass' Enantiornithes in 1981
(Cyril is one of those vehement anti-cladist types), he did not
create a hierarchical subdivision of the group, and only just about
made the taxon valid by including one species (_Enantiornis leali_)
as a caption in a table. Though this procedure allowed following
workers to then regard Enantiornithes as valid (i.e. because it had
at least one constituent taxon (arguments about monotypic higher
taxa being redundant are not universal (especially in ornithology)
and have only come in very recently anyway)), he did get a lot of
criticism from other palaeornithologists (see Olson 1985).
Conversely, he also got loads of credit as subsequent work has shown
how insightful he was in recognising a major new avian clade based on
only a few bits and pieces. Though Cyril and his long-time colleague
Colin Harrison have been getting a lot of stick recently (because
many of their proposed identifications are dubious or
non-supportable), it should always be recognised how tremendously
competent they were at recongising and identifying avian material,
often of very scrappy nature. Hats off to them.
A trend in the 1980s was to regard all or nearly all Cretaceous birds
as enantiornithines. This is carried to its extreme by Feduccia and
co-workers who have proposed a model whereby, though 'modern type'
(ornithurine) birds supposedly originated at the same time as
enantiornithines, it was the latter than dominated Mesozoic avian
diversity, supposedly thriving at massive diversity right up
to the Maastrichtian (see Feduccia 1994, 1995). _Confusiornis_ was
regarded as part of this assemblage (or very close to it) and
Feduccia (1996) even lists _Patagopteryx_ as worthy of an 'order'
within the Enantiornithes.
Working with empirical, rather than intuitive, data, other
palaeornithologists agree that Enantiornithes were a monophyletic
radiation of birds that represent the sister-group to ornithurines
(_Patagopteryx_, Hesperornithiformes, Ichthyornithiformes,
Neornithes) - this clade has been called Pygostylia. Pygostylia
+ _Iberomesornis_ is the clade Ornithothoraces. Confuciusornithids
are the sister-group to the Ornithothoraces (Ji et al. 1999).
The most important point I want to make here is the diversity of the
enantiornithines, much of which goes generally unappreciated.
Work by Chiappe and others has shown that avisaurid enantiornithines
(_Avisaurus_, _Neuquenornis_ etc.) have anisodactyl feet with
moderately long tarsomets, sharp, recurved pedal claws, and a large,
fully reflexed hallux. They look to have been good perchers and were
probably arboreal/scansorial. Meanwhile, Walker was the first to
propose that some enantiornithines - such as the_Enantiornis_ in his
original assemblage - had such short tarsometatarsi that they
appeared to be most like penguins, and perhaps may have been aquatic
and with limited or absent flight abilities. Larry Martin elaborated
on this idea in his big review of enantiornithines, though if I
remember correctly he did not reconstruct _Enantiornis_ as
flightless. _Yungavolucris_, a Chiappe taxon based on an unusual,
strongly asymmetrical tarsomet, has also been proposed as aquatic,
and as a foot-propelled diver. A diving enantiornithine! Why has
no-one other than Chiappe drawn attention to this? I find it amazing.
Chiappe further suggested that _Lectavis_, another tarsomet-based
taxon, was aquatic. In this genus, the tarsomet is incredibly
elongate and gracile, and would suit a wader (though I wonder if
tarsomet structure distinguishes long-legged cursors from waders
Bottom line here, then, is that enantiornithines may have been very
diverse, at least ecologically. They may have dwelt in trees, like
modern songbirds and rollers, may have waded in shallow waters, like
modern shorebirds, and may have dived and swam under water, like auks
and penguins. It has further been suggested that the enantiornithine
_Boluochia_ may have been a hawk-like predator (though the evidence
for this is open to other interpretations).
"...a theropod origin of birds is clearly inextricably coupled with
the biophysically difficult cursorial theory"