Iberomesornis
Name:
Iberomesornis
(Iberian intermediate bird).
Phonetic: I-beh-roe-may-sor-nis.
Named By: J. L. Sanz & J. F.
Bonaparte - 1992.
Classification: Chordata, Aves,
Ornithothoraces, Enantiornithes, Iberomesornithiformes,
Iberomesornithidae.
Species: I. romerali (type).
Diet: Uncertain.
Size: About 20 centimetres wingspan.
Known locations: Spain, Cuenca Province -
Calizas de La Huerguina Formation.
Time period: Barremian of the Cretaceous.
Fossil representation: Holotype (LH-22) of a
partial post cranial skeleton. Addiitonal remains of a left foot
(LH-8200) added in 1994. Skull unknkown.
The
name Iberomesornis is often translated to English
as ‘Spanish
intermediate bird’, but the first part of the name is actually in
reference to the Iberian Peninsula (also just called Iberia) which
includes not only Spain, but Portugal, Andorra and part of southern
France as well. At the time of writing however, Iberomesornis
fossils are still only known from Spain.
The
intermediate part of the name Iberomesornis is a
reference to how
Iberomesornis are considered to represent an
intermediate stage in bird
evolution before the appearance of, more advanced forms closer to
modern birds. The ancestors of birds are believed to have appeared at
least as far back as the late Jurassic with forms similar to
Archaeopteryx.
Although possessing wings, these even more primitive
forms had long dinosaur like tails, shoulder positions with a more
sideways alignment which limited the range of motion of the wings and
lacked a bony sternum (cartilaginous sternums in some are a
possibility but this cannot be definitely proven at the time of
writing).
The
most obvious difference between more primitive forms like Archaeopteryx
and Iberomesornis is that Iberomesornis
lacked the long tail, instead
having a short pygostyle that was the main support for the tail flight
feathers. The feet were also much better adapted for perching on
branches than earlier forms and the wing claws were also reduced to
just one, indicating an overall switch to perching as opposed to
clinging to trunks. The shoulders were also better aligned to provide
a far greater range of motion than that of more primitive forms,
meaning a much more efficient flight stroke of the wings.
One
area that is still lacking though is the skull which so far has still
not been found. Iberomesornis is usually
recreated with teeth in the
beak, and odds are that this is correct since most birds seem to
have retained teeth all the way to the late Cretaceous and even
beyond. But the lack of knowledge over a specific kind of teeth as
well as a lack of known stomach contents means that it is impossible to
say what Iberomesornis ate. The primitive nature
however, small
size as well as other known prehistoric birds suggests that an
insectivorous diet is probably the most likely. Insects would allow
for a high protein and therefore high energy diet, and most kinds of
advanced animals tend to select an animal protein diet before evolving
and diversifying towards other diets.
At
the time of the initial discovery and description of Iberomesornis
the
broader understanding of Mesozoic birds was still not that clear as
only a few key isolated birds, mostly from the late Cretaceous were
known. For this reason Iberomesornis was placed
outside of the
Ornithothoraces group, the same group that includes modern birds.
Since the 1990s however new fossil beds, particularly some from
the now world famous deposits in China where feather preservation is
common, has helped to fill in many evolutionary gaps, particularly
those of the early Cretaceous at roughly the same time and a little
before and beyond the time of Iberomesornis. As
such a much more
complete picture can be put together which has resulted in
Iberomesornis now being placed within
Ornithothoraces as a primitive
form.
Further reading
- Unusual Early Cretaceous birds from Spain - J. L. Sanz,
J. F. Bonaparte & A. Lacasa - 1988.
- A New Order of Birds (Class Aves) from the Lower Cretaceous of
Spain". In Jonathan J. Becker (ed.) - J. L. Sanz
& J. F. Bonarparte - 1992.
- An isolated bird foot from the Barremian (Lower Cretaceous) of
Las Hoyas (Cuenca, Spain) - J. L. Sanz & A. D.
Buscalioni - 1994.
- Iberomesornis romerali (Ornithothoraces,
Aves) re-evaluated
as an enantiornithine bird - Paul C. Sereno - 2000.
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