Brontornis

[Christoph Kulmann <kulmann@zfn.uni-bremen.de>]

Dear List Members,

I am making some comparative work on phorusrhacoid birds of South
America - sorry to post this here, but I found no other appropriate list
for it. And these birds were quite dinosaurian, at least secondarily.

Most of these large ground birds have been describes as very fast,
running predators, with laterally compressed beaks ideal for cutting
through their victim's flesh. This would be true for the later members
of this stock, inluding Andalgalornis (from Pliocene Argentina) and the
more fearsome Titanis (Florida, early Pleistocene). They lived in a
savannah-like environment, dominated by grassland, where their superior
speed probably gave them an advantage against the marsupial carnivores
of South America.

But from early Miocene times, there was a bird which would not fit into
this scheme - this is called Brontornis or "thunderbird", and it
differed from the other phorusrhacoids in some important aspects:

1. Though most phorusrhacids were large, Brontornis was a true giant,
although to date no one knows how large it really was. Best estimates
are between 2,40 meters and 3 meters. Long-bones indicate that it was
about one third larger than the two meter tall Paraphysornis.
(see the image at http://www1.uni-bremen.de/~kulmann/paraphysornis.jpg)

2. The Brontornis long-bones do not look to be from a fast-moving animal
(femur 45 cm, tibia 76 cm, tarsometatarsus 42 cm), and the bird may have
been from 237 kg to 320 kg in weight, depending on measurements for the
femur's diameter. General body proportions are assumed to be the same as
in Paraphysornis, except for the beak. Brontornis is always described as
a very strong, corpulent bird.

3. The lower jaw of Brontornis was broader, shorter, and higher than in
any other phorusrhacoid 
(image at http://www1.uni-bremen.de/~kulmann/kiefer.gif - scale bar in
centimeters).
For comparison (the smaller Tolmodus/Phorusrhacus inflatus): 
http://www1.uni-bremen.de/~kulmann/phorus_skull1.jpg
http://www1.uni-bremen.de/~kulmann/phorus_skull2.jpg

4. The bones of Brontornis' toes are rather broad and flat, and the
claws were shorter than in other terror birds.

5. The paleo-environment of Brontornis was not a strict savannah, but a
rather humid, open woodland, enriched by considerable areas of
grassland, inhabited by herbivores of great diversity (different kinds
of grond sloths, some glyptodonts, astrapotheres, as well as smaller,
more agile companions). This all changed towards the end of Miocene
times, when climate became increasingly arid, herbivore diversity
dropped, and all thunderbirds disappeared. Pliocene phorusrhacids were
still gigantic (Onactornis), but seem to be better suited for running.

So you know all about Brontornis than I do - and here comes the reason
for this posting. I am trying to describe the recognizable differences
among the known phorusrhacids, especially in respect to the birds'
lifestyle and possible ecological niche. I have already a detailed idea
for Brontornis, but I would like to know if other people would come to
the same conclusions than I do, or if my own idae is too speculative. 

So I have told you essentially all about Brontornis' morphology, and I
would be glad if someone else could briefly describe how this bird may
have lived, based on the data above. This is just a plausibility test
for my own idea, and for you too it may be interesting  :-)

Sorry to intrude this dinosaur list, but terror birds were not too
different...

Best Regards
Christoph Kulmann