[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: On rearing in sauropods



>> When asked about the blood pressure problem, the author
>> said that some kind of simple adaptation must have been
>> present anyway just to let the animal lower the head to
>> the ground and back to a horizontal neck posture.

To clarify: my answer at the conference what not very good, as I forgot to mention the obvious:

The whole problem of blood pressure, as detailed succinctly by Seymour (see http://www.sciencemag.org/cgi/content/full/323/5922/1671), only exists if blood flow to the brain is not separated from that going to the rest of the body. Much as the pulmonary system has a different blood pressure than the rest of the body in all(!) animals with parasagittal limbs, a similar pressure separation could allow extremely high pressures in the neck, on a limited blood volume. After all, how much blood do you NEED for a 1/4 pound brain?

I had a very nice chat with Roger Seymour in the course of the sauropod meeting in Bonn, and we agreed that
- a system as detailed below is feasible,
- could solve all problems,
- and sadly would be completely restricted to soft tissues. Thus untraceable in fossils. Sorry!


Here's what I imagine as a required preadaptation: Basal dinosaurs, and some of their archosaur ancestors, had fairly long necks. Cause, mainly, by the parasagittal limb posture in combination with a short skull, because the only way to stay flight-ready when drinking/feeding close to the ground is have a long neck. This neck has additional benefits, such as the ability to simply yank it out of the way of a predator's bite rather quickly. This, however, means a rough doubling of the height: bottom -> shoulder level plus shoulder level -> maximum height. Even for an animal with a 1.5m shoulder height, this evasive maneuver already results in a 3 m lift, and any adaptations such as rudimentary non-return valves formed by a simple wall duplication in the carotids would be beneficial.
Once these vales exist, the blood pressure in the carotids can be increased massively by simply creating a widening at their base, whcih gets filled with blood at normal systemic pressure (systolic phase), and using contractions of the muscular walls in phase with the diastole to pump blood up into the narrow parts, to the brain. A sauropod neck leaves ample room for such 'second heart' cavities, and arterial walls are muscular anyways.


Evidence? None, sadly!

So the question remains why we do not see anything similar in extant animals - but do we? AFAIK nobody has ever been able to measure blood pressure in giraffe necks during rapid up and down motions simultaneously with carotid diameter and blood flow velocity. I would not be surprised if they helped their blood along up the neck with peristaltic contractions of the carotid walls while they lift their necks up.

I guess we will never know how sauropods did it, but motion analysis based on articulation and manual manipulation of specimens indicates that most sauropods could lift their necks up high (Christian & Dzemski 2007). Stevens & Parrish (e.g. 1999) disagree, but I both distrust some of their assumptions (sorry, Kent!), and my imagination breaks down when I try to imagine a diplodocid attacked by theropods. The neck is a wonderful target, and I imagine the poor beast able to lift the head up high to avoid the attack, but fainting ASAP due to blood pressure problems and falling to the ground before the astonished hunters as 'meals on wheels'.


>> Outside of his talk, he also talked a lot about the rest >> of his recently finished PhD thesis. We can look forward >> to a lot of very interesting and very, very, very, very >> well argued papers.

Well thanks, David, for getting people's hopes high - it is now my turn to disappoint them ;)


lit cited:
Chrisitan, A. & Dzemski, G.** (2007): Reconstruction of the cervical skeleton
posture of *Brachiosaurus brancai* Janensch, 1914 by an analysis of the intervertebral
stress along the neck and a comparison with the results of different approaches - Fossil Record 10(1): 37-48.


Stevens, K.A. & Parrish, J.M. (1999): Neck posture and feeding habits of two Jurassic
sauropod dinosaurs. Science 284:798â800



btw, the meeting abstracts are available here: http://www.naturalsciences.be/science/colloquia/vertebrateevolution

direct link to PDF:
http://www.naturalsciences.be/common/pdf/science/colloquia/Darwin_Bernissart_abstracts_guidebook.pdf


--

Dr. Heinrich Mallison
Museum fÃr Naturkunde - Leibniz-Institut fÃr Evolutions- und
BiodiversitÃtsforschung an der Humboldt-UniversitÃt zu Berlin
Invalidenstrasse 43
10115 Berlin


Tel: +49(0)30-2093-8764 Email: heinrich.mallison@mfn-berlin.de