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

Re: [Re: Part 2: Terramegathermy (becoming longer again...)]



> There could always have been a counter-current heat exchange system     > in
long, thin tails (thick ones are not so much of a problem IMHO),     > like in
bird feet, no? (If it was necessary.)

++++++++++++++++++++++++++

Perhaps; though it does make me wonder why such a thing never seemed to have
evolved in weasels, but then, mammals always seemed to do things differently.

_________________________


> Well, birds _normally_ have nearly double resting metabolic rates of
> placental mammals. I have found numbers at the library in a book on
> marsupial biology, have yet to copy them.

+++++++++++++++++++++++++

Birds yes, but dinosaurs (please no cladistic pedantry >:)?

Again, early bird bone studies (for what good they are) seem to indicate
slower rates of growth than modern birds. It might not have been
bradymetaboly, but it didn't seem to be hyper-avian metaboly either.

__________________________


> > Ah, but varanids are highly active and still retain their sprawling
> > stance.

> Because they have evolved the famous gular pump that allows them to     >
breathe during running. Crocs have the famous hepatic-piston pump for   >
this, and ornithodirans apparently had neither, instead they evolved    >
erect stance.

+++++++++++++++++++++++++++++++

But an erect stance *doesn't* free one from this carrier's constraint. At
least one species of crocodylian today (_C.niloticus_) has an erect stance
(pers. observ) and yet this animal still walks in the same serpentine motion
seen in other reptiles. It is this motion that limits aerobic performance due
to alternate compression of the lungs with each move (and that the same trunk
muscles that power breathing are used in walking). Crocs bypassed this by
evolving their hepatic piston pump (and aerobic lizards did with their gular
pump, or by running bipedally; turtles mostly did with the stiffening of the
body cavity.), but if the crocs didn't have it, then regardless of their erect
stance, they would still be unable to move and breathe at the same time.
Something else must have been in place other than stance to allow for
dinosaurs to move and breathe at the same time.

_Protosuchus_ has a pelvic girdle that would indicate no diaphragm present (or
a very simple one) and they had an erect (at least semi-erect) stance. In fact
the development of more erect stances is a hallmark of all archosaurs. Perhaps
it has less to do with being big or moving easier, but more to do with trends
towards bipedality.

Of course, we have lepidosaurs that are bipedal and aren't erect, yet we also
have the erect and bipedal _Eudibamis_. The more I think about it, the more
confusing it becomes.

As for ornithodirans without these pumps, do we not have slight evidence of
diaphragms in ornithischians. And could it not be archaic to all dinosaurs; as
_Scipionyx_ *might* (note I did say ->might<- ) indicate. 

_______________________________


> A nonavian small archosaur? There is an unnamed coelophysid from Nova >
Scotia as big as a blackbird (20 cm long)... and there are always >
juveniles!

+++++++++++++++++++++++++++

True enough; I really don't know of any small (i.e. average lizard size)
archosaur barring the above mentioned example. I mean, _Longisquama_ might
count *if* it is an archosaur, but that's all I can think of.

____________________

> > Even small crocs (which may or may not stand erectly,
> > not that it matters much since crocs descended from erect             > >
ancestors)

> Depends on whether or not Sphenosuchia is paraphyletic.

+++++++++++++++++++++++++++

Does it?

We have numerous examples of erect crocodylomorphs all prior to modern
eusuchia (lots of those fun rauisuchians come to mind :). We also have erect
examples within eusuchia (the pristichampsines for one) and studies on modern
croc stance that indicates the sprawl is a derived one and not the norm for
ancient crocs (Reiley & Elias 1998).

______________________________

> > aren't all that small.

> Croc hatchlings?

++++++++++++++++++++++++++

True, but then we are talking about erect stance evolving in animals that grew
to larger sizes. That hatchlings are small wouldn't matter much if they grew
to be very large. I suppose, for the sake of argument, it might be interesting
to note that hatchling and young crocodylians all seem to have a far more
"semi-erect" if not inveterate sprawl to them than adults do.

___________________________

> > and how cold were these polar
> > nights?

> Around 0 °C AFAIK. Months of darkness don't allow for much more.

++++++++++++++++++++++++++++

I suppose they wouldn't; what about other flora and fauna?

I need to read more paleoecology papers :)

____________________________

> > The forearms [legs] in all large theropods
> > wouldn't allow for this type of quick turning, which would slow      > >
them down.

> Well... Are recent ratites capable of quick turning? And were          >
phorusracids (many of which were highly cursorial)?

++++++++++++++++++++++++++++

Perhaps; I've seen emus and they are capable of some quick turns. Of course
birds don't have long tails that counterbalance their bodies, nor do they move
from the hips like theropods. As such, they may not be the best examples for
these scenarios.

_______________________________

> That's news to me... Quite widely accepted, AFAIK, is today that        >
gracile *Australopithecus* and early *Homo*/*Kenyanthropus* did a lot   > of
scavenging (bone marrow is good for the brain!), running after the  > vultures
to get something before the lions and hyenas came...
> Slow tyrannosaurs? Someone has calculated offlist (last December
> IIRC) that *T. rex*, when walking with 2 steps per second, already     >
reached 36 km/h...

+++++++++++++++++++++++++++++

One of the few things about humanity that I *am* proud to boast about is that
we can outsweat any other animal on the planet (see the Natural History
Endpaper, April 2001). The Tarahumara tribe of Northern Mexico and the
Kalahari's Basarwa both captured ungulate prey (deer, zebra and gemsbok
respectively) by taking turns chasing them down until the animals died of heat
prostration. If _T.rex_ could keep a continuous pursuit, even if it was much
slower than that of its prey, it could kill in the same manner; as long as it
could stay cooler.

Anyway, it was just a thought.

_____________________________


Well, a tiger is 3 times as heavy as an ora, but requires 19 times as much
food, and 1/3 of a tiger, as heavy as an ora, still requires 5 times as much
food and not the same amount. Thus, the predator-prey ratio is quite
different, the tiger falling among endotherms and the ora among ectotherms.
Right?

++++++++++++++++++++++++++

Not with varanids; not easily that is. Auffenberg's calculations for the
amount of oras to the amount of ungulate prey (which differed from island to
island, but I do believe was averaged) shows that they had numbers that fell
in just behind lions (ora predator/ungulate prey ratio is 1:100, lion is
1:165) and with a difference of only 5 between an ora and a tiger food wise,
varanids would probably fall more inbetween the two (or possibly skew more
towards the endotherms).

_______________________________

> > Only if you are implying that dinosaurs had metabolism *higher*      > >
than mammals?

> 1. No, because there is still quite a considerable difference between   >
damned good reptiles and mammals.

+++++++++++++++++++++++++++++++++

All things considered the difference between the two is rather minimal at
best; enough to make it very hard (if even possible) to decipher among fossil
animals.

______________________________

> 2. Why not? Birds have much higher resting metabolic rates than         >
placental mammals on the average...

++++++++++++++++++++++++++++

Yes, and birds fly a lot as well. Since flying consumes more energy than just
being a big land lubber, the higher resting MRs aren't that surprising.

Anyway, the general consensus I seem to get among pro-L.C. endotherm
paleontologists today is that dinosaurs had higher MRs than reptiles, but were
still lower than or equal to mammals. I don't know of very many that believed
that they had avian metabolisms (which would seem extremely wasteful for such
large animals IMHO).

Jura

Jurassosaurus's Reptipage: A page devoted to the study of and education on,
the reptilia:

http://reptilis.net

____________________________________________________________________
Get free email and a permanent address at http://www.netaddress.com/?N=1