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

Re: Nocturnal crocs?



 Wow, this is my 3rd post for the day.  I don't think I have ever had more than two posts in a month.

 John Bois <jbois@umd5.umd.edu> wrote:





> > My mechanism
> > is not a "holocaust" at all; it is a slow, steady, abiding selecetion,
>
> ...beginning in the EK with the appearance of cat-sized mammals? Such a
> gradual extinction is a lot too much for me.

I'm not saying that. I am saying that mammals were tiny for most of the
Cretaceous--with some exceptions--and then there was an increase in mammal
size just before dinos became extinct. That is either true or
not. Lillegraven thought it was true. Even John Alroy's data show an
increase--though more modest, of course than post dino times--in mammals
size immediately pre K/T. My point is that the threshold for nest and
hatchling predation is fairly low. This is because dinosaurs had the
smallest offspring relative to parent size.

First, is larger size even indicative of greater nest robbery.  I have already stated that I doubt mammals had a sudden size boom.  They could very well have increased because of other reasons; new sources of food due to the angiosperm revolution, or perhaps they outcompeted the small land-living crocs of the Jurassic and were able to evolve larger predators.  Maybe the mammals just developed some new trait or adaption that allowed them to exploit various niches better?

> The present is not the key to the past. The past is the key to the present,
> and to the future.*


> It is well
> possible that all birds died out everywhere else and a few managed to
> survive in Antarctica.

Hardly. For such an unlikely scenario to be true, so must the
following: neos outcompeted enantis nowhere else but
Antarctica. Conditions were so different in A. that the competitive
advantage gained there did not apply anywhere e! ! ! lse. We are then at the
same old bugaboo: what was the critical adaptation that brought this very
unlikely thing to happen--some magic niche bullet? What? We are left
scratching our heads. OK, if it were some ground-bound beast we could
understand--isolation would be enough. But birds fly--there is no reason
why such competitive advantages shouldn't be shipped across the
planet. You need some reason for selection. Why only neos?

Thier is some reason to believe that neorniths had a southern origin.  If a few neorniths had the fortune of nesting at the southern pole, they may have pulled out of the extinction marginally better.  Today, many flying birds groups don't have global ranges.  For instance, thier are various familys that are confined to south America, or to Australia

> > Phorusrhacids evolved amid grasses.
>
> Stop -- wrong. Phorusracoids evolved in the Paleocene, means, before
> grasslands. They are known from IIRC Eocene France (*Ameghinornis*) and
> middle Eocene Messel (an AFAIK unnamed skull).

The biggest Phorusrhacids in grass--this is from Marshall. What size are
the sp. you're talking about?

> > Diatrymids may have depended upon
> > wetlands,
>
> How so? Apart from looking more like phorusracoids and tyrannosauroids than
> like, erm, flamingos, they had a pan-Laurasian distributio! ! ! n (*Gastornis* =
> "*Diatryma*" in NA and Europe, *Zhongyuanus* in China).

Gastornithids existed while NA and Europe were covered with
wetlands. When each continent drained, they disappeared--thopugh the
draining occurred at different times on each continent.

How about the Dromornithids?  they were all forest species IIRC

> > a mammalian refuge
>
> ???

In modern swamps, predator access is increased when water level is
lowered. Predator density is
greater in uplands surrounding wetlands than in the swamps themselves.
(Can cite both of these).

> > which was in short supply at the terminal Cretaceous.
>
> when the regression turned the Western Interior Seaway into one huge...
> wetland. Please correct me if I'm wrong.

Archibald (1996) says that they lost alot of their prime habitat--I
assumed...

> > > But I would argue that their wasn't really any "size relaxation" in the
> > > K/T. Merely a better represented fossil record.
> >
> > Then we have competing hypotheses. But I have positive evidence, you,
> > only negative.
>
> Your positive evidence is, see above, 2:1:1, maybe 3:1! ! ! :1. I won't call that
> "evidence".

No. It _is_ evidence. Not of the quality you like perhaps. And not
convincing, true. But it is supportive.

> > > And Ostriches aren't anyware near as predatory as some of the
> > >dinosaurs. If a modern day jackal tried to drive a dromaeosaur off a
> > >nest, it would be merely dinner.
> >
> > Only if it could _see_ the jackal.
>
> 1. Any reason to suppose the contrary?

Yes. See earlier citation--or ask me for it.

> 2. We're not talking of jackal-sized mammals, but of cat-sized ones.

I would argue it is not the aggressive abilities of the predator, but its
ability to crack the egg. Gape is irrelevant to this problem as
illustrated by hairy armadillo on rhea eggs--knocks them together to
account for high predation rate.

Ability to crack an egg is irrelevant, if it can't get the egg away from a watchful mother.



Do You Yahoo!?
Make international calls for as low as $0.04/minute with Yahoo! Messenger.