Re: Cretaceous taeniodont -- long, combined answer

["David Marjanovic" <david.marjanovic@gmx.at>]

Combined answer, starting with:

----- Original Message -----
From: "John Bois" <jbois@umd5.umd.edu>
Sent: Thursday, April 08, 2004 4:08 PM

> On Wed, 7 Apr 2004, David Marjanovic wrote:
>
> > The question is wrong. First we need to have a _good_ fossil record
> > that can tell us when big mammals _were absent_. Because _this_
> > is what we currently don't know, and what you assume _a priori_
> > in order to pose your question.
>
> Is there a fossil bias against finding large mammals?  I'm assuming not.

There is
1. a fossil bias against finding _anything_. That's a BIG bias.
2. a fossil bias against finding anything terrestrial. I know of 2 (two)
places in the _world_ that record the start of the Cretaceous on land, for
example -- Purbeck and Cherves-de-Cognac.
3. a fossil bias against finding anything terrestrial smaller than a
sauropod femur. Isn't all that big, but...
4. ... there's a perception bias against finding anything terrestrial
smaller than an *Allosaurus* skull.
5. There's also disarticulation. Very, very few Mesozoic mammal sites have
yielded mammal skeletons. The normal thing to find is "the tooth, the whole
tooth, and nothing but the tooth". You know the size of your molars. There
are sites where a tooth of half that size would shine dark brown-black (like
Cherves) or white (like the Gobi), but in many places such a thing is quite
inconspicuous.

In other words: We are _not_ going to be able to make statistically
meaningful statements about the abundance of big mammals over time.

> Is there a reason why Cretaceous large mammals are not found but other
> periods are?  I'm assuming not.

See above. End-Jurassic terrestrial fossils are relatively common,
start-Cretaceous ones are exceedingly rare... fossils are not equally but
_randomly_ distributed between the strata.

> We haven't found any Martians in the fossil record either.

Er... and so what?

> > Triconodonts and deltatheridians had been carnivores all the time.
> > Grain? Only the multituberculates.
>
> OK.  Let's be more specific.  There was apparently significant size and
> niche development in placentals in NA at this time.

Well, before that time, there were AFAIK almost no eutherians in NA. :-)

> The predator-relaxation hypothesis is at least
> supported by parallel size increases in multis and marsupials,
> though the latter two groups have isolated large species earlier.

We do not know if we have a parallel size increase, a parallel size
_decrease_, or _any_ trend _at all_ here. The sample is _too small_.

> I am arguing for the following scenario: predators of small mammals
> may have been excluded from some habitats by predation on
> themselves. _If_ bird species and pterosaurs were reduced by
> predatory birds--and I believe the predation hypothesesis explains
> at least pterosaur dcline better than competition--it is feasible that
> they could also threaten or make life miserable for terrestrial
> (or arboreal?) dinosaurs.

Predatory birds? Yes, there is evidence for a clade of predatory birds in
the Mesozoic. That clade is called Avisauridae, and it starts (at least)
with *Cuspirostrisornis* in the middle Early Cretaceous -- you would need it
_far_ later.

Pterosaur decline? First show me that there _was_ a pterosaur decline. The
fossil record of LK pterosaurs is simply too bad to judge _this_ at the
moment. In other words: There may not be anything for you to explain!

> > A _Microraptor_ or _Sinosauropteryx_ might go for a
> > shrew-sized mammal in a heartbeat, but would baulk at
> > pouncing on a mammal the size of a badger or wolverine.
> > Of course, the theropods could get bigger too, and so on...
>
> If size gave immunity from predation, why didn't this fuel an arms
> race as it seems to have done in dinosaurs.

Because there was a big heavy lid on this arms race. A too large mammal
would have had to evolve into an occupied niche. There may or may not have
been such an arms race, but it was finite if it was at all.

> But who says the only advantage to size is protection against being eaten.
> For example, if by increasing size this allows you to expand your diet
> (e.g., bigger insects and/or small mammals and lizards),

Not to expand it. To shift it. Play
http://dsc.discovery.com/cgi-bin/bigal/bigal.pl to find out why. =8-)

> you can sustain more predation because
> your reproductive success is better.

Your reproductive rate is _worse_ when you're bigger.

> Apart from the whole asteroid thing, evidence in support of this is:
> - decrease in pterosaur diversity

May not have taken place.

> (prime suspects being birds)

For you. :-)

> - eradication of old bird clade

At the K-T boundary, it seems... Enantiornithes and Ichthyornithidae, at
least, survived to the end.

> by new birds

Why?

> (unless one accepts
> Feduccia's lucky survivor of shorebirds theory)

Brace yourself, but here I agree with Feduccia. Except on the details. These
are that he has too few neornithine clades surviving -- Galliformes and
Gaviidae come to mind as being present in the LK and not being shorebirds.

> - decrease in dino diversity before the K/T

May not have taken place.

> - increase in mammal size just before the K/T (if this is demonstrated)

If. :-)

> - susceptibility of large egg layers to small racoon-size
> mammalian predators in extant communities

Susceptibility? They don't die out from this.

> - susceptibility of young of large egg layers to bird predation.

Ditto.