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Re: attack on dinosaur--horrific video



Catching up with the backlog...

----- Original Message -----
From: "don ohmes" <d_ohmes@yahoo.com>
Sent: Thursday, November 22, 2007 4:17 PM

"Crocodiles nest on unprotected riverbanks, with themselves as the only
protection. And indeed, their nests often are raided in the middle of
the day. How is it possible that they are still with us?" -- David Marjanovic


Not exaactly who is on which side here, and the above may be a rhetorical question. But anyhow --

Yes, it is rhetorical.

In the case of long-lived but fairly prolific animals it is not necessary to have a successful nesting season every year or possibly even every decade. Even where the tactical vulnerability of the prey to predation is very high (such as when dinos/crocs in egg form are the prey), annual boom/bust cycles in short-lived predator populations, perhaps driven by independent factors, can create windows of reproductive opportunity.

Or there simply may not be enough predators to eat all the prey, even in boom years, again due to factors that may be independent of the prey/predator relationship (the predator may have pathogen-driven reproductive issues of it's own, for instance). This would apply to long-lived predators.

I agree -- and all of these apply to dinosaurs as well.

Lastly, a question -- does predation really appear in the record to be a primary extinctor of taxa? It sometimes happens of course, transgression of island refugia by invasive species being a prime example, but isn't it usually more of a 'coup-de-grace' type of thing, in populations reduced by environmental change or niche competition?

I cannot think of a single instance in the fossil record where the predation hypothesis is both testable and unfalsified.


----- Original Message -----
From: "john bois" <jbois@verizon.net>
Sent: Thursday, November 22, 2007 4:41 PM

----- Original Message ----- From: "David Marjanovic" <david.marjanovic@gmx.at>
Sent: Thursday, November 22, 2007 8:02 AM


I repeat: By your logic the two species of *Repenomamus*, not to mention
*Gobiconodon*, triggered a mass extinction among egg-layers. How is it
possible that they didn't?

I am not arguing against the overwhelming evidence for environmental effects
at the boundary. However, though predation on nests and hatchlings has
always been present, the intensity of that predation has varied over time.
I am strongly arguing that traits of predatory species affect this
intensity; that placental mammals since their appearance have had a large
influence on the structure of communities through predation on unprotected
or unconcealed nests! And, finally, the co-occurence of placentals with the
extinction of large egg layers, the relaxation of size contraints on
placentals at that time, and their tyrannical ecological influence since
then--all these things keep the window open for suggestion of an additional
influence at the boundary. Yet, despite the controversial nature of that
claim, I am far more interested in the ecological inferences that may be
approached when comparing today's ecosystems with those of the Mesozoic.

Then let me summarize once again.

1. Not that it matters much, but _placentals_ remain unknown from the K.
2. We have fairly large eutherians in the Maastrichtian of North America. Good. But we have just as large metatherians in the Campanian of North America (and isn't there a "stagodontid" skull from Mongolia as well?). You don't have any evidence to argue for a relaxation of size constraints!
3. In the late Early Cretaceous, there were just as large gobiconodontids in Asia and North America. Yet, there was no mass extinction among egg layers. In the Yixian Fm, there are even larger gobiconodontids preserved, one of them with baby *Psittacosaurus* as stomach contents -- the only direct evidence for nest predation from the entire Mesozoic.


You have tried to get around the third point below:

If the answer lies somewhere in "very different creatures", then please
elaborate.

I hate to use such fait accomli arguments such as: placentals are the
dominant form of mammal; their characteristics may have had something to do
with this.

Or not.

In the "Tertiary" of South America, there was a nice partition: the herbivores were placentals (...or eutherians anyway... who knows), the grapple-and-bite predators were sebecid crocodiles, the grapple-and-slash predators were metatherians, and the pursuit-and-bite predators were birds. This situation remained stable till 3.3 Ma ago, when an impact happened just off the coast of Argentina and the metatherian predators died out, 300,000 years before they were replaced by placentals. (The sebecids are so rare that we can't tell when they died out.) There was a carnivorous armadillo. Why didn't it stop digging and take over? Or why didn't the monkeys take over in the Oligocene (the oldest known New World monkey was rather terrestrial)?

Sure, we have the dingo and the Tassie tiger as a probable case of extinction by competition, but here we're talking about the skull base, not about the reproductive system.

Ask any mother how she would feel about having to choose between
leaving baby home alone or never leaving home--then you have the choices
available to egg laying mammals (presumably *Repenomamus*);

Be careful with "presumably". Apparently, it was inside the crown-group, closer to us than to the monotremes, so it did not necessarily lay eggs. The marsupial mode -- perhaps minus the elaborate pouch -- is probably older than the placental one; we don't have any evidence on when egg-laying was given up. OK, it has been suggested that, because of their small pelvic outlets, the multituberculates must have reproduced the marsupial way; but the multis are probably closer to us than the triconodonts are. We have two possibilities for the way the triconodonts reproduced, and both are equally parsimonious.


then ask another mother how she would feel if her baby were
to be denied the extra time in utero for brain development

Who says brain development can't happen in a pouch?

Indeed, placentalism has its advantages: two that may relate to
current discussion: mothers and fathers can both forage (because neither has
to stay at the nest)

Same for marsupialism.

Crocodiles nest on unprotected riverbanks, with themselves as the only
protection.

Tres formidable, non? Like saying a tiger is no threat except for its teeth and claws.

That's my point, because it applies to most dinosaurs, too: reliance on nest defense does nowhere near guarantee extinction.


And indeed, their nests often are raided in the middle of the
day.

Yes...when the parent is away.

And even this does nowhere near guarantee extinction.

They are the exception...because they are exceptional: semi-aquatic niche
gives home-field advantage; they see at night (are practically nocturnal!!);

Are you sure seeing at night isn't normal?

many would-be predators don't like getting wet

The eggs are not laid in water.

or eaten (e.g., bird species
use tree above alligator nesting sites for protection against racoons);

That's my point again.

silt river banks make poor burrows for predators;

Predators are not required to have their own nests very close.

predatory threat can only come
from 180 degrees not 360;

No, croc nests are not directly next to the water.

each of many hatchlings represent less reproductive effort than
dinosaur hatchlings (and are therefore more expendable).

You are making this up. I submit that each croc hatchling represents _more_ of an effort than just about any sauropod hatchling.


I view archosaur success as dependent on parental investment,
and that an interesting hypothesis is that this PI increased from crocs to
dinosaurs with its zenith in birds--but that relative to most birds, croc
babies are low maintenance.  This allows excess production of offspring to
offset intense predation.

Yes, and this does not only apply to crocodiles. All known Mesozoic dinosaur nests are very large.


I still don't understand why you bother in the first place. We don't need an explanation for the extinction of "the dinosaurs" because we already have one for the _whole_ mass extinction, forams, ammonites, belemnites, mosasaurs, dinosaurs, pterosaurs, polyglyphanodontids, bennettites, everything! Why bothering to work against the most parsimonious hypothesis as long as it isn't falsified?