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Re: attack on dinosaur--horrific video



----- Original Message ----- From: "David Marjanovic" <david.marjanovic@gmx.at>
To: "DML" <dinosaur@usc.edu>
Sent: Sunday, November 25, 2007 8:47 AM
Subject: Re: attack on dinosaur--horrific video


I can't imagine the size difference between *Daspletosaurus* and *Tyrannosaurus* can be explained by selection for nest defense.

Hadrosaurs gang up on D...T is selected...classic arms race.

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!

Close relatives of the clades that adaptively radiated after the boundary increased in size before the boundary--their descendants have an observable major influence on the community structure of oviparous species via predation--again, not as a hypothesis for extinction at the K/T...but an influence, perhaps. And, yes...perhaps not.


You have tried to get around the third point below:

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

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.

Look at the forest for a minute: Continental placental predators and herbivores--and whatever else--are invading/have invaded global terrestrial niches. Rabbits, cats, rats, mice, dogs, pigs, goats--these animals are the subject of thousands of papers dealing mainly with how to ameliorate their influence. Yes...there are other invasive animals, e.g., _Boiga boiga_, zebra muscles, Burmese python, etc. but their effects are somewhat local, not global...and not clade-wide. Placentals enjoy a great diversity of morphologies--probably a greater diversity than is available to, for example, marsupials. It is a fact--ref. if needed--that marsupial embryos rush to develop craniofacial muscles and bone in order to be able to feed early...it is a fact that placentals spend this early phase developing neurosensory ability, i.e., laying down neurons. Thus marsupials start late and are therefore at an ontological disadvantage in this respect. Also, limb morphology diversity is far less in marsupials because the front legs are already chaneled into being legs, not wings, not fins, not arms. Point is, ontogenic imperatives are a limit on morhological diversity in marsupials. This is not going to go away for them.


...reliance on nest defense does nowhere near guarantee extinction.

Why has it not re-evolved in your view? I mean, crocs are it, right? So, what is the probablity that nest defence strategy in terrestrial biomes has been eliminated by chance alone? If it's a viable strategy, how come no one outside of crocs employs it?


They (crocs) 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?

It's dark outside...at night.

many would-be predators don't like getting wet

The eggs are not laid in water.

They are laid in swamps--many nests are accessible only by getting one's self wet.


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.

Yes...in fact, predators have to enter the crocs homefield...the one place where they are the only game in town.


predatory threat can only come
from 180 degrees not 360;

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

But a croc only has to scan 180 degrees--predators are not going to be accessing the nest from the water. And to do this a croc can lie as still as a log waiting to ambush. This is an exceptionally effective strategy whether animals are merely trying to get a drink of water or robbing a nest.


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.

Croc eggs are smaller relative to parent than dinosaurs of the same size, right?


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.

Remembering that parental investment includes: yolk mass, incubation time, quality of incubation--e.g., brooding, post-hatchling care, and not just clutch-size.


I still don't understand why you bother in the first place...Why bother to work against the most parsimonious hypothesis as long as it isn't falsified?

First, I am primarily interested in the ecological considerations of various strategies--regardless of their mass extinction impact...this interest doesn't go away and has fueled, I think, some interesting hypotheses: e.g., dinosaurs were big because they had to defend nests.


Second, I think extinctions are always very complex events.

Third--the only thing that is _falsifiable_ about your explanation, is the event itself. The event happened. But any specific cause of death for any clade, population, or individual is unfalsifiable. Also, complex synergy remains equally parsimonious.