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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: Thursday, November 22, 2007 8:02 AM
Subject: Re: attack on dinosaur--horrific video



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.

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. 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*); then ask another mother how she would feel if her baby were to be denied the extra time in utero for brain development and that the brief time spent inside her body had to be spent developing not sensory or info-processing ability, but rather arm strength and sucking power so that the little tyke could crawl out as an embryo and attach itself to her nipple for 9 months. 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)--this means twice the number of predators on your nest; greater brain size/processing power (see above for how this is possible) gives greater flexibility in decision making.


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.

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

Yes...when the parent is away.

How is it possible that they are still with us?

They are the exception...because they are exceptional: semi-aquatic niche gives home-field advantage; they see at night (are practically nocturnal!!); many would-be predators don't like getting wet or eaten (e.g., bird species use tree above alligator nesting sites for protection against racoons); silt river banks make poor burrows for predators; predatory threat can only come from 180 degrees not 360; each of many hatchlings represent less reproductive effort than dinosaur hatchlings (and are therefore more expendable). 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.


But today is Thanksgiving in the US...and I am an invested parent...I must attend to my reproductive output. One thing I will give thanks for as I sit down to dine on a dinosaur today, is my placental life style.