Re: Dinosaur egg-laying contributed to extinction?

[Jura <pristichampsus@yahoo.com>]

----- Original Message -----

> From: Erik Boehm <erikboehm07@yahoo.com>
>>  *Confuciusornis* and the Enantiornithes, at a minimum, were
>>  apparently able to fly at a _very_ young age and did most of
>>  their growing later.
> 
> Well... this could then support the argument - those which had young which 
> were 
> on their own before becoming close to adult size, were more likely to go 
> extinct. I'm not so sure about the premise that a difference between the 
> size of an adult and the size of an independent-adolescent implies different 
> ecological niches. It makes sense for large carnivorous, but I'm not so sure 
> about grazing animals and insectivores.

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Having young that are orders of magnitude smaller than the adults is pretty 
common for large reptiles (Shine 2005). Both alligators and leatherback sea 
turtles have young that are some 10,000th the size of the adults. Even in 
smaller reptiles a difference of 100-1000 is not uncommon. The resulting 
offspring do wind up moving through different ecological niches as they become 
better capable of handling different types of food. 

As for whether this would make sense for an herbivore, I would argue yes. One 
of the benefits of large size in herbivores is that one can digest less 
nutritious food. Young, growing herbivorous reptiles would not have that 
luxury, which is probably why young iguanas and even some tortoises have been 
known to "supplement" their diets with insects (Pough 1973, MacDonald and 
Mushinsky 1988). Herbivorous dinosaurs would have gone through an even larger 
body-size range as they moved from hatchlings to adults. I would fully expect 
the young to occupy different niches and tackle different food items (including 
insect and small animal "supplements") as they grew up.

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> Problem there is things like crocodiles. Or sharks. Or
> anything
> producing a larval form (which, very similarly to things
> like
> sauropods, had young that occup
think anyone is arguing that this was the sole criteria for
 survival. For example, such creatures generally have low metabolic 
rates

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Some/many/most dinosaur may have had low metabolic rates too. There is 
currently no way to tell this (nor is there a way to tell that all those 
Mesozoic crocs and sharks had low metabolic rates). What we can do is look at 
the K/T extinction event and see what passed through and what did not. We can 
see multiple species of egg-laying animals making it through. We also see many 
viviparous species make it through. We see animals with relatively big brains 
and relatively small brains, come through. Scaly integument and fuzzy 
integument, long tails and short tails, presumed parental care and presumed 
non-parental care, presumed temperature-dependent sex determination and 
presumed genetic sex determination. The K/T event doesn't seem to make much 
sense as an indicator of "failed" life history strategies. The one apparent 
trend that we do see is that things that lived in the open ocean or on dry land 
took the brunt of the event. Flora and fauna that lived at
 the junction of water and land (swamps, wetlands, maybe deltas) seemed to make 
it through okay. There also seemed to be a body-size variable that may have 
been in play too (I think it was nothing much bigger than 1 meter, or so).

The point is that the authors of this paper seem to be arguing that dinosaurs 
went extinct at the K/T event because they shared a life history strategy with 
other reptiles that made it through. It reminds me of the old argument about 
dinosaurs dying out because changes in Earth's temperature forced all the eggs 
to hatch out as one sex. All these ideas suffer from too much tunnel vision. If 
one is going to explain the K/T event then one has to remember that dinosaurs 
were only one small part of it.

As for why mammals were able to diversify into so many niches after the K/T, I 
think that part of the paper holds more merit

Jason

References

MacDonald and Mushinsky 1988. Foraging ecology of the gopher tortoise 
Gopherus polyphemus, in a sandhill habitat. Herptelogica 
Vol.44(3):345-353 

Pough, F.H. 1973. Lizard Energetics and Diet. Ecology. Vol.54(4):837-844

Shine, R. 2005. Life-History Evolution in Reptiles. Annu.Rev.Ecol.Syst. 
Vol.36:23-46