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Re: Mammal diversity takes a 20 milion year step backwards!
John Bois writes:
> I am tempted, especially with the additional "molecular clock"
>evidence which puts the branching of all placental lines back to 100
>million years....... Finally, I am tempted to speculate that all
>the pieces were in place for a multi-species onslaught on the one
>thing all the dinosaurs had in common and COULD NOT CHANGE: they
>laid eggs and could not hide them! But I won't speculate on these
>things because I want to do some serious speculation later in this
Whether the mammal diversity was low or high at the K/T seems to me
utterly irrelevant to dinosaur extinction in general or egg predation
in particular. There is simply only so much room for egg predators in
any community, and I can't for the life of me imagine that high
diversity would increase the amount of egg predation actually taking
place. More species of egg predators does not necessarily translate
into more individual egg predators - and the idea of a highly diverse
community suddenly focussing on a single food source to the extent
that it wipes it out seems quite unrealistic.
On the other hand, a SINGLE egg predator can affect species survival
in certain circumstances if ecological conditions change - we are
seeing that today with the Brown-headed Cowbird and such species as
the Kirtland's Warbler. However, it would be remarkable if, of all
the species that went extinct at the K/T on land or sea, only the
dinosaurs vanished because of such a limited cause.
Further - who says dinosaurs could not hide their eggs? We have no
idea what colour the eggs were, for example - were they
cryptically-coloured, as many ground-nesting bird eggs are? Did the
parents cover the eggs with vegetation for concealment when not
brooding them? Are we to suppose that animals which had shared their
entire evolutionary history with mammals of one sort or another, any
of which could have been potential egg predators, had not evolved
methods of protecting their eggs?
> However, the one thing we can say for certain is that mammals were
>not null species, inert until "ecologically released" by the
>accidental fall of their betters--the dinosaurs. Important
>functional branch points (dentition and reproductive "advances")
>were being attained a good 20 million years before the K/T "just so"
First of all, who says that Mesozoic mammals were "ecologically
inert"? There was considerable taxonomic diversity of mammals in the
mesozoic, including several lines (eg tritylodonts) entirely absent
today. However, there doesn't seem to have been the range of size and
niche diversity we see during the Cenozoic. All the discovery of a
protoungulate tooth means (assuming it is correctly identified) is
that there was one more line of mammals in the Mesozoic than we had
thought - it does not mean, necessarily, that the ecological diversity
of mammals was much greater than we had assumed. After all, it's not
as though this was a Mesozoic zebra or something. I presume from what
little I have read that we are not talking about something outside the
known size range of Mesozoic mammals, for example.
>But back to egg-laying vs. live-bearing. Perhaps, as I claim, the
>mammal's secure reproduction played a part in this increase in
Can we remind ourselves that mammals have been around since the
Triassic? What "increase in evolutionary creativity" are we talking
> Egg- laying in birds is preferable to live-bearing because of the
>need for lightness and, anyway, the eggs can be hidden in
>out-of-the- way places. Egg-laying in crocodiles works because,
>sovereigns in their swampy habitat, they can guard their nest with
>ferocious efficiency, on their own terms. Egg laying in most snakes
>is okay because the nest is either guarded by venomous parents, or
>the eggs are down some slithery hole which only the parent can
>reach. Lizards are small and can hide their eggs well. Turtles
>make a single stealthy trip to lay their eggs, then they forget
These are gross overgeneralizations. What about ostriches, for
example? Komodo dragons? Pythons, which brood their eggs? And may I
recall that most snakes are not venomous?
> But dinosaurs, big and parentally invested, had no means of
>avoiding detection. The only strategy they could resort to was
>defence of the nest. This is an expensive strategy and violates the
>first law of self-preservation: it is better to run than fight.
>Mammals holding the baby within could obey this law and have their
>offspring as well!
First of all - we only know what little we do about the parenting
strategy of a very few dinosaurs in a very few groups. We know
nothing about the parenting of such late K/T dinos as Triceratops and
Tyrannosaurus, for example. I see no reason to assume that the
egg-laying and egg-guarding strategies of dinosaurs were any less
diverse than the animals themselves. And what running vs fighting has
to do with defense by a parent weighing several tons against egg
predators weighing a few pounds I can't imagine - this is the
proverbial "what do you say to a 300-lb gorilla" issue.
>But he is stingy in his praise of mammalian reproductivity! He says
>the pregnant female is a liability for mammals.
If live birth were as advantageous across the board as is being
suggested here, we might ask why all lizards and snakes are not
live-bearers (not to mention all fishes), as the capacity to do so
exists in those groups. I prefer to believe that each strategy has
its advantage under differing circumstances, and that there is no
intrinsic advantage of egg-laying over live birth or vice versa
equally applicable to all forms. For all we know there could have
been live-bearing dinosaurs, too.
>True, such tactics were available to dinosaurs. But they were not
>as effective for a stationary offspring (the egg).
Who says eggs are stationary? We know that crocodilians will
transport their eggs in their mouths when necessary. Dinosaurs could
have done the same thing.
>Ultimately, the dinosaurs had to resort to blood and guts, expensive
Against something the size of a squirrel?
>Greg Paul, in _Dinosaur Eggs and Babies_ reports that dinosaurs were
>r-selected. That is, few reached adulthood. In contrast, many if
>not most mammal babies do reach adulthood.
I am curious as to how Greg Paul or anyone else can arrive at this as
an across-the-board generalization for all dinosaurs. What was the
clutch size for T. rex, say? The mortality in the first year? Don't
know? Sorry, you can't say it is r-selected, then. And certainly I
do not see why dinosaurs should differ from birds in this manner.
Egg-laying vs live birth has nothing whatever to do with r vs k
selection - consider the California Condor, an egg-layer that produces
one egg every other year!
And if most mammal babies reached adulthood we'd be drowning in
rabbits and mice (as has happened in Australia, for example - proof
that mortality is indeed high in such species when normal predators
> As a result, open-field mammals only need bear one or so offspring
>a year to maintain their populations at carrying capacity! Indeed,
>the very term used to describe this sort of reproduction is _K-
>selection_ and it is itself defined as low mortality among
Not exactly, but let that pass. However, let me point out that
r-selected and k-selected species do not necessarily differ in
reproductive investment. Producing ten or a hundred small young which
you abandon may require no further investment than producing one large
young whom you raise - perhaps less. The two strategies have to do
with resource availability and other factors, and are equally
successful for differing forms that practice them.
>Dinosaur age structures were like those of an oyster, bottom heavy.
>The strategy was to lay lots of eggs in the hope that some would
>live. This, however, is less than just an alternative strategy. It
>was, in a world full of increasingly adept egg and offspring
>predators, increasingly expensive.
Who says egg predators were "increasingly adept" in the late
Cretaceous? Increasingly diverse mammals does NOT mean increasingly
adept egg predators. For all we know the very earliest mammals may
have been just as good at it as the late Mesozoic ones. I also see no
reason to assume that an r-selected strategy (if that is what
dinosaurs really used) is any more expensive than any other. Any
strategy depends on a suite of adaptations and behaviours, most of
which we cannot hope to know about for dinosaurs. This is piling
speculation on speculation with a vengeance.
>As argued above, for most of the mammals gestation there is _no_
>increased risk. And there is little risk _at any time_ except
>during delivery (by the way, I know some plain animals deliver at
>night so that by the morning mother and baby can be off and
The "increased risk" of pregnancy is certainly a greater demand on
resources by the female during gestation - but this is compensated for
by specific behaviours. So is the "increased risk" of egg-laying
among those that use it, ranging from parental care at one extreme to
the abandonment of huge clutches of small eggs at the other (and no
one is, I hope, suggesting that dinosaurs were even remotely as fecund
as some fishes that lay eggs by the million!).
>Derek's claim also depends on the notion that egg defence was
>inexpensive and effective. Again the evidence suggests otherwise:
>1. dinosaurs were r-selected (see above); 2. dinosaurs were
What "evidence" there is for this is (a) limited and (b) confined to a
very few taxa. Besides, what about birds?
>Dinosaurs were precocial to _avoid_ their pre-hatching experience of
>being completely immobile--they had to get away from the stationary
>mode as quickly as possible!
Pure speculation. Even if true it applies to most living reptiles and
many birds today, and they seem to survive quite nicely. PS - the
major egg predators of birds are not necessarily mammals - other birds
and snakes may be much more important. Who says that the major egg
predators of dinosaurs were mammals anyway?
>Plain mammals _always_ herd!
Not true. Even if we restrict this to "large, herbivorous plains
mammals" it does not apply to white rhinoceroses, for instance. It
certainly doesn't apply to all carnivores, rodents etc.
> Moving now to speculation:
We've been there for quite some time now, I'm afraid.
> The size of a herd, notwithstanding the intense predation at all
>stages of development bar adults (which makes them an r-selected
Please cite the evidence for predation levels at various age classes
in any dinosaur.
>, would be determined by the productivity of the range within
>(probably) a six-hour forage (not a six-hour walk!) from the nest
>(in daylight hours only, six-hours out and six in to get back before
Firstly - living birds do not nest all year. Secondly - many species
of egg-layers do without food during the brooding period for extended
periods of time, trading off with mates who share brooding
responsibilities while the other heads off for long periods (eg
penguins). There is no reason I know of to suppose that dinosaurs
could not have done any of these things if they were colony-bound
during the nesting season. We know nothing, for example, about
pre-brooding fat deposition in dinosaurs, as far as I know.
>And now factor in the predators. Big predators, stealthy predators,
>small predators, flying predators, burrowing predators (perhaps),
>predators on juveniles, predators on eggs, predators on hatchlings,
>predators on adults. Again, the dinosaurs had no flexibility of
By this logic it is amazing that dinosaurs evolved at all! Are we to
assume that over tens of millions of years of evolution in a world
shared with mammals, birds, snakes, lizards, other dinosaurs and lord
knows what other potential egg predators there might have been (not to
mention predators on young or brooding adults) dinosaurs never came up
with compensating strategies to cope with these problems?
> Derek says that dinosaurs suffering intense predation at one
>site can simply abandon the nest and move to a new one. "After
>all," he says, "there's always next year." This sounds like one of
>those "Famous Last Words" jokes, perhaps said on the last day of the
Actually, it sounds like biology. Such things certainly happen in
colonial birds today, including opportunistic occasional breeders like
tha Banded Stilt of Australia. In fact it would have been an even
more useful strategy for species with long breeding lives. If a large
dinosaur had a breeding life of (say) thirty years, and could
potentially produce (say) twenty eggs a year - with the possibility of
double-cltutching in a single year, like many birds - we have a
potential of 600 offspring over a reproductive lifetime. That would
seem, for an animal the size of (say) a maiosaur, to leave a lot of
margin for error, and clutch abandonment might well have been a
flexible adaptive strategy - though, as in many birds today, it might
have been based less on predation than on response to food or water
shortages. It is not a "deficit in fitness" but a highly adaptive
>As in birds, nest sites are a limited resource and are vigorously
>defended. In addition, perhaps the range itself was defended. In
>any case, moving to a new home would first mean a costly eviction of
>the previous tenants!
This varies tremendously within birds - the generalization does not
apply. Many birds defend territories but, except when eggs or young
are actually present, do not defend the nest site to any degree.
>A carnivorous dinosaur has some of the same liabilities of its
>reproductive mode as an herbivorous dino has. It must forage yet
>provision. It still has stationary eggs and relatively defenseless
With the proviso noted above, so does a crocodile.
>Altricial mammal babies could cling to mummy's fur, or mummy could
>move baby by the scruff of its neck. A dinosaur trying this tactic
>would have scrambled babies.
On what is this based? Crocodiles can move their young in their
mouths, quite effectively. At least some dinosaurs could have done
the same thing.
>In my non-stealthy nest idea, small dinos are lumped in with the
Please cite your evidence that small dinosaurs had "non-stealthy
I'm sorry, but the idea that dinosaurs failed to evolve strategies to
protect their young or eggs, or to compensate for parental investment,
over 150 million years of evolution as diverse as in any vertebrate
group known, in a world full of potential predators and competitors
FOR THE WHOLE OF THAT TIME - something that could not be said for any
other line of creatures on earth - borders on the ridiculous as far as
I am concerned.
Ronald I. Orenstein Phone: (905) 820-7886 (home)
International Wildlife Coalition Fax/Modem: (905) 569-0116 (home)
Home: 1825 Shady Creek Court Messages: (416) 368-4661
Mississauga, Ontario, Canada L5L 3W2 Internet: firstname.lastname@example.org
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Toronto, Ontario Canada M5H 3P5