[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

was: Gigantism in Sauropods



> I am sorry to go back on my word here,

Heh.

>but this is COMPLETELY illogical.

> Just to be clear -- you are claiming that the association of giant these
> (predator) and giant thoses (prey) is actually evidence they did NOT evolve
> in tandem?  No typo here?
>
> Interesting...

OK, let's think about this.

If gigantism in sauropods was primarily a *response* to predation
pressure by theropods, then theropod predation *upon large
individuals* is the *cause* of sauropod gigantism, correct? In an
evolutionary arms-race sense? I.e. adults needed to be bigger to deter
predation, so that's what was selected for.

In order for theropod predation upon the larger/largest sauropods in
their ecosystems to be effective, we either need to postulate pack
behavior, for which we have no real evidence, or observe that the
local theropods had gained a size sufficient to consistently threaten
the larger/largest sauropods in their environment. As in other cases,
we should be able to observe the initial trigger before the we see the
full expression of the condition (excepting limitations of the record
itself). We see evidence for sizeable tracts of grassland well before
we see lots of highly cursorial ungulates. We see the onset of
glaciation at the poles well before we see large populations of
cold-adapted megafauna. We see flowering plants before we see evidence
for large-scale eusocial hymenopteran colonies. We see goodly-sized
cetaceans before we see gigantic sharks with morphologies to feed on
them. We see fairly large theropods well before we see thyreophorans
with the size and armor to resist them. We see robust ceratopsians and
giant hadrosaurs before we see a tyrannosaur with the firepower to
take them down in a single crunch. Trends have to start somewhere, and
where the two can be discerned, cause must and does come before
effect. Hence, I would expect to see proportionately large theropods
threatening proportionately small/medium sauropods before the
evolution of really super-huge sauropods relying on their quadropedal
posture to permit them to outstrip predators limited to bipedal size
constraints. If predation was a primary driver rather than a secondary
component, that is.

Now, the main limitation here is, again, the incomplete fossil record,
so anything I'm about to extrapolate is based only on a loose sense of
what's observed, but see if you follow the logic.

In Middle Jurassic China, we have 11-meter shunosaurs and 4-meter
gasosaurs. In Middle Jurassic Europe, we have 18-meter cetiosaurs and
critters like *Eustreptospondylus* that probably topped out at about 7
meters adult length (4-5 meters subadult). In Middle Jurassic
Argentina, we again have 18-meter patagosaurs and a predator
(*Piatnitzkysaurus*) that was about 4.5 meters as a subadult and thus
probably no more than 7 meters as an adult. If hunting sauropods were
such an incredibly metabolically advantageous capability, I would not
expect to see these couple-hundred-kilogram predators trying to cope
with 3- to 20-ton adult sauropods, I would expect to see
two-to-four-ton bruisers like the bigger allosaurs and torvosaurs.
Surely it's much easier to kill an 18-meter sauropod when you yourself
are a good 10 or 12 meters long instead of 6 or 7, and surely ten tons
of flesh is still entirely worth your while to support your metabolism
at that size, so why wouldn't such a serious advantage be rapidly
selected for? The ability to rapidly kill prey and thus reduce the
risk of serious injury or injury-plus-infection should be a strong
adaptive advantage, and if your prey is adult sauropods, you're at a
serious disadvantage if you go into that fight as a featherweight who
can be crushed underfoot.

Juveniles are always going to be easier targets, and IIRC we see
modern cheetahs, lions, etc going after juveniles both
opportunistically in general and *preferentially* during the seasonal
times when they're readily available, despite their small size and (in
the case of lions) having to share the kill. Modern predators make
clear that the ease of a kill frequently outweighs the preference for
a larger metabolic payoff, because hunts impose risks both of injury
and of wasted energy if the prey escapes, and easier hunts impose
reduced risk compared to more challenging or protracted ones. I would
expect large predatory theropods to follow at least broadly similar
patterns stemming from comparable rules. I can thus predict that if a
single big theropod could easily, routinely kill a single big
sauropod, I would expect either the theropod population to have been
*very* high given the ease with which so much meat flowed up the food
chain, or to see herd behavior in sauropods, making predation by a
single theropod ineffective when confronted by a mass of sweeping
tails or a wall of trampling torsos and feet. The latter case could
well be true, but it would still obviate the "giant predator pwns
giant prey" concept because the actual ecological reality would be
that the prey was still mostly inaccessible - just due to behavior
instead of size.

> Further -- YOU might expect to see a predator evolve to a size larger than
> it needs to be just because the kill itself is 'easier'(thereby outracing
> the prey in the predator-prey relationship), but I certainly would not.

So you don't believe natural selection selects for clear metabolic and
injury-avoidance advantages? Interesting. Personally I don't think
evolution "overshoots" anything, so that's a bit of a straw man from
my angle. "Overshooting" implies that directional evolution can
continue onward even once the prior input from natural selection
ceases, and that kinda violates causality as far as I can tell.

Somewhere, though, I got under the impression that the evolution of
predators is more usually in response to the emergence of a newly
available/newly modified food source, rather than some unexpected
novelty in the predator permitting it to take on previously present,
but previously inaccessible, prey targets (suddenly forcing the prey
to adapt in turn). If so, then the evolution of giant theropods would
be a *response* to bigger and bigger sauropods, not an evolutionary
novelty triggering a response in the prey (and this appears to be what
we observe in the fossil record). Further, the pattern doesn't even
hold ecologically: the most common sauropods in the Morrison were not
brachiosaurs or supersaurs, but comparatively small camarasaurs. The
most common sauropods in carcharodontosaur territory were not
brachiosaurs or the truly stupendous titanosaurs, but the considerably
smaller rebbachisaurs and/or non-incredibly-gigantic
titanosaur(iform)s.

Basic trophic-level reasoning suggests that predators in general
should be adapted mainly for tackling the most *commonly* available
prey that serves their metabolic needs, not the *largest* available
prey. That is the overwhelming trend we see in ecosystems today -
exceptions like wolves relying on snow to impede adult elk or lions
using swamps to corner and mire adult buffalo notwithstanding - and
both 1) easy-to-overtake juvenile sauropods would have been more
common than adults, lest the population crash from lack of replacement
since not everybody can reach adulthood, and 2) the most common
sauropods in an ecosystem were the smaller species, not the biggest
ones around. The most common prey that would have met the needs of big
theropods was significantly smaller than what said theropods might
technically have been capable of taking. Cue parsimony in ecological
underpinnings.

The extremely rapid growth of juveniles all the way from hatchling to
hugeness *in itself* is strong evidence that size alone was a major
survival advantage. If the adults were just walking meat storehouses
waiting to die the next time a predator wandered through the area,
then what advantage to their size would there be? Why would the
juveniles *keep growing so fast* once they got past the capabilities
of the small-predator guild (at what, maybe 7-8 meters out of an adult
20-30?), if they were only to then remain equally vulnerable to the
large-predator guild all the way through adulthood? The implication is
massive selective pressure from big predators who really preferred
going after juveniles instead of messing with the adults. I'd expect
some degree of arms-racing in terms of size - but the race would be
between theropods and *juvenile* sauropods, with sheer growth rate in
the latter being the main arena, and maximum size being less relevant
given that "smaller = easier target" in this case no matter what size
ratio is involved, as long as "smaller" still supplies the predator's
metabolic requirements.


Pressure to reaching breeding size and related factors must surely
have been involved, also, again showing that multiple factors need to
be accounted for. Sauropod gigantism simply is not best-explained
singularly by anything - but especially not by predator deterrence, in
light of their feeding strategy, which is much more plausible as the
real primary cause; and in light of their reproductive strategy, which
would have left multiple generations of juveniles in the size range of
ideal targets for big theropods at any one time in any one ecosystem -
even more so than in modern ecosystems, where juveniles are targeted
heavily (remember, the predator mantra is "young, weakened, or
injured"), even in cases where predator size exceeds *adult* prey
size. But even if it was not driven by predator deterrence, it still
would have had that effect at the upper size ranges, based on the
ecological parameters that these sorts of interactions follow, wherein
small inexperienced juveniles are lower-risk and more cost-effective
to hunt than adults.

> Could you clarify exactly who "we" is?

Certainly: those who participate in making observations of the
evidence. I.e., if the evidence is there, anyone should be able to see
it; and if the logic is there, anyone should be able to follow it,
barring communication problems or limits on the evidence it follows
from. Hence "we," those who consider such matters.



Belabored though it may be, this is not pirates-and-ninjas stuff.
There are a lot of unknowns, but that is no reason to dismiss the
knowns as insubstantial in favor of some blow-by-blow versus-debating
procedure. There are many possibilities, and any one person's pet
hypothesis is no more or less than that: a pet hypothesis. In the
absence of direct behavioral observations, the most conservative and
parsimonious course of action is to keep our speculation constrained
by real data from modern analogues, as seen through the lens of the
biomechanics and physiology of the extinct organisms in question. And
that process strongly suggests that theropods did not routinely kill
proportionately giant sauropods, even if their juveniles, the sick or
injured, or smaller sauropod species were targeted.