Re: [Part 2: Terramegathermy (very long, too)]

["Adam Britton" <abritton@wmi.com.au>]

Disaster. I found another spare 5 mins, and now this...

re: Deinosuchus growth, David Marjanovic cited the following paper:

> Gregory M. Erickson*, Christopher A. Brochu [who else?]: How the 'terror
> crocodile' grew so big. Nature 398, 205f. (18. 3. 1999)

and commented that the authors:

> counted growth rings in dorsal osteoderms,
> giving longevity of several specimens (estimated to have been 8.43 to 9.10
> m) of 50 and 51 years (+-2).

That paper contains some interesting research, but there are some issues
that aren't entirely satisfying to me. Firstly, it is known that osteodermal
laminations or growth rings in long bones can be highly unreliable
indicators of age in extant crocodilians, unless there is a significant
difference in growth rate between seasons. For example, growth rings in
_Alligator mississippiensis_ are reasonably well-defined because during
winter they cease growing altogether (and often estivate). However, the
climate experienced by _A. mississippiensis_ is not typical of most extant
crocodilians. Laminations in _Crocodylus porosus_ or _C. niloticus_ are
extremely difficult to resolve accurately because growth is too similar
between wet and dry seasons. Also, the older laminations (in long bones in
particular) can deteriorate in older animals, providing an underestimate
of actual age. Work on osteoderm laminations in _C. johnstoni_ (an
Australian species which does experience a considerable slowdown in growth
during the dry season), showed that even in this species the growth rings
were highly unreliable indicators of age. If the climate that _Deinosuchus_
experienced was similar to that experienced by extant tropical species, then
I'd be extremely cautious about divining age from growth rings, especially
from the apparently small sample size used in the paper. However, I don't
know much about the climate _Deinosuchus_ found itself in.

Also, the growth charts with _C. porosus_ need to be interpreted with
caution. The _C. porosus_ growth data come from a recovering population of
crocodiles where the population was highly skewed towards small individuals,
in an area with considerable seasonal differences in growth rate, where the
majority of food items taken were small (even in larger animals). I don't
think this was the best example they could have taken. Also, the idealised
growth curve they present seems more representative of mean female _C.
porosus_ growth than anything else. Although there's little chance that _C.
porosus_ could even approach the kind of sizes that _Deinosuchus_ could
attain, they certainly would grow faster and for longer if there wasn't such
a pronounced difference between seasons, and if high densities of large prey
were available for most of the year. It's no coincidence that the largest
known _C. porosus_ (over 6 metres) have come from such areas. Is that
scenario more applicable to _Deinosuchus_? Interestingly, growth rates
achieved by the largest known _C. porosus_ specimens in captivity (ideal
diet) appear to be on a par with _Deinosuchus_ up to 30 years of age at
least, but then they drop off dramatically.

The difference in growth rates between _C. porosus_ and _C. johnstoni_
(sympatric species) in identical conditions is quite different - the former
grows at over four times the rate of the latter, and reaches twice the
maximum size. Why is this, and does it shed light on why _Deinosuchus_ grew
at least twice the speed of the "average" modern crocodilians? I'd be
cautious about claiming the ages of _Deinosuchus_ specimens (50 and 51
years) exceed extant crocodilians - that statement is demonstrably untrue
for many species. Even a population of C. johnstoni (a relatively small
species) that I'm studying contains individuals that are at least 50 years
of age that show no readily apparent signs of senesence. I'd be surprised if
50 years was close to the limit for Deinosuchus either. It's interesting
work, but I'd like to see a few more _Deinosuchus_ datapoints thrown into
the mix with a more realistic sample of extant crocodilian populations from
fast-growing species. I think the most relevant finding is the increased
growth rates in the first couple of decades, which is similar to captive
crocodilians raised in "ideal" conditions rarely found in the wild.


> > Extrapolations from known-sized crocs would put the largest measured
crocs
> > (6.3 metres) closer to 1500 kg. [...]
>
> OK, but these are still semiaquatic...

True, large crocs are not very efficient predators on land, but then they're
not supposed to be. Small crocs can move rapidly for many minutes over land,
but adults cannot. Unfortunately, small crocs tend to get eaten if they try
that particular hunting strategy.


> Now extrapolating this to "megadinosaurs" indicates that the latter must
> have been tachyaerobic, doesn't it?

If crocs are good models for terrestrial megadinosaurs then yes, but I don't
believe they are.

Adam Britton