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MESOZOIC MARINE HERBIVORY....... and sea cows



Before I go into the body of this message - Nick, you keep mentioning flightless
bats on New Zealand. Can you (1) tell me where you heard this, and (2) confirm
that you have not confused 'flightless bats' with the ground-adapted
mystacinids? Thanks.

Nick Longrich says...

>  There was one completely marine kelp-eater, Steller's sea cow, 
> which unfortunately was wiped out in the 17th century by Russian 
> seafarers. I wonder if, however, it had not suffered previous hunting at 
> the hands of the Alaska natives, who are capable whalers, to say nothing 
> of their ability to hunt a slow-moving creature like the sea-cow which, 
> according to Steller, was not even capable of diving. There is almost 
> nothing today known of them except for a few fragments Stellar was able 
> to save. It would be interesting to see if there are any fossils, 
> however. 

The giant Steller's sea cow, _Hydrodamalis gigas_, was a sitting duck for human
hunters. After the wrecking of the Russian _Saint Peter_ in November 1741, it
took about 27 years for the entire population (perhaps only 2000 or so animals
anyway) to be killed off (though some 19th century scholars maintained that some
Steller's sea cow lived 'till the 1850s). The original shipwrecked crew of the
_Saint Peter_ were able to survive by eating sea cow meat and fat, and after
returning home in 1742, they spread the word. Thereafter hunting teams regularly
visited Bering and Copper Island to stock up on sea cow meat, and as early as
1743 hunting crews (hunting for fur-bearers like fox and otter) actually
wintered on Bering Island, subsisting also on sea cow meat. Hunting was easy -
 you just sneak up on a sea cow, spike it with a pole and then wait for the
carcass to float in on the tide - and incredibly wasteful. No sea cows left by
1768.

What Nick says about there being 'nothing known' is rather incorrect. Whole
skeletons have been retrieved from Bering Island at least, and I gather that
remains are fairly abundant. Of course, we rely on the observations of Steller
and others as to the behaviour and life appearance of these animals. They were
probably hunted by aboriginals, but I'm not aware of any real evidence for it
(probably out there though).

>  I hadn't heard the thing was supposed to be a kelp-eater though. 
> I would have thought something more along the lines of sea grasses?

As I understand, sea grasses (the Hydrocharitaceae and Potamogetonaceae) are
tropical-subtropical. They evolved in the late Cretaceous and are determinants
of sirenian diversity (see Domning 1981). A lineage of north Pacific dugongid
sirenians began in the Miocene: moving northward, the animals got bigger, cold-
adapted, and took to eating kelp. Plio-Pleistocene _Hydrodamalis cuestae_, at
over 9 m long (bigger than the Steller sea cow), lacked teeth and fingers, like
its probable descendant (_H. gigas_). Kelp are usually assumed to have appeared
in the Miocene, but the Miocene forms are rather derived - also, the Oligocene
radiation of (also kelp-eating) desmostylians in the cold north Pacific suggests
an earlier origin. I'd agree with Clayton (1984), she had hypothetical kelp in
the Upper Cretaceous. Makes for prettier elasmosaur pictures;-)

>  This seems to be another niche strangely absent from the 
> Mesozoic, the marine herbivore.  

Indeed. To exploit kelp as a resource, the hydrodamaline dugongids had to become
adapted for life in the turbulent surf zone. They did this by evolving very
thick, tough skin and a very high s.g. meaning that they could float without
effort. Steller wrote that the sea cows preferred estuaries and river mouths for
feeding, presumably as the risk of contact with abrasive substrates was less.
They must have had a truly massive effect on kelp populations as not only did
they consume loads, they also uprooted or killed large amounts of kelp which
just floated to shore. Modern relictual populations of manatee actually have
considerable impact on aquatic plants (some manatees have even been used to
reduce the amount of out-of-control alien weed species, like the water
hyacinth), so Steller's sea cows must have been major-league agents of kelp
destruction.

If kelp was around in the Cretaceous, then as to why marine reptiles didn't
adapt to it is anyone's guess. You might as well ask why there aren't aquatic
eagles or gliding ichthyosaurs... herbivory may simply be beyond the adaptive
range of marine reptiles (as it is for cetaceans and seals). It's all a bit of
a stretch, but I'm wondering if organisms have to be preadapted for marine flora
before adapting to an aquatic lifestyle. Marine reptiles, cetaceans and seals
all took to the seas as carnivores, whereas sirenians and the marine iguana have
gone in grazing.. let's just hope (via time travel of course) we can look at the
south-eastern Pacific in 10 million years, when a diversity of giant marine
iguanians will flourish (neomosasaurs). Perhaps.

"Call his friends up on the phone - except for the Pope maybe in Rome"

[ For that you get a 50%.  It should be "Nobody calling on the
  phone...".  -- MR ]

DARREN NAISH