Re: Speculative dino species

[christian farrell kammerer <cfkammer@midway.uchicago.edu>]

<<Christian Kammerer:>>Also, let us not get to reptiliocentric in
our understaning of the oceans--even before the K/T the teleost
radiation
was well underway, and big fishes would likely still be prevalent
(and likely to beat the reptiles to the "whale" ecomorph).<<
Oh, good point.>>

Thanks, and since the argument could easily arise that "although fishes
might beat reptiles to the whale ecomorph, that's no reason that reptiles
could still get there, as witness the co-habitance of the oceans by whale
sharks and whales", let me expound on this a little further. I think a
question we have to ask ourselves when postulating Cenozoic
whale-pliosaurs (or mosasaurs) is not whether they _would_ exist without
mammals, but rather why _didn't_ they exist without mammals
previously. The earliest baleen whales aren't known until the Miocene
(there are mysticetes from the Oligocene, but they're toothy forms like
Aetiocetus and Mammalodon--incidentally, the fishes did indeed
"beat" mammals to the Cenozoic whale ecomorph, as the fossil whale
shark Paleorhincodon is known from the Eocene, and fossil species of the
extant basking shark genus Cetorhinus are known from the Oligocene). Thus,
why did mammals go from purely terrestrial creatures to baleen whales in
35 million years, while, say, sauropterygians didn't in 150 million years
of purely marine existence. I see three possible explanations for this. 1,
the reptiles were outcompeted by giant filter-feeding fish
(e.g. Leedsichthys) when they were diversifying in the
Triassic/Jurassic. This is extremely unlikely, because for one thing,
plankton isn't exactly a resource that you have to compete over, and as
we've already seen, giant filter-feeding sharks and whales seem to coexist
just fine. 2, the reptiles had some morphological constraint that stopped
them from entering the "whale" niche. This has been addressed in detail by
Collin & Janis (1997) in the _Ancient Marine Reptiles_ volume. Although I
am somewhat skeptical of some of their argument, this is still an
extremely interesting paper and certainly something to think about when
postulating the existence of filter-feeding giant reptiles. 3, there were
filter-feeding sea reptiles, but they either haven't been found yet (if
they did exist, they would likely have been offshore, epipelagic animals
that only rarely would have entered the environments most sea reptiles are
preserved in, i.e. shallow water seas like the Niobrara) or haven't been
universally recognized as such (Stomatosuchus, filter feeder?). In any
case, working on known fossil evidence, it doesn't seem plausible for
marine reptiles to "go mysticete" in the Cenozoic when, as far as is
currently known, they didn't in the Mesozoic. (Not that I'm trying to put
an end to Cenozoic sea-critter speculation, heck, I'm toying with the idea
of inshore mosasaurs becoming extinct and being replaced by huge,
estuarine choristoderes, but I'm just presenting some things to think
about.)

Sincerely,
Christian Kammerer