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Since this thread started, I have been pondering long and hard about so-called
living fossils and, as several other listies (like it? Should it be listers?)
have pointed out, exactly what a living fossil IS just ain't that clear. So
here's some thoughts.

For starters, how do we define a 'living fossil'. If someone could give a strict
definition, I'd really appreciate it. As I see it, it means an organism still
living that has fossil relatives identical, or almost identical, to itself. But
this seems pretty dumb, as over 50% of living tetrapods have very
similar/identical fossil representatives. I had a book years ago that listed
cassowaries (want me to talk about them?), thylacines, baleen whales and platypi
as living fossils amongst horseshoe crabs, tuatara, lungfish etc. I guess that
their reasoning was that these things are all known from fossils, and the
fossils aren't different from the living types. As I've said, you could
therefore say this for just about everything. There are fossil Homo sapiens and
Canis lupus, Corvus corvus and Giraffa camelopardalis. Does that mean that
they're all living fossils too? And, yet, Sphenodon punctatus is unknown from
fossils. I'm getting confused now.

Perhaps this means that you can only say something is a 'living fossil' when it
has remained extant for  A CERTAIN PERIOD OF TIME (i.e. needs a record extending
well back before the Pleistocene [the cassowaries are Pliocene, thylacines etc.
are Pleis., baleen whales first appeared in the Oligocene]). And, over that
time, it must have remained 'unchanged'. Platypi are therefore ruled out, as the
fossil ones (Steropodon from the Cret., Obdurodon from the Miocene, giant
Ornithorhynchus from the Pleistocene) are quite different from the modern
Ornithorhynchus anatinus. I think that modern Horseshoe crabs also have no
fossil record, but have remained morphologically conservative since their debut
in the ????...?  The same can be said for lungfish - does Neoceratodus (Ozzie
lungfish) have a fossil record? Of course they have fossil ancestors and
relatives, but so does (just about) everything! I guess the thinking is that
they are considered L.Fs because they have remained conservative throughout
their history. 

Now that's just not fair - how do you know that they're not just really
successful in the mode of life that they've evolved into? Are they really, then,
living fossils?

(BTW, what's the name of the giant lungfish from the Wyoming 'Drinker' beds? Oh,
and it was MegaLOcoelacanthus, not Megacoelacanthus... Coelacanths... now
there's a slippery subject. New ones reported from ?Maastrichtian of U.S.,
coelacanth trinkets and scales turning up in Mexico and Spain, blood urea
evidence suggesting that they're related to sharks... Gee..) 
With these (poorly defined) points thus in mind, I would thus define 'living
fossil' as 'a genus - or species or subspecies - of organism that has a fossil
record extending back a longer-than-average length of geologic time'. But then,
who's to say what's longer-than-average? How do you know that Lingula isn't the
norm? Tyrannosaurs, cassowaries and thylacines are just short-lived exceptions.
Hmmmm, err...?

To hell with this, it was fun before I started thinking. 

I have amassed fascinating facts on cassowaries. Shall I bother posting them
for your amusement... and education? Please mail me dwn194@soton.ac.uk
(population of tame black thrushes on site - and I think the species name is
'melanurus' Colin [but I could be wrong]). Pleistocene thylacines were up to 25%
bigger than recent examples. With several thousand thylacines sightings (a few
blurry photos, some dubious footprints and some physical remains), the thylacine
is, of course, another 'living moa'. Funny how cryptozoology still rears its
head amongst dinosaurian discussion....(I write for cryptozoological mags see)

Lawn moa. Wished I'D understood it first time round. Derrr.

Do tapeworms have a fossil record? There's a thought.

"I didn't hit it very hard. Must have had a self-destruct"