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Re: MEGA-PLIOSAURS REVISITED (AGAIN)



[ A technical problem prevented this from going straight through -- MPR ]

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From: "Colin McHenry" <cmchenry@westserv.net.au>
To: <dinosaur@usc.edu>
Subject: Re: MEGA-PLIOSAURS REVISITED (AGAIN)
Date: Thu, 1 Mar 2001 00:05:59 +1100

Since watching Walking With Dinosaurs I have been curious to ask Mssrs Naish
and Martill if the blame for the 25 metre Liopleurodon can be traced back to
the suggestion that the large, isolated 'cetiosaurid' vertebrae at the
Peterborough Museum might actually represent a cervical vertebrae from a
very large pliosaur.  As the wide-eyed student who originally made that
suggestion, I feel I should explain.

In 1996, after starting my research project on _Kronosaurus_, I was in
England looking at comparitive pliosaur material.  Arthur Cruickshank took
me to the Peterborough Museum, which has some fantastic Oxford Clay material
from the Leeds collection, and whilst looking at the exhibits I noticed the
vertebra.  It was labelled as a cetiosaurid caudal; in overall shape,
however, it looked very similar to a _Kronosaurus_ cervical.  The largest
_Kronosaurus_ cervical I had seen was about 180mm in diameter - this thing
was about 240mm (from memory - my notes are still buried after my latest
house-move).  Anyway, it was big.  I pointed it out to Arthur, and we got
very excited.  We told Leslie Noe and Dave Martill (the other co-athours on
the resulting abstract), who also got excited, and then BBC radio got hold
of it, and it took on a life of its own....

Assuming that it was a cervical from a large pliosaur, our back-of-the
envelope calculations suggested that the offending beast might have been
16 - 18 metres in length, and 30 - 50 metric tonnes in mass.  This was based
on the comparisons with the Queensland _Kronosaurus_ material I was
studying, whilst using 12.9 metres as the overall length of _Kronosaurus_
(this estimate of the length coming from the infamous Harvard
reconstruction).  This seems to be the size class that worked its way into
Walking With Dinosaurs (on which Dave consulted), given a bit of obligatory
exaggeration (50 tonnes --> 150 tonnes!)

After that a couple of things happened which caused me to doubt our
interpretation.  Firstly, Dave and Arthur had a really good look at the
vertebrae and decided that it really was from a cetiosaurid dinosaur after
all.  Now, we don't have a great deal of sauropod material down here, and
when I'm overseas I'm generally too busy looking at marine reptile and croc
stuff to woory about sauropods, so I wouldn't know a cetiosaur bone if it
stamped on me - I'm more than happy to defer to their experience in that
matter.  There are features on the Peterborough vertebrae which would be
unusual for a pliosaur, in particular the pattern of the foramena and the
shape of the neural arch facet on the centrum.

Secondly, on my way home from the UK I stopped in the US and was able to
check out the Harvard _Kronosaurus_.  As Ben summarised in his article, I
regard that reconstruction as very suspect and I think the length of
_Kronosaurus_ has been over-estimated as a result.   So even if the
Peterborough vertebrae does represent a pliosaur, the basis for our original
size estimate is almost certainly invalid.  Having pieced together the
Queensland material over the last few years it is much more complete than I
had suspected - I'm now fairly certain that 8 - 9 metres is a good adult
size for _Kronosaurus_.


>Going back to the mega-pliosaurs, even if the Peterborough vertebra
>(formerly used as a doorstop, painted blue etc etc etc) is not from a
>pliosaur there are still some other mid-Jurassic English bits and pieces
>that clearly do indicate the presence of giants: the _Liopleurodon
>macromerus_ mandible is HUGE and there are some rostral tips and
>other fragments of mandibles in the BMNH that suggest individuals of
>15 m or so. Giant pliosaurs have been much discussed on this list
>before:

As for the other material that Darren mentions,  I remember looking at that
with Leslie on the same trip.  The NHM does have a large _'Pliosaurus'
macromerus_ mandible, but it's in the same size class as the large
_Kronosaurus_ material.  There are also some large isolated teeth referrable
to _Liopleurodon_ and _Pliosaurus_, but again they are no larger than
_Kronosaurus_.  There is also a monstrous mandibular symphysis in that
collection which caused Leslie and myself a great deal of head sratching; it
is certainly massive, but we couldn't work out what taxon it might belong
to.  It could be a from a _Liopleurodon_ type animal, but it is a very
different shape to the smaller, more complete _Liopleurodon_ specimens and
would thus require that these pliosaurs underwent a significant late
ontogenetic allometry, as some of the larger croc species do today - and we
simply have no other evidence of this.  The shape of the symphysis is closer
to that of another of the Oxford Clay pliosaurs, _Simolestes_, but it is
much, much larger than any material previously referred to that taxon.  It
could be a new taxon of which we know next to nothing.  So whilst this
symphysis at least is tantalising evidence of a hitherto unreported large
form of pliosaur (of whichever taxon), I would hesitate to make estimates of
overall size based on such scrappy material.

When it comes down to it, the largest pliosaur for which we have anything
like complete specimens is still _Kronosaurus_, even if it is not as big as
everyone thought.  As I said, the Queensland and Columbian material suggests
an adult size of 8 - 9 metres.  The _Liopleurodon_ and _Pliosaurus_
specimens that are at least partially complete suggest that these animals
got up to 5 - 8 metres, though some incomplete specimens suggest that both
may have acheived the same size class as _Kronosaurus_ and may even have
been slightly bigger.  North America's most beautiful fossil, the Upper
Cretaceous pliosaur _Brachauchenius_, was probably also in the 5 - 8 metre
range.

The claims for large size for _Pliosaurus (=Stretosaurus) macromerus_ and
_Megalneusaurus rex_ also seem to have been made through comparisons with
the Harvard _Kronosaurus_ (principally by comparisons between femur sizes).
The best estimate that I have been able to make is that they are in the same
size class as _Kronosaurus_  or slightly bigger.

After all that, the only material that supports the idea of mega-pliosaurs
(15 m+) is the Peterborough vertebrae (which might be / probably is a
dinosaur) and the BMNH symphysis (which we basically have no idea about).
Pretty thin evidence, I think.  I would have been happier if WWD had
downsized the _Liopleurodon_, but it was still a great episode, especially
for the _Cryptocleidus_ swimming.  My final suggestion on the subject is
that a 9 metre (~ 9,000 kg) pliosaur is still a formidable marine predator -
in engineering terms, the skull is the most powerful raptorial feeding
apparatus developed by any vertebrate, and the animal was probably very
manoevurable and fast.   Not that size matters, or anything.....

Sorry for the length of the post, but nice to see the list discussing marine
reps occasionally.

Colin



Colin McHenry
Age of Fishes Museum
P.O.Box 360, CANOWINDRA
NSW 2804, Australia
Ph: +61 2 6344 1040
Fax: +61 2 6344 2450
- museum email: cgrossi@westserv.net.au -
Mobile phone: 0428 131 858
- personal email: cmchenry@westserv.net.au
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