There is an unofficial 'Premier League' in vertebrate palaeontology which consits of the animals which attract a lot of public attention. Its members include T.rex, Seismosaurus, Argentinosaurus, Giganotosaurus and so on - the biggest and fiercest extinct animals. When the BBC broadcast 'Walking with Dinosaurs' they moved Liopleurodon ferox firmly into the Premier League. Here was an animal that made T.rex look like a kitten - 25 meters long and weighing 150 tons, an awesome predator that dwarfs anything before or since.
The problem is that Liopleurodon ferox was not 25 meters long, and did not weigh 150 tons.
Liopleurodon was a large predatory marine reptile. Its remains are found in the Callovian Oxford Clay of Eastern England and Northern France, and date from about 160 million years ago. It is a pliosaur, one of a family of plesiosaurs which includes larger and smaller relatives, such as Peloneustes, Pliosaurus, Kronosaurus and Brachauchenius. The big pliosaurs were the 'top predators' - the top end of the food chain - in the seas from the middle Jurassic, when they took over the role from large ichthyosaurs such as Temnodontosaurus until the middle of the Cretaceous, when they were supplanted by the Mosasaurs. Unlike the more familiar long-necked plesiosaurs such as Elasmosaurus, they have large heads and short necks, and a rather longer body. Typically they have formidable teeth, and the back of skull is extended to make room for the large and powerful muscles which drive those teeth into their prey.
It is hard to identify bones are being definitely those of Liopleurodon. Remains of top predators in general are rare - there aren't as many of them as there are of animals further down the food chain. Our knowledge of the pliosaurs of the Oxford Clay in general has recently been greatly refined by Leslie Noè's PhD thesis. His work has concentrated on skull morphology, so our knowledge of their postcranial anatomy is still rather limited. The largest skull definitely belonging to L.ferox are about 1.5 meters long, and if the head was about a seventh of the body length (as reconstructed by Tarlo) it would make the length a little over 10 meters. I have studied the bite marks left by Liopleurodon on the bones especially of Cryptocleidus, and by matching skull size to recognisable patterns of tooth marks would conclude that the normal size range was about 5 to 7 meters. Very rough calculations (based on measuring the volume of a plastic model of a pliosaur) suggest that the normal weight range was from about a quarter to three quarters of a ton, and up to around 2.5 tons for the biggest animals. This does not allow for changes in body proportion as animal increase in size, and it is not unlikely that larger animals were also proportionally bulkier, as is the case in modern crocodiles. Even taking this into account it seems unlikely that Liopleurodon reached weights of more than 5 tons.
These calculations assume that the proportion of head to body as reconstructed by Tarlo are correct, and this may not be the case. As I said before, we don't know much about the post-cranial anatomy of pliosaurs in general. In the past many extrapolations were made based on the famous mounted specimen of Kronosaurus in Harvard Museum of Natural History which is 12.8 meters long. A substantially complete and articulated specimen from Colombia was described by Oliver Haumpe in 1992 and assigned to the genus Kronosaurus, though with some reservation. In this animal the head is larger in proportion to the body than the Harvard specimen, and more recent work by Colin McHenry (as yet largely unpublished) has confirmed that the same is true for Kronosaurus queenslandicus - the Harvard animal. It is not unreasonable to assume that the model of pliosaurs we have been using tends to exaggerate the length, though the bulk may have been proportionally greater. A realistic assumption is that the head is between a quarter and a fifth of the overall length, not one seventh as the older reconstructions show.
There is evidence of a much larger pliosaur in the Oxford and Kimmeridge Clays. Very large isolated elements such as vertebrae and a large lower jaw have been found. These cannot be identified as Liopleurodon and it seems most likely that they belong to an unknown and very large taxon. A lower jaw, in the collection of the Oxford Museum is from the Kimmeridge Clay and reportedly measures 3 meters in length (though I haven't measured it myself). Leslie Noé thinks it is from an undescribed taxon. Scaling up on the same basis as before, this would suggest an animal as much as 12-15 meters in length, and weighing therefore in the region of 10 tons. There have been unconfirmed reports of a lower jaw found on the Dorset coast measuring as much as 4 meters, which could therefore measure 16 - 20 meters in length, and weigh as much as 20 tons.
A preliminary report on an pliosaur from Mexico was made in June of 2002 and gained some media publicity in December 2002/January 2003 (more about this here). This specimen is estimated, possibly rather conservatively at 15 meters in length based on the diameter of a pectoral vertebra being rather larger than that of Kronosaurus. This specimen is substantially complete, and there is some expectation that it will be possible to prepare the post-cranial material as well as the skull. Reports of this animal were somewhat exaggerated - its length increased to 25 meters and its weight to 150 tons. Although widely designated as such it's pretty certain that it isn't Liopleurodon ferox. The reports, drawing on the BBC's Walking with Dinosaurs site, have simply repeated and reinforced Liopleurodon's semi-mythical status as biggest and fiercest.
It has been said that science is not the search for truth, but the search for funding. Publicity helps to get funding. A medium-sized pliosaur, or a small, timid dinosaur will not attract much public attention no matter how interesting they may be to researchers. This can lead to a mathematically rather sloppy approach and tendency to treat the uppermost end of the size estimates as being the norm. Another argument which I have heard goes like this: "We have found an animal up to 12 meters long. We can realistically assume that the specimens we have found are not the largest in the population. Therefore, we can estimate the size of the largest animal in the population as rather larger than the largest we have found - say 20% longer. Therefore we can say with some justification that the largest animals of this type were 15 meters long."
The Aramberri animal was widely reported as having teeth that could 'chew through granite', though why any animal would want to do so was never explained. Somehow, I doubt it. Nevertheless: (I've run out of 'howevers') Pliosaurs were almost uniquely adapted for powerful biting. Estimates of biting force in excess of those produced by any existing animal have been made for (the kitten-like)T.rex based on marks left on the bones of its prey (or scavenged carcases). Pliosaurs were much bigger than T.rex, and better adapted for powerful biting. Think of a modern car, with a monocoque chassis: all relatively thin steel. A pliosaur was big enough to hold a medium-sized car in its jaws, and powerful enough to bite it in half!
It is important to take some basic mathematics into account when thinking of the size of animals. If the length increases, the volume and hence the weight goes up by the cube of the proportional increase in length. So if the length is doubled, the weight increase by a factor of eight. Too look at it another way, to double the weight of an animal we need to increase the length by fractionally over a quarter. A 15 meter pliosaur is twice the size of a 12 meter pliosaur, and an 18 meter animal twice as big again. If we add a meter to the length of a 15 meter animal, we are increasing its weight by a quarter. It's easy to add an odd meter to an estimate, or to round it up a bit, but this has a dramatic effect on the size.