From the Late Jurassic to the early Late Cretaceous, giant pliosaurs were the terrors of the sea. Like all members of the Plesiosauria, they had heavily constructed bodies, short tails, and four powerful paddles used to swim and steer. In the past, paleontologists applied the term "pliosaur" any type of plesiosaur with a short neck and a large head. The real evolutionary story may be more complex (a topic for a future Dino-Dispatch!), and some researchers now think the "pliosaur" design may have developed more than once. There is agreement, though, that the most fearsome of the short-necked forms-- Pliosaurus, Peloneustes, Liopleurodon, Brachauchenius, Polyptychodon, Kronosaurus--are close relatives. Each bore a huge skull that combined the biting power of killer whales and crocodiles. Exactly how big these great predatory sea dragons grew remains an intriguing question--and a source of confusion.
Oxford Giant Harpooned
Late in 1996, a published abstract (McHenry, et al.) and media stories hailed the discovery of a gigantic pliosaur forty percent larger the famous Harvard Kronosaurus. Extrapolating from Kronosaurus, a team of paleontologists estimated the new monster at 17 to 20 meters (around 56 to 65 feet) long--the size of a large sperm whale--with a weight 50 metric tons. Excitement centered on a single vertebra found years ago in the Oxford Clay and stored at the Peterborough Museum in Cambridgeshire, England. The long-neglected piece--for decades considered a fairly uninteresting anterior tail vertebra from a cetiosaurid sauropod dinosaur--was in rather poor condition and covered in blue paint for some unexplained reason. A number of plesiosaur experts reexamined the bone and detected similarities to a neck vertebra of a pliosaur--except that the Peterborough specimen was 245 mm across compared to 182 mm across in Kronosaurus! The reidentification raised a few problems--the specimen seemed to lack the characteristic plesiosaur foramina (small holes) on the underside of the centrum while the rib articulations had thin buttresses up to the neural arch, unlike in typical plesiosaurs. Now, after more study, the counterarguments have won the day, and with a bit of professional embarrassment, scholars have definitively harpooned the super-pliosaur--according to Colin McHenry, David Martill has shown that the bone belonged to a sauropod dinosaur after all.
Downsizing Kronosaurus
Kronosaurus has long enjoyed a reputation as the largest known plesiosaur, supposedly approaching 15 meters (50 feet) in length--based on the spectacular mounted specimen in the Harvard Museum of Comparative Zoology. Unfortunately, the fossil material on display was heavily restored, rendering many details suspect. Colin McHenry of Queensland University is currently restudying Kronosaurus, including important new material collected from Australia, and recently took a first-hand look at the Harvard specimen--with surprising results.
Despite its fame, Kronosaurus is currently a problematical genus. The type species Kronosaurus queenslandicus was based on jaw fragments found near Hughenden, central-western Queensland, Australia, in the 1920s. Later very large limb bones were added to the material. A Harvard expedition in 1931-1932 recovered two specimens of large pliosaurs in another region of Queensland: a giant incomplete skeleton, removed in part by dynamiting, from Army Downs, 35 miles north of Richmond, Queensland; and the rostrum (with upper and lower jaws) of a smaller individual from Grampian Valley, 30 miles north of Richmond.
Alfred Sherwood Romer helped mount the giant skeleton in the Harvard Museum of Comparative Zoology in the 1950s, but much of material was badly eroded. Consequently, Romer and his team were forced to fill in many details using plaster--and a rather generous amounts of imagination, earning the specimen the derisive nickname "Plasterosaurus" in some quarters. In addition to providing a heavily reconstructed skull, Romer gave the giant 30 dorsal vertebrae--a decision that stretched their Kronosaurus to an awe-inspiring restored length of 12.8 meters (42 feet), qualifying the genus for the Guinness Book of Records.
At the moment, where fact leaves off and fiction begins with Kronosaurus is not always clear--but the situation could soon improve. In recent years, Australian researchers have recovered new material belonging to giant pliosaurs from the Early Cretaceous, including a nearly complete skull about 2 meters (6 feet) long shown on the cable television program "Into the Unknown." Colin McHenry is currently studying the new finds for his Ph.D., and his work should clear up many questions about Kronosaurus. But Australia may not hold the whole story--a major find from South America is shedding important new light on giant pliosaurs.
In 1992 Oliver Hampe of the Frankfurt Museum described a very large short-necked plesiosaur found in Boyaca region of northern Colombia and dated to the upper Aptian of the Early Cretaceous. The specimen included a fairly complete skull and much of the skeleton except the tail. The animal was probably about 9 meters (30 feet) long in life. Though well aware of the current taxonomic uncertainties surrounding Kronosaurus, Hampe named his new species Kronosaurus boyacensis. He indicated that more research might eventually justify erecting a new genus for the South American form, however.
Kronosaurus boyacensis is unusual for its thickened ribs (not known in other specimens identified as Kronosaurus-- or in other large pliosaurs for that matter), though the bones need to be studied in cross-section to see if they were truly pachyostosed (made of heavy, dense bone) rather than spongy or hollow. A recently described short-necked plesiosaur from England Pachycostasaurus (Cruickshank, Martill & Noe 1996) had thickened ribs that were pachyostosed--a feature in this case thought to indicate a slow-moving form specialized for hunting arthropods along the sea floor. However, Pachycostasaurus was much smaller (3 meters (10 feet) long) than Kronosaurus boyacensis, and it is unlikely the two had similar lifestyles.
Since much of the backbone (except the tail) was preserved in Hampe's fossil, the specimen raises important questions about Romer's recontruction of Kronosaurus. Romer (1959) originally described the Harvard specimen has having "12 true cervical vertebrae, 2 'pectorals,' probably 30 dorsals, giving a total of approximately 44 presacrals, followed by 3 or 4 sacrals." Hampe gives a different count for K. boyacensis: 12 cervicals, 3 pectorals, 19 dorsals, 5 sacrals--for a total of 34 presacrals, 10 fewer than Romer's estimate. Inasmuch as Peloneustes, the only other closely related pliosaur known from nearly complete skeletons, had about 20 dorsals, Romer's estimate begins to look suspect. Colin McHenry has examined the Harvard skeleton and indicates that 10 restored dorsal vertebrae should be removed (pers. comm.). This drops the Harvard giant down to a more modest 8 to 9 meters (27 to 30 feet) in length, in line with Kronosaurus boyacensis.
Why did Romer overestimate the number of dorsals? While admitting that the "exact number of cervical plus trunk vertebrae is uncertain," Romer (1959) concluded that "the trunk is relatively long in the Australian giant, and the neck extremely short; although the number of presacral vertebrae in Kronosaurus and Peloneustes is approximately the same, the beginning of the dorsal series and the position of the shoulder appears to have been about 10 vertebrae farther forward in our specimen." Since Peloneustes had from 20 to 22 neck vertebrae compared to only 12 in Kronosaurus, unnecessarily adjusting for this difference apparently misled Romer into creating extra bones.
Bits of Behemoth, Pieces of Pliosaur
With the 60-foot Oxford monster sunk and the Harvard's giant kronosaur trimmed from around 50 to 30 feet, fans of truly gigantic pliosaurs need not despair. Colin McHenry reports that Leslie Noe is studying fragmentary pliosaur material from the Late Jurassic Oxford and Kimmeridge Clays of England that hints at genuine sea monsters in the possible 15+ meter (50+ foot) range. These bits and pieces could be from giant individuals of Liopleurodon ferox (Martill 1991), Pliosaurus macromerus, or maybe from a new taxon.
There were Jurassic sea giants in North America, too. Megalneusaurus ("great swimming lizard") was named by Wilbur C. Knight in 1898 and hailed as "the largest known animals of the Sauropterygia." Knight's published descriptions appear unreliable, however, and seem to confuse some basic details. The taxon is based mainly on a forelimb with paddle estimated by Knight at 2.209 meters (7.3 feet) in total length, though around 1.5 meters (5 feet) with a shorter humerus may be more realistic (Rufenacht 1997). (See the Tate Museum web site (http://www.cc.whecn.edu/tate/webpage.htm) for a photo of the specimen from the University of Wyoming). Robert Bakker is currently studying the material to provide a more detailed, updated description. Though the skull was not preserved, Bakker has suggested that the animal's head may have been 11 feet long-- at least based on the Harvard reconstruction of Kronosaurus.
The fossils of Megalneusaurus were found in the upper part of the Late Jurassic Sundance Formation of Wyoming. Williston (1903) stated that "A large portion of the type species is known; the parts so far described are the vertebrae and limbs." However, some of the original remains (ribs, vertebrae) mentioned by Knight and Williston now would appear to be lost, since the surviving specimen consists only of a fore-paddle, some vertebrae and fragments of a pectoral girdle--material many researchers (but not all) have considered inadequate to diagnose a genus and species. The relatively long and slender humerus (if the original description is correct) appears distinct in some details from the forelimbs of other Late Jurassic pliosaurs, supporting the idea that the Sundance giant represents a separate taxon and perhaps a particularly fast swimmer. Recently Weems and Blodgett (1996) have identified parts of a plesiosaur humerus from the Late Jurassic Naknek Formation of southern Alaska as belonging to Megalneusaurus, although the animal would have been only half as large as the type specimen individual. A full-grown Megalneusaurus may have been in the 11-to-12-meter (35-to-40-foot) range or larger--but such estimates should be greeted with caution until more hard facts are known.
For now, good specimens of these Late Jurassic mega-pliosaurs are proving as elusive as the equally large mega-ichthyosaurs from the Early Jurassic recently described by Chris McGowan (1996) from tantalizing, but still fragmentary evidence. With luck, museum-display-quality fossils of these spectacular sea dragons may yet come to light!
A special thanks to Colin McHenry for updated information on the "giant Oxford pliosaur" and for sharing some of his research into Kronosaurus. Also, many thanks to Ken Carpenter for much appreciated help at various stages in researching the topic.