Kronosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Plesiosauria

Family: Pliosauridae

Genus:Kronosaurus

Species:Kronosaurus queenslandicus

Introduction

Kronosaurus was a marine reptile that lived during the age of dinosaurs, and is often confused for a dinosaur itself. It is one member of the Pliosaurides family that arose in the Early Jurassic period and lived through the Cretaceous period, 201–66 million years ago. During that time, Kronosaurus is thought to have been a top predator of the seas for a period of about 25 million years. It is thought that the pliosaurs were able to withstand several extinction events because their worldwide distribution permitted them to find sufficient prey populations to maintain a surviving population. This undoubtedly contributed to the longevity and relative abundance of pliosaurs in the fossil record. They are characterized by long, massive skulls, shortened neck structures, and streamlined bodies equipped with two sets of large, oar-like flippers.

Pliosaurs have been found throughout the world, indicating that they were pelagic creatures. Throughout the span of their existence, the action of plate tectonics pulled apart the supercontinents of Pangaea to form Laurasia and Gondwana, which were then further fragmented into the continents more or less as they are known today. As the continents moved into their different positions, various oceans and seas were formed and unformed. The Tethys Sea split Laurasia and Gondwana between the regions that became Europe, Africa, and Asia. The North American continent was later bisected by a large inland sea, the Western Interior Seaway, which today corresponds to the plains of the Midwest. The relatively shallow waters of this sea were home to abundant sea life, including the pliosaurs and other plesiosaurs.

The pliosaurs were a diverse clade, spanning a wide variety of sizes. They are believed to have been deep water species that fed on fish, belemnites, and even other plesiosaurids. The pliosaurs also contained the largest marine carnivores ever known, including Liopleurodon, “Predator X” (Pliosaurus funkei), and Kronosaurus. These creatures were large enough to make a meal of Tyrannosaurus rex and even of Carcharodon megalodon, the giant ancestor of the present day Great White shark.

Classification

The prehistoric marine reptiles collectively known as plesiosaurs were traditionally divided into the subgroups Plesiosauroidea (plesiosaurs) and Pliosauroidea (pliosaurs). As a pliosaur, Kronosaurus possessed a streamlined, teardrop-shaped body designed to fly through the water. Its motion was driven by two sets of lateral flippers that clearly appear to be modified legs adapted to the aquatic environment. Pliosaurs are also characterized by a much shorter neck structure than the plesiosaurs, supporting a long, narrow flattened head with massive jaws that makes up fully one quarter of the overall length of the creature. This is in contrast to the long, narrow necks and small heads that are typical of the plesiosaurs.

In the Linnaean system of classification, the pliosaurs share certain features with plesiosaurs that identify them as a subset of the Plesiosauria. These include such things as general body shape and flipper structures. At the same time, the shortened neck structure and disproportionate head and jaw size of the pliosaurids immediately sets them apart from the plesiosaurids. The issue is somewhat confused, however, in that some Cretaceous pliosaurids are believed to have evolved from a branch of the long-necked Jurassic plesiosaurids. This, coupled with a scarcity of complete fossil pliosaurid remains, has made the identification and classification of the pliosaurs a somewhat less-than-exact science. Many scientists prefer to use of the terms "plesiosauromorph" and "pliosauromorph" to refer to the two classic body shapes rather than use physical appearance for any formal grouping.

The field of cladistic analysis, which became more common among scientists in the late twentieth and early twenty-first centuries, seeks to draw evolutionary relationships among creatures by placing them in clades or groups with shared and sometimes overlapping characteristics. In a cladistic analysis, some scientists place Kronosaurus in the Brachauchenidae clade, with Brachauchenius. Other scientists have placed Kronosaurus in a sister relationship with Peloneustes, a pliosaur with blunt teeth thought to feast on ammonites, a type of Mesozoic shellfish.

Anatomy

The pliosaurs have a very distinct anatomy. The overall torpedo shape suggested by their fossil remains would have allowed them to fly through the water, powered by two sets of flippers. Some are thought to have been the fastest of swimming creatures, although their shape and size would not have been conducive to high maneuverability. Their streamlined shape was complemented by relatively short neck structures, with fewer cervical vertebrae than those of the plesiosaurs. Nevertheless, there is considerable variation in the cervical structures of different species of pliosaurs. Macroplata, for example, had twenty-nine cervical vertebrae, while Kronosaurus had only thirteen.

The neck structure of the pliosaurs supported the creatures’ long head and the oversized jaws that more closely resembled those of crocodiles than of plesiosaurs. The upper jaw of Kronosaurus made up fully 25 percent of the creature's overall length and was as long as some other pliosaurs just by itself. Estimates of the bite strength of Kronosaurus have placed it as among the highest ever, at about 33,000 pounds per square inch (psi). Though massive, the jaws of the pliosaurs were long and relatively narrow, armed with an impressive array of triangular, dagger-like teeth that in Kronosaurus were the size of large bananas. The head shape blended smoothly into the neck structure and then the body, which tapered quickly to the point of a tail from behind the rear set of flippers. This provided an overall body shape that offered the absolute minimum of resistance to movement through the water. The two sets of flippers, situated at one-third and two-thirds of the length of the body, probably looked and functioned more like wings. In movement, it is likely that they worked in opposing motions and in the manner of wings.

Intelligence

One system for estimating animal intelligence is encephalization quotient (EQ). The EQ of a species is determined as a relationship between the size of the brain and the mass of animals of similar size. The trend is seen in the terrestrial dinosaurs that EQ and body size exhibit a reverse relationship, so that the larger the animal, the lower is its EQ. The long-necked sauropods such as Brachiosaurus and Apatosaurus had very small heads relative to their massive bodies. Correspondingly, their brain size was also very small, on the order of 0.05 as compared to about 7.8 for modern humans. The similarly built plesiosaurs, assuming that the definition of EQ applies to such aquatic species in the same manner that it applies to land creatures, would likely have had similarly low EQ values.

Pliosaurs such as Kronosaurus, on the other hand, were equipped with very large heads that may have housed similarly large brains. This would have produced a significantly higher value for the EQ, presumably corresponding to higher natural intelligence and problem-solving ability necessary for successful pursuit predators.

Reproduction and Population

Though it is not known with certainty, pliosaurs were likely viviparous because of their size and aquatic nature. No evidence has been found to demonstrate that they withdrew to land to deposit eggs in the manner of other reptiles. It is much more probable, given the body structure and size of the pliosaurs, that offspring were born alive in environments offering some protection from other predators in the open sea. However, this does not preclude the possibility that some of the smaller species of pliosaurs may have been egg-layers, capable of withdrawing to the shoreline to deposit eggs in the sand, as turtles and other reptiles do.

Pliosaur fossil remains have been found throughout the world. New discoveries expand the diversity of the pliosauridia family in many ways. In 2006, a previously unknown pliosaur called “Predator X” (later named Pliosaurus funkei), which may have been as much as 20 percent larger than Kronosaurus queenslandicus, was discovered on the Norwegian island of Svalbard. Smaller pliosaurs have also been discovered there and elsewhere. The known pliosaurs now cover a size range from about 1.5 meters (5 ft), as in Umoonasaurus demoscyllus, to about 15 meters (50 ft). The diversity and world-wide distribution of fossil pliosaur remains, as well as the extended period of time in which they existed, indicate that they were well adapted and may have been quite numerous, although there is little evidence to indicate their numbers.

Diet

The pliosaurs were distinctly carnivorous. Their bodies were built for high-speed chases through the water, and the teeth in their oversized jaws are designed only for catching and holding prey animals. A small number of pliosaurs have been identified by teeth that would function well to crush the shells of belemnites or ammonites and other shelled creatures, but none have ever been found that indicate an herbivorous component to the pliosaur diet.

The seas of the world throughout the span of the pliosaurs were abundant with fish and other creatures, especially the large, shallow inland seas that existed in various locations at various times. The sheer abundance of suitable prey ensured that the pliosaurs could find food somewhere and survive almost any extinction event, five of which are believed to have occurred throughout the Mesozoic age, before the Chicxulub impact that ended the age of dinosaurs.

Behavior

The pliosaurs were built for speed and agility in the water. Their motion would probably have been very similar to that of present-day penguins and seals, which have a very similar overall body shape. Although pliosaurs were fast and agile, some prey creatures were probably more maneuverable in short runs and more able to evade immediate capture. This might indicate that pliosaurs primarily fed on creatures within their guild, that is, their own size range. The identification of pliosaur fossil remains of different species and sizes in the same location tends to support this notion.

It is not known whether the pliosaurs shepherded their young until they were of an age to live independently. This does seem probable, however, as the behavior is more conducive to the long-term survival of a species in an inimical environment populated by predators.

Habitat and Other Life Forms

The habitat of the pliosaurs can be described simply as pelagic. The open sea was their realm. It is likely that they would have lived and hunted in the relatively shallow regions nearer to shore, where the variety of suitable prey would have been the richest and most abundant. The largest pliosaurs, such as Kronosaurus, Liopleurodon and Predator X, were undoubtedly pelagic hunters, living out their lives in the deeper waters. Because their size rivaled that of modern whales, stranding on shore would have been deadly. Smaller pliosaurs, however, would possibly have been able to work their way back to the open water by using their flippers to assist movement, as seals and walruses do today.

The waters of the Mesozoic teemed with a rich variety of other creatures, from jellyfish and ammonites to large fish and reptiles. The pliosaurs would have shared their environment with such contemporary creatures as Paleoniscum, Saurichthys, Aspidorhynchus, Pholidophorus, Henodus, Lariosaurus, Ceresiosaurus, Pistosaurus, various plesiosaurs and ichthyosaurs, and a great many other types of creatures. The Brachauchenius, which grew to a length of 13 meters (40 ft) in the Late Cretaceous period, is the last known pliosaur to inhabit the North American continent.

Research

The group of animals called pliosaurs has been known for almost 200 years, since the identification of fossil remains of Sinopliosaurus by paleontologist G. Young in 1820. Liopleurodon was described by Sauvage in 1873, Brachauchenius by Williston in 1903. Several other species were discovered and described during this same period, and many more since then. The first known Kronosaurus fossils were collected in the late nineteenth century in Australia, and entered the collection of the Queensland Museum. It was not until 1924 that Heber Longman, then director of the museum, identified the new species Kronosaurus queenslandicus, which would become the type species. Other examples have been found elsewhere in Australia, and a related species was found in Colombia.

The number of Mesozoic species, dinosaurs and non-dinosaurs alike, that have been identified to date is relatively low. Of these, only about one quarter are known by complete skeletons, and many are known only by a few identifiable skeletal fragments. The full extent of prehistoric life will probably never be known. Only more exploration and careful research will reveal what species are still to be discovered.

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