Plesiosaurs

The greatly expanded shoulder and pelvic girdle in plesiosaurs provided attachment areas for the well-developed muscles that moved the limbs when the animal swam. Some of them like Thalassomedon, hanging from the ceiling, evolved a very long neck with as many as 70 vertebrae.

So even though there have been many claims that the Loch Ness monster is a plesiosaur left over from the past (and that a single animal somehow managed to remain alive for 100 million years), this is not the skeleton of "Nessie."* It is

* It now appears that the original "Nessie" is a fraud. On March 1 3,1 994, the London Sunday Telegraph published this headline: "Revealed: The Loch Ness Picture Hoax." The front-page story detailed a complicated plot involving a filmmaker and big-game hunter named Marmadukc Arundel Wcthcrcll, his son and stepson, and Dr. R. Kenneth Wilson, the man who allegedly took the famous "surgeon's photograph" that has been the basis for all Nessie the plcsiosaur Thalassomedon, a representative of a group of marine reptiles that thrived for about 100 million years from the Triassic to the Cretaceous periods and then, for reasons not understood, became extinct. Some had long necks, some had short necks; some were petite, and some were gigantic. Some had sharp little teeth like a python, while others had 8-inch daggers that rivaled the fearsome dental equipment of T. rex. Like all reptiles, they breathed air, but some, unlike any living reptiles, probably gave birth to living young underwater. (Female sea turtles come ashore to lay their eggs.) Sea turtles are the only living reptiles that have four flippers, so we have to compare the four-finned plesiosaurs to them, but whereas the turtles arc slow swimmers, some of the plesiosaurs were aggressive predators and had to chase down their prey. Turtles are encased in a pair of shells, known as the carapace (top) and the plastron (bottom :, and although the plesiosaurs had no shells, they did have a set of belly ribs called gastralia. How they actually swam —whether they "flew," "rowed," or performed some combination of the two —has been a subject for much palcontological speculation. Plcsiosaur fossils have been found all over the world, including England (the first one was found in 1811 by the famous fossilist Mary Anning), Kansas, Wyoming, Colorado, Germany, Russia, Japan, Africa, the Middle East, Madagascar, New Zealand, and Antarctica. A plesiosaur fossil found at Coober Pedy in Australia was completely opalized.

In this discussion and those that follow, it will become obvious that none of these creatures has a common name. We are used to referring to familiar animals by their vernacular names, such as lion, tiger, fox, whale, hummingbird, jellyfish, and so on. These animals also have scientific names, based on a system developed by Carl von Linne (Carolus Linnaeus) in the mid-eighteenth century. The lion is Panthera leo, the tiger is Pantbera tigris, the red fox is Vulpes vulpes, the blue whale is Balaenoptera museulus, the ruby-throated hum-

hunting since 1933. Christian Spurling, WcthercH's stepson, revealed on his deathbed that lie had participated in the manufacture and photographing of a foot-high model mounted on a toy submarine, and they had coerced Dr. Wilson —otherwise a man of impeccable credentials —to claim that he had taken the picture, when in fact it had been taken by Wetherell (Langton 1994).

Lehthyostega

Skeletal reconstruction of

Plesiosaurus dolichodeirus, one ojthe first clasmosaurs jound in England.

mingbird is Archilochus colubris, and the box jellyfish is ChironexJlcckcri. The first of these names identifies the genus (lions and tigers are in the genus Panthera), and the second identifies the species. In all cases, the genus (generic) name is capitalized, and the species (specific) name is not. Lion, tiger, blue whale, and so forth anY the names of these animals in English; in other languages, of course, they are different. But whatever the language, the scientific name remains the same. In a Polish, Chinese, Swedish, or Sanskrit discussion of the lion, its scientific name still appears exactly as you see it here: Panthera leo.

There arc no equivalents of lions or tigers among the marine reptiles. Just as with the terrestrial dinosaurs, the scientific name is the only one used. Tyrannosaurus rex is sometimes shortened to T. rex, but it is still known only by its scientific name. The same is true for Triceratops, Stegosaurus, Apatosaurus, Veloeiraptor, and even Archaeopteryx. The names of the major groups of marine reptiles can be rendered into English, as in ichthyosaurs, plcsiosaurs, and mosasaurs, but each of these is also the generic name of a species within the larger category, such as Ichthyosaurus communis, Plesiosaurus dolichodeirus, and Mosa-saurus hoffmanni. Some of these names, like dolichodeirus, are more than a little difficult to pronounce (it should be pronounced DOL-ik-o-DIE-rus), but because nobody ever refers to this species as "longncck" (the meaning of dolichodeirus), whenever this species appears in print it is Plesiosaurus dolichodeirus. In this introduction to the marine reptiles, you will encounter jawbreakers like Ophthalmosaurus, Brachauchenius, and Pachycostasaurus, but most of the names are somewhat easier and more comfortable on the tongue. The lack of common names might even help in recognizing the most significant characteristic of all the animals in this book: they are all extinct, and have been for millions of years. We would like to become more familiar with them, but time and nomenclature still remain formidable barriers.

As with many things palcontological, the evidence for the existence of long-extinct creatures consists of bones. Anatomists have given names to these bones, and they are the same for living animals as they are for extinct ones. The bones in your arm below the elbow are the radius and ulna, and they are the same — although sometimes of greatly differing size and proportion —for whales, zebras, chipmunks, and dinosaurs. Because there are very few instances in which anything but bones is preserved — and in those cases, rarely completely —those who describe extinct animals often limit themselves to detailed descriptions of the bones. If, for example, one finds a fossilized ichthyosaur with an upper jaw longer than that of another known fossil, the long-jawed ichthyosaur might be described as a new species. It might also be a juvenile as opposed to an adult, but other measurements can confirm its similarity to or difference from other known specimens.

Many of the descriptions of the creatures in this book —all of which are extinct, and all of which arc known only from fossils —consist primarily of osteological terminology. (Osteology is the study of bones.) Even the size of the eye, so critical to the differentiation of various ichthyosaur genera, relies largely on the circle of bony plates in the eye socket known as the sclerotic ring. At least part of the behavior of a large-eyed animal can be postulated from the size of the sclerotic ring, and although we might suggest that such a creature hunted at night or in reduced light circumstances, we can only guess as to what it hunted. (Sometimes, remnants of its last meal are fossilized too; in some cases, squid beaks or sucker hooks have been found in the fossilized predator's stomach.) The ability to separate one species from another depends on these detailed descriptions, and when one fossil is compared with another and found to be different in its particulars, the result might be a new species. Comparative anatomy therefore is one of the cornerstones of paleontology, but it often results in complex technical descriptions not easily understood by the nonspecialist. Here, for example, is Edward Drinker Cope's

(1868a) description of a plcsiosaur fossil that had been shipped to him from Kansas:

The species represented a genus differing in important features from Plesiosaurus and its near allies. These were the absence of diapophyses on the caudal vertebrae, and the presence of inferiorly directed plate-like parapophyscs which took the place of the usual chevron bones, in the same position; also in the presence of chevron-like bones on the inferior surfaces of the cervical vertebrae; further in some details of the scapular and pelvic arches. The diapophyses of the dorsal vertebrae originated from the centrum, and not from the neural arch.

Cope was reading the vertebrae backward, which resulted in his reconstructing the skeleton with the skull at the wrong end, but the point is the same: "the presence of inferiorly directed plate-like parapophyscs" docs not help the layperson understand what Elasuwsaurus platyurus looked like. Most descriptions of fossils are like this, and only the most creative of paleontologists, or those attempting to make particular points about limb structure and movement or tooth structure and prey items, will extrapolate from the bones to the lifestyle. Because we are not all trained paleontologists, we would like to learn more about a given species than we can discover from the absence of diapophyses.

Although there are no "rules" governing the form of a scientific paper, most of them follow a recognized pattern, generally consisting of abstract, introduction, discussion, conclusion, and references. In a paleontological discussion, there is usually a section describing the location of the fossil find, its condition, and its eventual disposition; if possible, an attempt is made to place it in a recognizable phylogenetic category, such as ichthyosauridac or mosasaundae, so the reader will know the general nature of the fossil. (It is not always easy to place, say, a single tooth in a known category, and there have been instances when the description of the tooth was accurate but its designation was not.) Such organization is possible for scientific papers, but it is considerably more difficult with books. This book has certainly been organized into broad categories (ichthyosaurs, plesiosaurs, pliosaurs, mosa-saurs), but the very nature of paleontology, with new specimens being un earthed or descriptions of old ones being revised in the literature, makes for an uneasy chronology. Does the writer talk about the sequence of discovery or the geological sequence of the specimens themselves? Is the "earliest" ich-thyosaur the oldest, or the first one found by fossil hunters? The first discovery of a fossil mosasaur—in fact, the first discovery of a fossil marine reptile of any kind —occurred in a Belgian limestone mine in 1780. Later christened Mosasaurus hoffmanni, it was found to be one of the last of the mosasaurs; therefore, depending on who's doing the structuring, the story of the mosa-saurs can cither begin at the end or end at the beginning.

Many early paleontologists attempted to reconstruct the lifestyle of the marine reptiles or the dinosaurs, but because this exercise was speculative, often the safest thing to do was to describe the bones and leave the lifestyle to someone else. It is true that much can be learned from the size and shape of the bones and the muscle attachments, and from this evidence it can be ascertained how the animal might have moved, but what it did when it got there is often an enigma. Much can also be deduced from teeth. Just like today's animals, extinct large animals with big, sharp teeth were probably aggressive killers, and it is not difficult to imagine that a big marine lizard with teeth like a crocodile or a killer whale might have behaved in a similar fashion to these powerful predators. Animals with flattened teeth that look suitable for grinding were probably plant or shellfish eaters. Osteological descriptions are critical to understanding the relationships of extinct animals, but because we cannot observe them in action, we can only guess as to how the animals swam or hunted or gave birth. Whenever possible, therefore, I describe the animal's size, teeth, flippers, and tail (or where the fossil was found and by whom) and refrain from differentiating various species by the relative size and shape of the shoulder blade or pelvic girdle.

In many cases, there is enough fossil evidence to allow a fairly accurate reconstruction of the animal's size and shape, but except for the obvious — eves are for seeing, teeth are for biting or tearing, backbones arc for support — conclusions can rarely be drawn about how the animal actually used this equipment. As will be seen, the existence of four flippers in plesiosaurs has presented a virtually unsolvable question of how the flippers were used to propel the animals through the water, but we have a pretty good idea of the prey subdued by the enormous teeth of the giant pliosaurs. Most ichthyo-saurs had a downward tailbcnd at the end of the vertebral column, which suggests that their swimming was powered by flexions of the lower lobe, so the comparison to the tail of a shark —where the vertebral column extends into the upper lobe —is obvious. We know what the tails of living sharks look like, but in only a few cases in which the outline of the entire animal was preserved do we know what the upper lobe of an ichthyosaur's tail looked like. The mosasaurs had scales, not unlike those of a snake, and at least some ichthyosaurs had smooth skin like that of whales and dolphins (Lingham-Soliar 2001); otherwise, with so few preserved fossilized impressions, we do not know whether the rest of them —particularly the plesiosaurs —were smooth-skinned or had scales, ridges, lumps, or bumps. And there is one thing that we know nothing about: we have no idea what color the marine reptiles were.

In order to re-create them, I could have made them monochromatic (I was, after all, working in pen and ink), but this seemed to detract from their vitality. We know that many living reptiles are brightly colored — think of snakes and lizards —so there is no reason to assume that the reptiles that lived 100 millions years ago were drab and colorless. Because these reptiles lived in the water, does that mean that they would be only countershaded, like many fishes —dark above and light below? Of course not. Fishes come in all the colors of the rainbow, and for good measure, some of them even light up. Well, then, aren't whales dull and monochromatic? Not on your tintype. Killer whales are spectacularly patterned in black and white; various dolphins sport elaborate haberdashery; and the fin whale, with its complicated asymmetrical pattern of swoops and swirls, may be the most intricately colored animal on Earth. In my drawings, I made some of the reptiles plain, and I patterned some of them with stripes, spots, and countershadings. All these color schemes are imaginary, designed (I hope) to breathe life into long-dead ichthyosaurs, plesiosaurs, and mosasaurs.

The fossil record is tantalizingly incomplete. There are any number of creatures that lived on Earth for which we have found no evidence whatsoever, so paleontologists have had to make do with the comparatively small number of fossils found and extrapolate from there. In Atlas of the Prehistoric World,

Douglas Palmer wrote, "there should be in the order of 500 million fossil species buried in the stratigraphic record. So far, paleontologists have described only a few hundred thousand fossil species. At less than 0.01 percent, this represents a very small sample of the estimated total." (Palmer says that real numbers are not available and his figure is a "guesstimate," but it is probably in the right range.) In 1994, David Raup summarized the fossil record for dinosaurs as follows:

The dinosaur fossil record illustrates some of the more severe sampling problems. According to a review by Dodson, 336 of the named species of dinosaurs are taxonomically valid. Of these, 50% are known only from a single specimen, and about 80% are based on incomplete skeletons. The 336 species are grouped into 285 genera, and of these, 72% have been found in the rock formations where they were first discovered, and 78% have been found in only one country. These numbers are astonishing if viewed as if the data were complete.

In many instances, a single tooth, bone, or bone fragment has been found, and because there is no other possible explanation, the paleontologist identifies the animal that originally owned these bones and declares that it once lived (or died) here. How do they know? Comparison with specimens described in books and journals and those seen in museum collections, experience, and, of course, location. A great many species have been described from limited fragments of evidence, and the discovery of a complete or even partially complete specimen is a rare occurrence in paleontology. There are some notable exceptions where fossils are particularly plentiful, and they are occasionally even fairly complete. These special sites include the Burgess Shale in British Columbia, Holzmaden and Solnhofen in Germany, the seaside cliffs of Dorset in England, certain areas of the Gobi Desert, the Hell Creek Formation of Montana and the Dakotas, the Bear Gulch Formation in Montana, the Niobrara Chalk Formations of Kansas, and the Yixian Formation of the Liaoning province of China. Even when the fossils are relatively complete, an enormous amount of work is required to extract them and prepare them for study or exhibition.

Consider the fossils unearthed by New Zealand's "Dragon Lady," Joan

Wiffen. She and her colleagues (most of whom are amateurs) have been prospecting on North Island and have found "$i partial clasmosaur and 8 partial pliosaur specimens . . . collected over 10 years of summer ficldwork in the Mangahouanga Stream, inland Hawkc's Bay." Wiffen and Moisley's 1986 paper, which occupies 47 pages in New Zealand's prestigious Journal of Geology and Geophysics, describes one skull (of Tuarangisaurus keyesi, "the only elasmo-saurid skull... so far found in New Zealand"), and assorted teeth, vertebrae, pectoral girdle elements, and shoulder blades. From these fragments, Wiffen and Moisley have been able to postulate the existence of "young juveniles through to adult forms and provide a representative record of Late Cretaceous elasmosaurs and pliosaurs that lived in shallow cstuarine or local offshore waters on the east coast of New Zealand during the Late Cretaceous." Not a single complete skeleton of any species was found, so the previously unsuspected presence of marine reptiles in New Zealand has been deduced from these fragments alone. In her 1991 book Valley ofthe Dragons, Wiffen wrote, "Those first fossil bones . . . were later identified as plcsiosaur vertebrae, and plesiosaur remains proved to be the most common bone fossils found at Mangahouanga. Of these, the discovery of a complete skull in 1978 was by far the most exciting. It was the first found in New Zealand and one of less than a dozen complete elasmosaur skulls known anywhere." New Zealand is not unique; most of the world's fossiliferous locations have produced only scattered bits and pieces for paleontologists to work with. The history of the hominids — which includes us —is based largely on teeth and scraps of bone that have been found lying on the ground in Africa. Somewhere, perhaps buried deep in the earth or encased in impregnable rock, is the rest of the evidence, but it is reasonable to assume that it will never be found. Our understanding of ancient life-forms is often based on the skimpiest of evidence, but we arc grateful that the earth has revealed as much as it has; much of our understanding of the processes of extinction and evolution has come from these shards of Earth's history.

None of this is meant to imply that paleontologists do not do proper science. They cannot directly observe the biology of their animals, but that does not mean that paleontology is any less rigorous a discipline than, say, ornithology. Despite the "data-poor" nature of their studies (missing parts, missing lineages, or even missing taxa), there are still many ways that good, testable hypotheses can be developed in studying the fossil record. Of necessity, the evidence collected is usually more indirect and circumstantial, but that does not make it less worthy than direct evidence, if used properly. Because paleontologists do not have the opportunity to observe the living subjects of their studies, their hypotheses must be tested by techniques of comparative biology, based on a thorough, detailed, and broad knowledge of living animals. There are no living ichthyosaurs, but they shared certain characteristics with sharks and dolphins, and comparisons with the living animals have given us great insights into the modus Vivendi of the fish lizards. Mosasaurs resemble varanid lizards in some respects and crocodilians in others, and comparative studies have enabled paleontologists to make numerous assumptions about the lives and phvlogenv of the great seagoing lizards. Unfortunately, there arc few living creatures that resemble plesiosaurs (only today's sea turtles propel themselves with four flippers), and so much about their lives is still a mystery. The absence of living models, however, does not preclude creative analysis of the fossils. There is a substantial body of literature devoted to the locomotion of plesiosaurs, all based on the shape, structure, and relationship of the bony elements. Indeed, one does not have to compare fossil structures to analogous structures in living animals; the hydro-dynamic capabilities of plcsiosaur flippers have been compared to the wings of birds, bats, and even airplanes (O'Keefe 1001c).

Throughout this discussion of the marine reptiles, I cite the various chronological periods, which have been named by geologists and paleontologists so that they would have a consistent timetable with which to associate particular fossil faunas. The span under discussion here is generally known as the Mesozoic era, which lasted from 248 million to 6$ million years ago. The Mcsozoic is further broken down into three large periods, the Triassic (248 to 209 million years ago), the Jurassic (208 to 144 million years ago), and the Cretaceous (144 to 65 million years ago). These have been further compartmentalized into smaller, tighter groups; the late Cretaceous, for example, is subdivided into the Coniacian age (89.9 million years ago), the Santonian age (85.8 million years ago), the Campanian age (83.5 million years ago), and the

Maastrichtian age (71.3 to 65 million years ago). The dating of these periods is fairly firm —almost, but not quite, "written in stone." The dating of rocks by analyzing radioactive isotopes that decay at a known rate (the half-life) has provided geologists with an absolute scale of dating, and quite often, fossils themselves can be used to establish chronologies. Many invertebrates whose timeline is known can serve as "index fossils," and the particular period can be identified by the presence of these creatures. The coiled shells of ammonites, for example, are very common fossils, and because many species were incredibly numerous, we can identify a particular moment in time by the presence of certain ammonite fossils. Therefore, if we find fossilized animals (such as mosasaurs) alongside these cephalopod fossils, we can fairly safely assume that they lived (and died) at the same time. (But as we shall see, it is not so easy to figure out their interactions.)

Breaking down the chronology of the earth into convenient segments is enormously helpful in establishing an evolutionary sequence for groups of animals; if some arc found, say, in Coniacian deposits, and similar forms are found in deposits that can be dated later as Maastrichtian, it can be assumed that the former are earlier and (perhaps) ancestral to the latter. This may not be accurate, for evolution does not necessarily consist of an unbroken chain of creatures that gradually morph into their modified descendants. Rather, evolution has been described as a bush, with branches that occasionally lead to other forms but more often end abruptly. As Darwin wrote in On the Origin of Speeies, "Though Nature grants long periods of time for the work of natural selection, she does not grant an indefinite period; for as all organic beings are striving to seize on each place in the economy of nature, if any one species does not become modified and improved in a corresponding degree with its competitors, it will be exterminated." That we have been able to identify the ancestors of any living (or extinct) creatures is the singular triumph of investigative paleontology, for it has been estimated that 99.9 percent of all the species that have ever lived are extinct.

When we see references to the Campanian (83.5 to 71.3 million years ago and Maastrichtian (71.3 to 65 million years ago) ages, we might he able to understand the sequence, but this compression into comfortable categories erases the almost incomprehensible extent of time involved. Everybody is familiar with the comparison of the history of life on Earth to a 24-hour day, in which humans have been around for only the last few seconds, but such a construct diminishes the actual passage of time, probably in the ever-present interest of anthropocentrism —the belief that the world revolves around us and that we can understand things only in human terms. But human beings, as we know us, have been around for 100,000 years, a fleeting one-tenth of a million. The time of the mosasaurs, often described as "only" 25 million years, is 250 times greater than the total experience of Homo sapiens, from the moment he (or she) picked up the first rock and shaped it into an ax head to the moment you are reading these words. The ichthyosaurs lasted four times longer than the mosasaurs, so their time on earth was a thousand times longer than ours. To grasp the pace of evolution, we don't need to speed up the film, we need to slow it down. We must not be misled by the idea that a million years is a mere blink of the eye. That is the case only in geological terms; the planet is believed to be 4.5 billion years old, but a million years is a very long time indeed. If a human generation is 20 years, then 5,000 generations have passed in the entire history ofH. sapiens, and ifwe were to last a million years — a highly unlikely scenario — 50,000 generations would pass. If a generation of ichthyosaurs was also 20 years, then there were 7.5 million generations of fish lizards in their 150-milIion-year history. Evolution is a slow process, but during their "reign," the dinosaurs diversified into hundreds of different species, grew to enormous sizes, and even sprouted wings and feathers. Various ancestral reptiles took to the water, and through an inexorably slow and gradual process — comparatively speaking, glaciers move at the speed of bullets —became the aquatic reptiles that will be visited here.

The aquatic reptiles are all believed to have descended from terrestrial forebears, but those forebears were descended from animals that lived in the water. The first terrestrial tetrapods of the late Devonian period (circa 354 million years ago), such as Acanthostega and lehthyostega, emerged from the water with their limbs modified to walk on land. Recent discoveries (Coates and Clack 1990) indicate that these early tetrapods had eight digits on the fore-limbs and seven on the hind. This plan did not perdure, and the five-finger arrangement dominated the future of reptiles and mammals. (Stephen J. Gould wrote an essay on seven- and eight-fingered tetrapods, suggesting that

Polydactyly was a stepping-stone on the way to the normal five-finger plan that dominates vertebrate morphology. He was so taken with the idea that the 1993 book in which the essay appears is entitled Eight Little Piggies.) In their return to the sea, the ichthyosaurs, mosasaurs, and plesiosaurs exhibited what Michael Caldwell (2002) calls "an intriguing aspect of tetrapod limb evolution ... the fin-to-limb-to-fin transition." He wrote:

The recolonization of the water has occurred repeatedly in distantly related tetrapod lineages, and in each case involves a major morphogenic reorganization of the limb to a paddle-like or fin-like structure. Among living groups of tctrapods this process of secondary radiation and mor-phogenetic evolution has produced the specialized limbs of cetaceans, seals, sea lions, manatees, walruses, and sea turtles. The fossil record also provides evidence of aquatic adaptation and extreme morphological specialization in a number of extinct lineages of diapsid reptiles: mosasaurs, ichthyosaurs, plesiosaurs, pliosaurs, their basal sauropterygian cousins, and extinct crocodiles.

The marine reptiles all lived in the water. 1 hey all breathed air: some the ichthyosaurs) arc known unequivocally to have given birth to live young in the water, but the evidence is less convincing for the plesiosaurs and mosasaurs; and they were all descended from terrestrial reptilian ancestors. Some of them were contemporaries in time and place, but the ichthyosaurs finally went extinct at the Cenomanian-Turonian boundary, which was 93.5 million years ago, or 25 million years before the demise of the last of the plesiosaurs and mosasaurs. The final extinction of the plesiosaurs and mosasaurs is thought to be somehow connected to the event that took out the nonavian dinosaurs 65 million years ago, but how an asteroid impact and its consequences eliminated some of the seagoing reptiles while sparing the turtles and crocodiles is not clear.

Despite the apparent similarities in habitat and lifestyle, however, the three major groups of marine reptiles were quite different and were not closely related. The ichthyosaurs were more or less dolphin-shaped, but they had four flippers to the dolphins' two and a vertical tail fin where that of the dolphins is horizontal. The mosasaurs were also tail-powered swimmers, but their propulsion came from sinuous oscillations of the tail, quite different from the short power stroke of the ichthyosaurs. (Theagarten Lingham-Soliar believes that at least one mosasaur species — Plioplatecarpus marshi— "flew" through the water, using its fins as well as its tail, but not many agree with him.) And the plesiosaurs, with their short tails and four powerful flippers, moved through the water somehow, but the experts disagree as to how this might have been accomplished. However they propelled themselves, the short-necked plesiosaurs (known collectively as pliosaurs) met normal resistance from the water; the long-necked ones seemed to have problems that they were obviously able to solve (a long neck held out in front of a swimming animal would act as a rudder), but we still can't figure out how they did it.

Some 65 million years ago, at the geological boundary of the Cretaceous and Tertiary eras (known as the K-T boundary), a massive asteroid slammed into the earth at a place that would eventually be identified as Chicxulub, off the Yucatan peninsula in the Gulf of Mexico. Because this impact coincides with the last recorded terrestrial dinosaurs, there are those who draw a connection between the two events and claim that the environmental havoc caused by the impact led to the dinosaurs' extinction. Others hold that different variables, such as climate change, massive volcanic eruptions, and elimination of their food source, were at least partly responsible for the demise of the dinosaurs. Now it is believed that today's living birds are actually descended from terrestrial, feathered dinosaurs and that dinosaurs are not extinct after all.*

* In The Evolution and Extinction of the Dinosaurs (1996), Fastovsky and Weishampel answer the question. I low can .1 bird be a reptile? "( Heady we have a decidedly different Reptilia twin the traditional motley crew of crawling, scaly, nonmammal, nonbird, nonamphibian creatures that most ol us think of when we think of reptiles. If it is true that crocodiles and birds are more closely related to each other than cither is to snakes and lizards, then a monophyletic group that includes snakes, lizards, and crocodiles must also include birds. I he implication of calling a bird a reptile is that birds share the derived characters of Reptilia, as well as having unique characters of their own."

In a 2002 paper entitled "Extinction of Ichthyosaurs: A Catastrophic or Evolutionary Paradigm?" Lingham-Soliar wrote:

Lumping ichthyosaurs, plesiosaurs, mosasaurs, marine crocodiles and marine turtles into a marine reptile assemblage is fraught with problems, particularly when viewed over a range of functional attributes, e.g. reproduction, feeding and locomotion, irrespective of phylogeny. Jurassic and Cretaceous ichthyosaurs arc the only marine reptiles thought to have used a thunniform mode of locomotion, with sustained speeds that were probably greater than that of the other marine reptiles. Some plesiosaurs are also thought to have achieved reasonably fast speeds although they employed a novel form of locomotion, viz. underwater flight. . . . The peculiar hydrodynamics of underwater flight in plesiosaurs had a number of definitive effects on the lifestyle of this unique group of marine reptiles. Ichthyosaurs gave birth to live young, a form of reproduction typical ofmammals rather than reptiles. . . . Positions in the food pyramid would also presumably have differed in ichthyosaurs, plesiosaurs, mosasaurs, crocodiles and marine turtles. Thunniform ichthyosaurs were fast efficient predators that fed on fish and squid. They were secondary consumers in the food pyramid, comparable with e.g. present-day bottlenose and common dolphins. Mo-sasaurs in the late Cretaceous were ubiquitous archetypal ambush predators, feeding on a range of marine animals including sharks, other mosa-saurs, birds, fish etc., and were probably at the top of the food pyramid. Their predecessors, pliosaurs (and certain fast swimming plesiosaurs such as Cryptoclidus) were probably adapted to both ambush and pursuit predation.

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