In the Early Cretaceous period, just over 100 million years ago, Australia lay alongside Antarctica, which straddled the South Pole as it does today. Australia’s southeastern corner, now the state of Victoria, lay well inside the Antarctic Circle. At that time, the region hosted an assemblage of animals and plants that lived under climate conditions having no modern analogue. The average temperature appears to have ranged from frigid to low temperate. Through the long winter, the sun did not shine for weeks or months at a time. Many dinosaur lineages survived in this strange environment after they had died out in other places. At least one member of the group evolved an adaptation to the cold and to the dark that is interesting both in itself and for what it tells of the passing of a biological epoch. If global cooling indeed killed the dinosaurs, as many paleontologists have suggested, then Australia’s species were the ones most likely to have survived the longest. Did their adaptations to an already marginal climate help them survive a sharp cooling trend, one that caught species living on other continents unprepared? Although the Cretaceous fossil plants of southeastern Australia have been studied for more than a century, the animals remained mostly hidden until recently. In 1903 geologist William Hamilton Ferguson found two bones that have had a bearing on later paleontological work—the tooth of a lungfish and the claw of a carnivorous dinosaur, assigned to the theropod genus Megalosaurus. For the next 75 years, as no further finds joined them, these bones lay neglected in a cabinet in Museum Victoria. Then, in 1978, two graduate students at Monash University, Tim F. Flannery and John A. Long, discovered near Ferguson’s original site the first specimens of a trove of dinosaur bones embedded in sediments from the Early Cretaceous. These discoveries—only an hour and a half’s drive southeast of Melbourne—encouraged paleontologists to prospect other coastal sites. In 1980 we struck a rich lode in the Otway ranges, which the Victorian government, at our suggestion, has since named Dinosaur Cove. There, for a decade—with the help of Earthwatch and other volunteers, the National Geographic Society, the Australian Research Council and Atlas Copco, a manufacturer of mining equipment—we spent three months out of every year chiseling, hammering and on occasion blasting tunnels into the fossil-bearing strata. With Dinosaur Cove worked out in 1994, effort has since been concentrated at a site about 200 kilometers east, called Flat Rocks. The rocks there are about 10 million years older than those at Dinosaur Cove. Sediments at Flat Rocks, Dinosaur Cove and other sites of similar character were formed when violent, seasonal streams swept broad floodplains of their accumulated bones and plant life, depositing this flotsam and jetsam at the bottom of shallow stream channels. These deposits appear along the southern Victorian shore because only there could gnawing waves expose the sediments laid down in the rift valley that formed when Australia and Antarctica went their separate ways, as did the other fragments of Gondwana, an ancient supercontinent [see box on opposite page]. Only three fossil sites from the same period have been found inland, one in sediments laid down under far quieter conditions at the bottom of an ancient lake. This inland site has therefore yielded some uncommonly well preserved specimens. It must be noted that southeastern Australia’s dinosaurs are known from about 8,000 individual bones and four partial skeletons. Only a few hundred of the bones can be assigned to a given species or genus. What they lack in number, however, they make up for in scientific interest. All efforts at interpretation revolve around the estimation of temperature, for which three methods have been tried. Robert T. Gregory of Southern Methodist University and his associates infer Australian paleoclimate from the ratio of oxygen 18 to oxygen 16 trapped in concretions in ancient rocks. They find that mean annual temperatures probably approached zero degrees Celsius but might have reached as high as eight degrees C. Such values occur today in Hudson Bay, Saskatchewan (zero degrees C), and in Minneapolis and Toronto (eight degrees C). Work by Andrew Constantine of Origin Energy on structures preserved in the rocks near where the dinosaur bones are buried provides evidence for the former existence of permafrost and ice wedging, as well as patterned ground and hummocky ground. Such features are formed today in regions with mean annual temperatures of three degrees C below zero to three degrees C above zero. These structures are most obvious only three meters stratigraphically below the Flat Rocks locality where dinosaurs and mammals have been found. Evidence for the occurrence of permafrost had never before been reported in association with dinosaurs. Robert A. Spicer of the Open University in the U.K. and Judith Totman Parrish of the University of Idaho instead deduce temperature from the structure of ancient plants, arriving at the somewhat higher mean annual temperature of 10 degrees C. Their research with colleagues has demonstrated that polar Australia supported conifers, ginkgoes, ferns, cycads, bryophytes and horsetails but only a few angiosperms, or flowering plants, identifiable by a sprinkling of pollen. The angiosperms were then just beginning to spread into new niches. Perhaps they got their start by exploiting weedy ecological systems in the rift valleys that formed as the supercontinent split apart. Spicer and Parrish noticed that evergreens, which provided forage in all seasons, had thick cuticles and other structural features that indicate adaptation to cold or dryness (perhaps brought on by winter freezing). Deciduous plants offer another climatic clue: they seem to have lost all their leaves at once. These mass falls may have been triggered by darkness or cold. Drought, however, probably did not serve as a constant cue—the sedimentary record and the abundance of ferns and bryophytes argue for conditions that were moist in all seasons except perhaps winter. Surviving the Cold If the higher estimate of mean temperature is correct, Australia was both temperate and subject to a period of continuous darkness every year—a combination with absolutely no modern counterpart. The winter night lasted between six weeks and four and a half months, depending on the true paleolatitude. Because the lower extreme of temperature would then have fallen well below the mean, most of the vertebrates preserved as fossils must have lived quite close to their thermal limits. Some, such as lungfish, cannot now breed in waters colder than 10 degrees C. If, on the other hand, the lower estimate is correct, it becomes more than a typical scientific challenge to understand how this paleocommunity functioned at all. Before seriously attacking this problem, scientists will first have to demonstrate that it exists. To refine the temperature estimate, a multidisciplinary team is comparing floral, geochemical and other forms of evidence. Nothing in this fauna is quite so peculiar to the region as the koala is today, for although the species and genera were local, they belonged to cosmopolitan families. Yet their adaptations are striking, as is the fact that some survived beyond the time of demise for their families elsewhere. Among such anachronisms—or relicts—are the temnospondyl amphibians, possibly ancestors of modern amphibians. Most paleontologists had thought this group went extinct by some 160 million years ago, in the Jurassic. In the past few decades, however, Michael Cleeland and Lesley Kool of Monash University found three jaws from this group in Victorian sediments dating from the Early Cretaceous. Two of the jaws were unmistakable because their teeth had the labyrinthine infolding of the enamel that gives this group its common name: labyrinthodonts. At least one large species of temnospondyl lived in polar Australia about 120 million years ago, several million years after the group had died out elsewhere. How did they survive? We suspect that the cool weather preserved the animals from competition with crocodiles, which were probably poorly adapted to the conditions prevailing in southeastern Australia until the onset of climate warming during the last five million years of the Early Cretaceous. The hypothesis rests on the fact that contemporary crocodilians now live in waters no colder than 10 degrees C, whereas some modern frogs and salamanders can be active in meltwater from snow. At least nine different types of carnivorous theropod dinosaurs once lived in what is now Victoria. Australovenator is known from a partial skeleton, about 5.5 meters long, found in northeastern Australia about 2,000 kilometers north of Victoria. It is an allosauroid and thus a relative of the well-known Allosaurus from North America and Africa. An ankle bone from Victoria appears to belong to either this same genus or one closely related to it. A spinosaur, a type of theropod dinosaur with tall spines that projected above its back, might have been even larger. A single fragment of a vertebra is the only evidence that this primarily fish-eating animal once existed anywhere in Australia. Also present in Victoria were at least seven smaller-bodied theropods. The one best represented in the fossil record, by numerous isolated bones and teeth, is another species of allosauroid. In stark contrast, each of the other six smaller-bodied species is represented by only one or a few isolated bones. This meagerness of evidence for the former presence of so many taxa suggests that more of these smaller-bodied theropods will be found with further work in Victoria. Among the smaller theropods was Timimus hermani, known from only two bones. It was a tyrannosauroid distantly related to Tyrannosaurus rex and had unusually long and slender femora, suggesting it may have been particularly fleet-footed. This tyrannosauroid and two other, smaller theropods—the ostrichlike ornithomimosaur and a possible oviraptorosaur dubbed “the egg thief”—are otherwise not known from Gondwana but are found on the northern continents. Whereas tyrannosauroids in the Northern Hemisphere significantly predate their Australian counterparts, the ornithomimosaur and oviraptorosaur, if they are correctly identified, first appeared during the same geologic period in both hemispheres. Fossils of the other three smaller-bodied Victorian theropods—an allosauroid, a sickle-clawed dromaeosaurid and a horned ceratosaur—have been found elsewhere in Gondwana and the Northern Hemisphere. Besides the theropods, another dinosaur group that has been identified belongs to the neoceratopsians, or horned dinosaurs. Identification is tentative because it is based on just two ulnae (part of the lower arm), but the similarity of one of them to Leptoceratops, a browser the size of a sheep, is uncanny. Previously, all neoceratopsian records dated from the Late Cretaceous and, with the exception of a few bones from Argentina, came from the Northern Hemisphere. Reports indicate the existence of Early Cretaceous neoceratopsians in Utah and China. This dinosaur family may also have arisen in the southern supercontinent. In addition to dinosaurs, the region provides evidence for mammals that appear to be among the earliest members of their groups. The minuscule Bishops resembles the living spineless hedgehog Neotetracus. This animal may have been a placental. If so, it is as old as the oldest placentals reported from the Northern Hemisphere and twice the age of the oldest marsupial yet found in Australia. This age is surprising because the domination of Australia by marsupials today is typically explained as the result of land-dwelling placentals reaching the continent long after the marsupials. Another mammalian group, whose presence is no surprise, comprises the monotremes. An isolated limb bone of one of them has a structure suggestive of a more upright stance than either the echidna or the platypus. A second species is by far the smallest monotreme, weighing only 1 percent as much as any other living or fossil member of the group. The Australian Early Cretaceous also reshaped forms that continued to flourish in other regions. By far the most successful such group consisted of the “hypsilophodontid” dinosaurs, an informal subdivision of the basal Ornithopoda. These animals, most of them hardly larger than a chicken, were bipeds built for speed, with large hind legs, small but well-developed hands, substantial tails and—for the most part—herbivorous habits. They thus resembled wallabies in both shape and ecological role. The family Hypsilophodontidae was common throughout the world from the Middle Jurassic to Late Cretaceous times, but its prominence reaches an absolute and relative peak in the Victorian sediments. Not only do hypsilophodontids constitute most of the dinosaur remains, they are also represented by four to five genera, depending on the taxonomic criteria one uses, and five to six species. Other areas, some much more richly endowed with dinosaur species, never harbored more than three kinds of hypsilophodontids at a time. Something clearly favored the diversification of this group in polar Australia. Big-Eyed Foragers A particularly intriguing adaptation of at least one species of polar hypsilophodontid is suggested by the magnificently preserved brain cast of Leaellynasaura amicagraphica (named after our daughter, along with friends of the Museum Victoria and the National Geographic Society). The brain, unusually large for a dinosaur of this size, bears the marks of optic lobes, the relative size of which is easily the greatest ever documented in a hypsilophodontid. How is one to interpret these enlarged lobes? We hypothesize that they enhanced the animals’ ability to see in the dark, enabling them to forage effectively during the long winter months. There would have been no lack of food then, for those capable of seeing it: the herbivores could have lived off evergreens and deciduous leaf mats, and the carnivores could have hunted the herbivores. This hypothesis also explains why this group came to dominate the polar environment in the first place. Hypsilophodontids everywhere in the world had large eyes and, presumably, acute vision. That trait could have given them their foothold in polar Australia. Once established in this “protected” environment, the hypsilophodontids could have competed with one another to produce the observed diversity of genera and species, perhaps all sharing hypertrophied optic lobes. If the animals foraged at night, they must have been active at freezing or subfreezing temperatures. This feat goes far beyond the cold tolerance of any modern reptile, even the New Zealand tuatara, Sphenodon punctatus, which can remain active at five degrees C, provided it can sun itself. Leaellynasaura could have survived solely by maintaining a constant body temperature, eating frequently, as birds do in wintertime. Pterosaurs—the flying reptiles—and the heavily armored ankylosaurs also appear in the Victorian fossil record, but the remains are so fragmented that they tell us little about the animals’ lives. Much can be gleaned from one handful of teeth, however, for they come from plesiosaurs. These reptiles, not themselves dinosaurs, generally paddled the seas, but here two plesiosaur species quite different from one another inhabited freshwater in the ancient valley between Australia and Antarctica. They thus recall the Ganges River dolphin, one of the few cetaceans that live in freshwater. One of the two species may have been a pliosaur—a type of plesiosaur with a short neck and elongated skull—whose body was four meters or more in length. The other was a typically long-necked, small-skulled plesiosaur. This obvious difference in body plan may explain how the two plesiosaur species managed to coexist in the same environment, being adapted to feeding on quite different prey. The sauropods are one of the few major dinosaur groups that are absent. These giants, familiar from the example of Apatosaurus, lived at that time in Australia’s lower latitudes. Not one, however, has been found farther south. The apparent restriction of these large dinosaurs to lower latitudes in the Cretaceous of Australia may be real or merely an artifact of sampling. We worry about this question because the floodwaters that broke out of rain-swollen rivers would have collected small and medium-size bones but left large ones. The body of a sauropod would have stayed put rather than floating to a place where many specimens were concentrated in the small flood channels, which were no more than five to 10 meters in width and 20 to 30 centimeters in depth. This factor may explain the absence of the large bones of sauropods but not their tiny teeth. Thus, they did not seem to have reached polar Australia, although their vertebrae are known from polar New Zealand and the Antarctic Peninsula. Yet we suspect there was an underlying tendency toward small body size in these polar environs. None of the hypsilophodontids, it must be remembered, stood taller than a human, and most were barely knee-high. The ornithomimosaur is equally unprepossessing, and the protoceratopsid and the ankylosaur are each no bigger than a sheep. A single fragment of a claw constitutes our sole record of a large dinosaur—a carnivore, apparently similar to Baryonyx of England—which may have measured up to eight meters in length. The rare evidence for the largest dinosaur in polar Victoria is based on the smallest identifiable part of its skeleton. Thus, we cannot rule out the possibility that a systematic bias against the preservation of larger fossils is the reason that large dinosaurs rarely are found there. But although there is evidence for at least one large dinosaur in polar Victoria, the dinosaur assemblage does seem to have consisted primarily of smaller individuals. This pattern contradicts the classic scaling laws formulated by Carl Bergmann and Joel Allen in the 19th century. According to these laws, animals in a given lineage tend to become larger and more compact as the average temperature of their environment falls. This trend is exemplified by the comparison of mountain lions in Canada with pumas of Central America and of human populations in the subarctic and tropical zones. Other factors also determine body dimensions, especially the size of the territory in which a population lives. Individuals found on islands are often smaller than their mainland counterparts. For example, there were dwarf elephants on the ancient Mediterranean islands, and pygmy mammoths were found in 4,000-year-old sediments on islands off the north coast of Siberia. Dwarfism may be a response to selective pressure to increase the number of individuals so as to ensure a gene pool diverse enough for the species to survive in a restricted area. This effect has also been observed on peninsulas—and ancient southeastern Australia was a peninsula of the Gondwana landmass. The dinosaurs on that peninsula were trapped virtually at the ends of the earth. Their direct path north was blocked by a vast inland sea, which they could have passed only by going hundreds of kilometers to the west before wheeling about to the north. At the end of such labors, they would have been able to catch, at most, an hour of sun a day in winter. Migration would have made little sense for such small animals. Less formidable barriers sealed in the dinosaurs of the one other polar site that has yielded large quantities of fossils: the North Slope of Alaska. The dinosaurs there had a clear north-south corridor along which they could migrate with ease. It is significant that those dinosaurs were big—at least equal in size to caribou, wildebeest and other modern animals that migrate. Safe Haven in Gondwana One must question whether animals so superbly adapted to the cold and the dark could have been driven to extinction by an artificial winter, such as is supposed to have followed a cataclysmic event at the boundary between the Cretaceous and Paleogene. It is proposed that the cataclysm, perhaps a combination of a collision with a comet or asteroid and a series of volcanic eruptions, suffused the atmosphere with a blanket of dust, excluding sunlight and freezing or starving most animals to death. We suspect, however, that no such artificial winter could have killed the dinosaurs unless it lasted for a long time, certainly more than a few months. Otherwise at least a few of the polar dinosaurs would have survived the cataclysm. Of course, it is possible that a different development had already ended the reign of southern Australia’s dinosaurs by the end of the Cretaceous. English writer Arthur Conan Doyle once dreamed of a plateau in South America that time forgot, where dinosaurs continued to rule the land. Reports in the early 1990s that dwarf mammoths survived to early historical times, on islands off the coast of Siberia, give force to such speculation. If dinosaurs found a similar haven in which they outlived the rest of their kind, then we think polar Gondwana, including southeastern Australia, is a likely place to look.
Many dinosaur lineages survived in this strange environment after they had died out in other places. At least one member of the group evolved an adaptation to the cold and to the dark that is interesting both in itself and for what it tells of the passing of a biological epoch. If global cooling indeed killed the dinosaurs, as many paleontologists have suggested, then Australia’s species were the ones most likely to have survived the longest. Did their adaptations to an already marginal climate help them survive a sharp cooling trend, one that caught species living on other continents unprepared?
Although the Cretaceous fossil plants of southeastern Australia have been studied for more than a century, the animals remained mostly hidden until recently. In 1903 geologist William Hamilton Ferguson found two bones that have had a bearing on later paleontological work—the tooth of a lungfish and the claw of a carnivorous dinosaur, assigned to the theropod genus Megalosaurus. For the next 75 years, as no further finds joined them, these bones lay neglected in a cabinet in Museum Victoria. Then, in 1978, two graduate students at Monash University, Tim F. Flannery and John A. Long, discovered near Ferguson’s original site the first specimens of a trove of dinosaur bones embedded in sediments from the Early Cretaceous.
These discoveries—only an hour and a half’s drive southeast of Melbourne—encouraged paleontologists to prospect other coastal sites. In 1980 we struck a rich lode in the Otway ranges, which the Victorian government, at our suggestion, has since named Dinosaur Cove. There, for a decade—with the help of Earthwatch and other volunteers, the National Geographic Society, the Australian Research Council and Atlas Copco, a manufacturer of mining equipment—we spent three months out of every year chiseling, hammering and on occasion blasting tunnels into the fossil-bearing strata. With Dinosaur Cove worked out in 1994, effort has since been concentrated at a site about 200 kilometers east, called Flat Rocks. The rocks there are about 10 million years older than those at Dinosaur Cove.
Sediments at Flat Rocks, Dinosaur Cove and other sites of similar character were formed when violent, seasonal streams swept broad floodplains of their accumulated bones and plant life, depositing this flotsam and jetsam at the bottom of shallow stream channels. These deposits appear along the southern Victorian shore because only there could gnawing waves expose the sediments laid down in the rift valley that formed when Australia and Antarctica went their separate ways, as did the other fragments of Gondwana, an ancient supercontinent [see box on opposite page]. Only three fossil sites from the same period have been found inland, one in sediments laid down under far quieter conditions at the bottom of an ancient lake. This inland site has therefore yielded some uncommonly well preserved specimens. It must be noted that southeastern Australia’s dinosaurs are known from about 8,000 individual bones and four partial skeletons. Only a few hundred of the bones can be assigned to a given species or genus. What they lack in number, however, they make up for in scientific interest.
All efforts at interpretation revolve around the estimation of temperature, for which three methods have been tried. Robert T. Gregory of Southern Methodist University and his associates infer Australian paleoclimate from the ratio of oxygen 18 to oxygen 16 trapped in concretions in ancient rocks. They find that mean annual temperatures probably approached zero degrees Celsius but might have reached as high as eight degrees C. Such values occur today in Hudson Bay, Saskatchewan (zero degrees C), and in Minneapolis and Toronto (eight degrees C).
Work by Andrew Constantine of Origin Energy on structures preserved in the rocks near where the dinosaur bones are buried provides evidence for the former existence of permafrost and ice wedging, as well as patterned ground and hummocky ground. Such features are formed today in regions with mean annual temperatures of three degrees C below zero to three degrees C above zero. These structures are most obvious only three meters stratigraphically below the Flat Rocks locality where dinosaurs and mammals have been found. Evidence for the occurrence of permafrost had never before been reported in association with dinosaurs.
Robert A. Spicer of the Open University in the U.K. and Judith Totman Parrish of the University of Idaho instead deduce temperature from the structure of ancient plants, arriving at the somewhat higher mean annual temperature of 10 degrees C. Their research with colleagues has demonstrated that polar Australia supported conifers, ginkgoes, ferns, cycads, bryophytes and horsetails but only a few angiosperms, or flowering plants, identifiable by a sprinkling of pollen. The angiosperms were then just beginning to spread into new niches. Perhaps they got their start by exploiting weedy ecological systems in the rift valleys that formed as the supercontinent split apart.
Spicer and Parrish noticed that evergreens, which provided forage in all seasons, had thick cuticles and other structural features that indicate adaptation to cold or dryness (perhaps brought on by winter freezing). Deciduous plants offer another climatic clue: they seem to have lost all their leaves at once. These mass falls may have been triggered by darkness or cold. Drought, however, probably did not serve as a constant cue—the sedimentary record and the abundance of ferns and bryophytes argue for conditions that were moist in all seasons except perhaps winter.
Surviving the Cold If the higher estimate of mean temperature is correct, Australia was both temperate and subject to a period of continuous darkness every year—a combination with absolutely no modern counterpart. The winter night lasted between six weeks and four and a half months, depending on the true paleolatitude. Because the lower extreme of temperature would then have fallen well below the mean, most of the vertebrates preserved as fossils must have lived quite close to their thermal limits. Some, such as lungfish, cannot now breed in waters colder than 10 degrees C.
If, on the other hand, the lower estimate is correct, it becomes more than a typical scientific challenge to understand how this paleocommunity functioned at all. Before seriously attacking this problem, scientists will first have to demonstrate that it exists. To refine the temperature estimate, a multidisciplinary team is comparing floral, geochemical and other forms of evidence.
Nothing in this fauna is quite so peculiar to the region as the koala is today, for although the species and genera were local, they belonged to cosmopolitan families. Yet their adaptations are striking, as is the fact that some survived beyond the time of demise for their families elsewhere.
Among such anachronisms—or relicts—are the temnospondyl amphibians, possibly ancestors of modern amphibians. Most paleontologists had thought this group went extinct by some 160 million years ago, in the Jurassic. In the past few decades, however, Michael Cleeland and Lesley Kool of Monash University found three jaws from this group in Victorian sediments dating from the Early Cretaceous. Two of the jaws were unmistakable because their teeth had the labyrinthine infolding of the enamel that gives this group its common name: labyrinthodonts. At least one large species of temnospondyl lived in polar Australia about 120 million years ago, several million years after the group had died out elsewhere.
How did they survive? We suspect that the cool weather preserved the animals from competition with crocodiles, which were probably poorly adapted to the conditions prevailing in southeastern Australia until the onset of climate warming during the last five million years of the Early Cretaceous. The hypothesis rests on the fact that contemporary crocodilians now live in waters no colder than 10 degrees C, whereas some modern frogs and salamanders can be active in meltwater from snow.
At least nine different types of carnivorous theropod dinosaurs once lived in what is now Victoria. Australovenator is known from a partial skeleton, about 5.5 meters long, found in northeastern Australia about 2,000 kilometers north of Victoria. It is an allosauroid and thus a relative of the well-known Allosaurus from North America and Africa. An ankle bone from Victoria appears to belong to either this same genus or one closely related to it.
A spinosaur, a type of theropod dinosaur with tall spines that projected above its back, might have been even larger. A single fragment of a vertebra is the only evidence that this primarily fish-eating animal once existed anywhere in Australia.
Also present in Victoria were at least seven smaller-bodied theropods. The one best represented in the fossil record, by numerous isolated bones and teeth, is another species of allosauroid. In stark contrast, each of the other six smaller-bodied species is represented by only one or a few isolated bones. This meagerness of evidence for the former presence of so many taxa suggests that more of these smaller-bodied theropods will be found with further work in Victoria.
Among the smaller theropods was Timimus hermani, known from only two bones. It was a tyrannosauroid distantly related to Tyrannosaurus rex and had unusually long and slender femora, suggesting it may have been particularly fleet-footed.
This tyrannosauroid and two other, smaller theropods—the ostrichlike ornithomimosaur and a possible oviraptorosaur dubbed “the egg thief”—are otherwise not known from Gondwana but are found on the northern continents. Whereas tyrannosauroids in the Northern Hemisphere significantly predate their Australian counterparts, the ornithomimosaur and oviraptorosaur, if they are correctly identified, first appeared during the same geologic period in both hemispheres.
Fossils of the other three smaller-bodied Victorian theropods—an allosauroid, a sickle-clawed dromaeosaurid and a horned ceratosaur—have been found elsewhere in Gondwana and the Northern Hemisphere.
Besides the theropods, another dinosaur group that has been identified belongs to the neoceratopsians, or horned dinosaurs. Identification is tentative because it is based on just two ulnae (part of the lower arm), but the similarity of one of them to Leptoceratops, a browser the size of a sheep, is uncanny. Previously, all neoceratopsian records dated from the Late Cretaceous and, with the exception of a few bones from Argentina, came from the Northern Hemisphere. Reports indicate the existence of Early Cretaceous neoceratopsians in Utah and China. This dinosaur family may also have arisen in the southern supercontinent.
In addition to dinosaurs, the region provides evidence for mammals that appear to be among the earliest members of their groups. The minuscule Bishops resembles the living spineless hedgehog Neotetracus. This animal may have been a placental. If so, it is as old as the oldest placentals reported from the Northern Hemisphere and twice the age of the oldest marsupial yet found in Australia. This age is surprising because the domination of Australia by marsupials today is typically explained as the result of land-dwelling placentals reaching the continent long after the marsupials.
Another mammalian group, whose presence is no surprise, comprises the monotremes. An isolated limb bone of one of them has a structure suggestive of a more upright stance than either the echidna or the platypus. A second species is by far the smallest monotreme, weighing only 1 percent as much as any other living or fossil member of the group.
The Australian Early Cretaceous also reshaped forms that continued to flourish in other regions. By far the most successful such group consisted of the “hypsilophodontid” dinosaurs, an informal subdivision of the basal Ornithopoda. These animals, most of them hardly larger than a chicken, were bipeds built for speed, with large hind legs, small but well-developed hands, substantial tails and—for the most part—herbivorous habits. They thus resembled wallabies in both shape and ecological role.
The family Hypsilophodontidae was common throughout the world from the Middle Jurassic to Late Cretaceous times, but its prominence reaches an absolute and relative peak in the Victorian sediments. Not only do hypsilophodontids constitute most of the dinosaur remains, they are also represented by four to five genera, depending on the taxonomic criteria one uses, and five to six species. Other areas, some much more richly endowed with dinosaur species, never harbored more than three kinds of hypsilophodontids at a time. Something clearly favored the diversification of this group in polar Australia.
Big-Eyed Foragers A particularly intriguing adaptation of at least one species of polar hypsilophodontid is suggested by the magnificently preserved brain cast of Leaellynasaura amicagraphica (named after our daughter, along with friends of the Museum Victoria and the National Geographic Society). The brain, unusually large for a dinosaur of this size, bears the marks of optic lobes, the relative size of which is easily the greatest ever documented in a hypsilophodontid.
How is one to interpret these enlarged lobes? We hypothesize that they enhanced the animals’ ability to see in the dark, enabling them to forage effectively during the long winter months. There would have been no lack of food then, for those capable of seeing it: the herbivores could have lived off evergreens and deciduous leaf mats, and the carnivores could have hunted the herbivores.
This hypothesis also explains why this group came to dominate the polar environment in the first place. Hypsilophodontids everywhere in the world had large eyes and, presumably, acute vision. That trait could have given them their foothold in polar Australia. Once established in this “protected” environment, the hypsilophodontids could have competed with one another to produce the observed diversity of genera and species, perhaps all sharing hypertrophied optic lobes.
If the animals foraged at night, they must have been active at freezing or subfreezing temperatures. This feat goes far beyond the cold tolerance of any modern reptile, even the New Zealand tuatara, Sphenodon punctatus, which can remain active at five degrees C, provided it can sun itself. Leaellynasaura could have survived solely by maintaining a constant body temperature, eating frequently, as birds do in wintertime.
Pterosaurs—the flying reptiles—and the heavily armored ankylosaurs also appear in the Victorian fossil record, but the remains are so fragmented that they tell us little about the animals’ lives. Much can be gleaned from one handful of teeth, however, for they come from plesiosaurs. These reptiles, not themselves dinosaurs, generally paddled the seas, but here two plesiosaur species quite different from one another inhabited freshwater in the ancient valley between Australia and Antarctica. They thus recall the Ganges River dolphin, one of the few cetaceans that live in freshwater.
One of the two species may have been a pliosaur—a type of plesiosaur with a short neck and elongated skull—whose body was four meters or more in length. The other was a typically long-necked, small-skulled plesiosaur. This obvious difference in body plan may explain how the two plesiosaur species managed to coexist in the same environment, being adapted to feeding on quite different prey.
The sauropods are one of the few major dinosaur groups that are absent. These giants, familiar from the example of Apatosaurus, lived at that time in Australia’s lower latitudes. Not one, however, has been found farther south.
The apparent restriction of these large dinosaurs to lower latitudes in the Cretaceous of Australia may be real or merely an artifact of sampling. We worry about this question because the floodwaters that broke out of rain-swollen rivers would have collected small and medium-size bones but left large ones. The body of a sauropod would have stayed put rather than floating to a place where many specimens were concentrated in the small flood channels, which were no more than five to 10 meters in width and 20 to 30 centimeters in depth. This factor may explain the absence of the large bones of sauropods but not their tiny teeth. Thus, they did not seem to have reached polar Australia, although their vertebrae are known from polar New Zealand and the Antarctic Peninsula.
Yet we suspect there was an underlying tendency toward small body size in these polar environs. None of the hypsilophodontids, it must be remembered, stood taller than a human, and most were barely knee-high. The ornithomimosaur is equally unprepossessing, and the protoceratopsid and the ankylosaur are each no bigger than a sheep. A single fragment of a claw constitutes our sole record of a large dinosaur—a carnivore, apparently similar to Baryonyx of England—which may have measured up to eight meters in length. The rare evidence for the largest dinosaur in polar Victoria is based on the smallest identifiable part of its skeleton. Thus, we cannot rule out the possibility that a systematic bias against the preservation of larger fossils is the reason that large dinosaurs rarely are found there. But although there is evidence for at least one large dinosaur in polar Victoria, the dinosaur assemblage does seem to have consisted primarily of smaller individuals.
This pattern contradicts the classic scaling laws formulated by Carl Bergmann and Joel Allen in the 19th century. According to these laws, animals in a given lineage tend to become larger and more compact as the average temperature of their environment falls. This trend is exemplified by the comparison of mountain lions in Canada with pumas of Central America and of human populations in the subarctic and tropical zones.
Other factors also determine body dimensions, especially the size of the territory in which a population lives. Individuals found on islands are often smaller than their mainland counterparts. For example, there were dwarf elephants on the ancient Mediterranean islands, and pygmy mammoths were found in 4,000-year-old sediments on islands off the north coast of Siberia. Dwarfism may be a response to selective pressure to increase the number of individuals so as to ensure a gene pool diverse enough for the species to survive in a restricted area. This effect has also been observed on peninsulas—and ancient southeastern Australia was a peninsula of the Gondwana landmass.
The dinosaurs on that peninsula were trapped virtually at the ends of the earth. Their direct path north was blocked by a vast inland sea, which they could have passed only by going hundreds of kilometers to the west before wheeling about to the north. At the end of such labors, they would have been able to catch, at most, an hour of sun a day in winter. Migration would have made little sense for such small animals.
Less formidable barriers sealed in the dinosaurs of the one other polar site that has yielded large quantities of fossils: the North Slope of Alaska. The dinosaurs there had a clear north-south corridor along which they could migrate with ease. It is significant that those dinosaurs were big—at least equal in size to caribou, wildebeest and other modern animals that migrate.
Safe Haven in Gondwana One must question whether animals so superbly adapted to the cold and the dark could have been driven to extinction by an artificial winter, such as is supposed to have followed a cataclysmic event at the boundary between the Cretaceous and Paleogene. It is proposed that the cataclysm, perhaps a combination of a collision with a comet or asteroid and a series of volcanic eruptions, suffused the atmosphere with a blanket of dust, excluding sunlight and freezing or starving most animals to death.
We suspect, however, that no such artificial winter could have killed the dinosaurs unless it lasted for a long time, certainly more than a few months. Otherwise at least a few of the polar dinosaurs would have survived the cataclysm. Of course, it is possible that a different development had already ended the reign of southern Australia’s dinosaurs by the end of the Cretaceous.
English writer Arthur Conan Doyle once dreamed of a plateau in South America that time forgot, where dinosaurs continued to rule the land. Reports in the early 1990s that dwarf mammoths survived to early historical times, on islands off the coast of Siberia, give force to such speculation. If dinosaurs found a similar haven in which they outlived the rest of their kind, then we think polar Gondwana, including southeastern Australia, is a likely place to look.
