The Triassic Period
245 to 208 Million Years Ago
In many ways, the Triassic was a time of transition. It was at this time that the world-continent of Pangaea existed, altering global climate and ocean circulation. The Triassic also follows the largest extinction event in the history of life, and so is a time when the survivors of that event spread and recolonized.
The organisms of the Triassic can be considered to belong to one of three groups: holdovers from the Permo-Triassic extinction, new groups which flourished briefly, and new groups which went on to dominate the Mesozoic world. The holdovers included the lycophytes, glossopterids, and dicynodonts. While those that went on to dominate the Mesozoic world include modern conifers, cycadeoids, and the dinosaurs.
The mammals of today are but one branch of the Synapsida, a great vertebrate group with a 300 million year history. Pre-mammalian synapsids – including the famous "finback" Dimetrodon, shown below -- dominated the land vertebrate fauna of the Permian and early Triassic before losing ground to the diversifying dinosaurs and other archosaurs. These pre-mammalian groups of synapsids have at times been called "mammal-like reptiles". This term is now
discouraged because although many had characteristics in common with mammals, none of them were actually reptiles.
The third great therapsid suborder, Theriodontia ("beast tooth", referring to the mammal-like teeth) constitute the higher or more mammal-like of the Therapsida (protomammals). They include both carnivores and herbivores, and various lineages correlated to late Cenozoic wolves, weasels, otters, rodents, and shrews. Most diverse and successful during the late Permian and early Triassic, they were increasingly overwhelmed by the archosaurian radiation of the later Triassic. During the latest Triassic (late Norian to Rhaetic) at least one line of Eutheriodonts evolved into mammals. Even so, one family, the rodent-like Tritylodontids continued right into the late Jurassic (Oxfordian).
The Theriodonts evolved a feeding mechanism much more successful than that of the Dinocephalia. The quadrate bone in the jaw was reduced in size. When the jaws are widely opened (drawing, above) the articular bone (art in the diagram) toghtly grips the head of the small quadrate (q). The lower incisors pass entirely behind the upper incisors when the jaws are closed. The development of versatile jaw movements and a more precise type of dental occlusion also played a critical role in preadapting the elements of the theriodont articular complex for becoming the ossicles of the mammalian middle ear.
The small quadrate may have been loosely attached to the skull, probably by a fibrous pad in life; set in a squamosal recess (B) in the diagram above; the upper end occupies a hollow in the anterior face of the squamosal which also buttressed it from behind. ) so that, Hopson suggests, it was probably capable of a small amount of fore-aft movement at its lower end. Nevertheless the joint between the quadrate and articular was a tight one, with the articular firmly gripping the quadrate condyle, allowing only a hinge (and some transversal sliding) movement at the jaw joint, but no fore-aft sliding.
The theriodonts fall into three distinct groups, which can be considered infraorders or (if Theriodonta itself is considered an order rather than as is usually the case, a suborder) suborders: the Gorgonopsia; the Therocephalia, and the Cynodontia. A fourth group, the Bauriamorpha, are now generally included under the Therocephalia.
The cynodonts, or 'dog teeth', were the most successful and one of the most diverse groups of therapsids, constituting a latest Permian and Triassic evolutionary radiation including such forms as large carnivorous cynognathids, equally large herbivorous traversodonts, and small and extremely mammal-like tritylodontids and ictidosaurs. The Ictidosaurs (Trithelodontids) are almost certainly close to the direct ancestry of of the class Mammalia (note - the above diagram shows mammals evolving from an unknown early Triassic cynodont stem; perhaps a form like Thrinaxodon. This is almost certainly in error, for Cynognathus and later forms were more mammal-like. Recent cladograms relate the first mammals more closely to late Triassic cynodonts). The extremely mammal-like structure of cynodonts has been known for nearly one hundred years, but only within recent years have we learned enough about them and about the very early mammals to say with confidence that all mammals are indeed descended from a single group of cynodonts.
Even the earliest cynodonts - the Procynosuchidae of the Late Permian, already show many advanced mammalian chatacteristics, such as a reduced number of bones in the lower jaw, a secondary bony palate and a complex pattern or the crowns of their cheek teeth. It is likely that
Cynodonts were at least partially if not completely warm-blooded, covered with hair, which would have insulated them and helped to maintain a high body temperature.
By early Triassic times, cynodonts had diverged into large predaceous carnivores such as Cynognathus and moderate large omnivorous and herbivorous types such as Trirachodon and Diademodon. the Middle Triassic saw a major radiation of herbivorous forms included in the
family Traversodontidae. From this family evolved he highly specialized and extremely mammal-like Tritylodontidae of the late Triassic to Middle Jurassic, the "rodents" of the early Mesozoic and culmination of the herbivorous cynodont radiation. At the same time, the
descendents of Cynognathus evolved into medium-sized to small carnivorous and insectivorous forms. It is interesting that as the archosaurian reptiles were becoming larger, the cynodonts became smaller, perhaps in the end nocturnal. The hot arid Triassic conditions favoured the ectothermic reptilian metabolism of the archosaurs over the warm-blooded mammalian organisation of the cynodonts. (In his The Dinosaur Heresies, Bob Bakker has clamed that even the early thecodontian archosaurs like Erythosuchus were warm-blooded, and out-competed the cynodonts for this reason, but this posittion is almost never held nowdays)
In the end, the small advanced cynodonts and their mammalian descendents became nocturnal, depending on hearing and smell and leaving the day to the visual-orientated archosaurs (thecodonts and dinosaurs). Cynodont and early mammalian brains were larger than sauropsid (reptilian) brains not because they were more intelligent, but because of the enlarged olfactory and auditory bulbs. The small cynodonts and Mesozoic mammals owned the cool night, or lived in trees, the large thecodonts and dinosaurs ruled the day and the ground. It was to be some 150 million years before a combination of environmental stress and cometary or asteroid impact brought about the end of the dinosaurs and the other great reptiles, and allowed the mammals to emerge and take control of the Earth.
Mammals may have either a monophyletic or polyphyletic origin.
Monotreme early evolution is unclear.
Earliest mammals are from order Triconodonta, put into family Morganucodontidae.
Triconodonta, Dioconodonta, and Multituberculata were three orders common throughout Jurassic.
The Multituberculata, are entirely extinct, though they were once widespread and successful. Multituberculates are the only major branch of mammals to have become completely extinct, and have no living descendants. Although not known to many people, they have a 100 million-year fossil history, the longest of any mammalian lineage. These rodent-like mammals were distributed throughout the world, but seem to have eventually been outcompeted by true rodents.
Multituberculates first appeared in the Late Jurassic, and went extinct in the early Oligocene, with the appearance of true rodents. Over 200 species are known, some as small as the tiniest of mice, the largest the size of beavers. Some, such as Lambdopsalis from China, lived in burrows like prairie dogs, while others, such as the North American Ptilodus, climbed trees as squirrels do today. The narrow shape of their pelvis suggests that, like marsupials, multituberculates gave birth to tiny, undeveloped pups that were dependent on their mother for a long time before they matured.
Two other early orders, known from the Triassic, are Symmetrodonta and Eupantotheria. An early family of Symmetrodonts is Kuehneotheriidae.
The order Eupantotheria is represented by two families, Dryolestidae and Peramuridae, which likely gave rise to modern marsupials and placentals. Eupantotheres had tribosphenic teeth.
L O N D O N, Sept 1 1999— Tiny teeth from a prehistoric animal found in a jawbone in Madagascar are proof that mammals and dinosaurs roamed the Earth together more than 100 million years ago, researchers said today. Until now scientists had suspected mammals arose after dinosaurs had become extinct. But the new finding published in the science journal Nature showed that early mammals lived much earlier than previously thought. “Smack in the middle of the dinosaur’s heyday, our ancestors were living as well — it’s just that they were small and inconspicuous,” said Andre Wyss, a professor of geology at the University of California in Santa Barbara and co-author of the Nature study. Oldest Mammal from Madagascar The furry mouse-sized creatures are from a group of mammals called Tribosphenida that lived in Madagascar about 165 million years ago. The teeth are more than double the age of the oldest known mammal from the Indian Ocean island. “The most conspicuous and hence the most famous land animals of the Mesozoic Era were of course dinosaurs,” said Wyss. “Less widely appreciated is the fact that furry animals — mammals — and dinosaurs sprang on an evolutionary scene at about the same time; the two groups lived side by side for more than 100 million years.” The scientists said the findings also showed that a subgroup of mammals evolved in the southern hemisphere earlier than anything in the north. Wyss and his colleagues collaborated on the research with scientists from The Field Museum of Natural History in Chicago, Northern Illinois University and the University d’Antananarivo in Madagascar.
208 to 146 Million Years Ago
Great plant-eating dinosaurs roaming the earth, feeding on lush growths of ferns and palm-like cycads and bennettitaleans. . . smaller but vicious carnivores stalking the great herbivores. . . oceans full of fish, squid, and coiled ammonites, plus great ichthyosaurs and long-necked plesiosaurs. . . vertebrates taking to the air, like the pterosaurs and the first birds. . . this was the Jurassic Period, beginning 210 million years ago and lasting for 70 million years of the
Named for the Jura Mountains on the border between France and Switzerland, where rocks of this age were first studied, the Jurassic has become a household word with the success of the movie Jurassic Park. Outside of Hollywood, the Jurassic is still important to us today, both because of its wealth of fossils and because of its economic importance -- the oilfields of the North Sea, for instance, are Jurassic in age.
146 to 65 Million Years Ago
The Cretaceous is usually noted for being the last portion of the "Age of Dinosaurs", but that does not mean that new kinds of dinosaurs did not appear then. It is during the Cretaceous that the first ceratopsian and pachycepalosaurid dinosaurs appeared. Also during this time, we find the first fossils of many insect groups, modern mammal and bird groups, and the first flowering plants.
The breakup of the world-continent Pangaea, which began to disperse during the Jurassic, continued. This led to increased regional differences in floras and faunas between the northern and southern continents.
The end of the Cretaceous brought the end of many previously successful and diverse groups of organisms, such as non-avian dinosaurs and ammonites. This laid open the stage for those groups which had previously taken secondary roles to come to the forefront. The Cretaceous was thus the time in which life as it now exists on Earth came together.
54 to 38 mya
The Eocene epoch is part of the Tertiary Period in the Cenozoic Era, and lasted from about 54 to 38 million years ago (mya). The oldest known fossils of most of the modern orders of mammals appear in a brief period during the Early Eocene and all were small, under 10 kg. Both groups of modern ungulates (Artiodactyla and Perissodactyla) became prevalent mammals at this time, due to a major radiation between Europe and North America.
38 to 23 mya
The Oligocene epoch is part of the Tertiary Period in the Cenozoic Era, and lasted from about 38 to 23 million years ago (mya). The Oligocene is thus a relatively short span of time, though a number of major changes occurred during this time. These include the appearance of the first elephants with trunks, early horses, and the appearance of many grasses -- plants that would produce vast tracts of grasslands in the following epoch, the Miocene.
23 to 5 mya
The Miocene was a time of warmer global climates than those in the preceeding Oligocene, or the following Pliocene. It is particularly notable in that two major ecosystems first appeared at this time: kelp forests and grasslands. The expansion of grasslands is correlated to a drying of continental interiors as the global climate first warmed and then cooled.
Global circulation patterns changed as Antarctica became isolated and the circum-polar ocean circulation became established. This reduced significantly the mixing or warmer tropical water and cold polar water, and permitted the buildup of the Antarctic polar cap. Likewise, the African-Arabian plate joined to Asia, closing the seaway which had previously separated Africa from Asia, and a number of migrations of animals brought these two faunas into contact.
5 to 1.8 mya
The Pliocene was a time of global cooling after the warmer Miocene. The cooling and drying of the global environment may have contributed to the enormous spread of grasslands and savannas during this time. The change in vegetation undoubtedly was a major factor in the rise of long-legged grazers who came to live in these areas.
Additionally, the Panamanian land-bridge between North and South America appeared during the Pliocene, allowing migrations of plants and animals into new habitats. Of even greater impact was the accumulation of ice at the poles, which would lead to the extinction of most species living there, as well as the advance of glaciers and ice ages of the Late Pliocene and the following Pleistocene.
1.8 million to 11,000 years ago
Pleistocene biotas were extremely close to modern ones -- many genera and even species of
Pleistocene conifers, mosses, flowering plants, insects, mollusks, birds, mammals, and others survive to this day. Yet the Pleistocene was also characterized by the presence of distinctive large land mammals and birds. Mammoths and their cousins the mastodons, longhorned bison, sabre-toothed cats, giant ground sloths, and many other large mammals characterized Pleistocene habitats in North America, Asia, and Europe. Native horses and camels galloped across the plains of North America. Great teratorn birds with 25-foot wingspans stalked prey. Around the end of the Pleistocene, all these creatures went extinct (the horses living in North America today
are all descendants of animals brought from Europe in historic times).
It was during the Pleistocene that the most recent episodes of global cooling, or ice ages, took place. Much of the world's temperate zones were alternately covered by glaciers during cool periods and uncovered during the warmer interglacial periods when the glaciers retreated. Did this cause the Pleistocene extinctions? It doesn't seem likely; the large mammals of the Pleistocene weathered several climate shifts.
The Pleistocene also saw the evolution and expansion of our own species, Homo sapiens, and by the close of the Pleistocene, humans had spread through most of the world. According to a controversial theory, first proposed in the 1960s, human hunting around the close of the Pleistocene caused or contributed to the extinction of many of the Pleistocene large mammals. It is true that the extinction of large animals on different continents appears to correlate with the arrival of humans, but questions remain as to whether early human hunters were sufficiently numerous and technologically advanced to wipe out whole species. It has also been hypothesized that some disease wiped out species after species in the Pleistocene. The issue remains unsolved; perhaps the real cause of the Pleistocene extinction was a combination of these factors.
Many paleontologists study Pleistocene fossils in order to understand the climates of the past. The Pleistocene was not only a time during which climates and temperatures shifted dramatically; Pleistocene fossils are often abundant, well-preserved, and can be dated very precisely. Some, such as diatoms, foraminifera, and plant pollen, are both abundant and highly informative about paleoclimates. Today, there is concern about future climate change (e.g. global
warming) and how it will affect us. Paleontologists who work on Pleistocene fossils are providing a growing amount of data on the effect of climate change on the Earth's biota, making it possible to understand the effects of future climate change.
The Case of the Irish Elk
The Irish Elk, Megaloceros, is misnamed, for it is neither exclusively Irish nor is it an elk. It is a giant extinct deer, the largest deer species ever, that stoodup to seven feet at the shoulder (2.1 meters), with antlers spanning up to 12 feet (3.65 meters). The Irish elk evolved during the glacial periods of the lastmillion years, during the Pleistocene Epoch. It ranged throughout Europe, northern Asia and northern Africa, and a related form is known from China. Thename "Irish" has stuck because excellent, well-preserved fossils of the giant deer are especially common in lake sediments and peat bogs in Ireland. The skullon display at the old UC Museum of Paleontology (figured below) came from such a locality, 18 miles north of Dublin. Such skulls, with their enormous racksof antlers, adorn the walls of castles and hunting lodges throughout Ireland. On the other hand, the complete skeleton pictured at the top of the page, on displayat the Paleontological Institute in Moscow, was found at the other end of Europe, near the Russian town of Sapozhka.
Unable to adapt to the subartic conditions of the last glaciation or the marked transition that occured after the final retreat of the ice sheet, the largest deer that ever lived became extinct, the last one in Ireland dying around 11,000 years ago. Megaloceros may have possibly survived in continental Europe into historic times.
Beyond its arresting size and singular appearance, the giant deer is of great significance to paleontologists because of the way in which the animal has become involved in evolutionary debates down through the years.
Can Extinction Happen?
In the seventeenth and eighteenth centuries, it was becoming increasingly apparent that many fossils represented organisms that were not known to survive anywhere on Earth. But to scientists who believed in the Divine creation of the Earth and its life, this posed a jarring philosophical problem: why would a good, perfect God allow any of the animals in His perfect creation to die out completely? Many scientists denied the reality of extinction, and instead
suggested that animals known only as fossils would one day be found alive in some unexplored part of the globe. In the words of Dr. Thomas Molyneux, the first scientist to describe the Irish elk:
That no real species of living creatures is so utterly extinct, as to be lost entirely out of the World, since it was first created, is the opinion of many naturalists; and 'tis grounded on so good a principle of Providence taking care in general of all its animal productions, that it deserves our assent.
Molyneux erroneously identified the Irish elk with the American moose, while others thought the Irish elk was identical with the European reindeer. Not until 1812 did the great French scientist Georges Cuvier document that the Irish elk, along with other fossil vertebrates such as the mammoth, did not belong to any living species of mammal. Cuvier's study of the Irish elk was a key part of the documentation that extinction had happened in the past.
Source: Gould, S.J. 1977. The misnamed, mistreated, and misunderstood Irish elk. Pp. 79-90 in Ever Since Darwin. W.W. Norton, New York.