December 12. Cenozoic mammals again

There are lots of “theres” among the Cenozoic mammals. Brontothere, Uintatherium, Chalicothere, Indricotherium – the “there” or “therium” suffix is just Greek for “beast,” so it gets attached to the scientific names of lots of critters, normally to mammals. 

image by DagdaMor, used under Creative Commons license.

Mammalian groups that began in the Paleocene or early Eocene diversified and thrived during the Eocene, Oligocene, and later epochs of the Cenozoic. Paraceratherium – which may be the same as Baluchitherium or Indrichotherium or both – was the largest land mammal that ever lived. It was related to rhinoceroses but had a long neck, giraffe-like in its proportions. There are no complete fossils, one reason we aren’t sure which name goes with it, but it’s estimated to have been 20 feet high and 27 feet long. It lived in Central Asia pretty much throughout Oligocene time, 34 to 23 million years ago. Paraceratherium means “near the hornless beast,” because it was similar to a contemporary hornless rhinoceros. The rhino family was much more diverse in the Oligocene than it is today. Even though the incomplete fossils mean we can’t really reconstruct it, there is no doubt that it was a huge animal that browsed on foliage in trees.

The name titanothere – titanic beast – has been pretty much discarded in favor of brontothere, the “thunder beast.” Brontotheres were hoofed animals that were descended from the ancestors of horses, but if you met a brontothere on the street you’d probably call it a rhinoceros. They typically had horns and other bony protuberances on their heads and faces, and unlike rhinos whose horns are made of keratin, the same compound that makes fingernails and hair, brontothere horns were bone. Brontotheres appear to have become extinct just about at the end of the Eocene. Their demise is speculated to be related to the rise of grasses – tough plants that they could not adequately graze on and process – and the related increase in arid conditions as the Oligocene began. 

Uintatherium skull (public domain)

Uintatherium, the Uinta Mountains Beast, lived in what is now Utah in middle to late Eocene time, and it had a cousin that lived in China. It was another critter that looked a lot like a rhino but wasn’t all that closely related to them. With big bony bumps and saber-like canine teeth, it must have looked intimidating, but it was a herbivore. Uintatherium belonged to the mammalian order Dinocerata, a group that is entirely extinct. 

We aren’t sure how the Dinocerata related to other mammals, but I think the favored position is that they branched from an ancestor that was also ancestral to the ungulates, the hoofed animals like horses. But that’s not certain. All the Dinocerata went extinct before the end of the Eocene. Changing conditions, together with competition from brontotheres may have eliminated them. But the brontotheres survived only a few more million years themselves before dying out about at the end of the Eocene.

The Chalicothere, whose name means “gravel beast,” appeared during the Eocene, and survived right up until about 4 million years ago, a run of 50 million years, unusual for the early Cenozoic mammals which typically evolved but in many cases did not survive as a genus or even at the family or order level. Chalicotheres had short hind legs and long front legs, so they were probably knuckle walkers, like modern gorillas. They were the size of a horse, and ranged across Europe and North America as well as parts of Asia and Africa. It is supposed that they used their strong but short hind legs to rear up and use their long forelegs to strip vegetation from trees. Unlike its hoofed relatives, the Chalicothere had three claws on its feet.

Gastornis (public domain)

Birds too were becoming huge, occupying ecological niches that their cousins, the non-avian dinosaurs held until the end of the Cretaceous. Gastornis giganteus, which lived in very late Paleocene and early Eocene time, stood nearly 7 feet high, comparable to ostriches and moas today. It was flightless, and various species lived in Europe, North America, and Asia. The giant “terror birds” of South America, the Phorusrhacids, were developing contemporaneously with Gastornis, but they were in completely different orders of birds. The Terror Birds reached their maximum sizes later in the Cenozoic.

—Richard I. Gibson

Links:
Paraceratherium 

Comparison image by DagdaMor, used under Creative Commons license.
Uintatherium skull photo by Jebulon, given to public domain.
Gastornis (public domain)

December 10. Eocene mammals

The Cenozoic really is the Age of Mammals. They rose almost meteorically after the extinction at the end of the Cretaceous eliminated the dinosaurs, which must have preyed on the mammals, but probably even more important, they occupied ecological niches that in their absence mammals could fill. And especially with the Paleocene-Eocene Thermal Maximum that we talked about the other day, mammals began to diversify dramatically, and to grow in size. 

3-foot-high Coryphodon reconstruction (source)

Coryphodon was a browsing animal that appeared in the Paleocene and thrived in the Eocene. It was common in North America, where its fossils have been found well-preserved in the Wasatch Formation of Wyoming and Utah, dating to about 57 to 46 million years ago.

The Wasatch Formation is more or less between the Paleocene Ft. Union formation and its coals, which we mentioned on Dec. 4, and the Green River Formation that we talked about yesterday, but they all overlap to some extent in both time and space. Coryphodon ranged into what is now the Canadian Arctic, Europe, and China too. At about a meter high, it was one of the largest land animals at the time.  

Its name, Coryphodon, means “pointed tooth.”  It used those teeth to crunch leaves and other plant matter in the swampy places where it lived a life that must have been similar to the modern hippopotamus, but it was not closely related to hippos.

Paleocastor burrow (source)

By Oligocene time, about 25 million years ago, mammals were clearly expanding into diverse ecological niches. Paleocastor – whose name means “ancient beaver” – lived in and around lakes and swamps in what are now Nebraska and South Dakota. They made corkscrew-shaped burrows as much as 3 meters deep, which were first identified as giant freshwater sponges, but eventually one was found with the ancient beaver within it, and the marks in the burrow were tied to the animal’s teeth used to excavate the hole.

Paleocastor was the size of a large rat, but beavers and many other mammals had a tendency toward gigantism over the course of the Cenozoic. Castoroides was a beaver that lived during Pleistocene time, the glacial period, and became extinct as recently as 10,000 years ago. Castoroides grew to as much as seven feet long.

Stylinodon, another Eocene critter, had a short, pug-like face. Its name means “ribbon tooth,” for a ribbon of enamel around its canine teeth. Strong claws have been interpreted to indicate that Stylinodon dug roots and tubers for its food, but the claws and forelimbs might have been used to pull down branches and strip them of fruits. The animal grew to as much as 180 pounds (80 kg). It’s been called “an aardvark with the head of a pig,” (Janis et al., 1998) but it really doesn’t have much in common with any modern mammal. It was extinct by the end of the Eocene, one of many variations that did not survive.

* * *

We haven’t had any geological birthdays in a while, but today we have two. Israel C. Russell was born December 10, 1852, in Garrattsville, New York. He had a wide-ranging career with the U.S. Geological Survey and the University of Michigan, and he’s probably best known for his studies in Alaska and Mono Lake, California, as well as glaciers in the U.S. Fielding Bradford Meek was born December 10, 1817, in Madison, Indiana. His career as a paleontologist took him all over the United States, and he’s perhaps best known for his work in California and in the Badlands of the Dakotas.

—Richard I. Gibson

Paleocastor burrow photo from University of Nebraska and Agate Fossil Beds National Monument. Used under creative commons license. 

Coryphodon by Heinrich Harder (public domain)

Stylinodon

Evolution of Tertiary Mammals of North America: edited by Christine Marie Janis, Kathleen Marie Scott, Louis L. Jacobs, 1998, Cambridge University Press.

November 24. Mammals

Mammals diversified during the Cretaceous, so that by the end of the period, the two major modern groups, the placentals and the marsupials, were well established. Placental mammals include most modern types including rodents, primates – most every kind of mammal other than marsupials and the egg-laying monotremes, which are platypuses and spiny anteaters.

The mutituberculates were rodent-like mammals that got started during the Jurassic, thrived in the Cretaceous and survived the end-Cretaceous extinction, only to disappear in the Oligocene about 30 million years ago. Their 120-million-year run is the longest of any mammal lineage, and their diversity is reflected in at least 200 different species. Their name comes from their teeth, which have rows of little points, or tubercules. Although they occupied rodent-like niches, including burrows and trees, and were superficially much like squirrels and rats and other rodents, they are classed taxonomically in their own order, and they have no modern descendents. It looks like the true rodents displaced them in the early part of the Cenozoic Era, which we’ll talk about next month.   

Brian Switek has a post this week on a new Cretaceous mammal discovery, Vintana, a muskrat-like critter that lived in Madagascar a few million years after that island finally became separated from larger land masses. Madagascar had pulled away from Africa while it was still attached to India during the latter part of the Early Cretaceous epoch. India and Madagascar separated around 90 million years ago, and Vintana dates to about 70 million years ago, so it might be a reflection of evolution in the relative isolation of a small island continent. Check the Laelaps blog at phenomena.nationalgeogeographic.com for more information on this and many other fossil topics. 

Repenomamus with dinosaur bones in stomach –
photo by David Wong, used under Creative Commons license.  

The common view of Cretaceous mammals living in fear of dinosaur predators, creeping through the underbrush at night, was turned topsy-turvy by the discovery of a meter-long mammal in China, called Repenomamus. One specimen was found with fragments of a juvenile herbivorous dinosaur in its stomach. We don’t know if it scavenged, or chased small dinosaurs down, or maybe it preyed on the young. But it did eat dinosaurs. At a meter long, three feet or so, Repenomamus is the largest known Mesozoic mammal. Most Cretaceous mammals were around 3 to 10 inches long. Many are known only from a single tooth or a few bones, so they are not really very well known. Some spectacular exceptions exist, including an early Cretaceous mammal fossil from China in which the 4-inch-long animal’s fur can be clearly seen.

We’re coming up on the end of the Cretaceous in a week, and the last day of November will be devoted to the extinction event at the end of the period. I wanted to mention today some ideas that addresses the question of how mammals and birds survived the extinction event when, so far as we know, no dinosaurs did. One popular idea has been that birds and mammals in general were smaller in size than most dinosaurs, although there were a few exceptions at times. Smaller size would have allowed animals to maintain and regulate body temperature more easily, in the event of climatic extremes such as those that might have accompanied the end-Cretaceous extinction. And many mammals were burrowers, which would also afford protections from cold or other climate changes.

Mammal and bird brains might have become complex enough by the end of the Cretaceous, and more complex than the non-avian dinosaurs, so that as individuals and as groups they were simply more adaptable to the challenging conditions during the extinction event. Some mammals and birds did die off at the end of the Cretaceous, so it was most definitely not a case of all mammals and birds good, all dinosaurs bad.

The idea of long-term hibernation, say something like three-quarters of the year, has been offered as a way some mammals might have made it through a nuclear winter or other ecological catastrophe at the end of the Cretaceous. That might work for some, but certainly not for all mammals and birds. When it’s all said and done, a lot of the thought about mammal and bird survival at the end Cretaceous is largely common sense and reason – and nothing is certain yet to make it clear why they did survive.

—Richard I. Gibson

Repenomamus with dinosaur bones in stomach – photo by David Wong, used under Creative Commons license.  

Hibernation?

Surviving the extinction 

Laelaps blog

Mammals ate dinosaurs

October 24. Jurassic life of China

Today, let’s talk about another lagerstätte – not as well known as the Solnhofen Limestone, but pretty cool nonetheless. It’s called the Daohugou Bed or Daohuguo biota, and it’s found in northeastern China. That part of the world was warm and wet in late Jurassic time, about 160 million years ago or a bit older – but there is some controversy as to the age. Rocks deposited in this setting actually extend from the Jurassic up into the Cretaceous, where the fossils are called the Jehol biota. Whatever the age, and I think most researchers accept a late Jurassic age, a lush forest ecology developed. 

Preservation is outstanding because the area was occasionally covered by thick, fine ash falls that likely killed the animals and preserved them intact, including exceptional detail of soft parts. For example, there’s a bee whose proboscis is preserved. 

Pencil drawing of gliding mammal Volaticotherium
by ArthurWeasley; head based on skull image published by
Meng in Nature, Dec 2006. Used under Creative Commons license 

The Jurassic rocks contains the fossil of Juramaia – not the bullfrog, but the early mammal we talked about October 13.  Other life in that forest included undisputed feathered dinosaurs – the first one was found there in 1996. The first gliding mammal, which looked an awful lot like a modern flying squirrel, was described from these beds in 2006. Like Juramaia, it was an insectivore. 

Castorocauda was an aquatic mammal that lived in the lakes of the region. At 2 pounds and 17 inches, it was probably the largest Jurassic mammal, and shows that mammals were adapting to various niches including the water world early in their evolution. Castorocauda probably occupied ecological niches similar to those occupied by beavers and platypuses today. Its scientific name actually means “beaver tail,” and it did have a wide tail similar to those of modern beavers. It also had webbed feet. Castorocauda is another of those critters that may or may not have quite been true mammals. It depends on your definition and on the interpretation of fine details in the fossils.

Along with the small feathered dinosaurs, some of which appear to have been adapted to climbing trees, the fossils include several different types of pterosaur, the flying reptiles that were not closely related to the dinosaurs or their descendents, birds. Some of the pterosaurs had wingspans close to three feet, making them some of the largest animals in the Daohugou beds. The feathered dinosaurs, including Eosinopteryx, were generally small, perhaps a foot in length – but reconstructions of some of them look an awful lot like birds. No actual birds have been found in the rocks, but they’re still looking.

The forest where these animals lived was dominated by conifers, horsetail rushes, cycads and ferns, and gingko-like trees. No flowering plants of the modern sort. The insect life included lots of flies, various spiders including an orb-weaver, mayflies, and water beetles. Plenty of prey for the insectivorous mammals, salamanders, and small dinosaurs that lived there.

* * *

J. Tuzo Wilson was born October 24, 1908, in Ottawa, Canada. In many ways, he was the architect of the concept of plate tectonics, which grew out of the idea of continental drift and sea-floor spreading. He synthesized many of the disparate ideas into an overarching theory. He also recognized the nature of transform faults, like the San Andreas, and their role in plate interactions.

Tuzo Wilson explaining transform faults (video)

—Richard I. Gibson

Chinese lagerstatte

Pencil drawing of gliding mammal Volaticotherium by ArthurWeasley; head based on skull image published by Meng in Nature, Dec 2006. Used under Creative Commons license 

October 13. Real mammals, finally

We’ve been dancing all around the start of the mammals – mammal-like reptiles, synapsids, pelycosaurs, therapsids. All related, but, generally, not quite mammals. And depending somewhat on exactly how you define a mammal, and how you look at the fossils, we just might have had real mammals by late Triassic time – we talked about those critters on September 23, with a question mark because it’s not completely agreed that they really were mammals.  

By the Jurassic, the arguments are over. The eutherians, a group whose name means “true beasts,” include all the modern placental mammals – which means all mammals except the marsupials and their close relatives. Today, about 90% of all mammals, including humans, are placentals, and the rest are marsupials like opossums and kangaroos. The oldest eutherian is named Juramaia sinensis, a name meaning “Jurassic mother from China.” It was described and named in 2011 and dated to 160 million years ago, about the boundary between the middle and late Jurassic epochs.

Juramaia reconstruction drawing by Nobu Tamura (source)

The discovery and date of Juramaia pushed the age of mammals back 35 million years and also indicates that the placental and marsupial lines of mammals diverged at least that long ago.

Like so many early mammals, Juramaia was probably an insectivore and the bones in its feet indicate that it was probably arboreal, climbing through trees in search of prey. It was only four or five centimeters long – a couple inches – and it was still very much shrew-like in its basic plan. It appears that mammals maintained a pretty low profile – a small size – for many millions of years, and presumably that was a survival advantage at a time when the world was dominated by larger animals including dinosaurs. The image of little mammals scurrying through the underbrush and up in the trees, avoiding predators, is a pretty good one.

Another Chinese fossil from the Jurassic is a little more debatable as an early mammal. Haramiyids were herbivores that may be ancestral to extinct multituberculates, egg-laying mammals that in turn might be related to modern platypuses. Haramiyids are known from the late Triassic, and if it can be agreed that they were true mammals, that obviously pushes the origin of mammals back that far – but that agreement isn’t there, yet. Jurassic haramiyids from China were squirrel-like creatures with long, possibly prehensile tails like monkeys. Like Juramaia, they lived in trees. So it would appear that placental mammals and non-placentals were occupying the same ecological niches, at least in some places. Haramiyids were extinct by late Jurassic time, so far as we know.

You can imagine that preservation is challenging for mammal fossils. Generally, when an animal died, it might have been eaten by its predators or scavenged by other organisms, and even more likely, would have rotted away on a forest floor. So unraveling the early history of the mammals, during Triassic and Jurassic time, has been challenging.

I have links below to a couple really good articles by Brian Switek about mammals, including one on when mammals got fur. Spoiler alert – they probably got fur before they diverged into the modern branches of placentals and marsupials.

So mammals, true mammals, are finally definitely on the scene. This might be a good time to remind you of the scale of our calendar – it’s not a proper scale, because I want to spend a month with each of the periods of the Paleozoic and Mesozoic eras. If it was at a true scale, this first Jurassic mammal, 160 million years old, wouldn’t appear until December 19. The Precambrian wouldn’t end until mid-November, and everything we’ve talked about since February first would be concentrated into the last 6 weeks of the calendar.  

* * *

John William Dawson was born at Pictou, Nova Scotia, October 13, 1820. He was a prominent Canadian geologist, often considered to be one of the fathers of paleobotany because of his studies of Paleozoic plants in Canada. He was a professor at McGill University and was first President of the Royal Society of Canada.

—Richard I. Gibson

When did mammals get furry?

BBC report on Juramaia 

Haramiyids


Juramaia reconstruction drawing by Nobu Tamura http://paleoexhibit.blogspot.com/ http://spinops.blogspot.com/ http://www.palaeocritti.com , used under creative commons license. 

September 28. Karoo Supergroup

Map of the Karoo Supergroup in South Africa by
Oggmus, used under Creative Commons license.
Most of the Beaufort and Stormberg Groups are Triassic in age. 

The Karoo Supergroup is an extensive package of rocks in South Africa whose age extends from the Carboniferous into the Jurassic. As you probably recall, South Africa during most of that time was well inside the supercontinent of Gondwana and later Pangaea, distant from the sea, so it is no surprise that most of the rocks of the Karoo are non-marine, deposited in river systems, flood plains, lakes, deserts, and alluvial fans in uplands. The Karoo has some outstanding Triassic fossils.

Much of the late Triassic Stormberg group was deposited by a vast braided stream system that was home to abundant life by late Triassic time. Cycads and other gymnosperms including confers created diverse woodland habitats, from riparian along the rivers to marshes and meadows. The Molteno Formation in the Stormberg group is the primary coal producer in the eastern part of South Africa’s Cape Province – so we had clearly left the early Triassic coal gap behind.  

The coal-bearing sands of the Molteno Formation are overlain by younger red mudstones of the Eliot Formation, which spans the time from very late Triassic into the very early Jurassic, about 210 to 190 million years ago, with the Triassic-Jurassic boundary at 199 million years ago. The Eliot strata contain fossils of more than 40 species of dinosaurs, ranging from theropods like coelophysis which we mentioned a few days ago, to the multi-ton bipedal herbivore, plateosaurus.

The Stormberg group of the Karoo Supergroup also contains fossils of some of those almost-mammals that we discussed the other day, along with a remarkable array of fossil insects.

* * *

Triassic Lystrosaurus  drawing by Dmitry Bogdanov,
used under Creative Commons license

Edwin Harris Colbert was born September 28, 1905, in Clarinda, Iowa. He was a vertebrate paleontologist who discovered and described the coelophysis dinosaurs in New Mexico, and his discovery of Lystrosaurus in Antarctica in 1969 contributed to the acceptance of the theory of continental drift. The dicynodont Lystrosaurus, a mammalian ancestor, was the most common terrestrial vertebrate of the early Triassic. It’s abundant in the early Triassic part of the Karoo sediments.

—Richard I. Gibson

Image sources:
Map of the Karoo Supergroup in South Africa by Oggmus, used under Creative Commons license. Most of the Beaufort and Stormberg Groups are Triassic in age. 

Triassic Lystrosaurus  drawing by Dmitry Bogdanov, used under Creative Commons license. 

September 23. Early mammals?

You remember the therapsids – that’s the group that includes all mammals, together with their pre-mammalian ancestors. They got their start in the Permian, and only a few pre-mammals, cynodonts and dicynodonts, survived the extinction at the end of the Permian. And most of them were gone by the Late Triassic. The only group that survived that long is called the eucynodonts, meaning “true dog teeth.” They were still probably not true mammals, but they were becoming more and more like modern mammals. One variety, called Tritylodonts, lived in what is now South Africa in late Triassic time. They were small warm-blooded rodent-like critters and were initially considered to be the first mammals, but since the 1980s they’ve been seen as close relatives to the mammals. There is no good consensus as to whether that group, or one of several others, was the direct ancestor to modern mammals. 

All of the descendents of the cynodonts and dicynodonts that survived into the late Triassic were probably insectivores or herbivores, an interpretation based on their teeth and jaw structures. Some were burrowing animals and some may have been arboreal and nocturnal. They were only a few inches long, so the traditional comparison of early mammals and their ancestors to modern shrews is a good one. 

Eozostrodon was one of these animals, found in the very late Triassic rocks of England. It dates to about 201 to 205 million years ago, the last 4 million years of the Triassic. It may be a true mammal, but there is ongoing discussion about that question. Its teeth were very much mammalian. Bones of some of its close relatives, called Morganucodon, have been found around the world, from China to South Africa and in North America. Whether Morganucodon is a mammal or a pre-mammal almost seems to me to be a matter of semantics, and exactly how one defines a true mammal versus a close ancestor that wasn’t quite a mammal. It has some teeth that are absolutely mammalian, as well as hair, and some skeletal features that are not so mammalian. Its ear bones are closer to those of reptiles than to mammals, but are clearly evolved from the original reptilian anatomy. So it’s a little subjective, but I guess maybe I’d call it one of the earliest mammals. But I’m not an evolutionary paleontologist, so take that with a grain of salt.

Morganucodon, a possible early mammal from the Late Triassic. Length about four inches.
Drawing by FunkMonk (Michael B. H.) used under Creative Commons license.

In any case, we were getting very, very close to having true mammals on the scene by the very late Triassic. But the Triassic mammals or pre-mammals were certainly not as dominant as their ancestors, the cynodonts and their kin, had been several million years earlier. The Triassic favored the development of the archosaurs and their descendants, the dinosaurs, perhaps because they had better ways of managing water – mammals excrete urea, which includes a lot of water, while archosaurs excreted a paste containing little water. But the dominance of archosaurs might have been a blessing in disguise. If the only surviving pre-mammals in the late Triassic were small insectivores and herbivores, their small size might have pushed them into better modes of regulating temperature, and their likely nocturnal lifestyles might have pushed the development of senses like hearing, sight, and smell, as I suggested in the post on the Antarctic a couple days ago. Those developments in turn might have helped these animals develop larger brains.

So although the ancestors to mammals barely survived the Permian extinction, and then barely survived the rigors of the Triassic world, enough did survive to fill some specific ecological niches so that they could expand when the time was ripe – even if that time wasn’t for another 135 million years.

—Richard I. Gibson

New blog post about Morganucodon

Morganucodon, a possible early mammal from the Late Triassic. Length about four inches.
Drawing by FunkMonk (Michael B. H.) used under Creative Commons license.