These daily podcasts build upon previous episodes, so the best way to work through them is by starting with the oldest, January 1. But you don't have to do that.
Most episodes are two to 10 minutes long. It's November, so we're moving through the Cretaceous Period.

Wednesday, November 26, 2014

November 26. Birds



The Cretaceous Interior Seaway of western North America was a great abode for life, both in the waters and on the adjacent tropical or subtropical lands. You remember the chalk cliffs of Dover, England – the chalk that gave the name to the Cretaceous, which means “chalk-bearing”? The North American interior seaway also contained a lot of those microscopic coccolithophores whose shells accumulated to make chalk. 

Probably the most famous example is the Niobrara Chalk, a prominent part of the Cretaceous stratigraphic section especially in Nebraska and Kansas which were under the waters of the seaway. Most of the fossils from the Niobrara are, as you’d expect, marine fossils – fish, turtles, plesiosaurs. But it also contains occasional land-dwelling dinosaurs, whose bodies must have been washed offshore, and there are winged vertebrates as well.

Pteranodons, winged reptiles that are not dinosaurs, flew in the late Cretaceous skies above and near the western interior seaway and what is now the Gulf Coastal Plain around 85 million years ago. Despite their relatively short existence, probably not much more than two or three million years, they are well known because more than 1,200 specimens have been found, the most of any pterosaur. “Pteranodon” means “winged, toothless,” and they had typical wingspans of 6 meters, about 20 feet.

Pteranodons shared the skies with birds, which were becoming much more diverse during the Cretaceous. Ichthyornis was a small toothed bird that probably occupied ecological niches similar to gulls. Many specimens of Ichthyornis have been found in the Niobrara Chalk. Its time range is about 95 to 85 million years ago, and it is interpreted to be of a bird lineage that was near that of modern birds, but it was probably not a direct ancestor to them. Its name means “fish-bird,” and it’s among the best-represented birds in the fossil record – but “well represented” probably means a few hundred specimens, not thousands. 

Hesperornis fossil (Smithsonian) photo by Quadell, used under Creative Commons license

Hesperornis, whose name means “western bird,” was a flightless bird – based on its size, its wings were useless for flight. It was most likely a penguin-like, or more accurately loon-like, fully aquatic bird, demonstrating that birds had evolved and adapted to the marine realm as early as 84 million years ago. Hesperornis was pretty big, about 6 feet or almost 2 meters long. Its fossils have been found in Russia and Europe as well as many places in the western North American Interior Seaway, so as a group it clearly had a wide range.

Both Ichthyornis and Hesperornis were extinct well before the end of the Cretaceous, at about 85 and 78 million years ago, respectively.


—Richard I. Gibson

Ichthyornis
Hesperornis

Hesperornis fossil (Smithsonian) photo by Quadell, used under Creative Commons license

Tuesday, November 25, 2014

November 25. Tyrannosaurus rex



Obviously we can’t leave the Cretaceous without at least a mention of Tyrannosaurus rex. The tyrant lizard king – that’s what the name means – only lived during a relatively brief span of geologic time, a million years from 67 to 66 million years ago, just before the end of the Cretaceous. It was one of the largest land predators of all time, at 12 meters or 40 feet in length.

Henry Fairfield Osborn described the species from specimens collected at Hell Creek, Montana, in 1902 and 1908. Since then fossils from more than 50 individuals have been found. There’s enough individual variation that there are also specimens originally assigned to other species that were reassigned to Tyrannosaurus rex after further study, but there are also other species within the overall group of tyrannosaurids.

Photo of Sue, the most complete T. rex specimen known,
in the Chicago Field Museum,
by Connie Ma, used under Creative Commons license.  
They were bipedal carnivores, and the debate continues over whether they were active predators, scavengers, or both. Most researchers today seem to agree that Tyrannosaurus rex was an opportunist that both scavenged and caught live prey. It had a really strong bite that would force its teeth – up to 12 inches long – into pretty much whatever it wanted to bite.

Another current debate is whether T. rex was warm-blooded or cold-blooded. As near as I can tell, the jury is still out on that question.

The Hell Creek Formation, the rocks in which many T. rex fossils have been found, was laid down on the margin of the Cretaceous Interior Seaway that we talked about a few days ago. The setting was one of coastal flood plains and deltas, with rivers and some swampy areas. The climate must have been subtropical or tropical and clearly supported a wide range of life. There were lush forests of angiosperms, conifers, gingkoes, and cycads. Besides the famous T. rex, the Hell Creek Formation contains mammals, birds, pterosaurs, lizards, snakes, turtles, crocodilians, and a wide range of dinosaurs large and small. The more marine sediments contain fish, plesiosaurs, ammonites, sharks and rays, and there were snails, oysters, and clams in the marine and fresh waters. Lots of life, a long, tall food chain with carnivores like Tyrannosaurus rex at or near the top.

Tyrannosaurs only represent about 4% of the dinosaur fossils in the Hell Creek. The most numerous group were the Ceratopsians, horned dinosaurs, of which the most famous member is probably Triceratops, which we’ll talk about a little more in a few days.

There is of course a vast amount of information readily available about Tyrannosaurus rex. I have links below to a video and a podcast, both with Tyrannosaur expert Dr. Thomas Holtz who is at the University of Maryland, for more information.

—Richard I. Gibson

Video: The Life and times of Tyrannosaurus rex with Thomas Holtz 

Podcast link: New understanding 


Photo of Sue, the most complete T. rex specimen known, in the Chicago Field Museum, by Connie Ma, used under Creative Commons license.    

Monday, November 24, 2014

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