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 October, so we're moving through the Jurassic Period.

Tuesday, October 21, 2014

October 21. Stegosaurus

Drawing by Marsh (USGS, public domain). This is an inaccurate depiction – a single row of back plates (rather than two) and too many tail spikes (should be two pairs).

We really can’t go through the Jurassic without talking about stegosaurus, arguably one of the most recognizable dinosaurs. The first specimen of stegosaurus was found near Morrison, Colorado, in 1877, by O.C. Marsh during his “bone war” with Edward Cope. Its remarkably small brain was noticed immediately – something like 2½ ounces for a 10-ton body, and modern reconstructions depict a creature that may have moved slowly but that probably carried its spiked tail high and might have wielded it like a weapon. 

Stegosaurus, whose name means “roofed” or “covered lizard,” in reference to the bony plates on its back, was a herbivore that grew to 30 feet long. It couldn’t lift its head very high, so it probably grazed on ground cover or short bushes.

For all its fame, only about 80 individuals are represented in fossils. Most are from the Morrison Formation of western United States, but in 2006 one was found in Portugal. Ancestral stegosaurids – not the genus Stegosaurus – are known from the United States, China, England, Germany, and France, but Stegosaurus itself appears to have had a limited distribution in both time and space. There are some Cretaceous specimens that some researchers have attributed to Stegosaurus, but this is not generally accepted, and as far as we can tell with certainty, Stegosaurus was only on the scene for about 5 million years, from about 150 to 155 million years ago.

If you see an image of Stegosaurus juxtaposed with Triceratops or Tyrannosaurus rex, be very suspicious. That image is off by about 80 million years.

—Richard I. Gibson

Kung-fu Stegosaur

Drawing by Marsh (USGS, public domain). This is an inaccurate depiction – a single row of back plates (rather than two) and too many tail spikes (should be two pairs).

Monday, October 20, 2014

October 20. The Solnhofen Limestone

In several of this month’s episodes I’ve mentioned the Solnhofen Limestone, the rocks in Bavaria where spectacular fossils are found – from the lobster-like Eryon to the flying pterosaur Rhamphorhynchus to Archaeopteryx that we talked about yesterday. Why is this rock so special?

Crinoid from Jurassic Solnhofen limestone (source)
During Late Jurassic time, about 150 million years ago, central Europe was a string of islands, the high-standing areas along seaways defined by rifts that formed as Pangaea began to break apart. The area was close to the Jurassic equator. The array of islands made for some restricted lagoons between them, and in some locations, arid conditions together with restricted circulation made for anoxic conditions where life could not survive. No scavengers, no oxygen to decompose bodies. We’ve heard this story before.

But wait, you say – when we’ve heard this in previous episodes, those stagnant lagoons accumulated organic-rich mud that became black shale. You said this was limestone. What’s the deal? We have to infer that the islands were low-lying, and not shedding much in the way of clastic sediment into the lagoons. Clastics – sand, silt, and mud – are deposited typically in settings where the topographic relief is at least moderately high, so erosion can remove those materials from outcopping rocks on land and dump them into adjacent marine settings. Here in Bavaria during the Jurassic, I think we have to see the setting as something like the modern Bahamas – low islands, and even there the rocks on the surface were probably limestone, not granite or other rocks that would yield quartz sand, silt, and mud. The lagoons were carbonate-rich, so it was fine grained calcite, calcium carbonate, that precipitated out.

The resulting extremely fine-grained rock was also remarkably uniform, so much so that the Solnhofen is called a lithographic limestone – ideal for making lithographic plates – stones carved in fine detail to use in printing illustrations, including multi-color lithographs. It was the quarrying operation in the 19th century for lithographic uses that revealed the spectacular fossils of the Solnhofen.

Fossils are actually not all that common in the Solnhofen limestone, but when they are found, they are preserved in exquisite detail, some of the finest fossils ever found anywhere. In addition to the animals we have talked about, the fauna includes jellyfish with soft parts preserved, free-floating crinoids, beetles, cephalopods, horseshoe crabs, turtles, fish, dragonflies, crocodiles, and more. It is truly a world-class lagerst├Ątte, one of those rare natural collections of spectacularly preserved fossils.

—Richard I. Gibson

UC Berkeley on Solnhofen 

Photo by Ushakaron, used under Creative Commons license

Sunday, October 19, 2014

October 19. The first bird

Photo of the Berlin Specimen by
H. Raab (User:Vesta) under creative commons license
In 1861, the first complete skeleton of a bird was found in the Solnhofen Limestone of Bavaria, in southern Germany. It was a strange bird, with teeth, a long second claw, and a reptilian tail – but it had something displayed by no other reptile fossil discovered to that time – Feathers. The animal was named for a fossil of a single feather found the year before in the same rocks. It was called Archaeopteryx, meaning “ancient wing” or “ancient feather.” 

Since 1861 eleven more specimens have been found. While the animal is covered with feathers so that it is instantly reminiscent of a raven-sized bird, the differences were enough that almost immediately it was seen as a transitional fossil between reptiles and birds. As much as Archaeopteryx is ingrained in our imaginations as the first bird, just as we saw with the mammals, there’s a degree of disagreement as to exactly what constituted a bird back in the Jurassic. But I don’t think it’s anything like as complicated as the story of mammals.  

Archaeopteryx was certainly an early bird (or at least an extremely bird-like reptile), but whether or not it was truly ancestral to modern birds is debated. Discoveries of other avians in Jurassic rocks of China may be as much as 10 million years older than the Bavarian specimens, which are dated to about 150 million years ago. Other candidates have been suggested as early bird ancestors as well. Their existence leads to the idea that some other lineage might be the one that led more directly to modern birds.

Studies of the feathers have suggested that Archaeopteryx was incapable of true flight, but I think that’s a minority view. It lacks a backward-pointing toe, which modern perching birds have, so it might have been more of a ground-dweller, like a chicken. With only 12 specimens, some represented by only a few bones, it’s challenging to reconstruct the daily life of Archaeopteryx.

Many fossils have been found of dinosaurs that are definitely not birds, but which have feathers. So you can’t use the presence or absence of feathers as a simple criterion to draw the line between reptiles and birds. There must have been transitional species, and Archaeopteryx was probably one of them. If a scientist could go back to the Jurassic and examine a live one, he or she might well classify the creature as neither a reptile nor a bird, but as something else – like some of those early mammal ancestors we’ve heard about.

One evolutionary trend that seems clear among bird-like dinosaurs and birds themselves is their shrinking size and evolution of a lightweight skeleton, presumably to reduce weight for flight and to increase maneuverability generally. I have a couple links below to some recent papers on the topic of size in dinosaurs and early birds. Obviously there are some pretty big birds today, including ostriches, but the increase in size of birds may be a development of relatively more recent times, the past 60 million years or so as mammals and birds began to fill niches vacated by dinosaurs killed in the extinction event 65 million years ago.

Today, I think there is no doubt whatsoever that birds are descended from theropod dinosaurs. In some ways of looking at it, as I’m sure you have heard, birds ARE dinosaurs – evolved and modified, and it really becomes a semantic issue of definition – exactly where IS the line between them? It may be clear today, but it definitely was not clear back in the Jurassic.

And even though technically Archaeopteryx is generally seen as not really the ancestor to modern birds, if you want to think of it as the first bird, I’d say you’re only going to get in trouble calling it that if you get into a debate with a real specialist in this field.

And whichever particular group might be the ultimate ancestor to modern birds, there is no doubt that by the late Jurassic, we had animals that a modern time traveler would have called birds. Strange birds, maybe, but birds nonetheless.

—Richard I. Gibson


Shrinking dinosaurs

bird bones

Photo of the Berlin Specimen by H. Raab (User:Vesta) under creative commons license