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)

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. 

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

Links:

Shrinking dinosaurs

bird bones

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