November 12. Pterosaurs

To start with today, I have a correction. In the episode for October 29, when I was talking about cycads, I misrepresented some research I cited. I said the study indicated that cycads had gone extinct and then re-emerged 10 million years ago, but the study was actually talking about a radiation, a rediversification, of cycads 10 million years ago, following a decline from the end of the Cretaceous. I’m sorry for this mistake, which has been corrected in the online blog post. Thanks to an anonymous commenter for pointing out my error.

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Back to the Cretaceous.

Drawing of Quetzalcoatlus feeding by
Mark Witton and Darren Naish,
used under Creative Commons license.

We have to talk about pterosaurs again, because during the Cretaceous they became the largest flying animals in the history of the earth. Pterosaurs were reptiles, but not closely related to dinosaurs nor to the birds that were descended from dinosaurs. They were the first vertebrates to achieve true flight.

They were really quite diverse, and not all of them were huge. The smallest known pterosaur is a specimen from the Jehol Biota of northeastern China, a not-quite-adult pterosaur with a wingspan of about 25 centimeters, or 10 inches. It lived in early Cretaceous time, about 120 million years ago.

At the other end of the size spectrum, two specimens have been found that date to the last 4 million years or so of the Cretaceous Period, about 70 to 66 million years ago. Quetzalcoatlus was discovered in the Javelina Formation at Big Bend National Park, Texas in 1971. You’ll see estimates for its wingspan up to 16 meters or 52 feet, but the consensus today seems to be more like 11 meters or 36 feet – still huge, given that the largest wingspan of a modern bird is the albatross, at 3.5 meters, or 11 feet.

Fossil remains of a late Cretaceous pterosaur found in Romania were described in 2002 with the genus name Hatzegopteryx, but there is a lot of opinion among researchers that it may be identical with Quetzalcoatlus. It had a similar wingspan.

Because these fossils are quite fragmentary, it has been challenging to estimate the body weight of these huge pterosaurs. Some researchers speculate that they could not in fact fly, though I gather that that is a minority view. There’s even more debate about their mode of feeding. Some ideas suggest that they were skimmers, snagging fish out of the near surface of waterways and oceans, but others include the idea that they were actually quadrupedal on land. Most pterosaurs were. The bones of their wings, essentially their front legs, were strong enough for them to lumber around, and perhaps they did that as they scavenged land-based life. Arguments have been put forth against both the fish-eating and the scavenging modes of feeding, and a 2008 study suggests that they were terrestrial stalkers, preying on small land animals as modern storks do – bur remember that storks and other birds have no close affinities with pterosaurs – they just maybe occupy similar ecological niches.

—Richard I. Gibson

Drawing of Quetzalcoatlus feeding by Mark Witton and Darren Naish, used under Creative Commons license.

October 12. Jurassic pterosaurs

Pterosaurs, whose name means “winged lizards,” began during the Triassic but really took off during the Jurassic. They were the first vertebrates to attain true flight, and while they are often called flying dinosaurs, they are not closely related to either dinosaurs or birds. Their descent within the reptiles is ultimately from some basal archosaur, but because of poor and sporadic preservation, their ancestry is not well understood. 

The wing-flap in pterosaurs was stretched over a greatly extended fourth finger. Bats, in contrast, have all their digits extended. 

Rhamphorhynchus image source: Picture by M0tty or Antoine Motte dit Falisse, used under Creative Commons license.

Rhamphorhynchus was an exclusively Jurassic pterosaur that grew to have a wingspan of about two to three feet, with a diamond-shaped tail appendage that might have served as a rudder. The name means beak-snout, and that snout was full of teeth. Spectacular examples have been found in the Solenhofen Limestone of Germany that preserve the soft tissues including the wings. Although it might seem that an active flying animal like rhamphorhynchus would have been warm-blooded, this is not established, and some lines of reasoning, including a slow growth rate to adulthood, something like three years, argue for a cold-blooded metabolism. This may not have been true of all the pterosaurs, however.

There are more than 30 different genera of Jurassic pterosaurs, so they were quite diverse. Most were relatively small, with wingspans of a few feet. There’s been a lot of analysis to try to separate small pterosaurs from juvenile versions of larger species, and there probably were some pretty small pterosaurs, the size of small birds like sparrows.

Geographically, Jurassic pterosaurs have been found on every continent, although they are quite rare in Australia and Antarctica – but whether that is a measure of their distribution or lack of appropriate sediments to preserve their fossils is unclear.

Next month, the Cretaceous, we’ll revisit pterosaurs because that’s when they became gigantic, the largest flying animals in earth’s long history.

If you are in New York, there is an exhibit at the American Museum of Natural History on pterosaurs. It runs through January 4, 2015.

—Richard I. Gibson

LINKS:
Pterosaur skeleton
Rhamphorynchus life story
AMNH Exhibit 

Rhamphorhynchus image source: Picture by M0tty or Antoine Motte dit Falisse, used under Creative Commons license.

September 16. The First Pterosaurs

By late Triassic time, reptiles had diversified a lot, and were occupying more and more ecological niches. We’ve already talked about some of the aquatic reptiles, and today our topic is the first reptiles – in fact the first vertebrates – that learned to fly. Pterosaurs’ name means “winged lizard” and they first appear in the rock record in the first part of the late Triassic, around 220 to 225 million years ago. 

Eudimorphodon, Triassic flying reptile. Photo by Tommy from Arad, under Creative Commons license

The first fossils, discovered in Jurassic rocks in 1784, were thought to be aquatic, but in 1801 Georges Cuvier suggested that they were flying animals, and that idea has survived the test of time. In pterosaurs, generally the fourth finger of the front legs is tremendously extended to support the flap of skin that served as the wing. Triassic varieties mostly had teeth – in some cases lots of teeth, even as many as 110 in a jaw only 6 centimeters (2½ inches) long. They are generally seen as fish-eaters, but they might have eaten insects as well. There’s some evidence that their bodies were at least partially covered by hair or fur.

Most of the known Triassic pterosaurs are from Europe, with most species represented by just one or two specimens. The first Triassic pterosaur wasn’t described until 1973. Their wingspans were small compared to the giants that would evolve by Jurassic and Cretaceous times – Triassic pterosaurs had wingspans ranging from about 1½ to 4 feet. One possible pterosaur from Brazil was about the size of a sparrow.

Because of the paucity of Triassic pterosaur specimens, their early heritage remains uncertain. They are definitely not dinosaurs, nor are they related to birds, which descended from dinosaurs. It’s not even clear whether they are descended from the common archosaurs, perhaps from a gliding variety, or from some other reptile lineage. They do appear relatively suddenly in the fossil record, during the Norian stage of the upper Triassic, but whether this represents a true sudden appearance or is a reflection of the poor preservation isn’t certain.

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Today’s birthday is my professor of geophysics at Indiana University, Judson Mead, born September 16, 1917, in Madison, Wisconsin. He was part of the team during World War II that developed airborne submarine detectors using magnetometers. He taught geophysics at Indiana from 1949 to 1983, and was the director of the Indiana University Geologic Field Station in Montana from 1960 to 1980.

—Richard I. Gibson

Links:
Triassic pterosaurs 

Triassic pterosaurs (U of Bristol)

Photo by Tommy from Arad, under Creative Commons license