Today's heaviest flying bird, the 48-pound Kori Bustard, was a lightweight compared to dinosaur-era flying reptiles known as pterosaurs, according to a new study that presents a novel method for estimating pterosaur weight.
Calculations based on footprints left behind by pterosaurs, also known as pterodactyls, reveal these animals weighed up to 320 pounds. The study, published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, is believed to be the first to use fossilized footprints to infer the body weight of an extinct animal.
Prior studies utilized everything from pterosaur bone mass to volumetric modeling to figure out how much the prehistoric "winged lizards" weighed.
"My study provides good independent evidence that large pterosaurs were quite heavy," author Tai Kubo told Discovery News.
Kubo, a paleontologist at the Fukui Prefectural Dinosaur Museum in Japan, began his study by collecting trackways from 17 species of living reptiles housed at Ueno Zoo in Tokyo. These animals included crocodiles, lizards, tortoises and a frog.
"I went to the zoo every week for two years," he said. "Since some reptiles, such as crocodiles, are very reluctant to move, and often it is very difficult to predict which way they will move, I had to just keep waiting for a whole day to collect one trackway."
After collecting the footprints, Kubo noted the weights of each animal. He determined there was a connection between weight and a calculated relationship between the individual's fore and hind limb foot sizes measured from their tracks.
Kubo then took this mathematical formula and applied it to fossilized trackways that had previously been attributed to pterosaurs. He found that many pterosaurs were at least 10 times heavier than the heaviest modern flying birds.
Michael Habib, a Chatham University assistant professor of biology, told Discovery News that he thinks Kubo's new method for estimating pterosaur weight is "sensible and interesting." He indicated, however, that the jury is still out on just how heavy the largest pterosaurs really were.
Habib mentioned that prior estimates, using other methods, concluded that big pterosaurs weighed anywhere from 441 to 573 pounds.
"I suspect that the trackway method of body mass estimation will be made more precise in the future," Habib said.
Donald Henderson, curator of dinosaurs at the Royal Tyrrell Museum, wondered if Kubo's data might have been different if he had used footprints made by living birds instead of the other animals Kubo selected for his study.
"I think the largest pterosaurs -- things like Quetzalcoatlus -- reached several hundred kilograms," Henderson told Discovery News. "Unfortunately, we have nothing even beginning to resemble a complete skeleton for the largest pterosaurs, so their body shapes and volumes, and associated body masses, are not known with any certainty."
Kubo, however, holds that "footprints can be a good indicator of body weight."
In the future, he hopes to apply his weight estimation calculations to dinosaurs.
"Before applying the method to dinosaurs," Kubo said, "we have to obtain a lot of data about body weight and footprints of extant birds and mammals, since their postures are more similar to those of dinosaurs than those of extant reptiles and amphibians."
He added, "I think weight can be measured from any footprint, and the result can tell us a lot of things."