Posted on behalf of Ed Yong.
Travel down the body of a dinosaur, and our knowledge of its anatomy tails away past its hips. As Dave Hone from University College Dublin has discovered, the vast majority of dinosaur skeletons, even many that have been deemed ‘complete’, are missing parts of their tails.
These lost bones are important because tails are included in estimates of dinosaur length, which are often quoted, and sometimes used to estimate mass. “A fairly simple question of ‘How long in total was this dinosaur?’ could be really quite tricky to answer for a very good number of species,” says Hone, writing in his Guardian blog. If tails are telling tall tales, other important measures could be inaccurate.
Hone did a painstaking search for complete tails among the scientific literature, more than a dozen museums, photos and his colleagues’ memories. His search came up largely empty. Even many of the best-preserved fossils, which have feathers and skin imprints, have only partial tails.
“Despite having thousands of dinosaur fossils, including a good few hundred that could broadly be considered complete, we’ve got barely two dozen complete tails,” he says. His results are published this week in the Journal of Vertebrate Paleontology.
The problem with tails is that their bones are often tiny and easily disconnected from the main skeleton. Many specimens have lost the distinctive final bone, which has no protrusions and tapers to a rounded point. Without it, we have no way of knowing whether the tail continued beyond the point where the preserved skeleton finishes.
Other specimens are missing larger numbers of vertebrae and for some species, such as the infamous Spinosaurus, we have virtually no tail material at all. Some researchers claim it was 14 metres long, and others say it stretched to 16–18 metres, but Hone says that the argument “is a bit frivolous when the tail could have been anything from 5–10 metres in length”.
Hone’s survey reveals how hard it is to estimate total length without complete tails, especially because tail length varies greatly within dinosaur families. Epidexipteryx has a tail just 1.2 times the length of its thigh bone, one of the shortest of any dinosaur. But its closest known relative, Epidendrosaurus, has proportionally one of the longest tails, 7 times the length of its thigh and possibly even longer. Hone even found significant variation within single species. One ‘complete’ specimen of the horned dinosaur Leptoceratops has 38 tail bones, whereas a second has 48.
In some ways, this is unsurprising. In modern four-legged animals (tetrapods), tail length can vary between individuals according to health and environmental conditions, and most scientists leave it out when measuring specimens. Instead, they take the animal’s length to be the distance between the tip of its head to its anus — the “snout–vent length”. But the position of the anus is hard to determine through skeletal remains, so Hone suggests using the end of the hip bone instead.