Catching a Dinosaur by the Tail

How big was Spinosaurus? The croc-snouted, sail-backed theropod was heralded as being even bigger and more menacing than Tyrannosaurus rex thanks to Jurassic Park III, placing Spinosaurus among the ranks of Giganotosaurus and Carcharodontosaurus as challengers to the vaunted title of the biggest flesh-eater to ever walk the earth. Depending on who you ask, Spinosaurus was about 41 to 59 feet long, making it as large as–if not larger than–old T. rex.

Asking “Which dinosaur was the biggest?” isn’t very helpful, though. “Bigness” isn’t something scientists actually measure. Consider the contemporaneous sauropods Apatosaurus louisae and Diplodocus carnegii. So far as we know, both grew to about 80 feet long, but Apatosaurus was a much bulkier dinosaur. Which is the more important feature for deciding which dinosaur was bigger–mass, length or a combination of the two? In this case, Apatosaurus would seem to win out through its combination of bulk and length, but what if you have two dinosaurs that are about the same size, but the shorter one seems to be stouter than the longer one? What then?

Dinosaur comparisons are especially fraught when dealing with partial skeletons and scientific estimates. How hefty we think a dinosaur was depends on the techniques we use to reconstruct mass. Paleontologists can come up with a probable range that encompassed the variation of a dinosaur species, but, sadly, we can’t weigh an Apatosaurus or Carcharodontosaurus to find out if we’re on the mark.

Length would seem to be a better option for comparing dinosaur size. With a little mathematical work to fill in the extent of cartilage and soft tissues between dinosaur bones, paleontologists can turn to the fossils themselves to gauge dinosaur size. Only, many of the largest dinosaurs are only known from scrappy skeletons.

Very few dinosaurs are known from complete skeletons. This is especially true of the largest dinosaurs. With the exception of specimens like the T. rex “Sue“, one of the most complete large dinosaurs ever discovered, many giants are only known from bits of skull, spine and limbs. Despite being touted as an absolute giant, for example, very little of Spinosaurus has been described. We don’t know how long this theropod truly was–paleontologists can only estimate using more complete dinosaurs as guides for what to expect. And even in relatively compete dinosaur skeletons, few specimens are found with complete tails. The delicate bones near the tip of the tail, especially, are rarely found.

Paleontologist Dave Hone examines how tails–or lack thereof–contributed to dinosaur size in the latest issue of Journal of Vertebrate Paleontology. In his survey of museum collections and the literature, Hone only identified a few dinosaur specimens with tails complete enough to fully comprehend how the organ contributed to the dinosaur’s size. Specimens of the ankylosaur Dyoplosaurus, ceratopsian Centrosaurus and tyrannosaur Gorgosaurus, among others, have complete tails, while individuals of dinosaurs such as the sauropodomorph Lufengosaurus and the oviraptorosaur Caudipteryx have tails missing five vertebrae or less.

When Hone examined these informative fossils, he found that dinosaur tails complicated the question of how long certain varieties of dinosaur were. Tails varied in their proportions among members of the same evolutionary lineage–one species of dinosaur may have a very short tail while its closest known relative may have an exceptionally long tail. And, not surprisingly, individuals of the same species varied in their tail lengths. In essence, statements such as “Spinosaurus was 45 feet long” are rough estimates that are significantly complicated by both variation and a lack of complete tail specimens. On his blog, Hone explained that these estimates affect how we envision dinosaurs and study their biology:

This is not a facile question, aside from the obvious public interest (when was the last time you saw a report on a new dinosaur that didn’t suggest how long it was, if only in terms of double decker buses?). Total length is a measure that’s been used by various researchers (myself included) over the years as a proxy for the mass of dinosaurs. If we’ve been over- or underestimating these values it could potentially affect our results quite a bit, so knowing whether or not these measures are right is worth checking.

This problem isn’t unique to dinosaurs. Natural variation even complicates length estimates of extant species. Take crocodiles, for example. For a long time, herpetologists thought that you could multiply a crocodile’s skull length by seven to get a fairly accurate estimate of the animal’s full stretch. Simple enough. But this rule appears to break down among the biggest individuals, particularly thanks to variations in their tail length. Researchers face the same problem with other reptiles. In estimating the size of extinct, giant monitor lizards, for example, paleontologists consider the length of the snout to the lizard’s “vent” at the base of the tail. This is because tails are variable, and may make an individual animal longer or shorter based on how it is reconstructed. Considering size from the tip of the nose to the base of the tail is a less unwieldy way of measuring size and comparing individuals.

What’s a paleontologist to do? Hone suggests cutting the tail out of dinosaur length estimates. While total length figures will never go out of fashion in popular articles and books, researchers may be better served by estimating the snout-vent length, or similar measurement, that allows for more accurate estimates of dinosaur size. As Hone states, dinosaur bodies from the snout to the back of the hip seems to vary less than tails, so this measurement may present more reliable estimates for dinosaur size. Hone is not saying that paleontologists should totally abandon measurements of total length for dinosaurs, but instead suggests that “snout-sacrum length” would be a better measurement that would coincidentally bring examinations of dinosaurs into line with studies of other tetrapods. The “My dinosaur is bigger than yours” contests will never end, but Hone’s paper suggests a new way of measuring the size of the contestants.

For more, see Hone’s two posts, as well as Ed Yong’s commentary.

Reference:

David W. E. Hone (2012): Variation in the tail length of non-avian dinosaurs, Journal of Vertebrate Paleontology, 32:5, 1082-1089 DOI: 10.1080/02724634.2012.680998

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.