Dinosaurs are a well known class of prehistoric animals and have been studied for over two hundred years. With their impressive skeletons they soon found their way from paleontology to popular culture. Be it in movies, books or video games, dinosaurs have been a staple of the fiction genre over the last century. Most of the time the enormous scale of dinosaurs is conveyed by their length and most people will have heard about giants like the Brachiosaurus which would get up to 30m. 
This length is typically measured from snout to tail. However a recent paper by David Hone  casts doubt on the reliability of such measurements. In his study Dr. Hone looked at all complete dinosaur fossils he could find (which were surprisingly few). His analysis showed that tail length strongly varies among individuals of a species and between related species. These variations can be strong even if the species are closely related.
These findings should not come as a huge surprise, considering that it is well known that tail lengths is quite variable in extant species and even the number of caudal vertebrae can be different for individuals in the same species. For example one study looking at the number of vertebrae in a newt found variations of up to 30% between individuals with a given tail length .
Unfortunately paleontologists can not just measure the tail of extinct species, but rather have to rely on the vertebrae as the only hints to estimate the tail length. As most fossils are incomplete, especially when it comes to the last vertebrae of the tail, this means it is very difficult to guess the tail length of extinct animals. Dr. Hone estimates that hitherto quoted lengths for sauropods could be off by 50% or more in either direction. The Brachiosaurus could then have been between 26m or 34m. Similar large variations are expected for other dinosaurs.
This leaves us with two questions. Firstly: Is there a reliable way to estimate animal length from a typically incomplete fossil? And secondly: How strong is the variation in tail length and vertebrate number really?
 Hone, D.W.E. 2012.Variation in the tail length of non-avian dinosaurs.Journal of Vertebrate Paleontology, 32: 1082-1089.
 Moment, G.B. 1949. The number of caudal vertebrae in a caudate amphibian, triturus viridescens, in relation to growth, The Anatomical Record, 103 (4) 711-719