Growth thumb|left|Juvenile A. sp. mount, Sam Noble Oklahoma Museum of Natural History A 1999 microscopic study of Apatosaurus and Brontosaurus bones concluded the animals grew rapidly when young and reached near-adult sizes in about 10years. In 2008, a study on the growth rates of sauropods was published by Thomas Lehman and Holly Woodward. They said that by using growth lines and length-to-mass ratios, Apatosaurus would have grown to 25t (25 long tons; 28 short tons) in 15years, with growth peaking at in a single year. An alternative method, using limb length and body mass, found Apatosaurus grew per year, and reached its full mass before it was about 70years old. These estimates have been called unreliable because the calculation methods are not sound; old growth lines would have been obliterated by bone remodeling. One of the first identified growth factors of Apatosaurus was the number of sacral vertebrae, which increased to five by the time of the creature's maturity. This was first noted in 1903 and again in 1936.
growth thumb|left|juvenile a sp mount sam noble oklahoma museum of natural history a 1999 microscopic study of apatosaurus and brontosaurus bones concluded the animals grew rapidly when young and reached near-adult sizes in about 10years in 2008 a study on the growth rates of sauropods was published by thomas lehman and holly woodward they said that by using growth lines and length-to-mass ratios apatosaurus would have grown to 25t (25 long tons; 28 short tons) in 15years with growth peaking at in a single year an alternative method using limb length and body mass found apatosaurus grew per year and reached its full mass before it was about 70years old these estimates have been called unreliable because the calculation methods are not sound; old growth lines would have been obliterated by bone remodeling one of the first identified growth factors of apatosaurus was the number of sacral vertebrae which increased to five by the time of the creature's maturity this was first noted in 1903 and again in 1936