<P> The theory of their use in sexual display was first proposed by Davitashvili in 1961 and has gained increasing acceptance since . Evidence that visual display was important, either in courtship or in other social behavior, can be seen in the fact that horned dinosaurs differ markedly in their adornments, making each species highly distinctive . Also, modern living creatures with such displays of horns and adornments use them in similar behavior . A 2006 study of the smallest Triceratops skull, ascertained to be a juvenile, shows the frill and horns developed at a very early age, predating sexual development and thus probably important for visual communication and species recognition in general . The use of the exaggerated structures in dinosaurs as species identification has been questioned, as no such function exists for structures in modern species . </P> <P> In 2006, the first extensive ontogenetic study of Triceratops was published in the journal Proceedings of the Royal Society . The study, by John R. Horner and Mark Goodwin, found that individuals of Triceratops could be divided into four general ontogenetic groups, babies, juveniles, subadults, and adults . With a total number of 28 skulls studied, the youngest was only 38 cm (15 in) long . 10 of the 28 skulls could be placed in order in a growth series with one representing each age . Each of the four growth stages were found to have identifying features . Multiple ontogenetic trends were discovered, including the size reduction of the epoccipitals, development and reorientation of postorbital horns, and hollowing out of the horns . </P> <P> Torosaurus is a ceratopsid genus first identified from a pair of skulls in 1891, two years after the identification of Triceratops . The Torosaurus genus resembles Triceratops in geological age, distribution, anatomy and physical size and it has been recognised as a close relative . Its distinguishing features are an elongated skull and the presence of two fenestrae, or holes, in the frill . Paleontologists investigating dinosaur ontogeny (growth and development of individuals over the life span) in the Hell Creek Formation, Montana, US, have recently presented evidence that the two represent a single genus . </P> <P> John Scannella, in a paper presented in Bristol, UK at the conference of the Society of Vertebrate Paleontology (25 September 2009) reclassified Torosaurus as especially mature Triceratops individuals, perhaps representing a single sex . Jack Horner, Scannella's mentor at Bozeman Campus, Montana State University, noted that ceratopsian skulls consist of metaplastic bone . A characteristic of metaplastic bone is that it lengthens and shortens over time, extending and resorbing to form new shapes . Significant variety is seen even in those skulls already identified as Triceratops, Horner said, "where the horn orientation is backwards in juveniles and forward in adults". Approximately 50% of all subadult Triceratops skulls have two thin areas in the frill that correspond with the placement of "holes" in Torosaurus skulls, suggesting that holes developed to offset the weight that would otherwise have been added as maturing Triceratops individuals grew longer frills . A paper describing these findings in detail was published in July 2010 by Scannella and Horner . It formally argues that Torosaurus and the similar contemporary Nedoceratops are synonymous with Triceratops . </P>

What is the closest relative to the triceratops