Triceratops ontogeny paper

[Tim Williams <twilliams_alpha@hotmail.com>]

John R. Horner and Mark B. Goodwin (2006).  Major cranial changes during 
_Triceratops_ ontogeny.  Proceedings of the Royal Society B: Biological 
Sciences.  273: 2757-2761.

Abstract: "This is the first cranial ontogenetic assessment of 
_Triceratops_, the well-known Late Cretaceous dinosaur distinguished by 
three horns and a massive parietal?squamosal frill. Our analysis is based on 
a growth series of 10 skulls, ranging from a 38cm long baby skull to about 
2m long adult skulls. Four growth stages correspond to a suite of 
ontogenetic characters expressed in the postorbital horns, frill, nasal, 
epinasal horn and epoccipitals. Postorbital horns are straight stubs in 
early ontogeny, curve posteriorly in juveniles, straighten in subadults and 
recurve anteriorly in adults. The posterior margin of the baby frill is 
deeply scalloped. In early juveniles, the frill margin becomes ornamented by 
17?19 delta-shaped epoccipitals. Epoccipitals are dorsoventrally compressed 
in subadults, strongly compressed and elongated in adults and ultimately 
merge onto the posterior frill margin in older adults. Ontogenetic trends 
within and between growth stages include: posterior frill margin transitions 
from scalloped to wavy and smooth; progressive exclusion of the 
supraoccipital from the foramen magnum; internal hollowing at the base of 
the postorbital horns; closure of the midline nasal suture; fusion of the 
epinasal onto the nasals; and epinasal expansion into a morphologically 
variable nasal horn. We hypothesize that the changes in horn orientation and 
epoccipital shape function to allow visual identity of juveniles, and signal 
their attainment of sexual maturity."