[N] 2006 Variations from adult morphology in an subadult skull of Triceratops
Morales, M. (2006) Variations from adult morphology in an subadult skull of the Dinosaur Triceratops from the Hell Creek Formation of Montana. JVP 26(3) Abstracts pp.103
In recent years field crews from the St. Louis Science Center (St. Louis, Missouri) and Emporia State Univ. (Emporia, Kansas) have prospected for fossils in the Upper Cretaceous Hell Creek Formation in eastern Montana. The most commonly found dinosaur taxon is Triceratops, which is usually represented by large/adult individuals. One smaller skull, however, represents a subadult that shows some differences with typical adult morphology. The specimen came from a gray, cross-bedded sandstone channel deposit approximately ten meters below the K-T boundary [K/Pg boundary] , about 15 miles northwest of Jordan. Most of the frill of the subadult specimen is missing, but the main part of the cranium is complete enough to allow it to be reconstructed.
The skull is 70-80% of adult size, with relatively large orbits and some unfused sutures—indicative of a subadult stage of ontogeny. As expected, the diameter of the exoccipital is less than typical for adults or larger subadults. In contrast, however, the brow horns are nearly adult size, as is the braincase. Furthermore, the brow horns
come off the main part of the skull primarily in front of the orbits, rather than mainly above the orbits as in adults. The horns extend forward (not straight up or backward) at a 30-60 degree angle. The bases of the brow horns lie mostly anterior to the braincase, rather than mostly dorsal to it as in adults. The broken bases of the brow horns have large sinuses, which connect to sinuses at the attachment point on the main part of the skull.
As preserved, the horn sinuses are filled with a very dark mass of fossilized organic tissue that was probably rich with blood when the individual was alive. If this represents a highly vascularized area for bone growth, then the bony part of the brow horns may have grown outward from the base, rather than from the tip. Fragments identified as frill are smooth on both sides, without a rough reticulated anterior surface as in adults. The front side of the frill fragments is flat and has (blood) canals and tunnels, but the slightly convex back side lacks these.