Tyrannosaurus abundant in Hell Creek dino census

[bh480@scn.org]

From: Ben Creisler
bh480@scn.org

In case this paper in PLoS One has not been mentioned yet:

Horner, J.R., Goodwin, M.B., Myhrvold, N.(2011). Dinosaur 
Census Reveals Abundant Tyrannosaurus and Rare 
Ontogenetic Stages in the Upper Cretaceous Hell Creek 
Formation (Maastrichtian), Montana, USA. 
PLoS ONE 6(2): e16574. doi:10.1371/journal.pone.0016574
http://www.plosone.org/article/info%3Adoi%2F10.1371%
2Fjournal.pone.0016574

Dinosaur Census Reveals Abundant Tyrannosaurus and Rare 
Ontogenetic Stages in the Upper Cretaceous Hell Creek 
Formation (Maastrichtian), Montana, USA
A dinosaur census recorded during the Hell Creek Project 
(1999?2009) incorporates multiple lines of evidence from 
geography, taphohistory, stratigraphy, phylogeny and 
ontogeny to investigate the relative abundance of large 
dinosaurs preserved in the Upper Cretaceous Hell Creek 
Formation of northeastern Montana, USA. Overall, the 
dinosaur skeletal assemblages in the Hell Creek Formation 
(excluding lag-influenced records) consist primarily of 
subadult or small adult size individuals. Small juveniles 
and large adults are both extremely rare, whereas 
subadult individuals are relatively common. We propose 
that mature individuals of at least some dinosaur taxa 
either lived in a separate geographic locale analogous to 
younger individuals inhabiting an upland environment 
where sedimentation rates were relatively less, or these 
taxa experienced high mortality before reaching terminal 
size where late stage and often extreme cranial 
morphology is expressed.

Tyrannosaurus skeletons are as abundant as Edmontosaurus, 
an herbivore, in the upper Hell Creek Formation and 
nearly twice as common in the lower third of the 
formation. Smaller, predatory dinosaurs (e.g., Troodon 
and dromaeosaurids) are primarily represented by teeth 
found in microvertebrate localities and their skeletons 
or identifiable lag specimens were conspicuously absent. 
This relative abundance suggests Tyrannosaurus was not a 
typical predator and likely benefited from much wider 
food choice opportunities than exclusively live prey 
and/or specific taxa. Tyrannosaurus adults may not have 
competed with Tyrannosaurus juveniles if the potential 
for selecting carrion increased with size during ontogeny.

Triceratops is the most common dinosaur and isolated 
skulls contribute to a significant portion of this 
census. Associated specimens of Triceratops consisting of 
both cranial and postcranial elements remain relatively 
rare. This rarity may be explained by a historical 
collecting bias influenced by facies and taphonomic 
factors. The limited discovery of postcranial elements 
may also depend on how extensive a fossil quarry is 
expanded after a skull is collected.