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Clovery tyrannosauroid and polar Pachyrhinosaurus growth



From: Ben Creisler
bh480@scn.org

In case these new papers have not been mentioned:

Lindsay E. Zanno & Peter J. Makovicky (2011) 
On the earliest record of Cretaceous tyrannosauroids in 
western North America: implications for an Early 
Cretaceous Laurasian interchange event.
Historical Biology (advance online publication)
DOI: 10.1080/08912963.2010.543952 
http://www.informaworld.com/smpp/content~db=all~content=a9
34038947~frm=titlelink

Abstract 
The sudden appearance of Asian dinosaur clades within 
Lower Cretaceous strata of western North America has long 
been recognised as a biotic dispersion event related to 
initial establishment of a Beringian land bridge. To 
date, uncertainty exists regarding the timing of the 
Early Cretaceous Laurasian interchange event (EKLInE) and 
the pattern of associated biotic dispersal. Here, we 
report a tyrannosauroid premaxillary tooth (FMNH PR 2750) 
from the Cloverly Formation, Wyoming, USA, that pushes 
back the earliest Cretaceous record of the clade in North 
America. Although fragmentary, the tooth is consistent 
with mounting evidence for a pre-108 Ma initiation of 
EKLInE and earliest Albian emplacement of Beringia. 
Previous authors have considered the Aptian/Albian of 
western North America a depauperate dinosaur fauna, 
characterised by regional extinction and diversity 
decline. Documentation of Albian tyrannosauroids in the 
region indicates a more dynamic ecosystem than previously 
appreciated and marks an early start to faunal mixing 
between immigrant and endemic dinosaur clades. Finally, 
we find that the enamel microstructure of FMNH PR 2750 
conforms to the morphotype of tyrannosaurids, yet 
exhibits poor columnar differentiation. This morphology 
bolsters prior interpretations on the phylogenetic 
utility of enamel microstructure and suggests a trend of 
increasing enamel complexity within Tyrannosauroidea. 

 

Gregory M. Erickson & Patrick S. Druckenmiller (2011)
Longevity and growth rate estimates for a polar dinosaur: 
a Pachyrhinosaurus (Dinosauria: Neoceratopsia) specimen 
from the North Slope of Alaska showing a complete 
developmental record.
Historical Biology (advance online publication)  
DOI: 10.1080/08912963.2010.546856 
http://www.informaworld.com/smpp/content~db=all~content=a9
33897148~frm=titlelink

Abstract 
Our knowledge of growth dynamics in large ceratopsian 
dinosaurs is very poor, in part, due to the paucity of 
quantifiable age markers such as growth lines in their 
bones. We sought marker-based, osteohistological evidence 
for ceratopsid age structure from high Arctic 
paleolatitudes based on the observations that: (1) extant 
mammals from high latitudes better express growth lines 
in their hard tissues than those from lower latitudes, 
and (2) the occurrence of accentuated growth banding in 
teeth from Arctic dinosaurs. We examined the long bones 
in the specimens of Pachyrhinosaurus sp. from the early 
Maastrichtian of northern Alaska, and found conspicuous 
osseous banding. Histological analysis of the spacing, 
structure, pattern and numbers of these bands in the 
femur of a very large specimen suggests that they are not 
a taphonomic artefact, rather they appear to reflect 
annual growth cycling. Counts and measurements of the 
growth zones suggest that the animal showed rapid linear 
growth early in ontogeny, sexual maturity in perhaps the 
9th year of life, and that it died in the 19th year of 
life. Our data adds to a growing body of evidence that a 
genetically distinct northern dinosaurian fauna existed 
at high paleolatitudes in Alaska during the Late 
Cretaceous.