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Glishades, nomen dubium based on juvenile hadrosaurid material



From: Ben Creisler
bcreisler@gmail.com


A new online paper:



E. Campione, Kirstin S. Brink, Elizabeth A. Freedman, Christopher T.
McGarrity and David C. Evans (2012)
‘Glishades ericksoni’, an indeterminate juvenile hadrosaurid from the
Two Medicine Formation of Montana: implications for hadrosauroid
diversity in the latest Cretaceous (Campanian-Maastrichtian) of
western North America.
Palaeobiodiversity and Palaeoenvironments (advance online publication)
DOI: 10.1007/s12549-012-0097-1
http://www.springerlink.com/content/2512577152436484/


Glishades ericksoni was named on the basis of partial paired
premaxillae collected from the Late Campanian Two Medicine Formation
of Montana, and was described as a non-hadrosaurid hadrosauroid. This
interpretation of G. ericksoni has significant implications for
hadrosauroid diversity and distribution because it represents the
first occurrence of a non-hadrosaurid hadrosauroid in the Late
Campanian of North America, and therefore implies either a prolonged
period of sympatry between these forms and hadrosaurids or a biotic
interchange with Asia. Given its small size, and therefore potential
juvenile status, the taxonomic identity of G. ericksoni is
re-evaluated here. Comparison with similarly-sized, taxonomically
determinate, and coeval hadrosaurid specimens from the Two Medicine
Formation (Prosaurolophus, Gryposaurus, and Maiasaura) suggest that
the combination of characters used to distinguish G. ericksoni as a
non-hadrosaurid hadrosauroid are more widely distributed or
individually variable in hadrosaurids, or can be explained as the
result of ontogenetic variation. In particular, the unique combination
of characters used to diagnose G. ericksoni is also found in juvenile
individuals of Prosaurolophus, Gryposaurus, and Maiasaura. Inclusion
of juveniles of these taxa, scored on the basis of comparable anatomy,
in the original phylogenetic analysis recovers the juvenile
hadrosaurid specimens outside Hadrosauridae. Consequently, G.
ericksoni cannot be confidently differentiated from a juvenile
saurolophine, which are common in the upper and middle sections of the
Two Medicine Formation, and is thus considered a nomen dubium. Given
their absence in well-sampled Late Campanian and Maastrichtian
deposits, non-hadrosaurid hadrosauroids appear to have been completely
replaced by hadrosaurids in western North America by the Late
Campanian.