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Triceratops evolutionary trends in Hell Creek Formation, Montana
A new online paper:
John B. Scannella, Denver W. Fowler, Mark B. Goodwin, and John R. Horner (2014)
Evolutionary trends in Triceratops from the Hell Creek Formation, Montana.
Proceedings of the National Academy of Sciences (advance online publication)
The deciphering of evolutionary trends in nonavian dinosaurs can be
impeded by a combination of small sample sizes, low stratigraphic
resolution, and lack of ontogenetic (developmental) details for many
taxa. Analysis of a large sample (n > 50) of the famous horned
dinosaur Triceratops from the Hell Creek Formation of Montana
incorporates new stratigraphic and ontogenetic findings to permit the
investigation of evolution within this genus. Our research indicates
that the two currently recognized species of Triceratops (T. horridus
and T. prorsus) are stratigraphically separated and that the evolution
of this genus likely incorporated anagenetic (transformational)
change. These findings impact interpretations of dinosaur diversity at
the end of the Cretaceous and illuminate potential modes of evolution
in the Dinosauria.
The placement of over 50 skulls of the well-known horned dinosaur
Triceratops within a stratigraphic framework for the Upper Cretaceous
Hell Creek Formation (HCF) of Montana reveals the evolutionary
transformation of this genus. Specimens referable to the two
recognized morphospecies of Triceratops, T. horridus and T. prorsus,
are stratigraphically separated within the HCF with the T. prorsus
morphology recovered in the upper third of the formation and T.
horridus found lower in the formation. Hypotheses that these
morphospecies represent sexual or ontogenetic variation within a
single species are thus untenable. Stratigraphic placement of
specimens appears to reveal ancestor–descendant relationships.
Transitional morphologies are found in the middle unit of the
formation, a finding that is consistent with the evolution of
Triceratops being characterized by anagenesis, the transformation of a
lineage over time. Variation among specimens from this critical
stratigraphic zone may indicate a branching event in the Triceratops
lineage. Purely cladogenetic interpretations of the HCF dataset imply
greater diversity within the formation. These findings underscore the
critical role of stratigraphic data in deciphering evolutionary
patterns in the Dinosauria.