Name: Acristavus
(Non-crested grandfather).
Phonetic: Ak-riss-ta-vus.
Named By: Gates et al - 2011.
Classification: Chordata, Reptilia, Dinosauria,
Ornithischia, Hadrosauridae.
Species: A. gagslarsoni (type).
Type: Herbivore.
Size: Uncertain, but probably similar to
contemporary hadrosaurids of the time.
Known locations: USA, Montana- Two Medicine
Formation, Utah - Wahweap Formation.
Time period: Campanian of the Cretaceous.
Fossil representation: Two almost complete skulls,
possible other partial remains.
Acristavus
was a late surviving hadrosaurid
that stood out from amongst the others
of its kind because it had no head ornamentation. Although not strictly
unique, many of the other better known hadrosaurids such as
Parasaurolophus
and Lambeosaurus
had very elaborate and impressive head
crests. The fact Acristavus did not have one
suggests that it belonged
to a different group of hadrosaurids although it’s possible that it
could have been related to Maiasaura.
There
are two main thoughts
of reason about the lack of ornamentation. The first is that Acristavus
is descended from ornamented dinosaurs, but lost the ornamentation
through evolution, the result of it no longer being a desired trait in
choosing a mate. Essentially this would result in it being bred out.
The second is that Acristavus represents a more
basal form of
hadrosaurid and possible forerunner to the previously mentioned
Maiasaura and possibly the more numerous Edmontosaurus.
Further reading
- New unadorned hadrosaurine hadrosaurid (Dinosauria, Ornithopoda) from
the Campanian of North America. - Journal of Vertebrate Paleontology
31(4):798-811 - T. A. Gates, J. R. Horner, R. R. Hanna & C. R.
Nelson - 2011.