Achelousaurus
Name:
Achelousaurus
(Achelous's lizard).
Phonetic: Ah-kell-oo-sore-us.
Named By: Scott Sampson - 1995.
Classification: Chordata, Reptilia, Dinosauria,
Ornithischia, Ceratopsia, Ceratopsidae, Centrosaurinae,
Pachyrhinosaurini, Pachyrostra.
Species: A. horneri (type).
Diet: Herbivore.
Size: Skull up to 1.6 meter long.
Known locations: USA, Montana - Two Medicine
Formation.
Time period: Campanian of the Cretaceous.
Fossil representation: 3 skulls and partial post
cranial remains.
Achelousaurus
may represent something of a transition as even though it has two
distinct horns that rise from the top of its neck frill like in
Einiosaurus,
it has a large nasal boss as seen in Pachyrhinosaurus.
This is why Achelousaurus has been considered a
transitional form that
links these two ceratopsian
dinosaurs that goes on to show the gradual
development of the centrosaurine line. While not all palaeontologists
are convinced to this, it is the clearest line between the early
nasal horned centrosaurine ceratopsians like Centrosaurus
and the nasal
bossed (a large nasal lump as opposed to a pointed horn)
Pachyrhinosaurus.
The
lack of a nasal horn was the inspiration for the name Achelousaurus,
which references Achelous, the patron deity of the Achelous
River. In Greek mythology Heracles (sometimes spelt Hercules)
got in a fight with this deity and tore off one of his horns.
Achelous was also mentioned as having the ability to shape-shift,
and again this reflects upon Achelousaurus
seemingly shape-shifting
from Einiosaurus to Pachyrhinosaurus.
Achelous
has been combined with
the more common Ancient Greek for 'lizard’.
The
loss of the horn of Achelousaurus was probably not
as violent as the
mythological encounter as it seems to have been gradually formed by the
specialisation of Einiosaurus's nasal horn that
curved more and more
forwards. This was probably taken by members of the species to be a
sign of maturity with the most rounded horns belonging to the most
mature individuals. Given time it’s conceivable that the nasal boss
would form in place of the increasingly curved horn resulting in
Achelousaurus. The growth of a nasal boss as a
preferred sexually
selected characteristic may in turn have triggered a reduction in the
two large neck frill horns resulting in the form of Pachyrhinosaurus.
Further reading
- Two new horned dinosaurs from the Upper Cretaceous Two Medicine
Formation of Montana; with a phylogenetic analysis of the
Centrosaurinae (Ornithischia: Ceratopsidae) - Journal of Vertebrate
Paleontology 15(4): 743-760. - S. D. Sampson - 1995.
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