Prosaurolophus
Name:
Prosaurolophus
(Before Saurolophus).
Phonetic: Pro-sore-o-lo-phus.
Named By: Barnum Brown - 1916.
Classification: Chordata, Reptilia, Dinosauria,
Ornithischia, Ornithopoda, Iguanodontia, Hadrosauroidea,
Hadrosauridae, Saurolophinae.
Species: P. maximus
(type), P. blackfeetensis.
Diet: Herbivore.
Size: Up to around 9 meters long.
Known locations: Canada, Alberta - Dinosaur
Park Formation. USA, Montana - Two Medicine Formation.
Time period: Campanian of the Cretaceous.
Fossil representation: Multiple individuals.
The
name Prosaurolophus was decided for this genus of hadrosaurid
dinosaur
on the basis that it was believed to have been similar to but lived
before another genus called Saurolophus.
However, this
interpretation is now somewhat more controversial with different
palaeontologists citing Prosaurolophus as being
more closely related to
other varying genera other than Saurolophus. A
more basic and easier
to accept interpretation of Prosaurolophus is that
it is a saurolophine
hadrosaurid. This conclusion is reached upon the simple fact that
Prosaurolophus has a small and solid crest as
opposed to the hollow
(and often more elaborate) crests of the lambeosaurines. A
saurolophine hadrosaur is a member of the sub-group Saurolophinae, a
newer term for the hadrosaurids that were mostly previously included
with the Hadrosaurinae and described as hadrosaurines (at the time of
writing only Hadrosaurus
remains classified as a hadrosaurine). The
shapes of saurolophine hadrosaurids head crests can differ, but in
Prosaurolophus the head is small and triangular.
The
first Prosaurolophus fossils were discovered in
Alberta, Canada in
1915 by Barnum Brown. Later in 1916, Brown described the genus
based upon an incomplete skull that was actually incorrectly restored.
Later and better preserved discoveries revealed the skull to actually
be shorter than the reconstruction. The type species was established
as P. maximus, and while further remains were
attributed to the
genus, a new species would not be named until 1992. In this year
Jack R. Horner, a palaeontologist internationally recognised for
his work with hadrosaurid dinosaurs, described P.
blackfeetensis,
a second species described from fossil material collected from a bone
bed of several individuals. At the time of writing, the type
species is P. maximus is only found in Alberta,
while P.
blackfeetensis is known only from Montana.
There
are two key areas of difference between the two current species of
Prosaurolophus. The first is the morphological
difference between the
two, with P. blackfeetensis having a steeper
and taller face than
P. maximus when viewed from the side. The second
is actually the
analysis of the fossils sites, with P. maximus
inhabiting lowland
areas near the coastline of the Western Interior Seaway, and P.
blackfeetensis being in found in an area further inland and
at what
would have been a higher elevation at the time than the Canadian
deposits. This latter revelation about what would have been differing
ecosystems might also explain the differences in skull morphology
between the two.
Since
P. blackfeetensis essentially has a taller skull,
it would have a
greater amount of space for housing larger and hence stronger jaw
closing muscles, as well as having a proportionately closer placement
of food in the mouth to the point of jaw articulation. The latter
would allow for slightly greater focusing of jaw strength upon
processing plant material. Given that plants that are further inland
and at higher altitudes are usually growing in dryer conditions, they
are usually tougher so that they can preserve moisture. P.
maximus
by contrast with its location nearer a huge body of water was more
likely living and foraging in wetland ecosystems. With the vegetation
of such ecosystems being much softer than most inland varieties, a
stronger bite is just not necessary. The exact diet of what
hadrosaurid dinosaurs ate is a long running debate that is fuelled by
fossil evidence that supports the eating both soft wetland plants and
tougher inland plants. There is no good reason to assume however that
hadrosaurids, perhaps like Prosaurolophus could
over time adapt to
different conditions.
Since
the naming of P. blackfeetensis was born out of
the discovery of
several individuals collected in a bone bed, this raises the notion
that Prosaurolophus lived in groups. Although
this has been explained
as the possible clustering of several animals around a watering hole in
a drought (which dried up before the rains began and refilled it)
it does not entirely discount the notion that Prosaurolophus
lived in
groups the rest of the time. Not only could these dinosaurs have been
living in the same group when the draught began, it would make
ecological sense for herbivorous animals which are usually prey to
other animals to live in a group. This safety in numbers principal
means that several individuals are much more likely to detect a
predator than a single individual too engrossed in feeding or drinking
to notice. This of course assumes that dinosaurs lived in a similar
manner to many types of unrelated animals we know today, but there is
no solid evidence that they did not either.
Hadrosaurid
dinosaurs were physically well adapted to be generalists. Like its
relatives Prosaurolophus would have been quite
comfortable in either
bipedal or quadrupedal postures, enabling energy efficient locomotion
or versatility reaching different types of plants. The keratinous
beak that formed the front of the mouth could effectively snip off
plants while the batteries of grinding teeth in the mouth processed
plants to an easily digestible pulp. Analysis of scleral rings, the
bony supports of the eye, also indicates that Prosaurolophus
was
cathemeral. This means that Prosaurolophus was
active for short
periods throughout the day and did not need to wait for a specific time
of the day or night to be active.
Living
in North America during the late Cretaceous period, the two species
of Prosaurolophus would have still encountered the
same rough sets of
other dinosaurs. Most common of these would have been other
hadrosaurids like Maiasaura,
but also including lambeosaurines like
Parasaurolophus,
Corythosaurus
and Hypacrosaurus,
horned
ceratopsian
dinosaurs such as Centrosaurus
and Styracosaurus,
to
armoured dinosaurs like the ankylosaur Euoplocephalus
and the nodosaur
Edmontonia.
Predatory threats however may have come from large
theropods like the tyrannosaurs
Daspletosaurus
and Gorgosaurus,
but
even smaller theropods like Troodon
could have posed a threat,
particularly to smaller juveniles.
Further reading
- Cranial morphology of Prosaurolophus
(Ornithischia:
Hadrosauridae) with descriptions of two new hadrosaurid species and
an evaluation of hadrosaurid phylogenetic relationships, Jack R.
Horner - 1992.
- Dyoplosaurus acutosquameus, a new genus and
species of armoured
dinosaur; and notes on a skeleton of Prosaurolophus maximus,
William A. Parks - 1924.
- Taphonomy of three dinosaur bone beds in the Upper Cretaceous Two
Medicine Formation of northwestern Montana: Evidence for
drought-related mortality, Raymond R. Rogers - 1990.
- Cranial anatomy and variation in Prosaurolophus maximus (Dinosauria:
Hadrosauridae). - Zoological Journal of the Linnean Society. 167 (4):
531–568. - C. T. McGarrity, N. E. Campione & D. C. Evans - 2013.
- Description of juvenile specimens of Prosaurolophus maximus
(Hadrosauridae: Saurolophinae) from the Upper Cretaceous Bearpaw
Formation of southern Alberta, Canada, reveals ontogenetic changes in
crest morphology. - Journal of Vertebrate Paleontology: e1547310. -
Eamon T. Drysdale, Fran�ois Therrien, Darla K. Zelenitsky, David B.
Weishampel & David C. Evans - 2019.
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