Maiasaura
Name: Maiasaura
(Caring/Good mother lizard).
Phonetic: My-ah-sore-ah.
Named By: Jack Horner & Robert Makela -
1979.
Classification: Chordata, Reptilia, Dinosauria,
Ornithischia, Ornithopoda, Hadrosauridae, Saurolophinae,
Brachylophosaurini.
Species: M. peeblesorum
(type).
Diet: Herbivore.
Size: Around 9 meters long.
Known locations: USA - Montana - Two
Medicine Formation.
Time period: Late Campanian of the Cretaceous.
Fossil representation: Remains of over 200
individuals from newly hatched juveniles to adults including nests.
At
up to nine meters long Maiasaura was not amongst
the largest known
hadrosaurid
dinosaurs, though it still tipped the scale to be
considered big for its kind. Like with its hadrosaurid relatives,
Maiasaura is thought to have been a primarily
bipedal dinosaur,
though one that was at least capable of balancing if not walking upon
just its hind legs. The exact posture assumed was probably more
dependent upon what the Maiasaura in question was
actually doing at the
time. Maiasaura had a low crest on top of it
snout and because it is
solid, this indicates that Maiasaura was one of
the saurolophine
hadrosaurs.
Maiasaura
might appear to be a rather plain looking hadrosaurid, but its real
importance is not about its looks but upon the remains of nests that
members of this genus created. One of the most significant fossil
sites associated with Maiasaura is an area that we
know today as ‘Egg
Mountain’. Here a concentration of nests spread apart at distances
of around seven meters from each other, each containing around thirty
to forty eggs about fifteen centimetres across were found with multiple
remains of Maiasaura scattered around them. Some
of these remains
were of juveniles too large to have just hatched with underdeveloped
rear legs. The skulls of juveniles also have very different
proportions to those of the adults, though this is quite common in
vertebrates. In addition to these were remains of adult Maiasaura,
something that suggests that adults would not just lay their eggs and
wander off, but stay close to the nests and their young for some
time. Bearing in mind the underdeveloped rear eggs of the younger
juveniles it’s possible that a more precise scenario is that the adult
Maiasaura would move away from the nests for short
periods while they
foraged to bring food back to the nest while the hatchlings became
strong enough to walk around and forage for themselves.
The
sheer number of nests and Maiasaura fossils from
the Egg Mountain site
is proof that Maiasaura would congregate in large
numbers during the
nesting period, however it remains impossible to say for
certain how this genus lived when not nesting. It’s not impossible
that when Maiasaura were not nesting they moved
around in much smaller
groups to reduce the impact on the ecosystem in a given area, more
mouths in one place would after all cause more destruction to the
plants growing there, increasing the amount of time necessary for the
ecosystem to recover, although hadrosaurids like Maiasaura
might have
been continually on the move and only staying put for short periods
such as nesting.
A
modern analogy to this would be some species of birds (particularly
sea birds where the effect can most appreciated) where during the
year birds will form small concentrations with loose bonds between
individuals, but will once a year congregate upon a single area where
conditions are ideal for raising young. Due to the rarity of such
places, a higher number of birds move into a specific area that can
give observers who are not familiar with the life practices of the
birds during the remainder of the year the false impression that the
birds always live in such high concentrations.
It’s
probable however that Maiasaura, and other
hadrosaurids, were not
solitary creatures given that they have no obvious physical defences
from attacks by predators (such as the thick armoured plates of
ankylosaurs). The best bet for Maiasaura to
stay alive in a world
of large predators would be to stay in groups from a few individuals to
perhaps as many as dozens or even hundreds of individuals. This way
you would have more than one set of eyes for seeing predators, more
than one nose for smelling predators, and more than one pair of ears
for hearing predators, altogether making it significantly harder for
a predator to get in close for a surprise ambush. Multiple
individuals moving and calling out all at once would also make it
significantly harder for predators to single out a specific individual
for attack.
With
the exception of Maiasaura, another hadrosaurid
called Hypacrosaurus
has also allowed palaeontologists to look at nesting patterns within
this group of dinosaurs. Additionally this hadrosaurid along with
another named Prosaurolophus
appear to have been present at roughly the
same time and location as Maiasaura. It’s
possible that Maiasaura may
have shared its habitat with its close relative Edmontosaurus,
however the earliest known specimens of Edmontosaurus
are thought to
be around seventy-three million years old, while known Maiasaura
remains are dated one to two million years earlier. An alternative
theory is that the larger Edmontosaurus may have
replaced Maiasaura in
the ecosystems, especially in the advent of larger late Cretaceous
predators such as Albertosaurus
and Tyrannosaurus
dominating the
landscape.
Other
herbivores active around the same time and location as hadrosaurs
include centrosaurine
dinosaurs such as Styracosaurus,
Achelousaurus,
Einiosaurus
and Leptoceratops
as well as armoured
dinosaurs such as Euoplocephalus
and Edmontonia.
Dinosaurs that may
have been actual threats to Maiasaura however
include the Troodon
and
Daspletosaurus
and possibly Gorgosaurus.
Maiasaura
stands out as a name because it does not end with a ‘rus’ like the
more familiar saurus. This is because Maiasaura
is intended as a
feminine name which is why it ends in saura rather than the masculine
saurus.
Further reading
- Nest of juveniles provides evidence of family structure among
dinosaurs, J. R. Horner & R. Makela - 1979.
- Maiasaura, a model organism for extinct
vertebrate population
biology: A large sample statistical assessment of growth dynamics and
survivorship. - Paleobiology. 41 (4): 503–527. - Holly N. Woodward,
Elizabeth A. Freedman Fowler, James O. Farlow & John R. Horner
- 2015.
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