Hypacrosaurus

Name: Hypacrosaurus ‭(‬near the highest lizard‭)‬.
Phonetic: Hie-pak-roe-sore-us.
Named By: Barnum Brown‭ ‬-‭ ‬1923.
Synonyms: Cheneosaurus.
Classification: Chordata,‭ ‬Reptilia,‭ ‬Dinosauria,‭ ‬Ornithischia,‭ ‬Ornithopoda,‭ ‬Hadrosauridae,‭ ‬Lambeosaurinae.
Species: H.‭ ‬altispinus‭ (‬type‭)‬,‭ ‬H.‭ ‬stebingeri.
Diet: Herbivore.
Size:Up to ‬9.1‭ ‬meters long.
Known locations: Canada‭ ‬-‭ ‬Alberta‭ ‬-‭ ‬Horseshoe Canyon Formation.‭ ‬USA‭ ‬-‭ ‬Montana‭ ‬-‭ ‬Two Medicine Formation.
Time period: Campanian to early Maastrichtian of the Cretaceous.
Fossil representation: Multiple skulls and partial post cranial‭ ‬remains.‭ ‬Eggs and Juvenile specimens also known.

       Hypacrosaurus means‭ ‘‬near the highest lizard‭’‬,‭ ‬and in this context the‭ ‘‬lizard‭’ ‬was actually the dinosaur Tyrannosaurus,‭ ‬one of the largest dinosaurs in the ecosystems of late Cretaceous North America,‭ ‬but only about a third larger than Hypacrosaurus in the largest individuals‭ (‬twelve meters for large individual Tyrannosaurus compared to nine meters for Hypacrosaurus‭)‬.‭ ‬Interestingly Hypacrosaurus and other dinosaurs like it may have actually been prey to Tyrannosaurus and other related genera such as Albertosaurus.‭ ‬Evidence for this comes from a huge bite wound inflicted to the back of an Edmontosaurus that closely matches the shape of a Tyrannosaurus mouth.‭ ‬Because the bones in the wound actually healed afterwards,‭ ‬this proves that the Edmontosaurus in question was alive when it happened,‭ ‬and not a case of a tyrannosaur simply scavenging an existing carcass.
       The type species name of H.‭ ‬altispinus is a reference to the tall neural spines of the vertebrae.‭ ‬These tall spines would have significantly increased the height of the body and may have been to facilitate a display function,‭ ‬fat storage for leaner times,‭ ‬to perhaps being to help with activities for swimming‭ (‬as has been proposed for Magnapaulia‭)‬.‭ ‬H.‭ ‬stebingeri is significantly important since this is so far made up from the remains of juveniles and eggs that when combined with data from other hadrosaurid genera such as Maiasaura,‭ ‬allows for a serious insight into how these kinds of dinosaurs reproduced and grew up.
       Juvenile fossils display growth rings when examined in cross section‭ (‬similar to what you can see in a tree stump‭) ‬that indicate Hypacrosaurus achieved reproductive maturity in as little as two to three years while reaching full size in ten.‭ ‬This is significantly faster than in their potential predators the tyrannosaurs.‭ ‬Studies of these dinosaurs show that for the first ten years of a tyrannosaurs life growth would be pretty slow with the individual remaining small.‭ ‬Then in years ten to twenty,‭ ‬growth would rapidly accelerate until the individual reached almost full size and reproductive maturity,‭ ‬before a further ten years of slow and steady growth.‭ ‬To put this in perspective,‭ ‬in the twenty years it took for one generation of Tyrannosaurus to potentially pass,‭ ‬at least six generations of Hypacrosaurus could have happened.
       When you compare this to the twenty of so eggs in each Hypacrosaurus nest,‭ ‬you get the conclusion that Hypacrosaurus were breeding at a rate to compensate for‭ ‬high mortality levels.‭ ‬Assuming that environmental conditions were not so much of a factor,‭ ‬this could be because Hypacrosaurus was a viable and common prey species of the time,‭ ‬and tyrannosaurs withstanding,‭ ‬other predators of Hypacrosaurus may have included troodontids like Troodon.‭ ‬These small predators would have been a particular threat to the smaller individuals of Hypacrosaurus,‭ ‬reducing the numbers growing to adult hood.
       The crest of Hypacrosaurus is similar to that of its relative Corythosaurus,‭ ‬though wider and not as high.‭ ‬This crest was also hollow which confirms its establishment as a lambeosaurine hadrosaurid‭ (‬the group typified by Lambeosaurus‭)‬.‭ ‬Several theories have been made about the function of lambeosaurine head crests,‭ ‬though the one with the most support concerns visual display so that different species of hadrosaur can tell each other apart,‭ ‬probably in a similar fashion to how the differences in the forms of horns and neck frills allow different genera of ceratopsian dinosaurs to be identified.
       A theory more specific to lambeosaurine hadrosaurids however concerns the possibility that the hollowness of the crest may in fact be a resonating chamber for amplifying calls.‭ ‬Again the differences in crest form between different lambeosaurine genera could have produced different levels of amplification,‭ ‬and hence different sounding call specific to a distinct species.‭ ‬Although the resonating chamber theory is not supported by all palaeontologists,‭ ‬it is one that is still interesting in itself.
       It has already been said that Hypacrosaurus was similar in form to Corythosaurus,‭ ‬but other genera such as Olorotitan and Nipponosaurus are also thought to be closely related.

Further reading
-‭ ‬A new trachodont dinosaur,‭ ‬Hypacrosaurus,‭ ‬from the Edmonton Cretaceous of Alberta,‭ ‬Barnum Brown‭ ‬-‭ ‬1913.
-‭ ‬On Cheneosaurus tolmanensis,‭ ‬a new genus and species of trachodont dinosaur from the Edmonton Cretaceous of Alberta,‭ ‬Lawrence M.‭ ‬Lambe‭ ‬-‭ ‬1917.
-‭ ‬Taxonomic implications of relative growth in lambeosaurine dinosaurs,‭ ‬Peter Dodson‭ ‬-‭ ‬1975.
-‭ ‬Growth rate of Hypacrosaurus stebingeri as hypothesized from lines of arrested growth and whole femur circumference,‭ ‬L.‭ ‬N.‭ ‬Cooper‭ & ‬J.‭ ‬R‭ ‬.‭ ‬Horner‭ ‬-‭ ‬1999.
-‭ ‬First Evidence of Dinosaurian Secondary Cartilage in the Post-Hatching Skull of Hypacrosaurus stebingeri‭ (‬Dinosauria,‭ ‬Ornithischia‭)‬,‭ ‬A.‭ ‬M.‭ ‬Bailleul,‭ ‬B.‭ ‬K.‭ ‬Hall‭ & ‬J.‭ ‬R.‭ ‬Horner‭ ‬-‭ ‬2012.



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