Hypacrosaurus: Beast of the Week

This week we're checking out an important and well-studied plant-eater.  Make way for Hypacrosaurus!

Hypacrosaurus was a hadrosaurid ("duck-billed" dinosaur) that lived in what is now North America during the late Cretaceous period, between 75 and 67 million years ago.  When alive, like all members of its family, it would have eaten plants.  The largest adults measured about 30 feet (9.1 meters) from beak to tail.  The genus name translates to "Near the Highest Lizard" which is a seemingly odd genus name.  It's because the first Hypacrosaurus fossils uncovered were soon after the first Tyrannosaurus (the "highest" of dinosaurs at the time) fossils, and the two were similar in size, T.rex being a bit larger.  (This comparison made more sense at the time when we knew about WAY fewer dinosaurs.)

Hypcrosaurus altispinax life reconstruction in watercolors by Christopher DiPiazza.

Hypacrosaurus was a member of the lambeosaurine group within the hadrosaur family.  Lambeosaurines are known for having hollow bony crests growing from the tops of their skulls.  Parasaurolophus, and Hypacrosaurus' especially close relative, Corythosaurus are more famous members of this group.  Hypacrosaurus' crest was similar to Corythosaurus' in that it shaped almost like a sort of round helmet that starts about midway up the snout and ends at the base of the skull, behind the head.  Hypacrosaurus' crest was a bit shorter and wider than Corythosaurus', however.  Like its relatives, the crest was hollow and could have aided it in making specific kinds of sounds meant to communicate with members of its own species.  The crest also could have very well aided in visual display, since we know these dinosaurs grew the crests as they matured.  Different-sized and shaped crests would indicate the maturity of an individual when viewed by its peers.

Hypacrosaurus altispinax skull on display at the American Museum of Natural History in New York.

Hypacrosaurus, the genus, actually includes two distinct species that lived during different times.  Hypacrosaurus altispinax was discovered first and was found in Alberta, Canada.  Hypacrosaurus stebingeri was slightly smaller and had a lower crest than H.altispinax and was discovered in Montana, USA.

Hypacrosaurus is one of the few kinds of fossil dinosaurs scientists were able to find lots of eggs and babies from.  So much so, that they were able to study how these dinosaurs grew and aged throughout their lives.  Hypacrosaurus babies had only tiny, barely noticeable crests, and grew extremely fast for animals their size.  In fact, according to studies that examined multiple specimens of various sizes and looking at markings on the inside of their limb bones, paleontologists suggest Hypacrosaurus was sexually mature between only two to three years of age.  They didn't reach full adult size, however, until they were about ten to twelve.  The reasoning why an animal would have grown so quickly could have something to do with a need to avoid predation.  As the dinosaur grew, the variety of predators able to hunt it diminished.  The ability to reproduce early in life also probably played a part in its relationship with predators, since most dinosaurs likely never made it to adulthood.  

Skull of a baby Hypacrosaurus on display at the Museum of the Rockies in Montana.

Lastly, scientists were able to find preserved cells in beautifully preserved cartilage of a baby Hypacrosaurus specimen.  Not only that, but they were able to observe that these cells were in the process of dividing when the dinosaur died, and ultimately were able to extract traces of genetic material from it!  Not only was this cool for finding dinosaur genetic material, but it also proved that genetic material can survive MUCH longer than previously thought if the conditions are specific enough.

References

Bailleul, A. M.; Zheng, W.; Horner, J. R.; Hall, B. K.; Holliday, C. M.; Schweitzer, M. H. (2020). "Evidence of proteins, chromosomes and chemical markers of DNA in exceptionally preserved dinosaur cartilage". National Science Review. 7 (4): 815−822.

Cooper, Lisa N.; Lee, Andrew H.; Taper, Mark L.; Horner, John R. (2008). "Relative growth rates of predator and prey dinosaurs reflect effects of predation". Proceedings of the Royal Society B. 275 (1651): 2609–2615.

Erickson, G.M.; Zelenitsky, D.K.; Kay, D.I.; Norrell, M.A. (2017). "Dinosaur incubation periods directly determined from growth-line counts in embryonic teeth show reptilian-grade development" (PDF). Proceedings of the National Academy of Sciences. 114 (3): 540–545.

Horner, John R.; Currie, Phillip J. (1994). "Embryonic and neonatal morphology and ontogeny of a new species of Hypacrosaurus (Ornithischia, Lambeosauridae) from Montana and Alberta". In Carpenter, Kenneth; Hirsch, Karl F.; Horner John R. (eds.). Dinosaur Eggs and Babies. Cambridge: Cambridge University Press. pp. 312–336.