Probrachylophosaurus: Beast of the Week

This week we will be checking out a newly described plant-eating dinosaur.  Let's welcome Probrachylophosaurus bergei!  Probrachylophosaurus was a hadrosaurid (duck-billed) dinosaur that lived in what is now Montana, USA, during the late Cretaceous, 79 million years ago.  From beak to tail, an adult measured about twenty nine feet long.  The genus name translates to "Before the Short-Crested Lizard".  Who is this later "short-crested" guy?  We'll get to that...

Probrachylophosaurus by Christopher DiPiazza.

Probrachylophosaurus, as stated above, was a hadrosaurid, so it was related to dinosaurs like Anatotitan and Hadrosaurus, to name only a few.  That being said, it would have comfortably walked on all fours, but could have stood or ran on its powerful hind legs when needed.  It's tail would have been thick and stiffened with bony rods inside the body in life, making a nice counterbalance to it's front half, and possibly even an effective club-like weapon against predators and rivals.  With its broad beak, it would have been able to scarf down as much vegetation as possible to be pulverized thoroughly with its hundreds of tightly-packed teeth in the back of its mouth.  (Hadrosaurs had the most teeth in their mouths of any known animal!)

Probrachylophosaurus skull material from Fowler's 2015 description.

Brachylophosaurus is an important fossil because it gives us a clearer picture of a very specific instance of dinosaur evolution taking place.  In this case, Probrachylophosaurus strongly indicates that it was the direct ancestor of a different, yet closely-related kind of hadrosaur called (You guessed it!) Brachylophosaurus.  These two dinosaurs lived in the same area of North America but Probrachylophsoaurus was excavated from rocks about one million years older than Brachylophosaurus.  They both had flat, bony crests on their heads, above their eyes but Probrachylophosaurus' was shorter, telling us that natural selection shifted these animals towards longer crests as time went on.  The purpose of these crests is not totally clear, but it's always safe to say they were for display within the species, specifically mate selection.

One of the things scientists did with Probrachylophosaurus was cut open its leg bone to reveal how old it was when it died.  (You can count the rings on the inside of a femur or rib to see how many years old a dinosaur was, just like on the inside of a tree trunk.)  Turns out the Probrachylophosaurus specimen was fourteen when it died, which is still considered a juvenile, but past sexual maturity and very close to being full size going off what is known about hadrosaurs in general.  (Thanks Maiasaura!)

Cross section of Probrachylophosaurus' bone, showing the growth rings, which ultimately revealed that it was fourteen years old when it died.

So how old was the biggest known Brachylophosaurus when it died?  This could disprove the original idea that nature was selecting for longer crest lengths within that million years, if Brachylophosaurus was an older adult when it died, suggesting that the later juvenile Probrachylophosaurus simply had more crest-growing to do.  This is a question I asked Dr. Liz Freedman Fowler, who was one of the paleontologists who studied and ultimately described Probrachylophosaurus.  Turns out the original theory holds true.  Dr.Fowler explained to me that the biggest specimen of Brachylophosaurus with a longer crest, was found to have been younger than Probrachylophosaurus, with a shorter crest, when it died.  Both dinosaurs may have had a little bit more growing to do, but the crests still would have been longer, regardless, as generations moved on between 79 and 78 million years ago.

Quick doodlegram I did comparing the two.

It's always exciting when a new find ads a puzzle piece to a mystery.  Technically most organisms can be considered in the process or evolving somewhat, but rarely do we get such a clear picture of it taking place at different stages like this.  This is why paleontology is so important to biology.  It gives us a clear demonstration of evolution at work, a process that normally takes millions of years!

That is all for this week!  As always feel free to comment below or on our facebook page!  Special thanks to Dr. Fowler for talking to me before and during the making of this post!

References

Fowler, Elizabeth A. Freedman, and John R. Horner. "A New Brachylophosaurin Hadrosaur (Dinosauria: Ornithischia) with an Intermediate Nasal Crest from the Campanian Judith River Formation of Northcentral Montana." PloS one 10.11 (2015): e0141304.

Weishampel, David B.; Horner, Jack R. (1990). "Hadrosauridae". In Weishampel, David B.; Osmólska, Halszka; and Dodson, Peter (eds.). The Dinosauria (1st ed.). Berkeley: University of California Press. pp. 534–561. ISBN 0-520-06727-4.