Why E. Annectens Should Be Renamed

This is all just opinion from a hobbyist, and not a professional paleontologist, but I studied this intensely, and I think Edmontosaurus annectens should be called Anatosaurus, though Edmonotosaurus regalis can keep its name...

...and here is why.

First of all, I'm under the opinion that they shouldn't share a genus name because Edmontosaurus regalis and Edmontosaurus annectens were separated by nearly six million years of time and evolution.  Edmontosaurus regalis was excavated around Campanian layers, deeper, older rock, and Edmontosaurus annectens has been found around Maastrichian layers, about the same time the dinosaurs went extinct.  By then, the likelihood that they would still be biologically compatible is quite slim.  Typically, with living animals, while it's true that a "genus" and a "species" are at least in part convenient constructs, true, but they are also there for another reason: Typically, if an animal can breed with another animal, and they can produce offspring that will most likely grow up to be fertile, then they are of the same species.  If two animals can breed but the offspring isn't likely to be fertile, then they probably share a genus but aren't of the same species, like when [i]Equus caballus[/i] and [i]Equus asinus[/i] mate and produce a mule; same genus, different species.  If two animals probably can't breed in the first place, then they probably don't even share a genus. 

The significance of this is that six million years of evolution is usually sufficient time for most terrestrial warm-blooded animals to evolve beyond biological compatibility with their ancestors.  For example, if modern humans went back in time and tried to breed with Australopithecus-like apes, they likely wouldn't make any offspring in the first place.  According to Robert T. Bakker, one of the more reliable paleontologists (except for his extinction theory, a disease killing all the dinosaurs is unlikely because dinosaurs didn't just die out, so did sea life), rapid evolutionary turnover is a common trait of terrestrial warm-blooded animals, probably in part because with the major exceptions of humans and elephants, most terrestrial warm-blooded animals that we know of today hit sexual maturity faster in proportion to the adult body size than cold-blooded ones, which we know a lot of hadrosaurs did (if my memory serves me correctly, some hadrosaurs took about eight years to reach sexual maturity at the most, and we know this through growth rings on their bones, which were weak and widely spaced like warm-blooded animals today).  Another reason why rapid evolutionary turnover is common among warm-blooded animals is because when a metabolism is high, you need to eat a lot, and this can drive the animal to be an aggressive competitor with similar animals for its niche, like how African Lions and Spotted Hyenas end up competing with each other for food.  When faced with such dilemmas, you either adapt or you die out.

With these details in mind, if there is a mutation that can benefit the population, it will most likely spread fairly quickly, only over a course of thousands of years.  Given enough beneficial mutations, and enough time, a population will be sufficiently different from it's ancestors.  [i]Edmontosaurus annectens[/i] had more than enough time for that, it had nearly [i]six million years[/i]! Six million years is twice as much time as we took to evolve beyond most chances of being able to make offspring with our most probable ancestors, and the [i]Homo[/i] genus is one that's actually very good at [i]defying[/i] or [i]delaying[/i] evolution due to the fact that we take forever to reach maturity and are too intelligent to easily fall victim to the elements, we form societies basically made to protect us from some of the things that the Earth can dish out, like storms.  Edmonotosaurus didn't have these kinds of advantages, so it probably evolved quite a bit over six million years.

Moving past age and evolution, I also looked at the skulls, and observing the skeletal morphology has only gave me further cause to adhere to my opinion. 

These are, apparently, adult E. regalis skulls.

[img]https://upload.wikimedia.org/wikipedia/commons/e/ef/Edmontosaurus_regalis_skull_and_jaws%2C_Near_Drumheller%2C_Alberta%2C_Canada%2C_Late_Cretaceous_-_Royal_Ontario_Museum_-_DSC00020.JPG[/img]

[img]http://i.dailymail.co.uk/i/pix/2013/12/13/article-2523106-1A14687300000578-125_634x346.jpg[/img]

This is a reconstruction of an adult E. regalis skull based on the fossils we have here.

[img]http://www.paleofile.com/imges/Dinosaurs/Ornithopoda/Edmontosaurus%20regalis%20USNM%2012711%20s.JPG[/img]

These are fossils of adult E. annectens skulls:

[img]https://upload.wikimedia.org/wikipedia/commons/c/c8/Edmontosaurus_annectens_skull.jpg[/img]

[img]https://upload.wikimedia.org/wikipedia/commons/b/b8/Edmontosaurus_annectens_-_National_Museum_of_Natural_History_-_IMG_1968.JPG[/img]

This is a reconstruction of an E. annectens skull based on the fossils we have here.

[img]http://www.paleofile.com/imges/Dinosaurs/Ornithopoda/Anatosaurus%20UMP%20128374%20s.JPG[/img]

These differences seem to be far too numerous and obvious for them to be merely different species under the same genus.  See, this is how similar two skulls of two species under the same genus tend to be:

[img]http://www.equine-dentist-scotland.co.uk/wp-content/uploads/2013/02/horse-skull.jpg[/img]

[img]https://upload.wikimedia.org/wikipedia/commons/0/01/Cr%C3%A2ne_%C3%A2ne_donkey_skull_mus%C3%A9e_v%C3%A9t%C3%A9rinaire_Maisons_alfort_1a.jpg[/img]

One's a horse skull, and the other's a donkey skull.  The mandibles certainly are different, but obvious differences end there. They are far more similar to each other than the two "species" of Edmontosaurus.  This is normal for species with a shared genus; they tend to be alike because they have much of the same genetics. 

Meanwhile, the nasal cavity of E. regalis doesn't seem to grow as it matures, in fact, it tends to give the appearance of shrinking because the rest of the skull grows as it matures.  E. annectens, on the other hand, has a nasal cavity that continues to grow as its maxilla (upper jaw) stretches out with maturity, making it look more ducklike as the bill continues to stretch.  Such obvious differences seem to suggest that they are more distantly related than that.

It's also unlikely one is an adult version of the other, because we have documented the adolescent and young adult specimens of both, as seen here:

[img]https://evanslab.files.wordpress.com/2011/09/edmonto-skull-summary.jpg[/img]

Furthermore, It's also very unlikely that one is the female version of the other, again, due to the six million year separation.  And once more, it seems unlikely that they are different species sharing a genus, because, again, they're far too different and have been separated by over six million years of evolution.  E. regalis was a lot older than E. annectens (not to mention that a very likely sexual dimorphism – a small headcreast - has been found on E. regalis, but it sure isn't E. annectens!).

Since they likely shouldn't share a genus, which one should be renamed, and what should it be called?

Edmontosaurus regalis was the first species named "Edmontosaurus" found, so it can keep its name, only Edmontosaurus annectens needs to  be renamed.  What to call it though? Surely not Trachodon or Diclonius, those specimens were known only by teeth, and thus can't be confirmed to be connected to any dinosaur, so each of those names is a [i]nomen dumium[/i].  Surely not Claosaurus, that genus was already claimed by another hadrosaur rather distantly related to Edmontosaurus.  The oldest viable name is Anatosaurus.  That name was derived from more complete specimens.  It's an apt name too, because it means "duck lizard", which is an accurate description of the pronounced bill of the dinosaur, and as a lot of hadrosaurs are commonly called "duckbills", and as "Edmontosaurus" has been commonly used as the main example of hadrosaurs, it's probably for the best that it's named "Anatosaurus".