25 October 2016

The Most Wondrous Duckbill In All the West

In the in-development video game Saurian, which I wrote about recently far beyond recent memory, the world of the very-Latest Cretaceous environment preserved within the sedimentary rocks of the Hell Creek Formation is brought to life. One of the most abundant large animals in this ecosystem is a large, shovel-mouthed herbivore belonging to the group of dinosaurs known as hadrosaurs, or more commonly in the popular parlance, duckbills.

In Saurian, the Hell Creek duckbill is called Anatosaurus annectens. This not the name most people in the palaeo community use for this animal.  In just about all sources for the last few decades, the Hell Creek duckbill is called Edmontosaurus annectens. So what's up with the name Anatosaurus?

The duckiest duckbill, usually called Edmontosaurus annectens. Its scale
patterns are based on 'mummies' that preserve fossilised skin impressions.
[art by the author]

The developers of Saurian confronted the issue of the unfortunate arbitrariness of genera in taxonomy, and to keep up some internal consistency, decided that any species in the game that was separated in time from its nearest relative by more than one million years would be treated as its own genus. The Hell Creek duckbill is separated by more than one million years from its closest relative Edmontosaurus regalis, the type (originally-established) species for Edmontosaurus and thus the one to which the name is attached, and as such the Hell Creek duckbill is named Anatosaurus annectens rather than Edmontosaurus annectens in the game. While this could potentially present some interesting complications, as Matt Martyniuk details here on DinoGoss, I'm satisfied that at least for the purposes of this game this style of classification will serve. What I'm here to talk about is the alternate name for Edmontosaurus annectens.

As you may remember from my revisitation of the Brontosaurus resurrection, there are some rules for naming species when scientists decide to split them up into separate genera. The name Brontosaurus came back not because it's objectively one of the best names for any organism (though it is), but because it had priority as the name for a group of species found to be united as a natural group containing what was then called Apatosaurus excelsus. Apatosaurus excelsus was originally described as Brontosaurus excelsus, so as the next-oldest official name available for the animal, the rules of zoological nomenclature dictate that Brontosaurus is the next name in line.

This is the case for Edmontosaurus and Anatosaurus. Edmontosaurus regalis was originally named in 1917. The case for Edmontosaurus annectens is distressingly complicated, but without going into too much detail, suffice to say that the name Anatosaurus dates back to 1942, so it is the oldest available alternative valid name for this animal.


...Well, there is Thespesius occidentalis.

Thespesius was named in 1856 by early American palaeontologist Joseph Leidy; the name means 'western wondrous one,' hence the title of this blog post. Thespesius was based on remains from the Lance Formation (which is contemporary with the Hell Creek Formation and geographically nearby, so most animals are the same between the two); these remains were from a large duckbill. This is where arguments that Anatosaurus should be used because Thespesius isn't a valid name (essentially meaning that the remains it is attached to are demonstrably from a unique animal) get tricky. Thespesius isn't based on great material, but it is based on what are clearly hadrosaur fossils. A study from 1915 by Charles W Gilmore, in which he reviewed some of the scrappy dinosaur material from the American West, confirmed that Thespesius was indeed a large duckbill, very similar to other species known from more complete skeletons.

The original fossils of Thespesius occidentalis. By Joseph Leidy, in
Extinct Vertebrata From the Judith River and Great Lignite Formations of
Nebraska. Note the title refers to Nebraska Territory, not the modern state.
[image in the public domain]

In the past, the number of hadrosaurs in the Lance and Hell Creek Formations (collectively these represent a temporally-distinct unit of animals termed the 'Lancian', which I'll use from here on) generally hovered around three - Edmontosaurus/Anatosaurus annectens, the smallish E. saskatchewanensis, and the enormous Anatotitan copei. This would be a problem for the validity of Thespesius because it probably would be impossible to show beyond a shadow of a doubt which one of these duckbills the Thespesius bones came from.

In modern times, though, research has shown that the diversity of Lancian hadrosaurs is not so high - in fact, all evidence currently points to it being a single species, with the various duckbills representing not separate species, but rather a growth series of a single species, with the smaller Edmontosaurus saskatchewanensis representing young animals and the massive, super-duckbilled Anatotitan being physically-mature old adults. Since there's only one Lancian duckbill, the oldest name for it is... Thespesius occidentalis, beating Anatosaurus in terms of priority by 86 years, and since Thespesius remained in active usage by at least certain palaeontologists well into the 1920s-30s, no argument can be made for declaring it a nomen oblitum (forgotten name) and effectively having it stricken from the taxonomic record.

Therefore, I think the case is pretty solid that if you don't want to call it Edmontosaurus annectens (which I am perfectly happy doing), the Lancian duckbill's name should be Thespesius occidentalis, not Anatosaurus annectens, since Thespesius is the older name and based on material referable to the single hadrosaur in that time and place.

Up next: those museum pictures I mentioned back in July, some stuff drawn during 'Inktober,' and gearing up for that most wonderful time of the year... Dinovember!

-- M

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