New insights into the ornithopod manus

Duckbills,
like Edmontosaurus, and their kin are the ornithopod ornithischian dinosaurs, a clade I have been ignoring until now. Wikipedia reports, “[they] started out as small, bipedal running grazers, and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous world, and dominated the North American landscape.” 

Dryosaurus, Camptosaurus, Iguanodon and Edmontosaurus are genera within this clade and each has an interesting manus (Fig. 1). When one works in phylogenetic analysis it is imperative to compare homologous digits (apples to apples). In ornithopods, those homologies appear to be masked and perhaps misinterpreted by the appearances of new phalanges and the disappearances of old phalanges. Putting them all in one image (Fig.1) clarifies all issues (even without traveling to visit the fossils firsthand!). Hopefully the data are accurate to start with.

This all started with a phylogenetic analysis
that appeared to indicate that Edmontosaurus had a manual digit 1 with an extra digit that made it look like manual digit 2. Comparisons to other ornithopods ensued. A quick look through the Internet brought B. Switek’s article (see below) to the fore.

Science writer Brian Switek 
writing for Smithsonian.com reports,

1. “…the great herbivore Iguanodon had prominent thumb spikes.“
2. “The peculiar false thumb of Iguanodon was originally thought to set into the dinosaur’s nose.”
3. “But why should Iguanodon have a hand spike? “
4. “Though my own suggestion is not any better than those I have been disappointed by, I wonder if the Iguanodon spike is a Mesozoic equivalent of another false thumb seen among animals today—the enlarged wrist bones of red and giant pandas…  the Iguanodon spike was rigid.” Unfortunately that’s as far as journalist Switek has allowed himself to go, rather than proposing the homologies and comparisons demonstrated here.

Giving credit where credit is due,
Switek may be the first to suggest the spike was not a digit. I don’t know and was not able to find out the history of the spike. Given the text from his blogpost, you can see Switek’s choice of words actually evolves from “thumb spikes” to “false thumb” to “hand spike” to “enlarged wrist bone”. Like Brian, I also lack a PhD, but that doesn’t stop us from making contributions. If I’m duplicating earlier academic efforts, please let me know so credit can be given.

Here we’ll show
that the spike is indeed a wrist element… that digit 1 in Iguanodon and related taxa have one more phalanx, making it look like digit 2.

We’ll start with
the right manus of Dryosaurus, a basal ornithopod (at least in the large reptile tree it is, where only one other ornithopod, Edmontosaurus, is currently represented). During the course of this, I want you to focus on the the homologies of metatarsals 2 and 3 (colored in turquoise). These, I think, will guide us to correct interpretations of the other elements of the various ornithopod manus.

Now back to the manus of Dryosaurus:

1. Data comes form loose bones in a photo formed in the shape of a hand, not an in-situ articulated hand. Thus I do not know the identification or placement of the carpals
2. Five metatarsals are present.
3. Mt3 is the longest. Slightly shorter is mt2.
4. Phalangeal formula is 2-3-4-3-2, but digit 1 does not appear to be tipped with a sharp ungual. Is it missing? If so, that adds a phalanx to the formula 3-3-4-3-2.
5. Digit 3 is the longest. Slightly shorter is digit 2.
6. Unguals are lost in digits 4 and 5.

The manus of Camptosaurus

1. Is reduced (stumpy) by comparison to Dryosaurus
2. Mt 1 is a disk. M1.1 is a disk
3. M3.2 appears to fuse with m3.3
4. m4.3 and m5.2 are lost
5. The new phalangeal formula is 2-3-3-2-1

The manus of Iguanodon

1. is more robust and highly modified by comparison to Dryosaurus
2. Two wrist elements fill the wrist. Two others extend medially.
3. Digit 1 is longer and now sports an ungual
4. Ungual 1 is not sharp
5. Ungual 2 is a round hoof
6. Ungual 3 (m3.4) is lost along with m3.3
7. Mt4 is shorter. Two tiny phalanges are added.
8. Digit 5 is absent.
9. The new phalangeal formula is 3-3-2-4-0

The manus of Edmontosaurus 

1. is long and gracile by comparison to Dryosaurus.
2. Again, digit 1 has 3 phalanges, matching digits 2–4.
3. Digit 4 is a vestige
4. Mt 5 is again absent
5. As in Iguanodon, ungual 1 is not sharp and ungual 2 is a hoof
6. The new phalangeal formula is 3-3-3-3-0.

Always interesting to 
uncover little paradigm busters like these. Now back to phylogenetic analysis…