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Are the armored ornithischian dinosaurs diphyletic?
The Thyreophora are the armored ornithischians, a putative clade that includes Scutellosaurus (Fig. 1), Scelidosaurus (Fig. 2), ankylosaurs, nodosaurs and stegosaurs. This clade makes sense and is universally accepted because all the members are armored with ossified scutes and many members swung their clubbed or spiked tails to defend themselves. The Eurypoda is a derived clade within the Thyreophora that includes the ankylosaurs and stegosaurs. It does not include basal taxa like Scutellosaurus and Scelidosaurus.
According to Wikipedia
“Thyreophora was first named by Nopcsa in 1915. Thyreophora was defined as a clade by Paul Sereno in 1998, as “all genasaurs more closely related to Ankylosaurus than to Triceratops“. Thyreophoroidea was first named by Nopcsa in 1928 and defined by Sereno in 1986, as “Scelidosaurus, Ankylosaurus, their most recent common ancestor and all of its descendants”. Eurypoda was first named by Sereno in 1986 and defined by him in 1998, as “Stegosaurus, Ankylosaurus, their most recent common ancestor and all of their descendants”.
Figure 1. Scutellosaurus is a small armored ornithischian transitional between Lesothosaurus and Stegosaurus. This is a new reconstruction for the skull, which is largely unknown. This one is not as tall, in accord with the cervical vertebrae. This long whip-like tail may have been the precursor to a shorter spiked tail in the related Stegosaurus.
At odds with this
the large reptile tree splits two putative thyreophorans, Scutellosaurus and Scelidosaurus (Fig. 3). Readers have understandably wondered about and questioned this heretical split.
Plainly
the origin of the armored Ornithischia could use a few more fossils. Traditional paleontologists don’t yet recognize basal ornithischians like Chilesaurus and Daemonosaurus, which are relatively large taxa. Jeholosaurus and Haya are small and late survivors of a basal radiation. Scelidosaurus and ankylosaurs are closer to this base. More derived taxa include some smaller forms like Lesothosaurus and Scutellosaurus separated from Scleidosaurus by several taxonomic nodes that include the basalmost heterodontosaurs, ceratopsians and camptosaurs/iguanodonts/hadrosaurs.
Traditional paleontologists
like Holz 2011, nest Eocursor and Fabrosaurus (Lesothosaurus) at the base of the Ornithischia. By contrast, Butler et al. 2011 nested Pisanosaurus and Heterodocntosauridae at the base of the Ornithischia. Not sure what the outgroup is to either of these studies as neither employed Daemonosaurus or Chilesaurus. More here at Wikipedia.
Figure 2. Scleidosaurus is an armored basal ornithischian nesting between Pisanosaurus and Emausaurus + ankylosaurs.
Adding
the wide-bodied, short-skulled ankylosaur, Minmi, to the large reptile tree nests it with the large bulky armored Emausaurus and Scelidosaurus.
Adding
tall-bodied, long-skulled Stegosaurus nests it with the much smaller, short-legged, armored Scutellosaurus. So the armored ornithischian split remains here (Fig. 3). The Thyreophora appears to be diphyletic based on the few basal taxa included here.
Figure 3. Phytodinosauria. Purple nodes are traditional thyreophorans. Moving the two taxa back together increases the MPT score by at least 9 steps, a relatively low number which is a tribute to their many convergent traits.
There are problems
with scoring some members of the armored ornithischians. For instance, Scutellosaurus is known from disarticulated and broken bones and only a few skull bones. Perhaps some day more complete skulls will be found. On the other hand, ankylosaurs fuse skull bones and cover them with additional ossifications of armor. So sutures are difficult to determine and the temporal skull fenestra disappear.
Scelidosaurus has tall neural spines, but ankylosaurs do not. Scutellosaurus does not have tall neural spines, but Stegosaurus does. So there is a curious mix and match of individual traits.
Stegosaurs are tall and narrow in the torso, a trait further emphasized with those tall plates. Anklyosaurs are the opposite, with an extremely wide and low torso and generally low plates, but Crichtonsaurus (Fig. 4) has tall plates. It has not been added to the large reptile tree yet.
Figure 4. Crichtonsaurus has long legs, tall plates and long legs like a Stegosaurus, but a skull and other armor like an ankylosaur. It has not been added to the large reptile tree yet.
The addition of new basal taxa
might bring the armored ornithischians back together again, but at present they are separated on the larger reptile tree. Additional material for Scutellosaurus would be much appreciated and interesting to compare to the best guess in figure 1.
References
Butler RJ, Jin L-Y, Jun C, Godefroit P 2011. The postcranial osteology and phylogenetic position of the small ornithischian dinosaur Changchunsaurus parvus from the Quantou Formation (Cretaceous: Aptian–Cenomanian) of Jilin Province, north-eastern China. Palaeontology 54 (3): 667–683.
Holtz TR Jr 2012. Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages, Winter 2011
Source: https://pterosaurheresies.wordpress.com/2015/10/07/are-the-armored-ornithischian-dinosaurs-diphyletic/
