Establishing Aetosaurs, Ornithischians, and Tritylodontidae as Sister Species

Today we will use the wikipedia entry on Tritylodontidae, a sister clade to mammals,  to prove these Therapsids were also a sister clade to ornithischian dinosaurs.

Tritylodontidae ("three knob teeth", named after the shape of animal's teeth) is an extinct family of small to medium-sized, highly specialized and extremely mammal-like cynodonts, bearing several mammalian hallmarks like erect limbs, and endothermy. 

Ornithischians

Is endothermy thus a theropod trait? The fact that many small ornithopod ornithischians, such as the hypsilophodonts, have fibrolamellar bone (an endothermic characteristic) has been used to argue that they too were endothermic. 

Tianyulong from China appears to preserve filamentous integument which has been interpreted to be a variant of the proto-feathers found in some theropods. These filaments include a crest along its tail. The presence of this filamentous integument has been used to suggest that both ornithischians and saurischians were endothermic  

https://en.wikipedia.org/wiki/Heterodontosauridae

The ornithischian pelvis is "opisthopubic", meaning that the pubis points down and back (posterior) parallel with the ischium (Figure 1a).^[2] Additionally, the pelvis has a forward-pointing process to support the abdomen.^[2] This results in a four-pronged pelvic structure. In contrast to this, the saurischian pelvis is "propubic", meaning the pubis points toward the head (anterior), as in ancestral reptiles (Figure 1b).^[2] Ornithischians, wikipedia

The archosaurs are characterized by numerous synapomorphies that lend strong support to the hypothesis that they form a monophyletic group (clade) exclusive of other Reptilia. First of all, the "stem archosaurs" (properly termed Archosauromorpha), including Champsosauridae and Euparkeria, have a calcaneal tuber. This is a bony process projecting posteriorly from the ankle joint that serves as an attachment point for some of the lower leg flexor muscles. If you feel your heel, the bone that forms it is your calcaneal tuber. This is an example of convergent evolution; synapsids and archosaurs evolved these features independently..........This restricts the posture to a more erect orientation, so the gait can be called parasagittal

http://www.ucmp.berkeley.edu/diapsids/archomm.html

Tritylodontidae 

They were the last known family of the non-mammalian synapsids. Tritylodontidae probably descended from a Cynognathus-like cynodont. Most tritylodontids are thought to have been herbivorous, feeding on vegetation, such as stems, leaves, and roots. A recent study indicate some may have had more omnivorous diets.^[1] Tritylodontid fossils are found in the Americas, South Africa, and Eurasia - they appear to have had an almost global distribution, including Antarctica.

Ornithischians

Ornithischia (/ɔːrnɪˈθɪskiə/ or-ni-THISS-kee-ə) is an extinct clade of mainly herbivorous dinosaurs . Ornithischia, Wikipedia

Tritylodontidae 

"The tritylodont's skull has a high sagittal crest. They retained the reptilian joint between the quadrate bone of the skull and the articular bone of the lower jaw^[2] - the retention of the vestigial reptilian jawbones is one of the reasons they are technically regarded to not be mammals, but are instead mammaliaformes.^[3] "

Ornithischians

The left and right upper temporal fenestrae were separated by the sagittal crest, which would have provided lateral attachment surfaces for the jaw musculature in the living animal.[10]Heterodontosaurus, wikipedia

Amphibians, reptiles, and birds[edit]

The jaw in tetrapods is substantially simplified compared to fish. Most of the upper jaw bones (premaxilla, maxilla, jugal, quadratojugal, and quadrate) have been fused to the braincase, while the lower jaw bones (dentary, splenial, angular, surangular, and articular) have been fused together into a unit called the mandible. The jaw articulates via a hinge joint between the quadrate and articular. The jaws of tetrapods exhibit varying degrees of mobility between jaw bones. Some species have jaw bones completely fused, while others may have joints allowing for mobility of the dentary, quadrate, or maxilla. The snake skull shows the greatest degree of cranial kinesis, which allows the snake to swallow large prey items.

Mammals[edit]

In mammals the jaws are made up of the mandible (lower jaw) and the maxilla (upper jaw). In the ape there is a reinforcement to the lower jaw bone called the simian shelf. In the evolution of the mammalian jaw, two of the bones of the jaw structure (the articular bone of the lower jaw, and quadrate) were reduced in size and incorporated into the ear, while many others have been fused together.^[3] As a result, mammals show little or no cranial kinesis, and the mandible is attached to the temporal bone by the temporomandibular joints. Temporomandibular joint dysfunction is a common disorder of these joints, characterized by pain, clicking and limitation of mandibular movement.^{[4]  Jaw, wikipedia}

^{Ornithischians were also distinguished by an extra bone at the tip of the lower jaw called the predentary.} Ornithischian teeth were leaf-shaped, and the jaw joint was located well below the occlusal plane (where the teeth met during chewing).

https://www.britannica.com/animal/ornithischian

I posit the theory that the mammalian jaw bone is an indication of specialization and not necessarily evidence of a shared ancestor. It is largely absent from the fossil record until the late Cretaceous, when grass appears for the first time and many plant species begin to go extinct.

Tritylodontidae 

"The back of the skull had huge zygomatic arches for the attachment of its large jaw muscles. 

Ornithischians

The zygomatic arch or cheek bone is formed by the zygomatic process of temporal bone (a bone extending forward from the side of the skull, over the opening of the ear) and the temporal process of the zygomatic bone (the side of the cheekbone), the two being united by an oblique suture (zygomaticotemporal suture);^[1] the tendon of the temporalis passes medial to the arch to gain insertion into the coronoid process of the mandible. The jugal point is the point at the anterior end of the upper border of the zygomatic arch where the masseteric and maxillary edges meet at an angle. The jugal point is the anterior end of upper border of the zygomatic arch where it meets the process of the zygomatic bone.zygomatic arch, wikipedia

So, What If Ornithischians Did Have “Cheeks”?

Except in a couple of very primitive genera the cheek teeth of ornithischians are inset with a lateral space that was roofed by the overhanging maxilla and floored by the massive dentary.
http://onlinelibrary.wiley.com/doi/10.1111/j.1502-3931.1973.tb00873.x/pdf

In hadrosaurs and ceratopsians at least where there was a coincidence of cheeks, dental batteries, and the evidence of tremendous jaw musculature, food was being processed in the mouth to a far greater extent than is found in living ectotherms such as snakes, lizards, crocodilians, and turtles.

https://books.google.com/books?

The Evolution and Extinction of the Dinosaurs(Cambridge University Press)

By David E. Fastovsky, David B. Weishampel

The jugal bone also formed a "blade" that created a slot together with a flange on the pterygoid bone, for guiding the motion of the lower jaw. Ventrally, the antorbital fossa was bounded by a prominent bony ridge, to which the animal's fleshy cheek would have been attached
 Heterodontosaurus, wikipedia

Tritylodontidae

They also had a very well-developed secondary palate. 

Ornithischians

Ankylosaurs and hadrosaurs both bear well-developed secondary palates, suggesting that these animals had the benefit of being able to breathe and chew

The Evolution and Extinction of the Dinosaurs(Cambridge University Press)

By David E. Fastovsky, David B. Weishampel

Tritylodontidae 

 The tritylodont dentition was very different from that of other cynodonts: they did not have canines, and the front pair of incisors were enlarged and were very similar to rodents of today.^[2] 

Ornithischians

"An unusual feature of the skull was the different-shaped teeth (heterodonty) for which the genus is named, which is otherwise mainly known from mammals. Most dinosaurs (and indeed most reptiles) have a single type of tooth in their jaws, but Heterodontosaurus had three. The beaked tip of the snout was toothless, whereas the hind part of the premaxilla in the upper jaw had three teeth on each side. The first two upper teeth were small and cone-shaped (comparable to incisors), while the third on each side was much enlarged, forming prominent, canine-like tusks. These first teeth were probably partially encased by the upper beak. The first two teeth in the lower jaw also formed canines, but were much bigger than the upper equivalents" Heterodontosaurus, wikipedia 

My hypothesis - the beak was actually a gum covering and is also seen in basal mammals of many groups. The cavity in the jaw evolved sometime after 235 million years ago, which is my proposed divergence point of Tritylodontidae and Ornithischians. It may have occurred in the transitional phase between Tritylodontidae and Ornithischians, also known as Aetosaurs. See previous two blog entries for more on this.

Tritylodontidae 

Tritylodontids had a large gap, called a diastema, that separated the incisors from their square-shaped cheek teeth. The cheek teeth in the upper jaw had three rows of cusps running along its length, with grooves in between. The lower teeth had two rows of cusps which fitted into the grooves in the upper teeth. The matching of the cusps allowed the teeth to occlude more precisely than in earlier cynodonts. It would grind its food between the teeth in somewhat the same way as a modern rodent, though unlike rodents tritylodontids had a palinal jaw stroke (front-to-back), instead of a propalinal one (back-to-front).^[2] The teeth were well suited for shredding plants matter; however, there is evidence that some tritylodontids had more omnivorous diets, much in the same vein as modern mammals with "herbivore dentitions" like modern rats.^[1]

Ornithischians

Ankylosaurus was herbivorous and had approximately 72 small, leaf-shaped teeth. The teeth were thin and made up of a series of cusps that looked like large serrations and a swollen base or cingulum.  https://www.fossilera.com/pages/dinosaur-teeth

Like other ankylosaurs, Ankylosaurus had small, phylliform (leaf-shaped) teeth, which were compressed sideways.

Ankylosaurus, wikipedia

Tritylodontidae 

Like Mammaliformes, tritylodontids have epipubic bones, a possible synapomorphy between both clades,^[4] and this suggests they may also have laid eggs, or produced undeveloped fetus-like young like modern monotremes and marsupials.

Ornithischians

Ornithischians did not have epipubic bones, just like placental mammals. Evidence exists that Late Jurassic and Cretaceous Ornithischians gave live birth to large offspring. See the last two blog posts for more on this.

Tritylodontidae 

Tritylodonts were active animals that were probably warm blooded and probably burrowed, though in Kayentatherium these supposed burrowing adaptations may be indicative of semi-aquatic habits.^[5] For example, Oligokyphus could be compared to a weasel or mink, with a long, slim body and tail.

Ornithischians

Elsewhere in Ankylosaurus, how about that weird giant skull? 

Ankylosaurus shares a lot in common with Laramidian ankylosaurins like Euoplocephalus, Anodontosaurus, and Zuul, but when it comes to the nose Ankylosaurus is doing something very different and weird. Instead of having forward-facing nostrils, the nostrils are pulled backwards and roofed over by cranial ornamentation so that you can’t even see them when you look at the skull face-on. Why has Ankylosaurus done such a weird thing to its face? We can’t say for sure, but when we look at other animals with somewhat similar faces, the closest comparison we could come up with were the unusual subterranean amphisbaenians and scolecophidians. Some of these lizards have narial openings that look a lot like those of Ankylosaurus, broadly speaking. Ankylosaurus wasn’t a fossorial, burrowing dinosaur – but maybe it was foraging around in the earth, eating tubers, roots, and insects, instead of relying more on ferns and leaves for its diet. It’s speculative for now, but I think there’s still a lot to investigate around ankylosaur diets!

So you think you know Ankylosaurus

If you'll check out the wikipedia page on Aetosaurs, you'll find they also share the majority of these traits. They could even be said to be a transitional stage between the direct ancestor of Tritylodontidae and Ornithischians. The morphology and anatomy of Aetosaurs and Ankylosaurs is extremely similar, and both possess a well-formed osteoderm, or shell. The last Aetosaurs and first undisputed Ornithischians have coinciding dates, converging at the Triassic/Jurassic boundary 200 million years ago. 

The earliest undisputed Ornithischians belong to 

Thyreophora(Stegosaurs and Ankylosaurs).

 The teeth of Late Aetosaurs and early Ankylosaurs are nearly identicle:

During the Triassic, large tunnels akin to "Paleoburrows" are associated with Aetosaur activity. There is little evidence of such large, long burrows during the Jurassic and Cretaceous. However, good evidence exists that Ankylosaurus burrowed. During the Paleogene, paleoburrows are attributed to large members of Xenarthra, namely Glyptodonts and giant sloths.

It's remamrkable that these three animals connected by so many traits, skull shape, morphology, and lifestyle are also the only members of their Superfamily who burrowed. Aetosaur was unique among archsaurs in having a full -shell-like osteodern exoskeleton nearly identicle to Ankylosaurus, burrowing, and being a herbivore (among other traits). During the Jurassic, the same can be said of  Ornithischians (or more specifically, 

Thyreophora) in contrast to other dinosaurs. During the Paleogene, the same can be said of Xenarthra in regard to the other placentals.

If Ankylosaurs were indeed burrowing animals, it may not only explain these strange underground tunnels found in S. America but also those in Eurasia and Antarctica. The South American variety are usually attributed to giant sloths or giant armadillos. However, researchers have often noticed similarities in the claw marks between S. American paleoburrows and European claw marks supposedly made by cave bears.. Certain recently discovered tunnels in China of unknown date may also be a clue.

The Chinese tunnels have obviously seen the hand of man. But did the workers already find most of their work cut out for them? Or "clawed out," so to speak?The claw or tool marks on the walls bear a striking resemblance to paleo-burrows in S. America.

Tetrapod and Large Burrows of Uncertain Origin in Triassic High Paleolatitude Floodplain Deposits, Antarctica

This Massive Tunnel in South America Was Dug by Ancient Mega-Sloths

Get Lost in Mega-Tunnels Dug by South American Megafauna

Having spread over the globe before Gondwanaland broke up, Ankylosaurus had been present on all three continents by the time the tunnels were made. And in our last two blog posts, we learned of a striking possibility that descendants of Ankylosaurus lived into the Eocene.

https://prehistoricfantasy.blogspot.com/2017/11/convergent-evolution-between-aetosaurs.html

For more exciting possibilities from Dr. Eugene Anklesloths ongoing research, click the link below!

https://prehistoricfantasy.blogspot.com/2017/11/the-mother-of-all-placental-mammals.html

Establishing Aetosaurs, Ornithischians, and Tritylodontidae as Sister Species