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Friday, November 17, 2017

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. 


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

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

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 stemsleaves, and roots. A recent study indicate some may have had more omnivorous diets.[1] Tritylodontid fossils are found in the AmericasSouth Africa, and Eurasia - they appear to have had an almost global distribution, including Antarctica.

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

"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] "

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 (premaxillamaxillajugalquadratojugal, and quadrate) have been fused to the braincase, while the lower jaw bones (dentarysplenialangularsurangular, 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.


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 jointsTemporomandibular 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).

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.

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

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 latera! space that was roofed by the overhanging maxilla and floored by the massive dentary.

In hadrosaurs and ceratopsians at least where there was a coincidence of cheeks, dental batteries, and the evidence of tremendoes 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.

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


They also had a very well-developed secondary palate. 

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)

 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] 

"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 phasebetween Tritylodontidae and Ornithischians, also known as Aetosaurs. See previous two blog entries for more on this.

Tritylodontidshad 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]


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.

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

Ankylosaurus, wikipedia

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 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.

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.

Elsewhere in Ankylosaurus, how about that weird giant skull? Ankylosaurus shares a lot in common with Laramidian ankylosaurins like EuoplocephalusAnodontosaurus, 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!

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 Ornithischians have coinciding dates, converging at the Triassic/Jurassic boundary 200 million years ago. The earliest Ornithischians belong to 

Thyreophora(Stegosaurs and Ankylosaurs).

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

If Ankylosaurs indeed were burrowing animals, it may explain these strange underground tunnels found in S. America as well as 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. 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 wall 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.


Convergent Evolution between Aetosaurs, Ankylosaurs, and Armadillos Explained at Last!

According to the textbook version of the Triassic/Jurassic Extinction, not much survived into the Jurassic:

"The Triassic–Jurassic extinction event marks the boundary between the Triassic and Jurassic periods, 201.3 million years ago,[1] and is one of the major extinction events of the Phanerozoiceon, profoundly affecting life on land and in the oceans. In the seas, a whole class (conodonts)[2] and 34% of marine genera disappeared.[3] On land, all archosaurs other than crocodylomorphs(Sphenosuchia and Crocodyliformes) and Avemetatarsalia (pterosaurs and dinosaurs), some remaining therapsids, and many of the large amphibians became extinct."

However, the above statement contradicts what we supposedly know of dinosaurs. The first Sauropodomorpha, from which all dinosaurs supposedly evolved, arose around 231 million years ago and continued through the Triassic/Jurassic Boundary. A clear evolutionary transition from Sauropodomorpha to the  Saurischia dinosaurs. Both groups were mostly bipedal, and this explains the difference in hips between Saurischia and other reptiles. No matter how big reptiles quadrupedal reptiles get, they maintain a sprawling posture. A recent study on Varanid lizards confirmed this observation.

But there is no clear path from Sauropodomorpha to non-Saurischia dinosaurs, the large herbivores known as Ornithischians. Most Ornithischians walk on all four legs, though a few species divergent from the basal quadrupedal form are known. Quadrupedal ornithischians walked totally differently quadrupedal Saurischia, and bipedal ornithischians walked totally differently than bipedal Saurischia. The difference in their hips gave them their names.

Scientists believe ornithischians evolved from bipedal Sauropodomorpha, but no explanation exists as to why they should have evolved separate walking styles and features. In fact, a recent study found that the common ancestor of Saurischia and Ornithischians should be pushed back to before the time of Sauropodomorpha. The study also placed Theropod dinosaurs closer to Ornithischians but confirms no bipedal ancestor for the two groups.

By now you should realize that they don't really know whether the ancestor of all three groups walked on two legs. A bipedal ancestor for Ornithischians is nothing more than an assumption. They could have evolved from a different archosaur from Sauropodomorpha for all we know, since other archosaurs share more traits in common with Ornithischians than Sauropodomorpha.

Take Aetosaurs for instance. Superficially, they resemble the first ornithischians, quadruped dinosaurs with armor plates that later gave rise to stegosaurs and ankylosaurs. The diet and morphology of the two groups is the same, and Aetosaur and ankylosaur teeth are nearly identical.

No Sauropodomorpha or Saurischia possessed teeth similar to these.

In order to explain ornithischian armor, scientists suppose that some species of Sauropodomorpha evolved an osteoderm, an exoskeleton made of intricately formed bone and scutes (primitive scales). They have no transitional fossils of anything like this occurring. There's no evidence that any Sauropodomorpha even wore scutes. For aetosaurs, a common ancestry whith crocodylomorphs is assumed since they were both protected by the same osteoderm.

Aetosaur fossils end exactly at the Triassic/Jurassic Extinction boundary, but soon afterward  Thyreophora begin to the same time fossils begin to appear again for other animal lineages.

After each major extinction event in Earth's history, fossils become scarce. This isn't only because species died out, it's also because surviving species experienced bottlenecks. Bones only fossilize by rare chance, and when a species is reduced to a small population fossils may not be left at all. Imagining ornithischians and Thyreophora evolved from a  proto-Sauropodomorpha requires a ghost lineage of over 30 million years, whereas imagining an Aetosaur ancestor requires less than half that time.

Superficially, anatomically, and behaviorally, Ornithischians resemble Aetosaurs more than Sauropodomorpha.

Is this just a coincidence? Should we discard a simple explanation of Ornithischian armor for a complex and unsupported one? Should we ignore the lack of proof for a bipedal ancestor of Ornithischians, and forgo an explanation as to why Ornithischians and Saurischia evolved completely different hips and walking styles?

Should we dismiss the teeth as a coincidence too?

Ankylosaurs survive and diversify through the Jurassic and Cretaceous, only going extinct 66 million years ago at the time of the KTg extinction.

After the KTg extinction, fossils don't appear again with frequency until around 55 million years ago. Major groups in South America don't appear until around 46 million years ago. This is about the same time as the Antarctic/South American exchange, so many of these animals may have migrated from S. America as the continents reconnected.

A paleoisthmus linking southern South America with the Antarctic Peninsula during Late Cretaceous and Early Tertiary

Chapter 6 Paleogene

Since Antarctica is covered with ice, we find few fossils there. However, because of the few fossils we have found there and because of what we know of continental drift we can deduce that many S. American animals lived in Antarctica during the Late Cretaceous and early Paleogene.

One of the animal groups that appears at the time of the first exchange is Xenarthra, considered the most primitive of the four major groups of placental mammals. The earliest Xenarthrans possess exoskeltons made of the same intricate osteoderms, but again no transitional fossils show how or why the osteoderm formed.

Often when school children notice the striking similarity between Ankylosaurs and early Xenarthrans, they are told the resemblance is only superficial and it's just a case of "convergent evolution." But the resemblances are anything but superficial. The skeletons of the creatures show the same fused bones, the same number of vertebrae and teeth, the same bizzarre cheek flanges, and the same pair of tiny horns in the supraorbital position of the skull. The youngest Ankylosaurs even show the same number of temporal fenestra as early mammals, which is one of the main ways we tell reptiles and mammals apart. The skulls of Ankylosaurus and Megatherium, another early Xenarthran, are so close that one could be a subspecies of the other.



Other than crocodylomorphs and turtles, Aetosaurs were the only animals with osseoderms in the Late Triassic.
Interestingly, Ankylosaurs and their close kin were the only animals with full body osteoderms in the Jurassic and Cretaceous. Today. only Xenarthrans have full body osteoderms. The pangolin's armor is not a true osteoderm, as it does not contain bone matter. However, pangolins were first placed within Xenarthra, before DNA tests linked them with Carnivora.

So while crocodylomorphs and turtles remained mostly unchange for 300 million years or more, retaining the same osteoderm, three unrelated animal groups evolved similar osteoderms in succession, one right after the other. As soon as Aetosaurs died out, Ankylosaurs rapidly evolved osteoderms and Aetosaur teeth. As soon as Ankylosaurs died out, Xenarthra rapidly evolved the same osteoderm and skull.

Since this happens so quickly in such different animals, with no transitional fossils or explanation for the change, we would think the formation of an intricate full-body osteoderm should be quite common. Yet it only seems to happen to one animal group per time period, at the very beginning of the time period directly following a global disaster. During all of the 100 million years of the Jurassic and Cretaceous, for instance, not a single other group of animals with full body osteoderms arose.

Since exoskeletons are present in ancient fish and even amphibians, it's not hard to imagine why crocs and Aetosaurs may have had an osteoderm. Perhaps the condition was present in the first terrestrial amniote, and Aetosaurs and crocs retained the trait. The osteoderm could have evolved in the ancestor of Aetosaurs and crocs during the Permian and Triassic.
However, there is no explanation at all for the similar osteoderms of Ankylosaurs and Xenarthrans.

Is this just a big coincidence? Does nature keep recreating the same type of creature from scratch, but only once per 100 million years, directly after the disaster which ended the previous age?

Seems a bit hard to fathom. But there's another possibility few have considered. If you'd like to learn of a more plausible and probable answer for this highly unlikely sequence of events, you can find my full report on the matter here:

The Mother of All Placental Mammals Discovered: Identification of the first fossil Placental Mammal from the Cretaceous Period