Remember - although we are interested in the way these animals lived and interacted with the changing Earth, we are grouping the animals based on shared derived anatomy, NOT on outward similarities. So will all the four-legged animals group together? No. Will all the plant-eaters group together? No.
The more derived long-neck dinosaurs are proper sauropods.
Surprising things about Sauropods include:
Quadrupedal posture (walking on four legs) is a derived trait in Sauropods.
Basal dinosaurs, including basal sauropodomorphs, walked on two legs.
Derived Sauropods have thick, dense leg bones, like pillars of a huge suspension bridge.
Sauropods did NOT chew their food!
They certainly ate plant material, and a LOT of it!
But they did not grind up the plants in their mouth. We can tell because:
They don't have cheeks to keep the food in their mouth while chewing!
They don't have flat grinding teeth.
When their teeth DO show wear-marks, it's from repeated crashing together of the teeth in the upper-and-lower jaw.
Think of their jaws like hedge-clippers, not like a wood chipper.
Sauropods have five toes on their back feet, with big claws.
Their ankle would be a little off the ground, but not high up like theropods.
There would probably be a pad of fat under the foot, like an elephant.
UNLIKE an elephant, Sauropod feet have big claws. It seems weird for walking but they figured it out.
This is why they're called "lizard foot"; living lizards today have five long toes with claws.
Sauropod vertebrae have big air pockets!
Think of their spine like the archways of a suspension bridge crossing a river or bay.
Complex struts extend between the elements in the archway to keep it from collapsing, folding, or bending at too sharp an angle.
Air pockets probably helped reduce the overall mass in a Sauropod body.
Air pockets in bones are a basal dinosaur trait (and probably basal to ornithodira - including the flying pteranodon), but big air pockets are a derived trait in Sauropods AND Theropods.
Within Sauropods we can consider two sister clades.
Welcome to a very important dinosaur category that has a very silly name!
Ornithischians include:
The last common ancestor of a spiky Stegosaurus and a horned Utahceratops, plus all of its descendants.
Most of the classic, easy to recognize "plant eating" dinosaurs EXCEPT the long-necked Sauropods.
At least two different independent lineages of quadrupedal gait!
What traits distinguish an Ornithischian dinosaur from other dinosaurs?
The pre-dentary bone.
This is a "beak"-like tip on the lower jaw. It is an additional little bone that makes the jaw come to a tip.
Once you know what to look for, it's pretty easy to recognize on the skull of ornithischian dinosaurs.
The predentary bone is present in basal ornithischians,
AND the predentary bone is critical to the fancy feeding structures that appear in derived ornithischians.
File:AnkyloSkullObliqueNHMU.jpgThat funny little tip on the lower jaw is a derived expression of the PREDENTARY bone, an ornithischian trait.A rear-facing projection on the pubis bone.
The dinosaur pelvis is made of three bones: illium, ischium, and pubis.
The illium runs parallel to the spine. This is the part that gets all expanded like a big shield in the derived armored Thyreophoran dinosaurs like ankylosaurids.
The ischium frames the rear-end plumbing of the animal. (Rather than the specialized variety of pelvic anatomy found in most mammals, we can guess dinosaurs had a simple multi-purpose outlet channel, called a cloaca, like frogs and birds have today.)
The pubis bone produces the forward corner of the pelvis socket where the femur fits in.
In ornithischians, some portion of the pubis bone is pointing backward, toward the plumbing side of the animal's pubis.
This can look very obvious in derived animals, where the rear-facing pubis bone is almost parallel to the ischium.
One proposed advantage of the rear-facing pubis bone is that it would leave more space for a big, gassy belly in these plant-eating animals, allowing them more space for digestion aided by bacteria and other microbes. Contrast this to the giant pubis bone that jams into the belly of long-necked Sauropod dinosaurs.
Derived thyreophorans are easily recognized by their gnarly body armor: Stegosaurus, Ankylosaurs, etc.
Dinosaur body armor is made from scutes (pronounced "scooooooooooots", as in "Hey let's get some of those scooters and go to Yoko."), which are bones that grow right out of the skin!
Scutes are what make a crocodile look bumpy!
Scutes are basal in archosaurs, so they pop up in a lot of different groups
Scutes appear on some sauropods and theropods, but not as big or as many as on thyreophorans.
Our favorite basal thyreophoran is Scutellosaurus, aka "scute lizard" aka "skin bone lizard".
Fossils of Scutellosaurus are rare
Reconstructions of the animal's skeleton, posture, and capabilities are constantly revised & debated.
General observations & interpretations include:
The overall body is small; it is small like other basal dinosaurs.
the hindlimbs are longer and stouter than the forelimbs; this was a bipedal, or primarily bipedal, animal
the teeth are simple, spread across the whole jaw, and not very pointy; this animal ate plants or plants + bugs and stuff
the body is lined in little scutes; the skin was protected by this tough exterior
This paper by Benn Breeden has a beautiful reconstruction of a Scutellosaurus.
"wide foot" Honestly I don't understand this name because their feet aren't that wide, but I guess compared to the more basal dinosaurs like Scutellosaurus it fits.
Lots of traits distinguish this clade but the coolest one is their bony eye sockets. Although scutes grow out of skin, on the skulls of these animals that scutes can tend to collide and grow along with the skull bones. Also a lot of the skull bones fuse together and get thick and wide, which is very different from the theropods and sauropods. A consequence of this that's easy to see on a skeleton in a museum is the fused bony socket around the eye - it's serious.
Also - obligate quadrupedalism. Eurypods walk on four legs, unlike basal thyreophorans and other basal dinosaurs.
But then the two main groups of eurypoda go very weird and different directions with the whole scute business.
"roof lizard" Stegosaur dinosaurs have big plate-shaped scutes on their back.
Different species of stegosaurids are distinguished easily by their wacky combinations of plates and spikes. Some have spikes on the tail, some have spikes on the shoulders, and all look like no one was taking their lunch money easily.
General observations and interpretations about stegosaurids:
Long femur and robust lower humerus; Stegosaurids were obligate quadrupeds, always walking on four legs.
Big plate-like scutes on the back and tail; Stegosaurids are really derived and these features might have served many different functions.
Stegosaurs are typically found as individual fossils, rather than big piles of bones from many individuals; Stegosaurids probably lived alone, like Pandas, rather than in a herd, like Reindeer.
Ankylosaurs look a little less flashy than stegosaurs from a distance, but they have lots of wacky derived features with ecological interpretations.
Scutes line their back and sides; they were probably hard to bite.
Their top hip bone flares out to make a big sheild-like flat lower area along their back; they were probably less nimble than other dinosaurs.
Their tail bones can be fused together, making a stiff feature like a baseball bat.
Ankylosaurids include two sub-clades. Regular Ankylosaurs – that includes Ankylosaurus, a classic, and Acainocephalus, a new one from Utah – had clubs on the end of the tail.
Members of their sister clade Nodosaurids ("knot lizard") do NOT have the club on the tail, but they do rock AMAZING big shoulder spikes. Very cool.
Ankylosaurs have fancy nose chambers that might have helped them maintain their body temperature.
Ankylosaurs have decent teeth and cheeks more inset than Stegosaurids; they could probably chew tough plants better than other Thyreophorans.
What's this? An extra clade that is a NODE joining the branches that lead to duckbills and to horn-faces.
Duck-bills and horn faces have these features in common, which they get from a last-common ancestor:
rear-facing pubis extension
pre-dentary bone on lower jaw
cheeks
diastema
The name horn-foot can refer to the hoof-like toes that form in the derived members of this clade.
By now we are starting to notice that a lot of clades are named for DERIVED traits that show up in their famous members, so sometimes the name of the clade doesn't match to the trait that shows up on the branch leading to that node and joining all those animals via mutual inheritance.
Better-known as Duck-Billed Dinosaurs!
Includes the last common ancestor of an Iguanadon and a Parasaurolophus, and ALL of its descendants.
These animals could CHEW, and are members of the genosaurs.
This means they share a closer common ancestor to Stegosaurus than to, say, a Diplodocus.
Duckbills and iguanadontians have feet with THREE TOES. (hence: bird foot, ornithopod)
Duckbills and iguanadons have derived features for eating plant material.
Iguanadon teeth have ridges that would help gnash plant material.
Derived duck-billed dinosaurs, like Parasaurolophus, had packed-together batteries of teeth that could slap together to grind food.
Cheeks set the teeth in closer to the tongue, allowing muscles to hold food in the mouth for extra processing.
Even fancier, these mouths have a diastema!
This is an open space in between a snipping front and crunching back.
It leaves space for the tongue to move food around for more chewing time.
Lots of derived mammals have evolved a diastema, too.
Duckbills and Iguanadons probably mostly walked on two legs, but could also support their weight on their front legs.
Their toes on the hindlimbs developed hoof-like toenails.
Iguanadons also had hoof-like toenails on their forelimbs, PLUS a wacky stabby pointy nail on one digit.
The ecology of derived duck-bill dinosaurs is really well known from some excellent fossil examples of nests, and even from whole-body fossils from settings where the soft-parts preserved a bit, like a mummy.
