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Tetrapods

We propose that the last common ancestor of a cat and a frog had a pelvis.

  • We will call that last common ancestor, and ALL of its descendants, tetrapods.
  • Literally translated, tetrapods means "four feet".
  • But do all animals included in this clade STILL have four FEET? Nope.

What's the deal with tetrapods?

  • A pelvis forms a bridge between the legs and spine.
  • Similarly, shoulder blades and collar bones form a bridge between the arms and spine.
  • Each of these (the pelvis and the shoulder stuff) is easily considered as a package: a girdle.
  • The pelvic girdle manages the legs.
  • The pectoral girdle (think about lifting weights to get swole and have huge pecs) manages the arms, or fore-limbs, or front legs, or wings, or whatever you've got up there.

Fish already have a pectoral girdle, which is how their pectoral fins are linked to the spine.

By adding the pelvis, and with it the hind limbs, this new lineage of animals can generally be imagined as animals with FOUR LIMBS.

Tetrapods have been around for a LONG time, based both of fossils and DNA observations.

  • Because different subclades within tetrapods have been evoloving for such a long time, a lot of them are now very DERIVED.
  • Fossils of basal tetrapods include Acanthostega, and Ichthyostega.
  • Many derived lineages of tetrapods do not currently walk on four legs.
    • Birds
    • Humans
    • Whales
    • Snakes

Next time you see a rattlesnake while hiking, you probably don't want to shout, "Hey! Watch out for that tetrapod!" because this will be confusing to the average person. But you can always give the snake a respectful nod and say, "Nice rattle, there, fellow tetrapod. Look at us, being so derived."

Amniotes

We interpret that the last common ancestor of a cat and a bird had an amnion. We refer to this clade as amniotes.

  • An amnion allows an animal to reproduce away from water, by protecting the embryo in a special layer within an egg (i.e., lizard, platypus, chicken) or womb (cat, rat, etc.).

Synapsids

"With a window". The traits that help us diagnose membership in this clade are often hidden on derived, living animals, so let's dig into the puzzle.

Today, we can easily observe that lots of mammals have a mouth full of differently-shaped teeth. Consider a human and a wolf.

  • Humans have teeth that can cut, crush, and grind.
  • Wolves have teeth that can stab and crush.

Taking advantage of these teeth to do interesting things (bite into food, crack it open, chew it up) requires muscles to move the jaws, and cheeks to keep the food from falling out of the mouth!

  • The skulls of humans and wolves have a large arch of bone (on each side of the skull) that allows the muscles to pass between the skull and lower jaw.
  • The arch of bone also provides attachment points for muscular cheeks.
  • In biology, we call this the "zygomatic arch" which is fun to say!
  • We can look at this arch and consider it as a window for the passage of chewing muscles.

Now, let us consider the fossil animal Dimetrodon.

  • Dimetrodon means "two size teeth", named for the differently sizes teeth in it's mouth.
  • Dimetrodon has a skull with a large round hole placed behind each eye socket.
  • Paleontologists interpret that this hole allowed for muscle attachments for chewing.
  • Our current interpretation is that Dimetrodon is a synapsid - "with a window" - and that it is a basal member of our clade.

The clade synapsid contains the last common ancestor of a monkey and a Dimetrodon, and all of its descendants.

Casually, in this class, we will ignore synapsids.

  • So you'll often see us group Dimetrodon with some familiar mammal, to really drive the point home.
  • For example, see our Dinosaur Time Tree. We leave synapsids alone in their little lineage, and show a basal Dimetrodon back in the Paleozoic Era, and a fuzzy cat in the Cenozoic era.
  • Were synapsids around during the Mesozoic? Of course!
  • Will we learn much about them? Not in this class!

Diapsids

"Two windows". Above, we considered synapsids, which have a single archway in the skull to allow chewing muscles.

Diapsids have TWO archways in the skull behind each eye socket.

  • We interpret that one set of holes aided in muscles to work the jaw.
  • We interpret that the set of holes on TOP of the skull aided in temperature regulation. Let's explore!

In 2019, a team of scientists published peer-reviewed observations of heat-exchanging organs in the skulls of crocodiles and birds.

  • Living crocodiles have holes in the top of their skull.
  • So does the living tetrapod Tuatara.
  • Derived birds don't show these holes in the top of the skull. But many feathered dinosaurs related to birds DO show the holes.

Dr. Ritterbush agrees with the interpretation that the skull-top-holes were used to house heat-regulating organs. Dr. Ritterbush likes to call these holes the HotPockets.


AND, special bonus, there is another useful trait that we can observe in living tetrapods that we group together as diapsids.

Corneous beta protein is a tough material that builds outer body coatings.

  • Corneous beta protein is found in living lizards, alligators, turtles, and birds.
  • Corneous beta protein is composed of tiny protein strings woven together.
  • Corneous beta protein allows construction of strong, flexible materials, such as feathers.
  • It is NOT found in mammals or other synapsids. Mammals make keratin, a different set of proteins.

Observations clarifying this situation are fairly new, such that they haven't made it into mainstream teaching practices or textbooks.

  • Beta proteins were long recognized in microscopes, but were just named beta because they looked different.
  • Recent observations include the microscopic structure of the protein,
  • the number of different beta proteins made by different living animal groups,
  • and the placement of the genes that produce the protein on the DNA patterns of these living animals.

The result is a compelling interpretation that the last common ancestor of birds and lizards had corneous beta proteins.


Diapsids are a case study in the connections between paleontology and biology.

  • Biologists can observe the detailed chemical structures and DNA patterns of living animals.
  • Paleontologists can observe the shapes, sizes, structures, and microscopic appearance of extinct animals.
  • Interpreted relationships between living and dead animals are always changing, because biologists and paleontologists are continually making new observations.

Very important note!

Diapsids and synapsids are SISTER clades, both within the larger clade of amniotes. Diapsids are tetrapod amniotes. Synapsids are tetrapod amniotes. We don't have compelling evidence that either skull configuration is more basal or derived. We typically show them diverging within the amniote clade, and we mark the traits (two windows; one window) on each separate diverging branch.

Archosaurs

"Old lizard". The last common ancestor of a bird and an alligator, and all of its descendants, are a clade we call archosaurs.

Archosaurs are diapsids.

  • Basal archosaurs show a large lump on the rear face of their thigh bone (femur).
  • The lump allows stronger attachment for a muscle that runs between the tail and the thigh (caudal-femoral muscle).
  • The fancy name for this is the "fourth trochanter",
    • but we won't use that term in this class
    • because it implies we are going to learn all the other trochanters,
    • which we definitely will not!

The size of this muscle attachment spot varies in different archosaurs.

  • On fossils, it's easiest to see in crocodiles and dinosaurs.
  • In live animals, it's easier to see in crocodiles.
  • In modern birds:
    • the tail is so reduced and stumpy that,
    • although the muscle is still there
    • and still attaches to the femur,
    • the big lump is not so prominent or easy to see.

Fossils reveal an amazing variety of archosaurs, some of which were enormous and probably terrifying. That's super fun to explore, but we aren't making time in this class.

Dinosaurs

"Scary lizard". The clade dinosaur includes the last common ancestor of a bird and a Triceratops, and all of its descendants.

In this class, we will track two features that dinosaurs acquired.

Dinosaurs have smooth ankles.

  • Crocodile ankles are twisty, which is smooth in the water but sloppy on land.
  • Crocodile ankles concentrate bending motion by oblique shearing of two bones: the calcaneum and the astragulus.
  • In dinosaurs, the astragalus is big and sticks tightly to the shin bone (the tibia).
  • In dinosaurs, the calcaneum is tiny, and just sort of rides alongside the astragulus.
    • I like to remember them this way:
      • The astragulus looks like a big A with a pulley on the bottom.
      • The calcaneum? Barely know 'em!
  • The result in dinosaurs is a smooth hinge ankle that flaps forward and back, like a dog door.
  • Along with dinosaurs, smooth ankles are also found in pterosaurs.
    • Pterosaurs ("wide lizards") are flying archosaurs, like the pterodactyl ("wide finger") and pteranodon ("wide toothless")
    • The clade that includes the last common ancestor of a bird and a pteranodon, and all of its descendents, is called Ornithodira ("bird neck").

Dinosaurs have smooth hips.

  • Crocodile hips are twisty, which is subtle in the water but hilariously awkward on land.
    • Remember that tail-to-thigh muscle that's so prominent in crocodiles?
    • On land they have to really pump that muscle to yank the leg back and prep for a new step.
    • In the water, crocodiles can glide and sneak and creep with subtle adjustments, and their legs are kind of at an angle for little adjustments.
    • On land, crocodiles stagger around with big awkward steps.
    • Crocodile thigh bones have a top notch that fits into the pelvis at a funny angle.
  • Dinosaur hips allow the leg to swing like a clock pendulum. [click link for video of reconstruction!]
    • The dinosaur thigh bone has a top with a sharp 90 degree handle on top, like an upside down golf club.
    • The dinosaur pelvis also has a nice round socket hole for the femur to slide into.
    • The result is a nice smooth swinging motion allowed by the leg,
    • and an upright stance allowed by the whole skeleton.

Check out more clades within the dinosaurs!

Pterosaurs are NOT dinosaurs because they did not inherit the smooth hips.

Birds

The last common ancestor of a crow and an ostrich, and all of its descendants, is the clade of birds.

Birds are a clade within dinosaurs.

A key trait that appears in birds, but not other dinosaurs, is the stumpy tail bone, the pygostyle.

Lots of features we see on a chicken are shared by other dinosaurs:

  • Smooth ankles and smooth hips
  • Upright posture, with the body supported above the ankles.
  • A stout tibia and reduced fibula (shin bones)
  • Long forelimbs that do not walk on the ground.
  • Reduced numbers of fingers and toes.
  • Feathers

Other features we see on a chicken are NOT shared by other dinosaurs:

  • The pygostyle, a stumpy tail bone.
  • The pelvis and sacral spine are so fused together that it's hard to tell what you're even looking at.
  • Compared to their body size, most birds have HUGE eyes and HUGE brains!
  • The adult bird beak has no teeth.
  • The finger bones are fused into a creepy superfinger at the end of the wing.
    • Think about this next time you eat buffalo wings.
    • Or don't. It's disconcerting.


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