Lecture 4 - Tetrapods

Question 1

Name the 4 classes of tetrapods

Answer 1

1. Amphibians
2. Reptiles
3. Aves (birds)
4. Mammals

Question 2

Name the 2 (Major) subclasses of amphibias and describe their significance.

Answer 2

1. Labyrinthodontia - Extinct toothy lizard-like amphibians believed to be the ancestor to modern vertebrates
2. Lissamphibia - Modern amphibians (Frogs, Toads, Salamanders, caecilians)

Question 3

Name the 3 subclasses within the Reptile Group

Answer 3

1. Anapsids - Most simplistic reptiles (Possibly first subclass to have membrane-bound fetuses…I think), includes turtles, tortoises, etc…
2. Diapsids - Develop with two “holes” in the skull (lost later), this subclass includes Lizards, Snakes, Crocodiles, and dinosaurs
3. Synapsids - Not as clear - Temporal holes for better bite, believed to be the class of reptiles that would first develop mammal-like features…i dunno, more on this later

Question 4

Briefly describe the two subclasses of mammals

Answer 4

1. Prototheria - Early mammals

2. Theria - Modern mammals. Pretty easy

Question 5

Describe which subclass/group that were believed to be the first tetrapods

Answer 5

• Devonian Tetrapods (Called that because they emerged around the Devonian Era) were believed to be labyrinthodonts, named for their complex-folded enamel tooth formations.
• Labyrinthodonts are grouped with amphibians, though that’s somewhat disputed.
• The amphibian order, Temnospondyli are believed to have arisen during the later Carboniferous period and are said to be the ancestor for modern amphibians

Question 6

What order of amphibian, Devonian Tetrapods are the best studied today? Describe their characteristics

Answer 6

• Icththyostega were an order best represented by Lepospondyls, which were smaller than labyrinthodonts
• Vascular gas exchange on their skin (like frogs?), bony scales, and a tail with fin rays. Had basic sensory canals, including a nose. Some had horns.
• Was able to coexist with devonian tetrapods, but no modern variation is currently in existence

Question 7

What were some other characteristics of early Devonian tetrapods?

Answer 7

• Still probably spent most of their time in the water, but they had well-developed lungs
• Early tetrapods had fish scales, but these would become bony plates, with tougher skin…In the Carboniferous era, these may have become full-on armored skulls
• Had notches on the sides of their head that had a brand new feature: A Tympanic Membrane (eardrum)

Question 8

One more time now, describe the group of Devonian tetrapods that gave rise to modern amphibians

Answer 8

Temnospondyls - Arose during the Mississippian period, over 6 feet long, Heavily Boned with a 6-feet long skull, and probably quite carnivorous.

Question 9

Name the 3 modern orders of amphibians and describe their characteristics. Give an example animal for each order

Answer 9

• Caudates (Urodeles) - Had tails (into adulthood?) and include modern salamanders (first appeared in Jurassic Period)
• Anurans - Lacked a tail, includes frogs and first appeared in Jurassic Period. Back then, the vertebra was longer, and the spine was shorter.
• Apodans - Didn’t have any limbs at all, includes the modern-day caecilian. They’re worm-like and burrow like subterranean amphibeans.

Question 10

What are some characteristics that all modern amphibians have in common?

Answer 10

Pedicellate teeth, cylindrical vertebral centra, and two auditory ossicles, suggesting they belong to a single subclass, but some workers think otherwise.

Question 11

List and describe the 3 reptilian skull types

Answer 11

(Based on holes and arches on the back sides of the skull)
Anapsids - No holes in their skull apart from orbital and nasal
Synapsid - One additional hole
Diapsid - Two additional holes

Question 12

Describe anapsids

Answer 12

• Used to include cotylosaurs and chelonia (turtles). But now the turtle part is being second-guessed because they’re more likely modified diapsids.
• Cotylosaurs are the most primitive (stem) reptiles and resembled seymourian tetrapods. They might’ve have been several feet long and probably had some cool dermal armor.
• Their skulls are more laterally flattened than other reptiles.
• They didn’t have much serious jaw strength, started out pretty small and skinny (

Question 13

List some basic characteristics about diapsids.

Answer 13

• Two openings behind their orbits. Each opening initially has a bony bar. But lizards and snakes have lost one or both bars.
• Divided into two infraclasses:
• Lepidosaurs: Includes snakes, and lizards
• Archosaurs: Includes crocodiles, flying reptiles, dinosaurs, and ancestors to birds.

Question 14

Dinosaurs were sub-categorized into two different groups. Name them and describe the characteristics of each.

Answer 14

• Ornithishcians: Bird hipped dinosaurs. These had a tetraradiate pelvic girdle and could include both quadripedal and bipedal types. This included the hadrosaur, stegosaur, and triceratops.
• Saurischians: Lizard-hipped dinosaurs. Triradiate pelvic girdle. Included both quadripedal and bipedal forms, this included the Tyranosaurus, Allosaurus, and Apatosaurus.

Question 15

What are the 3 subgroups of lepidosaurs?

Answer 15

• Sphenodon: Has a “true” diapsid skull and a primitive pectoral girdle. It looks like a lizard, but it’s not. They are/were/not sure/ found in New Zealand.
• Snakes
  Lizards

Question 16

Why do dinosaurs form a paraphyletic group?

Answer 16

First off, since you probably forgot, paraphyletic means a group or selection of species that descended from a common ancestor of another group or species, but doesn’t include them all collectively.

• Case in point, birds are closely related to saurischians, birds are grouped into their own class (Aves)
• Some people refer to birds as feathered dinosaurs. Sounds lame.

Question 17

What kind of reptillian characteristics remained in early birds

Answer 17

• They first emerged in the Jurassic period. They had a bony tail (not yet feathers), clawlike fingers (not yet talons), and teeth. Mind you, even these weren’t the “first birds.”
• By the Cretaceous period, more modern aquatic birds were being found. They probably “dove” into and out of water.

Question 18

State some of the theories on the origin of bird flight

Answer 18

• Birds evolved from saurischian (not confirmed).
• May have evolved as a means of thermal insulation.
• May have evolved as an insect-catching net.
• May have allowed for better pursuit of prey or escape from predation.
• May have allowed early archosaurs to roost in trees at night.

Question 19

Describe synapsids

Answer 19

• Included the class pelycosaurs (earliest) and cynognathids. These appeared in the Permian period about 310 million years ago.
• Synapsids were reptiles believed to have been the bridge between reptiles and mammals. (Somewhere between Permian and Triassic - 230mya)
• Later synapsids were known as therapsids.

Question 20

Describe pelycosaurs

Answer 20

Generally only a few feet in length.

• NOT contemporary with dinosaurs.
• Some had a large sail-like structure on their back: This structure was highly vascularized. This was thought to have been a thermoregulatory device.

Question 21

Describe therapsids:

Answer 21

Make up a group of several clades, Cynognathous being the most common one.

• Elbow and knee swung in toward the body.
• Many features of the skull were more mammal-like.
• Transition to mammal-like jaw articulation.

Question 22

Long one incoming: What are characteristics of mammals that are shared with therapsids?

Answer 22

sigh (1. Secondary Plate Bones), (2. An enlarged dentary bone), (3. Differentiation of Teeth Types), (4. Regional differentiation of ribs and vertebrae), (5. Legs are pulled in and ilium is extended forward), (6. Sacrum is elongated and feet well-developed), (7. Mammal-style Jaw is articulated between mandible and temporal bone), (8. Reptile-style Jaw is articulated between quadrate and articular bone…One therapsid species had both?),

Question 23

What mammalian characteristics are exclusive from therapsids?

Answer 23

(1. Live Birth), (2. Homeothermy), (3. Respiratory Diaphragm), (4. 4-chambered heart), (5. Neocortex), (6. Double Occipital Condyle), (7. Single Mandible), (8. Bony secondary palate), (9. Loss of inter clavicle {except in egg-laying mammales}), (10. Hair/Fur), (11. Mammary Glands)

Question 24

What were the earliest mammals documented, and when did they emerge?

What are the two subclasses of mammals?

Answer 24

• Morganucodonts - From the late Triassic Period. They had the denture patterns of a mammal.
  1. Prototheria - Egg laying mammals. Extinct except for the platypus and the spiny anteaters.
  2. Theria - Marsupials and placentals; Had trituberculate molars. Whatever those are.

Question 25

Why did terestrialism first occur?

What evolutionary changes would have been required to become terrestrial?

Answer 25

• May have been necessary to find new environments after existing freshwater and ponds were drying up. And also to start eating those delicious things called Insects
  Main Requirements:
• (1. Dessication), (2. Resistance to Gravity), (3. Changes in organs for special senses), (4. Excretion of nitrogenous wastes), (5. Changes in reproduction)

Question 26

Describe Dessication Control

Answer 26

• Amphibians would dry up like prunes if they couldn’t avoid dehydrating. Most amphibians depend partially or entirely on their skin for gas exchange. They inhale has through the skin and not the lungs. The integument mus be kept moist to allow diffusion of respiratory gases and must be relatively thin and well vascularized. Some amphibians, such as plethodontid salamanders, depend entirelyon their integument for gas exchange and lack lungs.
• ADH (vasopressin) had to be secreted in the pituitary gland to alter permeability of the skin and water retention.

Question 27

Describe resistance to gravity

Answer 27

• Terrestrials need the skeletal system and abilty to resist gravity pull on land.
• Vertebral column becomes more dorsal. Hypaxial muscles become less dominant, pectoral and limb muscles become more advanced
• Pelvic girdle attaches to vertebral column

Question 28

Describe the evolution of the vertebral column:

Answer 28

In the devonian tetrapod, the vertebral column is made of discs, rings, composed into something called the centra.

• In early amphibians, the vertebra transformed into a series of interlocking bones with attachments for muscles and ligaments.
• It’s still pretty flexible in salamanders, but less so in tetrapods who rely on limbs to locomote. Salamanders have more buoyant limbs useful for traveling underwater.

Question 29

Describe the changes in organs for special senses

Answer 29

• The Lateral Line System: Seen in fish, they have an electromagnetic “sense” in water that helps them detect prey and predators alike.
• Ear: Possibly a successor to the LLS, allowing the new organ to detect soundwaves in air rather than water. The Lagena became the Cochlea.
• Eye: The evolution of the cornea allowed for course adjustment, while the lens allowed for fine adjustment.