8/31 - 9/2: Evolution & Dental Characteristics

Tetrapods, Amniotes, Synapsids, Early Mesozoic Mammals

What are tetrapods?

A group of four-finned fish that started working its way out of water and onto land, developing legs in the process.

What clade are tetrapods in?

Petroglyphs

What species is evolving to be on land like tetrapods did? What characteristics do they posses?

Mudskippers; Modified gills to breathe air, fins allow it to walk

After tetrapods invaded the land, what monophyletic group did they become? What animal group is excluded from this monophyletic group?

Amniotes; Amphibians

During what era did the tetrapods become the amniotes?

Early Carboniferous

What are the amniotes?

A monophyletic group that arose from amphibian-like tetrapods

What is the cleidoic shell?

The shell around an egg that allows amniotes to lay their eggs on land

What are the four amniote groups, and what are their groupings based on?

Anapsid, Diapsid, Euryapsid, Synapsid; Lateral temporal fossa

Which amniote group did not develop a lateral temporal fossa? What animal is included in this group?

Anapsid; Sea turtle

Which amniote group developed two lateral temporal fossa? What animal is included in this group?

Diapsid; Crocodile

Why did amniotes develop more lateral temporal fossa?

Muscle attachment

What is the mechanical advantage for lateral temporal fossa?

Stronger jaw because the muscle comes straight up and has more of a surface area to attach to as well as more range of motion

Which amniote group did developed a single temporal fossa above the post-orbital and squamosal suture? What animal is included in this group?

Euryapsid; Ichthyosaur

Which amniote group developed a single lateral temporal fossa below the suture between the squamosal and post-orbital bones? What animal is included in this group?

Synapsids; Dog

Describe the muscle attachment of synapsids.

The muscle attachment is along the dentary bone and comes underneath the zygomatic arch to attach all along the side of the skull

When did the synapsids develop?

Late Paleozoic Era

For how long were the synapsids the dominant land animals?

70 million years

What defines the official “mammal” designation?

The articulation squamosal and dentary bones

What are the two important bones that form the inner ear?

Quadrate and articular

Describe the evolution of the quadrate and articular bones into the inner ear bones.

The dentary bone forms an articulation with the squamosal bone on the cranium. The original articulation between quadrate and articular become inconsequential to jaw movement and moves farther back to become the incus and malleus.

What was the small, shrew-like creature that lived 225 million years ago that is classified as the most ancient mammal?

Brasilodon quadrangularis

What were the three mammal groups throughout the Mesozoic era that we discussed?

Primitive mammals, prototherians, and therians

Describe the primitive mammals.

Primary jaw joint composed of dentary and squamosal bones; Diphyodont dentition with complex occlusion; Rare and insignificant part of fauna; Small, likely nocturnal; Little

Describe the early prototherians.

Numerous and diverse; Still small; Some distinct mammalian structural features such as the mammal stance and inner ear bones that have not fully moved into the ear yet.

When did early therians develop?

Cretaceous period

Early therians had tribosphenic molars. What do these molars do?

Shear and grind

The asteroid caused sudden climate change, allowing mammals to do what?

They could use some features that they have developed over time, such as heat generation via metabolism

What started happening to mammals after Pangea split?

Their niches diversified

What changes took place as synapsids evolved into mammals? (7)

Development of a larger, heavier dentary bone; Enlargement of temporal opening; Maxillary and palatine bones extend posteriorly and medially, forming a secondary palate and making nares more posterior; Development of two occipital condyle; Limbs rotated directly beneath body; Loss of cervical and lumber ribs; Reduction of carpal and tarsal bones; Heterodont dentition

What did the development of a larger, heavier dentary bone allow for mammals?

A broader diversity of diet

What did a larger temporal opening allow for facial muscles?

The temporalis muscle could come around and attach to the outside of the skull

As the temporal opening enlarged, the zygomatic arch broadened and went out sideways. What did this provide?

Many masseter muscle attachment sites

What does a larger temporal opening provide?

A stronger bite

What did the widening of the temporal fossa into a large zygomatic arch allow for the muscle?

The muscle could come up to the temporalis region and attach to the zygomatic arch itself

What two advantages did the maxillary and palatine bones’ movement into a secondary plate provide?

Breathing while eating; Improved ability to suckle

What is the occipital condyle?

Where the cervical vertebrae articulate with your skull

What are the advantages of having a double condyle?

More strength in joint to hold a bigger, heavier skull; Allows finer head movement as there are two points of the joint

What are the disadvantages of having a double condyle?

Decreases lateral motion

What four different types of teeth does a heterodont dentition provide?

Little front teeth; Canine; Pre-molars; Molars

What can the little front teeth do?

Nip things

What can the canines do?

Tear into flesh and pierce the skull/jugular vein

What can the pre-molars do?

Slice flesh off of things

What can the molars do?

Grind food

What is the benefit of having limbs rotated 90 degrees directly under the body?

Improves locomotion efficiency

What does cervical rib loss allow? Lumber rib loss?

More range of motion in cervical vertebrae; More mobility in torso

What percentage of the population occasionally grows cervical ribs?

1-2%

What is the standard phalangeal formula in most mammals?

2-3-3-3-3

Over time, ____ phalange(s) were lost in the middle finger and _____ were lost in the ring finger.

One, two

What are the skeletal characteristics of mammals?

Single dentary bone, 3 ossicles of the middle ear, 2 occipital condyles, epiphyseal plate on many long bones

What is the epiphyseal plate?

The growth plate of the bone

What happens if a bone is broken near the epiphyseal plate and it is not set correctly?

The bone stops growing

What is determinate growth?

Growth that occurs until the epiphyseal plate closes (i.e., humans)

What is indeterminate growth?

Constant growth throughout an organism’s life (i.e., kangaroo)

What are the soft anatomical characteristics of mammals?

Hair, four-chambered heart with a functional left aortic arch, biconcave and enucleate erythrocytes, mammary glands, and a muscular diaphragm

What does biconcave mean?

Concave on both sides

What does enucleate mean?

Lacks a nucleus

What mammal has functional mammary glands regardless of the organism’s sex?

Indonesian fruit bat

What is a diaphragm?

A sheet of muscle that goes all the way across your torso and helps you breath

What is the basic pattern that teeth have? Why are the teeth shaped like this?

Tribosphenic; Mammals must eat constantly to maintain homeostasis

What are some secondary roles of teeth?

Burrowing, grooming, defense

Teeth can occur in mammals in what three bones?

Premaxilla, maxilla, dentary

What part of the tooth is included in the crown?

Anything above the gum line

What part of the tooth is included in the root?

Anything below the gum line

What structures are found within the tooth?

Dentine, pulp cavity, cementum

What does the dentine do?

Absorb shock

What does the pulp cavity do?

Contain blood vessels and nerves that maintain dentine

What does the cementum do?

Anchors teeth to the bone

What happens if a tooth is close-rooted?

Blood vessels are restricted and the tooth will die if the roots close

What happens if a tooth is open-rooted?

It continues to grow

What is the bone that holds the tooth, is the outgrowth of the dentary/maxillary/premaxillary bone, and helps determine what tooth was lost?

Alveolus

What kind of teeth do most mammals have?

Heterodont

Which teeth are anterior-most, rooted in the premaxilla/dentary bone, deciduous, has a lot of adaptations, and are important based on what you eat?

Incisors

Which teeth are posterior to the incisors, has only one pair on the upper and lower jaw, is generally unicuspid with a single root, and can be highly modified?

Canines

Which teeth are posterior to the canines and anterior to the molars, are more complex than the anterior teeth, are generally smaller than the molars, have two roots, have deciduous counterparts, and can be uni- or multicuspid?

Premolars

Which teeth are multicuspid, do not have deciduous counterparts, and have three roots?

Molars