Lecture 18

What are the major evolutionary trends in the synapsid lower jaw and associated muscles?

• Lower Jaw: Reduction from multiple bones to a single bone (dentary).

• Musculature: The single adductor mandibulae muscle differentiated into the temporalis and masseter (superficial and deep).

• Articulation: Shift from the quadrate-articular joint to the dentary-squamosal joint.

Describe the role and insertion of the Adductor mandibulae in basal synapsids.

It is a single large muscle that originates in the temporal region and inserts on the coronoid bone of the lower jaw, pulling the jaw up and back in a simple scissor-like motion.

What are the specific roles of the Temporalis and Masseter muscles in jaw movement?

• Temporalis: Pulls the lower jaw up and back.

• Superficial Masseter: Pulls the jaw up and forward.

• Deep Masseter: Pulls the jaw up, back, and sideways (enabling complex chewing).

Contrast Homodonty vs. Heterodonty.

• Homodonty: All teeth are a similar conical shape (ancestral condition).

• Heterodonty: Teeth are differentiated into types (incisors, canines, premolars, molars) with unique functions (derived mammalian condition).

Contrast Polyphyodonty vs. Diphyodonty.

• Polyphyodonty: Multiple sets of replacement teeth throughout life (ancestral).

• Diphyodonty: Only two sets of teeth—"milk" teeth and permanent adult teeth (mammalian trait).

What are the two primary layers of the mammalian epidermis?

1. Stratum corneum (outer layer).

2. Stratum germinativum (deeper germinal layer).

What structural protein characterizes mammalian integumental appendages?

Alpha-keratin (used for hair, whiskers, claws, nails, hooves, and horns).

Define the components of a Pilosebaceous unit.

A unit of the pelage (fur) comprising one guard hair and multiple underfur hairs.

What is the function of the Arrector pili muscle?

A smooth muscle that causes piloerection (goose-bumps) for insulation or communication in response to cold or stress.

Name the three types of mammalian exocrine glands and their primary products.

• Sebaceous glands: Produce sebum (oil) for lubrication and waterproofing.

• Apocrine glands: Produce volatile secretions for chemical communication/scents (and modified into milk/earwax).

• Eccrine glands: Produce watery sweat for evaporative cooling (primates) or friction/tactile interaction.

Which glands are associated with hair follicles?

Sebaceous and apocrine glands. Eccrine glands are generally not associated with hair follicles.

What is the evolutionary origin of Mammary glands?

They are highly modified apocrine glands.

How does the secondary palate facilitate suckling?

It allows a mammal to create a seal to swallow liquids (like milk) while simultaneously breathing, decoupling the paths of food and air.

What characterizes mammalian integument?

1. Thick, differentiated epidermis

2. Keratinous appendages

3. Epidermal glands

What are the locations of the mammalian skin glands?

1. Sebaceous glands - distributed over entire body (close to base of follicle) and commonly found in lanolin (sheep wool)

2. Apocrine glands - more restricted distribution than sebaceous glands

3. Eccrine glands - for most mammals they are found only on sole of feet/hands, prehensile tails, and other surfaces that interact with directly with environment but found all over the body in primates

What are the two functions of milk?

1. provide nutrition

2. provide immunity

What are the advantages of lactation?

• production of offspring separated from seasonal food supply

• mammals can store fat when food is abundant and convert it to milk later

• level of parental care can be reduced

Contrast Isognathous vs. Anisognathous jaws.

• Isognathous: The upper and lower jaws are the same width. When the mouth closes, the teeth on both sides of the jaw meet simultaneously. This is common in many reptiles and some basal mammals.

• Anisognathous: The upper jaw is wider than the lower jaw. Teeth can only meet on one side of the mouth at a time, requiring a lateral (side-to-side) chewing motion. This is the derived condition in most modern mammals, especially herbivores like ungulates.

Contrast Scent glands vs. Sweat glands.

• Scent Glands (Apocrine): Primarily used for communication (marking territory, defense, or social signaling). They produce thick, oily secretions that are often associated with hair follicles and are broken down by bacteria to produce odors.

• Sweat Glands (Eccrine): Primarily used for thermoregulation. They produce a watery secretion (mostly salts and water) that evaporates on the skin surface to dissipate heat. In many mammals, these are limited to specialized areas like the soles of feet or the nose.