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Lecture 21 - Class Mammalia.docx

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McGill University
Biology (Sci)
BIOL 111
Suzanne Gray

Mammals I: Reproduction Class Mammalia:  ~ 5,000 species  Endothermic (produce heat inside of them)  Advanced nervous system  Internal fertilization (females don’t lay eggs outside the body)  Unlike reptiles and fish, mammals also show heterodonty  different teeth specialized for different tasks o Example: molars and premolars for grinding things down  Specialization in teeth among mammals reflect varied diets Clicker Question: Who evolved first, mammals or birds? Mammals 4 characteristics: 1. Hair: insulation, camouflage, sensory and defense (some mammals have fur or blubber instead of hair) 2. 4-chambered heart: not unique to animals, apparent in birds and crocodiles as well  independent evolution 3. Sweat glands 4. Mammary glands: milk production Divergence between reptilian and Divergencemammalian-like amniotes occurred duringmniotes ocgurred during Carboniferous (~350 MYA) Carboniferous (~ 350 MYA) • SSkullll morphology is key characteristic used to denntfyf relatedness among amnnotes Skull morphology differences in amniotes Differences in Skull Morphology in Amniotes: a. Anapsids: turtles eye Skull types of b. Synapsids: therapsids and mammals nostriill amniotes c. Euryapsids: extinct marine reptiles d. Diapsids: birds and dinosaurs a. Anapsids (turtles) Key difference between skulls is presence, size b. Synapsids (incl. and number of temporal fenestrae = holes that Therapsds & provide more surface area for muscle attachment Mammals) Humans have arches attached by muscles, instead of holes. difference between skulls is presence, size & number of c. Euryappsdss temporal fenestrae = holes (extinct marine reptiles) thattprovide moore surface Mammals I: Reproduction The Therapsid (an early synapsid) gave rise to mammals:  Outwardly looked like a reptile  Powerful jaws  Teeth are differentiated into frontal incisors for nipping, canines for puncturing and tearing and molars for shearing and chopping food  Legs are positioned more vertically beneath their bodies than are the sprawling legs of reptiles Mammalian Evolution:  Brain vs. body size  larger the body size, the larger the brain  True mammals evolved ~225 mya (Triassic) o Early mammals were small, aboreal (lived in trees), shrew-like insectivores, nocturnal o Improvement to middle ear o Mammals coexisted with dinosaurs as little shrews with fast past life and feeding all the time  Example: Hadrocodium wui (smallest mammal) o ~195 mya (Jurassic) o Found in China o Size: 12mm, 2g o Significantly larger brain for its size o Has middle ear bones derived from jaw Mammals did not radiate until extinction of “dinosaurs”  Coexisted with dinosaurs  Mammals became a dominant group upon their extinction (~65 mya)  Diversified  Filled empty terrestrial and marine niches o Dinosaurs were taking over the niches, preventing the mammals from diversifying  Mammals evolved in the terrestrial environment and then colonized back in their marines Hermaphroditism: Simultaneous Hermaphroditism: capable of producing egg and sperm at the same point in their life cycle (either at the same time or different times) Sequential Hermaphroditism: only capable of producing egg and sperm at different points in their life cycle (i.e. organism changes sex  start life as one sex and end life as a different sex) Sexual Dimorphism: sexes appear different (i.e. size, color, morphology) Mammals I: Reproduction Mammal Reproduction: Asexual Reproduction Sexual Reproduction
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