Mammalian Evolution

•      Early Mammals

•      Mammals arose during the Mesozoic Era around 250 million years ago (mya).

•      Prior to this, the lineage known as Synapsida contained primitive organisms that led to the mammals we know today.

 

•      Problem with defining mammals

•      No single feature is useful for placing fossils with mammals

•      Instead use a suite of traits:

–     D-S jaw joint

–     Heterodont dentition

–     Complex molar surfaces

–     Complex jaw musculature

–     Well developed inner-ear

–     Various axial skeletal traits

 

•      Advantages of Mammalian traits

•      Evolution of endothermy likely generated the selective forces that favored mammalian traits.

–     Ex: Specialized jaw musculature and secondary palate was favored to increase efficiency in food processing.

–     Before mammals: Synapsida

 

•      Synapsid means “together arch,” which is a description of the skulls in this grouping.

–     Anapsid: primitive condition in which the lower jaw muscles anchored to bone and no opening at temples of skull

–     Early synapsids evolved an opening in the skull (temporal fenestra) which allowed a solid anchor for the lower jaw muscles.

–     Later synapsids show even greater expansion of this opening as well as an enlarged braincase, which is the state of modern mammals as well.

–     Pelycosaurs: primitive synapsids

•      Arose prior to mammals

•      Relatively large with dorsal sail

•      Small temporal fenestra

•      Weakly heterodont

•      No secondary palate

•      Jaw was formed by quadrate and articular bone (Q-A)

–     Therapsids: advanced synapsids

•      Arose 265 mya to dominate primitive synapsids

•      Relatively large in size

•      Diverse in form

•      Enlarged temporal fenestra

•      Partial secondary palate

•      Strongly heterodont

•      Middle ear bones developed

•      Cyanodonts: advanced therapsids

•      Arose in the late Permian and existed 70 my

•      Survived mass extinction

•      Complete secondary palate

•      Gradual enlargement of dentary bones

•      Expansion of temporal fenestra and braincase

•      Heterodont dentition

•      By late Triassic, all forms were relatively small

•      Likely represent ancestral stock of modern mammals

•      Throughout the Jurassic and Cretaceous, mammal groups were relatively unimportant.

 

•      Mammals radiated at end of Cretaceous.

•      Size-Refugium Hypothesis

–     Early therapsids: large ectotherms that evolved to become homeothermic.

–     In the Triassic, dinosaurs became dominant, which put selection pressure on mammals to become smaller in size.

–     As size decreases, heat loss increases. Selection favored endothermy (ability to produce own heat).

•      Energetically costly

•      Demands increased cardiopulmonary efficiency

 

Early Mammals

•      Mesozoic mammals include several important groups:

–     Morganucodontids

–     Triconodonts

–     Multituberculates

–     Pantotheres

•      Morganucodontids

–     Earliest known mammals

–     Small in size

–     Skull with large nasal cavity with pits and openings

–     Well developed inner ear regions

–     Large eye sockets

–     Heterodont and diphyodont

–     Primitive pectoral and pelvic girdle

–     Hadrocodium

•      Triconodonts

–     Similar to Morganucodontids except with linear cusps on molars

–     Jeholodens had a mammalian pectoral girdle with only a scapula and clavicle (coracoids and interclavicle are lost).

–     Ancestral pelvic girdle

–     Repenomamus giganticus: largest known mammal from Mesozoic

•      About 1 m long

•      Carnivorous

•      Multituberculates

–     Diverse group first seen in upper Jurassic

–     Named for unique dentition:

•      Chisel-like incisors with large diastema

•      Complex, grinding molars

–     Likely arboreal

•      Pantotheres

–     Seen during mid-Jurassic

–     Earliest mammals with bifunctional teeth

–     Tribosphenic molars