mammalogy mammalian origins. what is a mammal and why did they evolve? we have a number of problems:...
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Mammalogy
Mammalian Origins
What is a mammal and why did they evolve? We have a number of problems:
Ectothermy is very efficient. Reptiles experience an explosive
adaptive radiation. How can we explain the emergence
of the mammals. What is it about mammals that enabled them to persist?
Mammalian Characteristics Mammals are endothermic, at
least to some extent (but then, so are some reptiles).
Mammals have fur, presumably to increase the size of the boundary layer. However, the evolution of fur is problematic.
Mammals nurse their young
Mammalian Characteristics Mammals posses a synapsid skull
rather than a diapsid or anapsid skull. Mammals have a single bone in the
lower jaw (Dentary), while reptiles have 7.
Mammals have 3 inner ear bones rather than the reptilian number of 1.
Mammalian Origins What selective pressures might
have produced the mammals? Mammal-like reptiles appeared before
the end of the Permian, during the first amphibian adaptive radiation.
This amphibian radiation was very different from the modern amphibians of today.
Mammalian Origins The vegetation, climate, and tectonic
plate positions were very different as well.
The mass extinction at the end of the Permian (95% of all species lost) might have resulted from the formation of Pangea, and concomitant climate/vegetation change.
Some mammal-like reptiles survived the extinction event.
Mammalian Origins
Following the end-Permian mass extinction, we have the emergence of the Dinosaurs.
Pelycosaurs did not survive the mass extinction, but their descendants, the Therapsids persisted.
Pelycosaur (dimetrodon -Sphenacodontia)w/ reflected lamina of angularbone.
and
Therapsid.
Mammalian Origins Some Pelycosaurs had significant
sails, and were also quite large. Implications?
There is vascularization of the vertebral elements associated with the sails.
Large size promotes inertial homeothermy.
Mammalian Origins 3 suborders of Pelycosaurs:
Ophiacodontia - semiaquatic & piscivorous
Edaphosauria - terrestrial herbivores Shenacodontia - carnivorous.
Sphenacodontia was the parent group for the Therapsida
2 Therapsid suborders Anomodontia
Herbivores 3 groups
Dicynodontia (only group to last through Triassic)
Dinocephalia Eotitanosuchia
Theriodontia Carnivores 3 groups
Gorgonopsia Therocephalia
(secondary palate and complex cheek teeth.
Cynodontia (secondary palate and complex cheek teeth).
Theriodont Cynodonts Secondary palates Complex cheek teeth AND, mammal-like origins and
insertions of jaw musculature. Only Cynodonts survived into the
Jurassic. This lineage gave rise to the Mammalia in the Triassic.
Mammal-like Cynodont characters. Secondary palate, Jaw musculature,
complex cheek teeth. Tricuspid, double-rooted cheek
teeth. Increased size of dentary and
reduction of post-dentary bones. Development of glenoid fossa on
the squamosal.
Early & late CynodontsPelycosaur & Therapsid
Cynodont characters Regionalization of the vertebral column. Atlas/axis complex. Modified pectoral and pelvic girdles. Restriction of ribs to thoracic vertebrae. Erect posture. Evoution/modification of masseter. Development of zygomatic arch.
Cynodont characters.
Modification of neurocranium to relieve stress on jaw joint - increase bite force.
Once again, what is a mammal? A synapsid with a dentary -
squamosal jaw articulation. (Reptiles have a quadrate - articular articulation).
What happened to the quadrate and articular?
Mammalian Jaw Articulation Consider the tremendous implications
of the new jaw articulation. What does it mean for hearing? What does it mean for bite force?
We still see evidence for the role of the jaw in sound conduction in modern mammals (whales).
Again, what selective pressures led to mammals?
We still do not understand the transition from heterothermy (ectothermy/heliothermy) to endothermy (heterothermy/homeothermy).
Evolution of endothermy What came first, high metabolism or
insulation? If metabolism came first, then there
would be significant heat loss to the environment, and endothermy would be truly expensive.
If insulation came fisrt, Bartholomew has shown that they would never be able to heat up via conduction or convection.
Polyphyletic or monophyletic? There are 2 major lineages of extant
mammals: Prototherians and Therians.
They share some typical mammalian characters (single dentary, fur, endothermy) but differ markedly in others (teeth, girdles, urinogenital ducts, and development.
Polyphyletic or monophyletic? If mammals are polyphyletic, then
the defining characters for mammals evolved twice independently.
If mammals are monophyletic, then prototheres are clearly very unique.
Unfortunately, the fossil evidence is very fragmentary, and monotremes have no teeth as adults.
Early Mesozoic Mammals So, modern mammals have a
dentary/squamosal jaw articulation. They are diphyodont, rather than
the reptilian condition. Mesozoic mammals were mouse-
size, they are relatively rare in the fossil record, and they were probably nocturnal. WHY?
Mesozoic mammals Prototheria
Triconodonta Morganucodontidae Amphilestidae
Docodonta Docodontidae
Allotheria Multituberculata
Mesozoic Mammals Morganucodonts (Triconodonta)
Dentary / squamosal w/ some involvement of the quadrate and articular.
Heterodont w/ premolars and molars. Large cochlear region 2 cervical vertebrae 2 occipital condyles. Mammalian vertebrae, girdles, and
posture
Mesozoic Mammals Triconodonts
were carnivorous. Molars had 3
cusps arranged in a single row.
Mesozoic Mammals
Triconodont Amphilestids Teeth very much like
morganucodonts Deciduous molars.
Mesozoic Mammals Docodonta may
be derivatives of Triconodonts. Known only from
teeth and jaws. Retained reptilian
jaw articulation. Complex molars -
omnivores.
Mesozoic Mammals Multituberculata
Radiation coincides w/ angiosperms.
Herbivores w/ single pair of procumbent lower incisors.
Molariform teeth w/ as many as 8 conical cusps.
Mesozoic Mammals Multituberculata
For anterior molars, cusps arranged in triangular patterns.
Posterior molars had cusps arranged in rows.
Lower posterior molars were large & used for shearing (as in some Burramyids).
Early Therians These are the ‘Pantotheres’. There
are 2 orders: Symmetrodonta
Kuehnoetheriidae Eupantotheria
Dryolestidae Peramuridae
Early Therians Symmetrodont Kuehnoetheriids
were small carnivores and insectivores in the Triassic, but radiated during the Cretaceous.
Molars has 3 main cusps in a triangular arrangement. This is the precursor of the tribosphenic tooth of modern mammals.
Tribosphenic Teeth It is triangular. This
upper molar is the ‘Trigon’ and shows the Protocone to be medial, and the Paracone to be lingual and anterior. The ‘conules’ are smaller cones.
Tribosphenic Teeth The lower molar is
the ‘Trigonid’ - the id suffix denotes the dentary. Here, the Protoconid is lateral, not labial. The tooth also has a ‘Talonid’ or heel.
Tribosphenic Teeth When upper and
lower molars occlude, the Protocone meets the Talonid of the lower molar. Thus, food is both crushed and sheared by the cones and conids.
Tribosphenic Teeth The tritubercular teeth are the basis
for all modern mammalian teeth, and can still be seen in insectivores and marsupials.
Humans (and many other mammals) have quadritubercular teeth through the addition of a hypocone posterior to the protocone.
Tribosphenic Teeth
Things get really weird when we look at the teeth of rodents and a nubmer of other groups. However, these patterns are highly derived, and homologies can be established.
Cenozoic Radiation of Mammals Major radiation of mammals occurs
after the end-Cretaceous mass extinction.
Did an asteroid wipe out the Dinosaurs?
Did continental drift and climate change wipe out the Dinosaurs?
Cenozoic Radiation
Most modern mammal orders are extant by the Eocene, and most modern families are there by the Miocene.
Clearly, the mammalian adaptive radiation has just gotten underway.
Didelphidae Chironectes minimus
Didelphidae Chironectes minimus: HF
Didelphidae Didelphis aurita
Didelphidae Lutreolina crassicaudata
Diprotodontia: Petrogale brachyotis
Acrobates pygmaeus
Petrogale brachyotis
Petrogale brachyotis
Fisher: Martes pennanti
Fisher: Martes pennanti
Hyaena paraoccipitals
Hyaena upper tooth row
Hyaenidae: ardwolf
Hyaenidae: ardwolf ltr
Hyaenidae: ardwolf skull
Hyaena: lack of allisphenoid
Otariidae: Stellar sea lion
Otariidae: Zalophis californicus
Otariidae Zalophis californicus allisphenoid
Zalophis californicus UTR