Mammalogy
Mammalian Origins
What is a mammal and why
did they evolve?
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We have a number of problems:
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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
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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
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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
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What selective pressures might have
produced the mammals?
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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
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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
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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 angular
bone.
and
Therapsid.
Mammalian Origins
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Some Pelycosaurs had significant sails,
and were also quite large.
Implications?
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There is vascularization of the vertebral
elements associated with the sails.
Large size promotes inertial homeothermy.
Mammalian Origins
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3 suborders of Pelycosaurs:
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Ophiacodontia - semiaquatic & piscivorous
Edaphosauria - terrestrial herbivores
Shenacodontia - carnivorous.
Sphenacodontia was the parent group
for the Therapsida
2 Therapsid suborders
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Anomodontia
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Herbivores
3 groups
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Dicynodontia (only
group to last through
Triassic)
Dinocephalia
Eotitanosuchia
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Theriodontia
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Carnivores
3 groups
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Gorgonopsia
Therocephalia
(secondary palate
and complex cheek
teeth.
Cynodontia
(secondary palate
and complex cheek
teeth).
Theriodont Cynodonts
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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.
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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.
Pelycosaur & Therapsid
Early & late Cynodonts
Cynodont characters
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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.
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Modification of neurocranium to relieve
stress on jaw joint - increase bite force.
Once again, what is a
mammal?
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A synapsid with a dentary - squamosal
jaw articulation. (Reptiles have a
quadrate - articular articulation).
What happened to the quadrate and
articular?
Mammalian Jaw Articulation
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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?
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We still do not understand the transition
from heterothermy
(ectothermy/heliothermy) to
endothermy
(heterothermy/homeothermy).
Evolution of endothermy
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What came first, high metabolism or
insulation?
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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?
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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?
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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
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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
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Prototheria
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Triconodonta
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Docodonta
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Morganucodontidae
Amphilestidae
Docodontidae
Allotheria
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Multituberculata
Mesozoic Mammals
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Morganucodonts (Triconodonta)
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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
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Triconodonts were
carnivorous.
Molars had 3 cusps
arranged in a single
row.
Mesozoic Mammals
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Triconodont Amphilestids
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Teeth very much like morganucodonts
Deciduous molars.
Mesozoic Mammals
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Docodonta may be
derivatives of
Triconodonts.
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Known only from
teeth and jaws.
Retained reptilian
jaw articulation.
Complex molars omnivores.
Mesozoic Mammals
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Multituberculata
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Radiation coincides
w/ angiosperms.
Herbivores w/ single
pair of procumbent
lower incisors.
Molariform teeth w/
as many as 8 conical
cusps.
Mesozoic Mammals
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Multituberculata
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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
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These are the ‘Pantotheres’. There are
2 orders:
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Symmetrodonta
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Kuehnoetheriidae
Eupantotheria
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Dryolestidae
Peramuridae
Early Therians
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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
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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
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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
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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
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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
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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
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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
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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.
Otariidae: Stellar sea lion
Otariidae: Zalophus californicus
Otariidae: Zalophus californicus
Otariidae: Zalophus californicus
Fisher: Martes pennanti
Fisher: Martes pennanti
Hyaena: paraoccipital
Hyaena: upper tooth row
Hyaenidae: Ardwolf
Hyaenidae: Ardwolf
Hyaenidae: Ardwolf
Hyaena allisphenoid (lack thereof)
Manis pentadactyla
Manis pentadactyla
Monotremes and Marsupials
Monotremata
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There are 2 families, 3 genera, and 3
species.
Currently restricted to Australia, New
Guinea, and Tasmania.
Fossils date to the early Tertiary of
Australia, but there is a Paleocene
platypus from Argentina.
Monotreme morphology
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Cranial sutures are indistinct.
Zygomatic arch is composed of the
maxilla and squamosal (instead of
squamosal and jugal).
Reduced dentary.
Adults are edentate.
Cochlea is not coiled.
Monotreme morphology
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Epipubic bones present.
Males have hornlike medial spur (w/
poison gland in platypus).
As in marsupials, there is no corpus
callosum.
Males have a baculum, but lack a
scrotum and thus have permanently
abdominal testes.
Monotreme morphology
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Produce shelled eggs, but the eggs are
not cleidoic.
The placenta is choriovitteline instead of
chorioallantoic.
Cleavage is meroblastic instead of
holoblastic.
There are mammary glands, but no
teats.
Monotreme morphology
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Platypus has no pouch, but the
echidnas do. Thus, eggs are incubated
by female curling around.
There is a cloaca, and large reptilian like ovaries. Shells are laid down in the
oviduct, which is also where the
placenta occurs. The placenta does not
contain villi.
Monotremata
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Body temperatures
are considerable
lower than in other
mammalian groups
(about 32 instead of
37). Pelage in
platypus is very
luxurious, but does
body temps still
drop quickly in
water.
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Echidnas appear to
be convergent w/
porcupines and
hedge hogs, at least
in terms of antipredator tactics.
Marsupiala
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Most (not all) marsupials have a
marsupium.
Young are born after very short
gestation periods - at very small size (c.
1% of female mass). Can you imagine
why this might be? Feldhammer
suggests this is a consequence of lack
of villi in a choriovitteline palcenta. (
Marsupiala
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Note: 2 groups have a chorioallantoic
placenta.
Is there an alternative explanation to
Feldhammer explanation?
Kangaroos also have delayed
implantation. Other marsupials may do
this also.
Marsupiala
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It is often noted that marsupials have
little diversity compared to eutherian
mammals. There is usually a
suggestion that marsupials have been
‘out-competed’ by eutherians.
Consider the range of sizes and
locomotor/foraging modes in the 2
groups, as well as the sizes of the
groups.
Marsupiala
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It is important to remember that the
marsupial fauna was once much more
extensive than it is today. In fact, the
SA marsupial fauna contained truly
fantastic forms prior to the great faunal
interchange.
Consider Thylacosmilus atrox and
Smilodon.
Marsupiala
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To understand the curent status of
marsupials, we must understand their
history.
Current distribution is essentially NA,
SA, New Guinea, Tasmania.
All new world forms belong to 3
families, of which 2 are ‘Ameridelphia’.
Marsupiala
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How did this happen?
Marsupials probably had their origin in
North America, w/ fossils dating to the
Cretaceous. WOW.
Fossils have been found in NA, SA, and
Europe.
By the Paleocene, there were at elast
12 genera in NA.
Marsupiala
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Consider the what was happening to
the tectonic plates at the end of the
Cretaceous.
Marsupiala
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So, marsupials and monotremes
became isolated in Australia before the
eutherians got there. They did well in
the new world until the great faunal
interchange. They persist today w/ 1
species in NA, and quite a few species
in SA.
Marsupial morphology
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No corpus callosum.
Epipubic bones.
Pouches.
Paired sperm in new world forms.
Marsupial morphology
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Teeth are tribosphenic, w/ extra
heels/shelfs.
Hypoconulid and entoconid are
‘twinned’ in marsupials. They are not
twinned in eutherians.
Marsupial morphology
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Vagina is bipartite - so is the penis.
(this has resulted in some confusion
amongst the local bubbas)
Choriovittiline placenta has no villi.
Scrotum is anterior to the penis (in
Notoryctes typhlops -marsupial moletestes are permanently abdominal).
Marsupial morphology
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Numbats, some didelphids and
caenolestids, and some dasyurid mice
lack pouches.
Marsupial morphology
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There are several morhphological
themes amongst marsupials:
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Teeth - diprotodonts and polyprotodonts.
Feet - didactylous and syndactylous.
Didelphimorphia
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15 genera and 63 species: only 1 is N.
of Mexico. All in didelphidae.
Contains Chironectes minimus, the only
true aquatic marsupial: females pouch
becomes water-tight during dives.
Chironectes minimus
Chironectes minimus
Didelphis aurita
Didelphidae Lutreolina crassicaudata
Paucituberculata
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Contains the Caenolestidae, which are
the shrew opposums. They have no
marsupium.
Microbiotheria
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Contains only 1 species: Dromicops
gliroides. With previous 2 orders, this
group is new world.
Store fat in tail prior to hibernation.
Primarily insectivores.
Dasyuromorphia
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3 families of Australasian marsupials.
Small to medium size, w/ many
carnivores.
Most are in family Dasyuridae.
Dasyuromorphia includes the thylacine
(now extinct). Largest marsupial
carnivore is now dasyurid tasmanian
devil.
Dasyuromorphia
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Includes Dasyuridae, Thylacinidae, and
Myrmecobiidae
Thylacinidae and Mymecobiidae are
monotypic.
Thylacines are extinct, and Numbats are
endangered.
Dasyuridae
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Includes the genus Antechinus
Shown here is the fat-tailed antechinus:
Pseudantechinus macdonnellensis.
Also shown is the Spotted-tailed Quoll:
Dasyurus maculatus.
Paramelemorphia
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These are the banidcoots and bilbies.
They are omnivorous, they are good
diggers, have short compact bodies.
They lack a clavicle (unlike all other
marsupials) and have a good patella.
They are polyprotodont.
Note modification of the pes.
Diprotodontia
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Includes 8 families, primarily herbivores.
Includes kangaroos, koalas, wombats, and
Cercartetus (pygmy possums).
All are diprotodont. Many arboreal forms
have schizodactylous forefeet.
The halux on the hind foot is usually
opposable as well.
They lack lower canines.
Diprotodontia: Phascolarctidae
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These are the koalas.
Occupy Eucalyptus woodlands. Closely related
to wombats - have vestigal tail even though
arboreal.
Cheek teeth are closed-rooted. Also unusual
for an herbivore.
Extremely long cecum. With bandicoots and
bilbies, have chorioallantoic placenta w/o villi.
Diprodontia: Vombatidae
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2 genera and 3 species of wombats.
Similar to rodents in dental morphology,
and often considered convergent w/
marmots (should be terrestrial beaver).
Single pair of upper and lower incisors
and a diastemma.
Diprotodontia: Phalangeridae
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Includes 3 species of brush-tail
possums, 1 species of scaly-tailed
possum. Note difference between
possum and opossum, and 4 genera
and 14 species of cuscuses.
Arboreal, w/ prehensile tails.
Dactylopsila trivirgata
Diprotodontia: Potoridae
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4 genera and 7 species, including
potoroos and rat kangaroos.
Very small versions of macropodids.
They have carnassial premolars.
They have a sacculated stomach, like
macropodids.
They are omnivores/herbivores.
Diprotodontia: Macropodidae
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Largest family: 11 genera and 50 extant
species.
Kangaroos and wallabies.
Browsers and grazers, some are arboreal.
Upper and lower incisors do not occlude (where
else do you see this?) They chew their cud.
Molars are hypsodont, with mesial drift as in
elephants.
Petrogale brachyotis
Diprotodontia: Petrogale brachyotis
Diprotodontia: Burramyidae
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2 genera and 5 species.
Smallest possums: 7 - 50g.
Includes Burramys parvus, and
Cercartetus.
These are the pygmy possums.
Diprotodontia: Acrobatidae
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2 genera and 2 species.
Acrobates pygmaeus is found in
Australia, and Distoechurus pennatus is
found in New Guinea.
Diprotodontia: Acrobates pygmaeus UTR
Diprotodontia: Petrogale brachyotis
Diprotodontia: Acrobates pygmaeus LTR
Diprotodontia:
Pseudocheiridae
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5 genera and 14 species. Slow moving,
arboreal folivores w/ long cecum.
Includes Petauroides volans, a very
large elbow glider.
Also includes Hemibelideus lemuroides.
Diprotodontia: Petauridae
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5 species of wrist gliders, and 4 species
of striped possums.
Dactylopsila, and Gymnobelideus
leadbeateri .
Includes Petaurus breviceps,
norfolcensis, gracilis, and australis.
Exudate feeders.
Diprotodontia: Tarsipedidae
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This is the honey possum, Tarsipes
rostratus.
They have reverse sexual dimorphism.
Nectivorous.
Notoryctemorphia
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Contains 1 family (Notoryctidae), and 1
species (Notoryctes typhlops).
Marsupial mole.
Clearly convergent w/ eutherian moles.