MAMMAL EVOLUTION
Under the new cladistic classification system, mammals are members of the
synapsid clade. They include all animals that have or have had a synapsid skull.
In general usage the term “synapsids” refers to the long-extinct species. They
were reptile-like animals but were believed to have had fur, rather than scales.
Paleontologists sometimes describe them as being “more dog than frog.”
Synapsids were the dominant terrestrial animals in the middle to late Permian
Period, 280-251 million years ago. The massive Permian extinction event resulted
in most becoming extinct, but a few survived. The reptilian archosaurs radiated to
fill the ecological vacuum, and dinosaurs quickly became the dominant animals.
THE BEGINNING
Then the tables were turned. The Cretaceous-Tertiary extinction event
wiped out the dinosaurs 65.5 million years ago. The synapsids radiated
to fill the then-vacant ecological niches.
This poster shows how modern-day mammals evolved from the first
tetrapods. This is an outstanding example of adaptive radiation, a process in which organisms diversify rapidly into a multitude of new forms,
particularly when a change in the environment makes new resources
available.
Permian
Carboniferous
299 MYA
THERAPSIDS
TETRAPODS
Parareptiles were the first known amniotes. Radiant
evolution resulted in four clades. All were extinct by
the end of the Triassic Period, 248 million years ago.
PARAREPTILES
Hylonomus lyelli
EUREPTILIA
AMNIOTES
The Eureptilia are another example of
adaptive radiation. These small lizardlike animals became extinct during
the mid Permian Period, around 260
million years ago
DIAPSIDS
Adaptive radiation resulted in several clades of basal tetrapods. The
Lissamphibia evolved from them. They first appear in the fossil record
during the mid Triassic Period, around 200 million years ago. Modernday frogs, salamanders, and other amphibians descend from them.
Anapsid Skull
The amniotic egg was one of the most
important developments in vertebrate
evolution. Eggs have a semi-permeable
shell and contain amniotic fluid that
protects the embryo from drying out.
The yolk provides a food source. It was
this self-contained means of protecting
and feeding the unborn that permitted
tetrapods to live in environments far from
water. Amniotes are a major clade. It
includes all reptiles, mammals, and birds.
The first tetrapods had an anapsid skull. It
was very solid. The only openings were for
the nasal passages and eyes.
Cynodontia continued
into the Middle Triassic.
Triadobatrachus was one of the earliest Lissamphibia. It was
a very primitive frog. Due to lack of necessary fossils, its exact
relationship to earlier anapsids has not yet been established.
Anomodontia
Dinocephalia
The synapsid skull has a single opening
(temporal fenestra) behind each eye. This
skull is the primary characteristic used to
define this clade of animals.
Synapsids are also characterized by having differentiated teeth. These include the
canines, molars, and incisors.
Diapsid Skull
Biarmosuchia
This was a group of very primitive Permian Therapsida. They may be the stem
group of an early off-shoot of the main therapsid tree. They are characterized by
weird bones on their skulls and the purpose of these bones is not known. They may
have been used as a display or maybe they were covered with glands.
This was a clade of large, early therapsids that flourished during the Middle
Permian but became extinct, leaving no descendants. There were among the largest animals of the Permian period. All dinocephalians are distinguished by having
interlocking incisors, allowing a shearing contact between upper and lower teeth.
SYNAPSIDS
The diapsid skull had two holes (temporal
fenestra) in each side of the skull. Diapsid means
“two arches”. The diapsids first appear in the fossil
record during the late Carboniferous Period, about
300 million years ago. There are at least 7,925
species of diapsid reptiles living today. They include
snakes and lizards. When the birds are added, the
total jumps to over 18,000 species.
Amniotic Egg
Triadobatrachus massinoti
The first diapsids radiated into eight clades. Only
two have species that survived to modern times. The
Lepidosauria evolved into tuataras, lizards, snakes,
and amphisbaenians. The Archosauromorpha, or
“ruling lizards,” radiated into many forms, including
dinosaurs and modern-day crocodiles. All are extinct
except the crocodiles and birds, since they descend
from the dinosaurs.
Heterodontosaurus tucki
Petrolacosaurus was a small, 16-inch long
reptile. It is the earliest known diapsid.
LISSAMPHIBIANS
The Permian extinction event wiped out the
pelycosaurs and gorgonopsia, and severely
reduced the number of species in the other
groups. Most died out during the Triassic.
These animals were mostly toothless herbivores. During the Middle Permian, they
were very diverse and included both primitive and advanced forms. Of these, only the
dicynodonts survived to the Late Permian, and they became the most successful and
abundant of all Permian herbivores.
Westlothiana lizziae
Ichthyostega is an early tetrapod genus that lived in the Late
Devonian Period, 367-362.5 million years ago. For all practical
purposes, it was an intermediate form between fish and amphibians.
MAMMALS
Cynodontia
Therocephalia
Gorgonopsia
Anomodontia
251 MYA
PELYCOSAURS
Permian Extinction Event
Biarmosuchia
Triassic
Dinocephalia
199 MYA
Ichthyostega watsoni
Around 360 million years ago, some fish began changing.
Gills became lungs and fins turned into legs. These amphibiantype animals left the sea and began to live in tidal pools and
along seashores. They were the first tetrapods. All terrestrial
vertebrates descend from them and are members of the huge
tetrapod clade. The basal (first) tetrapods reproduced through
external fertilization. The female spawned her eggs in water
where they were fertilized by the male.
Adaptive Radiation
Life is forever reinventing itself. Natural mutations result in species constantly changing into slightly different forms. The earth is also changing,
and environments replacing one another have had dramatic impacts on
the species that inhabit our planet. Some animals developed features that
permitted them to survive. Other animals lacked those features, so they
perished. Over a period of time, one species effectively replaced another.
In the case of the terrestrial vertebrates, no single species is known to
have survived for more than a few million years. This process is called
adaptive radiation. It boils down to “survival of the fittest.”
The synapsids provide a wonderful example of this process. During
the Permian Period, they quickly radiated into many new forms. This
poster shows how one group evolved into the next, reflecting the constant
changes. One group did not necessarily replace another, as some groups
coexisted. Of course, the species within the groups did change, sometimes dramatically.
Petrolacosaurus kansensis
Synapsid Skull
THERAPSIDA
Human Teeth
PELYCOSAURS
These were the more advanced
synapsids. They are divided into the
groups that follow.
These were the first synapsids. They were more
primitive than the therapsida that evolved from them.
The Nature of the Beast
Titanosuchus ferox
Kannemeyeria simocephalus
Kannemeyeria was one of the first large herbivores of the Triassic. It
was about 10 feet long, about a third larger than an ox. It had a powerful
beak and strong jaw muscles built for shearing plant material. Various
species are known from South Africa, Argentina, India, and China. Like
many animals of its time, it probably had worldwide distribution.
Dinodontosaurus turpior
Dinodontosaurus was a pig-like, 7-foot long animal that had two
large tusks in the upper jaw, an unusual feature for a plant eater. It
lived in the Middle Triassic but disappeared in the Late Triassic. It is
the most common dicynodont of the period. Many have been found
in southern Brazil. One discovery was a group of ten pups found
together, demonstrating that these animals had strategies for coexistence in a group and in caring for their offspring.
Estemmenosuchus
uralensis
Burnetia mirabilis
Fragmentary remains of Titanosuchus were found
in South Africa in 1879. It was about eight feet long.
So little was then known about the synapsids, it was
misidentified. Its name means “ferocious giant crocodile.” Scientists now believe that it was a peaceful
plant-eater.
Very little is known about this group of carnivorous therapsids. Burnetia is known only from a single holotype skull
lacking the lower jaws. It was found in South Africa in 1923.
Burnetia was characterized by bumps on the nose, above
the eyes, and at the back of the head.
Moschops capensis
Estemmenosuchus meaning “crowned crocodile,” is a genus
of large, early omnivorous therapsid. It was the largest animal of
its day, and is characterized by distinctive horns-like structures,
probably for intraspecific display. Two species of Estemmenosuchus are known, both from the Perm (or Cis-Urals) region of
Russia.
Many of the synapsids predated the dinosaurs by a hundred million years or more, and there are far fewer fossils.
Many species are known from only a skull and a few bones,
but they are enough to link them with related species. This
makes it possible to draw some conclusions. There is no
doubt that all of the synapsids walked on four legs. The size
of the skull and bones can be used to establish the size of
the animal and provide an idea as to its weight.
The first synapsids - the pelycosaurs - were very reptilelike. They were cold-blooded, had scales, and their arms
went out from their body and then down, just like modernday lizards. The therapsids that followed stood upright, like
modern-day mammals, so they were capable of running.
This suggests that they were warm-blooded. Interestingly,
some paleontologists have speculated that some of the fast,
two-legged dinosaurs were warm-blooded.
Modern mammals are covered with fur, and it is known to
have existed very early in the fossil record as fully developed, completely modern hair. But no one has been able to
determine exactly when and how it evolved.
As an unproven working premise, some paleontologists
assume that all synapsids, beginning with the therapsids,
were warm-blooded and had at least some fur.
Moschops are known from several skulls and postcranial elements
found in South Africa. It had a very thick skull which may have been
used for head-butting between competing males. There may have
been more than one species.
Archaeothyris florensis
Known from a nearly complete skeleton,
Archaeothyris is the earliest known undisputed synapsid. It was found in Nova Scotia and dates to the Carboniferous Period,
306 million years ago.
Ophiacodon is known from a
partial skeleton. It was at least
six feet in length, but various
species grew longer throughout
the Early Permian Period until
reaching twelve feet. Its skull
was deep, and it had long jaws
and many sharp teeth. It may
have eaten fish.
Dimetrodon grandis
Dimetrodon flourished during the Permian Period. There are 15 known species. It
was an apex predator that grew up 10 feet
in length. The name Dimetrodon means
“two-measures of teeth” because it had
both sharp shearing teeth and canine
teeth. Its most distinctive characteristic
was the spectacular sail on its back. It was
dense with blood vessels and was probably used to regulate body temperature.
Ophiacodon retroversus
Deuterosaurus biarmicus
MAMMALIAFORMES
By the mid-Triassic, there were many synapsids that
looked like mammals, but looks can be deceiving. The defining characteristic of mammals has long been the mammary
gland found in the females. It produces milk used to feed
babies. It was this unique feature that resulted in the name
“mammals.” Soft tissue, such as skin and glands, do not
fossilize. Modern-day mammals are the only animals to have
three middle ear bones, so they have become the defining
characteristic used to distinguish modern mammals from
earlier synapsids.
Mammals are classified into three groups: the egglaying monotremes, the pouched marsupials, and the placental mammals. They appear to have developed in that
order.
Robertia broomiana
Stahleckeria potens
Stahleckeria was a Middle Triassic dicynodont, living about 240 million
years ago. The remains of this herbivore were found in Brazil. Stahleckeria was over 12 feet long and weighed around 900 pounds. It was a
contemporary to the more common Dinodontosaurus.
THERIODONTS
The theriodonts are a major group of therapsids. They lived from
the Middle Permian to the Middle Cretaceous. There are three main
groups: gorgonopsia, therocephalians, and cynodonts.
Only eight inches in length, Robertia was a small,
primitive dicynodont and among the earlier members
of this group. Fossils were found in the South African
Karoo. Robertia had a moderately wide skull roof,
small postcanine teeth, and the palatine bone in the
roof of its mouth. It had a notch immediately in front
of the tusk-like canines on the upper jaw, which would
presumably have held tough plant matter, such as
stems and twigs, before the animal severed them with
its horny beak.
Gorgonopsia
Gorgonopsia evolved in the Middle Permian. Their name “Gorgon face” refers to the
vicious Gorgon monsters of Greek mythology. The early species were no larger than
a dog. The extinction of dinocephalians (which dominated the Middle Permian world)
led the gorgonopsians to become the dominant predators of the Late Permian. Some
approached the size of a rhinoceros. They were the only theriodont group then living,
that was totally wiped out by the Permian mass extinction. The Gorgonopsia were the
theriodont line immediately lost to the Permian extinction event.
Ivantosaurus ensifer
Jonkeria ingens
Jonkeria was a very large herbivorous (although sometimes
thought to be carnivorous dinocephalian) from the Tapinocephalus Assemblage Zone, Lower Beaufort Group, of the South
African Karoo. The overall length was 12 feet.
Deuterosaurus was the size of a modern grizzly bear. With its
long tail, it had an adult length of 15-18 ft and weighed around half
a ton. Judging from related therapsids, the short but massive legs
sprawled, much like a modern crocodile. When walking, the tail
would have swung sideways, like in modern reptiles.
Ivantosaurus is an extinct genus of therapsid that lived in
Russia during the Permian Period. It is known only from the
fragmentary remains of its jaw. It was carnivorous and may have
grown to a length of 18 feet. Two canine teeth are set side-toside in its jaw. It is possible that this therapsid had a unique
dentition (no other known animal has two sets of canine teeth),
but it is more likely that a replacement tooth was growing in next
to the old tooth about to be lost.
Therocephalia
These animals lived from the Middle and Late Permian into the Triassic 265-245
MYA, existing for approximately 20 million years. The therocephalians (“beast-heads”)
are named after their large skulls, which, along with the structure of their teeth, suggest
that they were successful carnivores.
Biarmosuchus tener
Biarmosuchus was a relatively common animal from the
Middle Permian of Eastern Russia. It is known from several
partial skeletons. They provide the basic body plan for all the
species within the group. Although present, the skull bones are
not as prominent as those of other species.
Cynodontia
Human Mammary Gland
Cynodontia or cynodonts ("dog teeth") first appear in the Early Triassic, 256 MYA. Fossils have been found on
all seven continents. The clade / group includes modern mammals and their extinct close relatives. It is likely that
cynodonts were at least partially, if not completely, warm-blooded and covered with hair, which would have insulated them and helped to maintain a high body temperature. The mammal-like structure of cynodonts hints that all
mammals have descended from a single group of eucynodonts.
During their evolution, cynodonts’ teeth changed. They were first used for catching and holding prey and swallowing whole. Then they developed specialized teeth, including molars, designed for better mastication of food,
allowing for quicker digestion. Improved hearing gave these creatures a better awareness of their environment.
The monotremes are the most primitive
mammals. They lay eggs instead of giving birth to live young like marsupials and
placental mammals. The existing monotreme
species are the platypus and four species of
echidnas.
Gorgonops whaitsi
Gorgonops were therapsids that lived about 255-250 million years ago, during the latest part of the Permian Period. They ranged from 2 to 2.5 meters long from nose to tail.
There are six known species. G. whaitsi is known from a large number of specimens
collected from the Karoo Basin in South Africa, but there is little information about the
species. Some paleontologists joking refer to it as the “what’s it?”
This little gorgonopsid was about the size of
a fox. It had a stocky build and an extremely
broad skull with a short snout.
Eutheria
Placentals
Metatheria
Marsupials
Monotremata
Egg-laying Mammals
Joey marsupial
Newly-hatched platypus
Clelandina rubidgei
Human Ear Bones
Newborn marsupials are essentially in a fetal
state. They are blind, furless, and about the
size of a jelly bean. They are called a “Joey.”
The Joey crawls across its mother’s fur to a
pouch, where it latches onto a teat for food. It
will not re-emerge for several months, during
which time it develops fully.
Human fetus
in placenta
The placenta is an organ that connects the
developing fetus to the uterine wall to allow nutrient
uptake, waste elimination, and gas exchange. The
mother’s blood not only provides food, but fights
against internal infection. The placentals are the
only animal to have this organ.
Purlovia maxima
Euchambersia mirabilis
Euchambersia was a unique therocephalian. It apparently had venom
glands connected to its canine teeth. Like snakes, the teeth of Euchambersia were adapted to inject venom to kill its prey. A recess behind each
canine tooth housed the venom gland, while grooves down the outer
sides of the canine teeth delivered the venom into the puncture wound.
When combined with its powerful jaws, it was a very efficient predator. It
hunted herbivorious dicynodonts and pareiasaurs.
Purlovia is an extinct genus of therocephalian therapsid
from the Late Permian of Russia. The type species of Purlovia
maxima was named in 2011. In comparison to other therocephalians, Purlovia has a very wide skull due to a widened
temporal region. Viewed from above, it looks roughly triangular.
It had large canine teeth and smaller buccal, or cheek teeth,
along the thick upper and lower jaws.
Cynognathus crateronotus
Cynognathus means “dog jaw.” C. crateronotus is the only known
species in the clade. It was closely related to mammals. It had an
almost worldwide distribution. It was a heavily built animal and measured a little over three feet long. Its hind limbs were placed directly
beneath the body, but the fore-limbs sprawled outwards like reptiles.
Procynosuchus delaharpeae
Procynosuchus (Greek: “Before dog crocodile”) was a one of
the first cynodonts. It dates from the Late Permian. It has many
primitive features, including some unique ones that suggest it was
aquatic. The alligator-type tail could have easily been used for
propulsion, and the relatively flat foot bones could have resulted
in paddle-like feet. The wide vertebra would have permitted eellike undulations. In short, it may have been an excellent swimmer,
an unusual trait for a terrestrial animal.
Sinodelphys szalayi
Teinolophos trusleri
Teinolophos is the first known monotreme. It appears in the fossil record two million years earlier
than the marsupials and eutherian. Their fossil
record is very sparse. This animal is only known
from a single jawbone, found in Australia.
The earliest known marsupial appeared
125 million years ago. The only known
fossil was found in China. Sinodelphys
was less than six inches long.
MODERN MAMMALS
Juramaia sinensis
Juramaia is the first known eutherian
(member of placentals’ “parent” group). It
also first appeared around 125 million years
ago. Its fossils were recently discovered in
China. It was less than four inches long.
Inostrancevia alexandri
attacking a young pareiasaur.
Inostrancevia was a genus of gorgonopsid
found in Russia. There are four known species.
I. alexandri lived 251 million years ago during
the Late Permian and is known from an almost
complete skeleton. It was the largest gorgonopsid. It had an upright stance and was as big
as a bear. The teeth were quite large. It was a
formidable predator.
Olivierosuchus parringtoni
The pareiasaurs were medium to
large herbivorous anapsid reptiles that
flourished during the Permian Period.
A311 Mammal Evolution
Copyright 2014 Feenixx Publishing
All rights reserved. May not be reproduced in whole or in part
without prior written consent.
A311 Mammal Evolution 131 as of 2015-01-14.indd 1
Olivierosuchus lived in the Early Triassic. Its
fossils were found in South Africa. It was a top
predator of the lower Lystrosaurus Assemblage
Zone (LAZ) and lived alongside other large therapsids such as Moschorhinus. A burrow cast described in 2010 from the LAZ has been attributed
to Olivierosuchus or a related therocephalian.
Carnivorous tetrapods typically create straight
burrows and often store food in them.
Pristerognathus baini
Pristerognathus is an extinct genus of therocephalian named
after the Pristerognathus Assemblage Zone of the Beaufort
Group of South African geological strata. They were discovered in 2004, and there are three species. These animals were
roughly cat-sized and are characterized by long, narrow skulls
with large canines. They are likely to have preyed on smaller
therapsids and millerettids of the time
Exaeretodon frenguelli
Exaeretodon is a genus of traversodontid cynodont. Six
species have been named, a in Brazil. These animals were
herbivores up to six feet long. They had deciduous teeth, which
means that babies could not chew, so they required specialized
parental care. Only older juveniles had permanent teeth.
Original Illustrations by Gabriel Lio and David Wenzel
Massetognathus pascuali
Massetognathus was a plant-eating cynodont known to
have lived in Brazil and Argentina during the Middle Triassic period, around 237 million years ago. It was about a foot
and a half long and had cheek teeth specially adapted to
chewing on vegetation. Massetognathus had clawed feet
and a long dog-like tail.
Macropus rufus
Ornithorhynchus anatinus
Bison bison
Printed in U.S.A.
131
1/13/2015 7:15:26 AM
© Copyright 2024 ExpyDoc
