the Early Carboniferous amniotes split into two lineages the
synapsids and the reptiles or sauropsids (anapsids & diapsids).
Traditionally, synapsids have been referred to as mammal-like
reptiles. Synapsids did not evolve from reptiles, but both
groups share a common
ancestry with basal amniotes. The non-taxonomic
term protomammals is preferred over mammal-like reptiles
1998, p. 379). Synapsids underwent an adaptive radiation
during the Permian to become the dominant land animals. Synapsids
(Class Synapsida) are traditionally divided into the pelycosaurs
(a paraphyletic group unless it includes all synapsids) and
the Therapsids (a parahyletic group unless it includes higher
synapsids and mammals) (Prothero, 1998. pp 382-383).
Pelycosaurs (Order Pelycosauria) were basal synapsids that
became important during the Early Permian. The predatory finback
Dimetrodon and the herbivorous finback Edaphosaurus are among
the best known pelycosaurs. Finbacks had large sails along
their backs made from long neural spines covered with vascularized
skin. The fins were used for thermoregulation allowing the
animal to absorb heat energy from the Sun or radiate heat from
the body. The sails of finbacks may indicate that synapsids
were not yet endotherms.
Dimetrodon means “two measures of teeth” and
refers to its heterodont condition. Unlike reptiles Dimetrodon had
shearing teeth as well as canines for tearing. Dimetrodon had
four legs and a long tail. The four limbs of Dimetrodon were
splade to the sides of the body giving it a sprawling gate.
Edaphosaurus was a primitive pelycosaur and represents one
of the earliest known tetrapod herbivores. Edaphosaurus had
teeth specialized for chopping up plant material. Herbivory
represents an important evolutionary innovation in digestion,
as it requires hosting a community of bacteria within the gut
that can help chemically process the cellulose. Prior to organisms
like Edaphosaurus all vertebrates were carnivores and detritivores.
The Edaphosaurus and Dimetrodon genera would evolve into different
species during the Permian. Both genera would evolve into species
that reached over 3 meters in length. This made Dimetrodon one of the largest predators of the time.
Pelycosaurs became extinct by the Late Permian, but a second
radiation of synapsids, the therapsids (Order Therapsida) would
come to dominate the landscape. Therapsids are more advanced
synapsids from which all mammals evolved. Evolutionary trends
included a more upright posture, with legs tucked more directly
beneath the body, enlarged temporal fenestrae to accommodate
larger, more powerful jaw muscles, and increased heterodonty
with teeth differentiated into incisors, canines, and molars.
Therapsids evolved into a variety of carnivorous and herbivorous
forms. We will focus on several groups.
Dicynodontia) were the dominant herbivores in the late Permian.
Dicynondont means “two dog teeth” and
refers to their two large tusk-like canines. Dicynodonts had
an almost toothless mouth equipped with a strong beak-like
structure. A sliding jaw joint allowed them to process tough
vegetation. The body was barrel-shaped with short tails and
legs. Dicynodonts had a semi-sprawling gate and dimensions
ranged from rat to hippo size. At a length of approximately
1 meter, Lystrosaurus was a pig-sized dicynodont of the Late
Permian and Early Triassic. Lystrasaurus is the most famous
survivor of the Late Permian mass extinction. Lystrasaurus spread worldwide and made up to 95% of the faunas in some areas.
This low biodiversity is a sure sign that a major crisis had
taken place. Lystrosaurus fossils of the same age are found
in Africa, India, China, and Antarctica providing biological
evidence that the continents were once joined into the landmass
Pangea. Dicynodonts continued to radiate in the Triassic. Kannemeyeria was a 3-meter long ox-sized dicynodont. Kannemeyeria specimens
are found in Africa, India, and South America indicating these
landmasses were once connected. Members of the family Kannemeyriidae
were the dominant herbivores of the Triassic.
Gorgonopsians (Suborder Gorgonopsia) were the dominant carnivores
of the Late Permian. Gorgonopsians were wolf and bear-sized
predators with large canines and strong jaws. Arctognathus,
a saber tooth-like gorgonopsian had the ability to open its
jaw 90 degrees. The bite of this animal secured prey with large
canines. The jaw could then shift forward, allowing the incisors
to meet and remove chunks of flesh (Benton, 2005, p. 129).
Cynodontia) form a clade if mammals are included. Cynodont
means, “dog tooth” and
refers to their dog-like teeth. Cynodonts are the most derived
having differentiated teeth, larger braincase, secondary palate,
a more upright posture, as well as an ear and jaw structure
like mammals. Cynodonts like all protomammals laid eggs. The
cynodonts were weasel to dog sized predators, although Cynognathus was
bear-sized (Prothero, 1998, p. 383). Thrinaxodon from
the Early Triassic of Africa and Antarctica was a highly derived,
carnivorous, cat-sized therapsid. Thrinaxodon had
an erect posture with strong hindlegs and was probably capable
fast. Only the thoracic vertebrae bore ribs, which had broad
flanges, so the body was divided into a thoracic and lumbar
region. The location and structure of the ribs indicate that
Thrinaxodon breathed with a diaphragm. Thrinaxodon had
a secondary palate and its teeth were set only in the margins
of the jaw
and almost fully differentiated into incisors, canines, and
molars. Thrinaxodon also possessed small pits on their snouts,
which may indicate the presence of whiskers or hair (Prothero,
2007, p. 276). So, multiple lines of evidence suggest that Thrinaxodon was
Becoming a Mammal
A variety of synapsid fossils document the evolution of early
amniotes to mammals as reflected in changes to skeletal structure.
Many of these changes in skeletal structure may reflect the
development of a new method for controlling body temperature
(Dixon, 1988, pp. 184-185). Reptiles and early protomammals
were cold-blooded or ectotherms. Ectotherms rely on external
sources for body heat. Ectotherms may seek the sun, shade,
or different water temperatures to control body temperature.
Endotherms also exhibit behaviors to adjust body temperature
but also generate heat from within the body. Endotherms like
birds, mammals, and the later therapsids rely on a fast metabolism
fueled by the quick and frequent processing of food for their
internal source of heat energy.
Teeth became increasingly differentiated a condition called
heterodont dentition. Reptiles replace their teeth continuously
throughout their life. Therapsids evolved to replace teeth
less often. Today mammals replace deciduous teeth once, while
molars are never replaced. Changing teeth less often allows
opposing teeth on the upper and lower jaws to form crests and
valleys creating a precision bite. The temporal fenestrae increased
in size and the back of the skull became larger with the sides
bowed out, which accommodated larger jaw muscles. Mammal teeth
and jaws could process food more efficiently. Well-chewed food
is digested more quickly helping to fuel a warm-blooded metabolism.
Biting, chewing and
hearing in synapsids were affected by changes in jaw structure.
Early amniotes, synapsids, and reptiles had several
making up their jaw. These organisms
hear with their lower jaws as the sound is transmitted from
joint into the middle ear. In the lower jaw the dentary bore
the teeth and was connected to
a jaw hinge with the quadrate bone of the skull. The quadrate
and articular bones not only formed the jaw hinge they also
made contact with the stirrup or stapes of the inner ear.
in synapsids include an enlargement of the dentary and a
reduction in the size of the articular and quadrate bones. The
of the dentary continued until it came into contact with the
squamosal bone, forming a new joint. The
dentary bone also developed the coronoid process, which became
an extra attachment point for jaw muscles. The
dentary/squamosal jaw joint is a characteristic of mammals. The
transition from the
articular/quadrate joint to the dentary/squamosal joint is recorded
in both the fossil record and embryological development of mammals.
Diarthrognathus (“two jaw joint”)
is a synapsid that had both jaw joints functioning side by
side, the articular/quadrate and the dentary/squamosal.
eventually became detached from the
jaw to form
ear. The quadrate came into contact with the stapes or stirrup,
which had been in the ear since the early tetrapods. The quadrate
became the incus or anvil and the articular became the malleus
or hammer. The malleus, incus, and stapes make up the auditory
ossicles that transmit vibrations from the tympanic membrane
(eardrum) to the oval window of the inner ear. A recent fossil
find provides further evidence for this sequence of events. Yanoconodon,
from the lower Cretaceous of China, had the middle ear bones
still connected to the lower jaw. So, this organism transmitted
sound with its jaw/middle ear bones (Prothero, 2007, p. 280). Early
in our embryonic development these bones start out as part of
the ear later in ontogeny (Prothero, 1998, p. 381). Interestingly,
the layout of bones in the ear of Yanoconodon is the
same as cartilage precursors in mammalian embryos before the
ear and jawbones separate. The ontogeny (development of an organism)
can sometimes reflect the
(evolutionary history) of a species.
The idea that ontogeny
recapitulates phylogeny was first formulated as the Biogenetic
Law by German zoologist
in 1866. Haeckel’s law stated that an organism’s
development followed the same path as its evolutionary history.
Although the original form of his law is now rejected, embryological
development is used to help build evolutionary histories of
A faster metabolism not only requires more efficient methods
of processing food, it also requires better breathing. An abrupt
reduction in the rib cage of later protomammals indicates that
a diaphragm closed off the front part of the body cavity, which
houses the lungs and heart. The diaphragm and upper body cavity
allows larger lungs to be filled and emptied more rapidly.
Thus, greater amounts of oxygen enter the bloodstream. Quicker
digestion and increased availability of oxygen are needed to
accelerate metabolism. Breathing was also enhanced by the evolution
of a secondary palate, which is a shelf of bone that separates
the air passage from the mouth. In reptiles and early amniotes,
the nasal passage opens in front of the mouth, so they must
hold their breath while swallowing. Later therapsids and mammals
were able to breathe while retaining food in their mouth, allowing
them to chew food for longer periods of time. Thus, the secondary
palate helps animals to better process their food for quicker
Therapsid limbs and joints were modified to change from a
semi-sprawling gait to an erect gait. The pelvic and pectoral
girdles along with the limbs became structured to tuck the
limbs under the body. The backbone came to support an up and
down flexure instead of a side-to-side bending. These limb
improvements had a greater potential for fast movement. Several
mammalian characteristics are not so well preserved in the
Mammals are often defined
as homeothermic endothermic amniotes with hair. Mammals have
a high metabolism, a four-chambered
hear, a diaphragm, and a sophisticated brain with an enlarged
neocortex. Most mammals, except for the egg-laying platypus
and echidna, bear live young, which females nurse with milk.
Mammals require greater parental care than reptiles. Mammals
grow rapidly after birth, but slow to a terminal adult stage.
This is in contrast to most other animals, which grow continuously
throughout their lives. These features do not fossilize well,
but skeletal structures may provide indirect evidence for
mammalian physiology and reproduction (Prothero, 1998, p.
380). The braincase
can be a clue to the enlargement of the neocortex. Fossil
bones can leave indicators of an organism’s growth
rate. And as we have already discussed, clues to metabolism
be revealed by skeletal structures.
The largest mass extinction
occurred at the end of the Permian around 250 million years
ago. Seventy five
percent of tetrapod
families went extinct. At the same time 50% of marine families
died out. These family level losses indicate that 80 to 96%
of all species went extinct (Benton, 2005, p. 133). Some believe
the mass extinction could have been caused by an asteroid impact,
like that hypothesized for the KT event. The Siberian Traps
occurred at this time and represent a flood basalt that may
have changed the Earth’s atmosphere and temperature.
Some believe that an asteroid impact could have triggered the
massive volcanic activity. Whatever the cause therapsids, like
other organisms, were hit hard. A few therapsids would recover
from this mass extinction and come to dominate the landscape
until the Mid-Triassic when rapidly evolving archosaurs, especially
the dinosaurs, would displace them. Therapsids declined and
went extinct in the Cretaceous, but not before giving rise
to the first mammals. Mammals would stay shrew size throughout
the 150 million years of the dinosaur reign. As the dinosaurs
declined at the end of the Cretaceous mammals would diversify
and come to dominate the landscape, giving their synapsid ancestors
the “last ghostly laugh” (Dixon, 1988, p. 184).