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Reptilia
A Very Brief History
Reptiles: What are they?

Possess an amniotic
/cleidoic egg.
–
–
–
–
–
Amnion
Chorion
Allantois
Yolk sack
Reptilian/Monotreme
vs. eutherian
placentas.

Cleidoic eggs.
– Leathery egg which
is often pervious to
water.
– Egg becomes
calcified and
impervious to water.
– Necessitates internal
fertilization.
Reptiles: What are they?

Possess an
intromittent organ.
– All sauria and
serpentes have
hemipenes.
– Other groups
(Chelonians,
Crocodylians, and
Sphenodon) have a
different structure.
– Intromittent organ
increases chance of
fertilization.
– Reduces sperm loss.
– Increases probability
of reproductive
success.
– Increases cost of
reproduction for
female.
Reptiles: What are they?

Pulmonary
respiration.
– Increased surface
area in lungs.
– Changes in
circulation: 3.5
chamber heart.
– Increased rigidity of
ribs?

One occipital condyle!
– (amphibians have 2).
– Atlas and axis, with
organization of
centra.
– Pleurocentra instead
of intercentra.
– Maintain osseous
strength while
enhancing cranial
mobility
Reptiles: What are they?


2 sacral vertebrae.
Epidermal scales
– Arise as dermal evaginations.
– Contain both alpha and beta keratin. Beta
keratin is found only in reptiles.

Nerve endings in epidermis are
expanded at tips, not tapered as in all
other vertebrates.
Reptiles: What are they?





Nictitating membranes over eyes.
Intraocular muscles of iris are striated,
not smooth.
Tabular bone of skull is separated from
opisthotic.
Suborbital fenestra
Posttemporal fenestra
Reptiles: What are they?




Absence of anterior coronoid bone in
lower jaw.
Cervical vertebrae have midventral
keels.
In adults, second intercentrum fuses to
the axis.
Evolution within the skull.
What are the consequences of a
reptilian mode?

Divorce from water.
– Increased foraging options.
– Increased exposure to thermal variation.
– Higher operating temperatures.
– Increased metabolic efficiency.
Radiation of Early Amniotes

Earliest fossils come from midPennsylvanian of Nova Scotia.
– Includes relatively small aminals:
Pelycosaurs (Archaeothyris) and romeriids
(Hylonomus and Paleothryis).

Pelycosaurs became important in the
Permian: This is the lineage that
ultimately leads to the therapsida, and
then the mammals.
Westlothiana lizziae: E. Carboniferous of
Scotland - earliest known sistertaxon to
amniotes.
Radiation of Early Amniotes

The Synapsida:
– These pelycosaurs were very successful in
the Permian. There were 6 families.
Initially they were small, but later became
much larger (c. 3m+). Included good
carnivores and herbivores as well. Initially,
very lizard-like. Included Ophiacodon and
Dimetrodon.
Radiation of Early Amniotes

The Synapsida:
– Some Pelycosaurs (Ophiacodon,
Dimetrodon, and Edaphosaurus) were sail
backed. The sails were supplied with
blood, as evidenced by grooves on neural
arches – presumably for blood vessels.
How would the sails have worked as
thermoregulatory structures? What are the
other hypotheses (since not all large
pelycosaurs had sails)?
Radiation of Early Amniotes

The Synapsida:
– Pelycosaurs died out at the end of the
Permian, at the same time that the
Therapsids had an adaptive radiation.
Characters

1) Sacrum w/ more than 1 vertebra
Radiation of Early Amniotes

Paleozoic Reptiles:
– The anapsida are the first offshoots of the
reptilia that we understand (Parareptiles fit
in somewhere, but where?).
– Anapsida includes the turtles and the
Captorhinids. We have an extremely
limited fossil record of the turtles. We
know that the Testudines must have been
present, but we have no fossils. This may
be related to their habits, and consequently
poor chance of fossilization.
Radiation of Early Amniotes

Cont:
– Captorhinids are lizard-like, they are not
turtles. They had relatively big heads, and
they had teeth. Turtles do not have teeth.
Is it possible that ‘turtleness’ had not yet
evolved? What would be required for a
turtle to evolve its shell?
Radiation of Early Amniotes

Pre-diapsid Romeriids:
– Petrolacosaurus (Araeoscelida). Lizardlike. Probable good carnivore. Fore and
rear limbs of near equal size. Dissapear in
early Permian. Diapsid radiation really
takes off in the Triassic.
Radiation of Early Amniotes

Parareptiles:
– Origin is uncertain. This seems to be a
garbage group. It is possible that all forms
represent independent origins for the
Romeriids.
– Lumped with Sauropteryginas and
Ichthyosaurs.
– Adaptive radiation off all parareptiles
occusrs in the Mesozoic.
Radiation of Early Amniotes

Parareptiles:
– Mesosaurs are from the early Permian.
Marine, gharial-like, with a length of about
1m. Undulatory swimmers with laterally
compressed tails. (Think about how to
swim with a reptilian body plan).
– Millerosaurs are from the late Permian.
Lizard-like, small, similar to modern
Iguanids.
Radiation of Early Amniotes

Parareptiles:
– Pareiasaurs and Procolophonoids.
Pareiasaurs are from the mid to late
Permian, while the Procolophonoids are
from the late Permian through the late
Triassic.
– Pareiasaurs were up to 3m, columnar
limbs, robust bodies. Laterally
compressed, closely spaced teeth with leaf
shaped crowns. Probable herbivores.
– Procolophonoids were smaller and lizardlike, similar to Sauromalus in appearance.
Widely spaced crushing teeth.
Quadrupedal Ornithischians
Bipedal Ornithischians
Saurischian Theropods
Saurischian Sauropodomorphs