Mesozoic Reptiles
Origin, Evolution, and Demise
Origins

Presumably, Mesozoic reptiles are
derived from early amniotes.
– Oldest amniotes are from Mississipian of
Nova Scotia: Westlothiana lizzae was
found in 1991.
– Archaeothyris (Pelycosaur) and
Hylonomus (Romeriid) were around before
the end of the Permian.
– Generally small: 15cm SVL.
Origins

Pelycosaurs were the basal stock of
synapsids. The synapsids are the
mammal-like reptiles (a single temporal
fenestra instead of 2).
– Synapsids radiated into about 24 genera.
– Small (30cm SVL) and lizard-like.
– Radiated into both carnivorous and
herbivorous lineages.
Carnivorous Synapsids

Ophiacodon and Dimetrodon.
– Both had elongated neural spines,
presumably for a large sail tham may have
been used for thermoregulation.
– Both were over 3m long.
– Ophiacodon had a narrow snout and may
have fed on fish.
– Dimetrodon was broad jawed and likely a
terrestrail predator.
Herbivorous Synapsids


Edaphosaurus was sail finned like
Dimetrodon and Ophiacodon.
It had peg-like teeth indicating
herbivory.
Synapsids


Pelycosaur lineage disappeared by the
end of the Permian.
Disappearance is perhaps a
consequence of the therapsid radiation
in the late Permian.
Paleozoic Reptiles

There are 2 anapsid lineages.
– Testudines – there is a complete lack of
fossils in the late Paleozoic.
– Captorhinids – medium sized, lizard-like
reptiles, with broad heads.
• Some level of heterodont dentition.
• Limbs are slender and long, suggesting agility.
Romeriids

These are pre-diapsid, but are not on
the anapsid line.
Diapsid Origins



Essentially, this is a consideration of the
Diapsida (Crocodilians, birds,
Rhyncocephalians, and squamates).
It is a monophyletic group.
Diapsids have 2 temporal openings or
fenestrae, separated by a single bony
arch.
Diapsid origins


The upper arch is formed by the 3pronged postorbital bone and a 3pronged squamosal bone. The lower
arch is formed by the jugal and
quadratojugal.
Both arches have been lost repeatedly
in various groups - an important point
because this loss permits kinesis.
Lepidosaur
-omorphs:
A) Permian
petrolacosaurus.
B) Sphenodon. E)
Crotaphytus. G)
Snake, G)
Amphisbaenian.
Diapsid Origins



Derived forms also have a sub-orbital
fenestra.
Earliest known diapsid is
Petrolacosaurus from the late
Carboniferous of Kansas (60-70cm TL).
Derived diapsids can be split into the
Archosauromorpha (Crocs, birds,
pterosaurs, and dinosaurs) and
Lepidosauromorpha (squamates,
sphenodon, and some marine reptiles).
Araeoscelidia

These are the first true diapsid reptiles.
– Lizard-like in body proportions.
– Dentition is homodont and suggets
carnivory.
– Dissapear in the middle Permian.
Parareptiles

Difficult to classify.
– They contain both primitive and advanced
characters.
– They do not fit neatly with either the
Romeriids, the Diapsids, or the Anapsids.

Include the following:
– Mesosaurs
• Early Permian, miniature (1m TL), marine,
gharial-like.
Parareptiles
– Millerosaurs
– Pareiasaurs
– Procolophonoids


All four of these groups appear in the
mid-Permian.
Only the Procolophonoids persisted to
the end of the Triassic.
Ichthyostegidae
Nectridia
Microsauria
Temnospondyla
Caecilia
Anura
Urodela
Crassigyrinus
Anthracosauridae
Seymouriamorpha
Captorhinidae
Procolophonidae
Pareiasauridae
Testudines
Diapsida
Dinosauria
Synapsida
2
1
3
4
Ichthyostegidae
Nectridia
Microsauria
Temnospondyla
Caecilia
Anura
Urodela
Crassigyrinus
Anthracosauridae
Seymouriamorpha
Captorhinidae
Procolophonidae
Pareiasauridae
Testudines
Diapsida
Dinosauria
Synapsida
Characters




1 = Five digit hand and foot
2 = Pedicellate teeth
3 = Astragalus and Calcaneum in Ankle
4 = Two temporal openings in skull.
5
6
7
Youngiformes
Sphenodontidae
Sauria
Serpentes
Crocodylia
Pterosauria
Theropda
Aves?
Sauropodomorpha
Ornithopoda
Ceratopsia
Stegosauria
Ankylosauria
Ichthyosaurs
Plesiosaurs
Characters



5 = Kinetic Skull
6 = Antorbital Fenestra
7 = Simple Hinge-like Ankle.
Albertosaurus
Characters for phylogeny:

1) Diapsida: Skull w/ upper and lower
temporal fenestrae w/ triradiate
postorbitan and squamosal, suborbital
fenestra, ossified sternum, complex
ankle joint between tibia and astragalus,
and 1st metatarsal less than half the
length of the 4th metatarsal.
Characters for phylogeny:

2) Sauria (Neodiapsida): Anterior
process of squamosal narrow,
squamosal mainly restricted to top of
skull, tabular absent, stapes slender,
cleithrum absent, 5th metatarsal
hooked, trunk ribs mostly singleheaded.
Characters for phylogeny:



3) Archosauromorphia: Cervical ribs w/
2 heads, concave-convex articulation
between astragalus and calcaneum.
4) Archosauria: Presence of antorbital
fenestra; triangular shaped orbit; and
serrated, laterally compressed teeth.
5) Elongated pubis and ilium, 4th
trochanter on femur.
Characters for phylogeny:



6) Crown group Archosauria: Parietal
foramen absent; no palatal teeth on
pterygoid, palatine, or vomer.
7) Crurotarsi: Astragalus forms a peg
that fits into a socket on the calcaneum.
8) Crocodylomorpha: Secondary palate
fromed from at least the maxilla.
Characters for phylogeny:

9) Ornithodira: Anterior cervical
vertebrae longer than mid-dorsals,
interclavicals absent, clavicles reduced
or absent, tibia longer than femur,
clacaneal tuber rudimentary or absent,
metatarsals bunched together and 2-4
elongated.
Characters for phylogeny:


10) Pterosauria: Hand w/. 3 short
fingers and elongate 4th finger
supporting winglike membrane, pteroid
bone in wrist, short trunk, short pelvis w/
prepubic bones.
11) Dinosauria: S-shaped swan-like
neck, forelimb less than half the length
of hindlimb, hand digit 4 reduced, etc.
Characters for phylogeny:


12) Lepidosauromorpha: Postfrontal
enters border of upper temporal
fenestra, supratemporal absent, teeth
absent on lateral pterygoid flanges, etc.
13) Sauropterygia: Elongation of
postorbital region of skull, enlargement
of upper temporal fenestra, elongate
and robust mandibular symphysis,
curved humerus, radius and ulna equal
length.
Characters for phylogeny:


14) Lepidosauria: Determinant growth
with epiphyses on articulating surfaces
of long bones, postparietal and tabular
absent, fused astragalus and
calcaneum, etc.
15) Squamata: Loss of lower temporal
bar (and quadratojugal), kinetic skull,
reduction or loss of squamosal, nasals
reduced, etc.
Petrolacosaurus and
Euparkeria.
Note the
difference
in limb
proportions
of these
early
Diapsids.
Euparkeria
a) Prolacerta w/
incomplete lower
temporal arch.
b) Typical Archosaur
Carnosaur
Ornithosuchus.
c) Femur of
Thescelosaurus
showing 4th trochanter.
Evolution
Mesozoic aquatic
reptiles. All but (d) are
Lepidosauromorphs.
a) Ichthyosaur - Upper
Jurassic
b) Plesiosaur – Upper
Jurassic
c) Nothosaur - Triassic
d) Thallatosaur –
Middle Triassic
e) Placodont – Middle
Triassic
f) Hupehsuchus –
Middle Triassic.
Pterosaurs: a)
Jurrasic
Rhamphorhynchus,
b) Cretaceous
Pteranodon, c)
Anuroganthus
(insectivore?), d)
Eudimorphodon
(small verts), e)
Dorygnathus (fish),
f) Pteradaustro
(plankton), g)
Dsungaripterus
(mollusks – prying
teeth as well as
molariform for
crushing).
Phytosaurs: become
Aquatic & croc-like
Solve breathing problem
differently than crocs –
move nares to top of skull
Ornithischia
(Bird Hips)
Thecodonts
(Teeth implanted
in alveoli, rear legs
longer than fore
Legs).
All later
Archosaurs
Saurischia
(Reptile Hips)
Theropoda: good prop & leg forward.
Saurischia
Plateosaurs
(bipedal, small heads,
long necks, long tail)
Palaeopoda
(4-legged gait)
Die out at end of
Triassic
Palaeosaurs
(bipedal – predaceous)
Good propulsion, but slow
Getting leg forward.
Sauropoda
Start in Jurassic.
Sauropoda

Most fantastic of all dinosaurs.
– Jurassic to end of Cretaceous.
– Lasted 120my
– All basically 1) robust bodies, 2) long tail,
3) long neck, 4) pillar like legs.
– Length up to 90’
• Ex Brontosaurus 70ft., 30 tons, hind leg 3
claws, foreleg 1 claw.
• Cope and Marsh
• Diplodocus 90ft., slender & only 10 tons.
• Camarasaurus, 15ft., short neck & tail.
• All quadrupeds, but rear legs longer than fore.
Derived from bipeds?
Sauropoda cont.
• Exception to the rule: Brachiosaurus – Forelimb is longer than hind-limb. TL = 75ft.,
shoulder = 20ft., head at 40ft., nostrils on top of
head therefore under water it could breath
easily. Weight = 78 tons to 100 tons (= giant
whale).
• Remember, strength of bone increass in
proportion to crossectional area, but weight
increases as the cube of the length.
• Therefore, as length doubles, limbs are 4x as
strong, but mass increases 8x.
• Implications? Tend to minimize weight or
become aquatic.
Sauropoda cont.

Did they grow rapidly?
– Large pituitary body then you can get rapid
growth.
– There is a paucity of 3m individuals.
– Endothermy? Small heads (and a wealth
of other evidence) does not support the
endothermy hypothesis.
Theropoda

There are 2 lineages:
– Coelurosauria
• To end of Cretaceous, small and medium size
(to 10ft), bipedal, hollow bones w/ air sacs.
• Split in late Cretaceous: ostrich Dinosaurs, eg.
Struthiomimus - toothless jaws, look like
ostrich, horny bill as in birds. Feed on eggs?
Theropoda cont.
– Carnosaurs
• Predators of early Jurassic.
– Antrodemus c. 30 ft.
– Megalosaurs
– Allosaurs
» large head, large jaws. Extreme fenestration. 2
tons.
– Ceratosaurus (horned - 15 ft.)
– Spinosaurus (sail)
– Deinodonts
– Gorgosaurus
– Tyrannosaurus 47 ft, 7 tons.
Ornithichians




These are herbivorous
jaws are toothless, at least in front.
Probably a horny beak.
Tend to be bipedal.
Ornithopoda
– Hypsilophodon (small, high enamel folds in
teeth, late Jurassic to Cretaceous)
– Camptosaurus (17ft., 1600 lbs, well
developed cheek teeth,
– Iguanodontidae (Iguanodon 25 ft.)
– Heterodontosaurus (has tusks, skull only
4”)
– Hadrosaurus (duck-billed dinosaurs - 30ft,
3 tons). Some had up to 600 teeth in each
jaw half, and therefore up to 2000 teeth.
Diet consisted of terrestrial vegetation,
based on mummies.
Ornithopoda cont.
– Stegosaurs (2nd group of ornithichians).
They go extinct before Tyranosaurus
shows up.
Ornithopoda
(bipedal)
Ornithichians
Ceratopsia
(quadrupedal forms)
Ankylosaurs
Stegosaurs
(quadrupedal)
Ornithopods
Ceratopsians
Ankylosaurs
Thecodonts
Aves???
Stegosaurs
Sauropods
(Brachiosaurus)
Theropods
(T. rex, Oviraptors)
There is a general trend for hind legs longer than fore.
Ornithich
-ians
Stego
saurs
&
Ankyl
osaurs
S
a
u
r
o
p
o
d
s
T
h
er
o
p
o
ds
Demise
Deinony
-chus
Archeo
-pteryx,
Ornitho
-lestes,
and
Aves.