Mesozoic Reptiles

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Mesozoic Reptiles

• Origin, Evolution, and Demise

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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.

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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.

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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.

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Herbivorous Synapsids

• Edaphosaurus was sail finned like Dimetrodon and
  Ophiacodon.
• It had peg-like teeth indicating herbivory.

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Synapsids

• Pelycosaur lineage disappeared by the end of the
  Permian.
• Disappearance is perhaps a consequence of the
  therapsid radiation in the late Permian.

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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.

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Romeriids

• These are pre-diapsid, but are not on the anapsid
  line.

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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.

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Diapsid origins

• The upper arch is formed by the 3-pronged
  postorbital bone and a 3-pronged 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.

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Lepidosaur-omorphs A) Permian petrolacosaurus.
B) Sphenodon. E) Crotaphytus. G) Snake, G)
Amphisbaenian.
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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).

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Araeoscelidia

• These are the first true diapsid reptiles.
• Lizard-like in body proportions.
• Dentition is homodont and suggets carnivory.
• Dissapear in the middle Permian.

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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.

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Parareptiles

• Millerosaurs
• Pareiasaurs
• Procolophonoids
• All four of these groups appear in the
  mid-Permian.
• Only the Procolophonoids persisted to the end of
  the Triassic.

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Ichthyostegidae Nectridia Microsauria Temnospondyl
a Caecilia Anura Urodela Crassigyrinus Anthracosau
ridae Seymouriamorpha Captorhinidae Procolophonida
e Pareiasauridae Testudines Diapsida Dinosauria Sy
napsida
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Ichthyostegidae Nectridia Microsauria Temnospondyl
a Caecilia Anura Urodela Crassigyrinus Anthracosau
ridae Seymouriamorpha Captorhinidae Procolophonida
e Pareiasauridae Testudines Diapsida Dinosauria Sy
napsida
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Characters

• 1 Five digit hand and foot
• 2 Pedicellate teeth
• 3 Astragalus and Calcaneum in Ankle
• 4 Two temporal openings in skull.

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Youngiformes Sphenodontidae Sauria Serpentes Croco
dylia Pterosauria Theropda Aves? Sauropodomorpha O
rnithopoda Ceratopsia Stegosauria Ankylosauria Ich
thyosaurs Plesiosaurs
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Characters

• 5 Kinetic Skull
• 6 Antorbital Fenestra
• 7 Simple Hinge-like Ankle.

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Albertosaurus
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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.

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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 single-headed.

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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.

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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.

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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.

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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.

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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.

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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.

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Petrolaco-saurus and Euparkeria.Note the
difference in limb proportions of these early
Diapsids.
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Euparkeria
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a) Prolacerta w/ incomplete lower temporal
arch.b) Typical Archosaur Carnosaur
Ornithosuchus.c) Femur of Thescelosaurus showing
4th trochanter.
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Evolution
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Mesozoic aquatic reptiles. All but (d) are
Lepidosauromorphs.a) Ichthyosaur - Upper
Jurassicb) Plesiosaur Upper Jurassicc)
Nothosaur - Triassicd) Thallatosaur Middle
Triassice) Placodont Middle Triassicf)
Hupehsuchus Middle Triassic.
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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).
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Solve breathing problem differently than crocs
move nares to top of skull
Phytosaurs become Aquatic croc-like
Ornithischia (Bird Hips)
Thecodonts (Teeth implanted in alveoli, rear
legs longer than fore Legs).
All later Archosaurs
Saurischia (Reptile Hips)
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Theropoda good prop leg forward.
Saurischia
Plateosaurs (bipedal, small heads, long necks,
long tail)
Palaeopoda (4-legged gait)
Die out at end of Triassic
Sauropoda Start in Jurassic.
Palaeosaurs (bipedal predaceous)
Good propulsion, but slow Getting leg forward.
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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?

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Sauropoda cont.

• Exception to the rule Brachiosaurus Fore-limb
  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.

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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.

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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?

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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.

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Ornithichians

• These are herbivorous
• jaws are toothless, at least in front.
• Probably a horny beak.
• Tend to be bipedal.

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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.

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Ornithopoda cont.

• Stegosaurs (2nd group of ornithichians). They go
  extinct before Tyranosaurus shows up.

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Ornithopoda (bipedal)
Ceratopsia (quadrupedal forms)
Ornithichians
Ankylosaurs
Stegosaurs (quadrupedal)
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Ornithopods Ceratopsians Ankylosaurs Stegosaurs
Sauropods (Brachiosaurus) Theropods (T. rex,
Oviraptors)
Thecodonts
Aves???
There is a general trend for hind legs longer
than fore.
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Ornithich-ians
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Stegosaurs Ankylosaurs
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Sauropods
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Theropods
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Demise
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Deinony-chus
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Archeo-pteryx, Ornitho-lestes, and Aves.
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