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 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.

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.

IchthyostegidaeNectridiaMicrosauriaTemnospondylaCaeciliaAnuraUrodelaCrassigyrinusAnthracosauridaeSeymouriamorphaCaptorhinidaeProcolophonidaePareiasauridaeTestudinesDiapsidaDinosauriaSynapsida

IchthyostegidaeNectridiaMicrosauriaTemnospondylaCaeciliaAnuraUrodelaCrassigyrinusAnthracosauridaeSeymouriamorphaCaptorhinidaeProcolophonidaePareiasauridaeTestudinesDiapsidaDinosauriaSynapsida

1

2

3

4

Characters

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

YoungiformesSphenodontidaeSauriaSerpentesCrocodyliaPterosauriaTheropdaAves?SauropodomorphaOrnithopodaCeratopsiaStegosauriaAnkylosauriaIchthyosaursPlesiosaurs

5

6

7

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 single-headed.

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.

Petrolaco-saurus 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 Jurassicb) Plesiosaur – Upper Jurassicc) Nothosaur - Triassicd) Thallatosaur – Middle Triassice) Placodont – Middle Triassicf) 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).

Thecodonts(Teeth implantedin alveoli, rear legslonger than foreLegs).

All laterArchosaurs

Saurischia(Reptile Hips)

Ornithischia(Bird Hips)

Phytosaurs: becomeAquatic & croc-like

Solve breathing problemdifferently than crocs –move nares to top of skull

Saurischia

Palaeopoda(4-legged gait)

Palaeosaurs(bipedal – predaceous)

Plateosaurs(bipedal, small heads,long necks, long tail)

Die out at end ofTriassic

SauropodaStart in Jurassic.

Good propulsion, but slowGetting leg forward.

Theropoda: good prop & leg forward.

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 – 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.

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.

Ornithichians

Ornithopoda(bipedal)

Ceratopsia(quadrupedal forms)

Stegosaurs(quadrupedal)

Ankylosaurs

Ornithopods

Ceratopsians

Ankylosaurs

Stegosaurs

Sauropods(Brachiosaurus)Theropods(T. rex, Oviraptors)

There is a general trend for hind legs longer than fore.

Thecodonts

Aves???

Ornithich-ians

Stegosaurs & Ankylosaurs

Sauropods

Theropods

Demise

Deinony-chus

Archeo-pteryx, Ornitho-lestes, and Aves.