Vertebrate Paleontology

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Angdredd
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Vertebrate Paleontology
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2014-03-21 13:10:35
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The age of Dinosaurs
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    • faulty stegosaurus reconstruction
    • plates flattened over back
    • single row of plates (Marsh 1891) Dorsal plates offered little protection - not attached to toher bones. High position, overlapping arrangement, and vascular nature suggests thermoregulation
    • Colorado skeleton with plates in life position, showing degree of overlap
  1. Stegosaurus
    Plates on backs of Stegosaurus may have served as temperature-regulating devices, i.e., "radiators" to dissipate body heat, or as "solar panels" to catch sun's rays.
    • old world stegosaurids
    • Dorsal plates of Huayangosaurus and old world stegosaruids spiked-probably armor rather than thermoregulatory.
    • Spiked tails probably defensive in all stegosaurs.
  2. Ankylosaurs
    • slowest, most heavily armored
    • Fused vertebral rod in backs
    • Low, heavily armored heads
    • Leaf-shaped, non-interlocking teeth
    • Wide body supported by expanded ribs
    • Round or square armor plates covered body
    • Robust limbs, quadrupedal
    • Originated in early Jurassic, most diverse and abundant during Cretaceous
    • two ankylosaur groups
    • Nodosaurids (top) - narrow skull, less posterior armor, lack of ossified tail tendons or tail club.
    • Some had spines in their armor.
    • Ankylosaurids (bottom)
    • nodosaurids
    • some nodosaurids had spikes in their back armor
    • Gastonia - early Cretaceous of Utah
    • exhibits a mixture of nodosaurid and ankylosaurid characters
    • ankylosaurid features
    • wide, flat skull with horns in the posterior corners
  3. Ankylosaurid characters
    • ankylosaurids - complex nasal passages, unlike primitive condition in nodosaurids. May have been vocalization (like hadrosaurs).
    • Tail clubs composed of two large (anterior) and two small (posterior) dermal plates.
    • Swinging club would deter predators.
  4. Thyreophoran Dinosaurs
    • Stegosauria
    • -Huayangosaurids
    •   -Huayangosaurus - middle Jurassic, China
    • -Stegosaurids
    •   -Stegosaurus - Late Jurassic, North America
    • Ankylosauria
    •   -Gastonia - Early Cretaceous, North America
    • -Nodosaurids
    •   -Nodosaurus - Late Cretaceous, North America
    • -Ankylosaurids
    •   -Ankylosaurus - Late Cretaceous, North America
  5. Marginocephalia
    • Ceratopsians and Pachycephalosaurs
    • Characters-
    • Frill - bone shelf projecting from back of skull
    • Primarily cretaceous close relatives of Ornithopods
  6. Ceratopsian characters
    • Unique rostral bone
    • Narrow beak
    • Vaulted palate
    • Deep flaring jugals
    • Frill on skull rear
    • Psittacosaurus ("parrot lizard")
    • most primitive ceratopsian (Early Cretaceous of Asia)
    • still bipedal as are ornithopods
    • Protoceratopsids
    • Intermediate between Psittacosaurus and Ceratopsids
    • Significant frill development, mostly quadrupedal
    • Very abundant in late Cretaceous of Mongolia
    • sexual dimorphism
    • Two skull and frill types in Protoceratops
    • Fossils abundance in desert suggests gregariousness and the same species.
    • ?=males and females different
    • Ceratopsians show tendency for variation in skull shape
    • Ceratopsids large - especially large heads, large nostrils, and great variety of large frills and horns.
    • Known only from late Cretaceous of North America.
  7. Two ceratopsid groups
    • long, low face                 Short, high face
    • long frill                         Short frill
    • Long postorbital              Long nasal horn
  8. Ceratopsians
    • Triceratops a ceratopsine
    • Monoclonius a pachyrhinosaurine
    • Protoceratops a protoceratopsid
    • Triceratops
    • one of the the last surviving dinosaurs
    • Nearly thirty feet long, nine foot long skull
    • Typical large ceratopsian - best known
    • Unusual in having no openings in frill
    • First four vertebrae fused to support large skull
    • Ten sacral vertebrae and ossified hip tendons
  9. Function of Frills
    • (ceratopsian cheek teeth formed a complex battery similar to hadrosaurs)
    • Attachment of large jaw muscles
    • Display for identifying species, finding mates
    • Possibly thermoregulatory
    • Defense against predators or competitors
    • Highly vascularized with blood vessels like Stegosaurus plates - may have served to absorb or dissipate solar energy.
  10. Ceratopsian stance
    • Forelimbs of ceratopsians - sprawling or semi-sprawling posture.
    • But only known trackways suggest forelimbs were upright
    • Pachycephalosaurs
    • Known mostly from inland areas away from water bodies.
    • Previously grouped with ornithopods
    • Retain short forelimbs and long hindlimbs
    • Thickened skull bone (good preservation!)
    • Head oriented down, dome pointed forward
    • Long, low ilium in the pelvis
    • Unique locking vertebrae
    • Long tail with ossified tendons
  11. Pachycephalosaurids
    • high, thick, domed skulls
    • supratemporal openings filled in with bone
    • Cretaceous of Asia, Europe, and North America
  12. Homalocephalids
    • flat skull roof of even thickness
    • Late Cretaceous of Asia
  13. Features of pachycephalosaurs adapted to resist impact
    • thickened bone in dome of skull to protect brain
    • Shortened skull base
    • low inclination of skull relative to neck vertebrae
    • Back of skull widely expanded
    • Strengthened vertebral column
    • Reinforced upper lip of acetabulum
  14. Head butting
    • head structure suggests head-butting and flank-butting much like modern sheep.
    • i.e., social interaction and hearding behavior
    • Typical behaviors for inland, plains animals

    other explainations: if dome not strong enough to support head butting may have supported head ornament for display.
  15. Marginocephalans
    • Ceratopsia
    • --Psittacosaurus - early Cretaceous, Asia
    • -Protoceratopsians
    • --Protoceratops - late cretaceous, Mongolia
    • -Ceratopsids - late Cretaceous, N. Am.
    • --Subfamily Pachyrhinosaurinae (centrosaurus)
    • --Subfamily Ceratopsinae (Triceratops)
    • Pachycephalosauria
    • -Homalocephalids
    • --Homalocephale - Cretaceous, mongolia
    • -Pachycephalosaruids
    • --Stegoceras - Late Cretaceous, North America
  16. Dinosaur skin
    • Skin impressions known from theropods, ornithopods, and ceratopsans.
    • Scales similar to modern reptiles.
    • A few small theropods from China appear to have feathers.
    • Color speculative, not preserved in fossils.
  17. Feathered Dinosaurs?
    • Arguments for feathered dinosaurs pervasive based on circumstantial evidence:
    • -Birds=dinosaurs closest living relatives, and they have feathers
    • -lack of skin impressions in many dinosaur groups argues fro feathers because feathers are far less likely to be preserved than scales.
    • Feathers on dinosaurs for insulation and possibly display. This bears on the endothermy question because it would be a waste to invest in endothermy without insulation.
  18. Dinosaur soft anatomy
    • speculation on dinosaur soft tissues based on muscle attachment points can aid in reconstruction of musculature.
    • Collection and study of hadrosaurs
  19. Dinosaur weight and proportion
    • ornitholestes - small coelurosaur from late Jurassic.
    • Tyrannosaurus - large coleurosaur from late Cretaceous.
    • When similar-shaped animals of different body size scaled to same size, their skeletons look very different!
    • Body mass increases by cube of linear size; bone strength increases by square of linear size.
    • So bones must be relatively larger in larger animals to support the mass of the body.
    • Weight estimated by measureing x-sections of limb bones to see how much weight they could hold.  All limbs must be accounted for. Posture also a factor and can introduce uncertainty. Dinosaurs large rthan living land animals, so also uncertain whether comparisons with modern species are accurate.
    • Living animal limb bone cross sections good means to estimate body weight because bones are just strong enough for support.
    • Another estimate of dinosaur weights is to make scale models to see how much water they displace to provide body volume. volume=multiplied by 0.9kg/liter (density of living crocodiles) to give weight extimate.
    • Colbert and Bakker published dinosaur weights using this method. Muscles and other soft tissues volumes must be estimated for each dinosaur. Their extimates differ. Bakker believes in lean, fast dinosaurs!
  20. Growth Rates
    • indeterminat growth vs determinate growth
    • Tyrannosaurus bone histology suggests rapid growth rates as in mammals and birds
    • Large sauropods would require over a century to reach adult size at typical reptilian growht rates.
    • Growth rates of living vertebrates closely related to metabolic rates.
    • Endothermic birds and mammals with high metabolism have highest growth rates for their body size.
  21. indeterminate growth
    modern reptiles with low metabolism grow throughout life but at a slow rate.
  22. Determinate growth
    modern mammals and birds rapidly grow to adult size then stop.
  23. Altricial
    born naked, helpless and with eyes closed.
  24. Precocial
    born more mature with insulation (fur or down) and eyes open.
  25. Feeding habits
    • easy to distinguish carnivores from herbivores using teeth.
    • -Carnivore teeth serrated blades - killing and slicing prey.
    • -Herbivore teeth adapted for plucking and/or grinding tough plants.
    • -A few are harder to classifly - ?=omnivorous.
    • Except in cases where stomach contents have been examined, it is difficult to tell exactly which plants or animals dinosaurs fed on.
    • Neck structure can b used to estimate the vertical feeding ranges of herbivorous dinosaurs.
  26. Locomotion
    • Virtually all dinosaurs were ground-dwelling walkers and runners.
    • Speculations about aquatic dinsaurs have been mostly discarded, through webbed feet on late Cretaceous hadrosaurs from alberta suggest they could swim.
    • The dromaeosaur Microraptor from the early Cretaceous of China may have been arboreal (lived in trees).
  27. Dinosaur eggs
    • Discovered in France in 1869
    • Many discovered in Mongolia in 1923
    • Up to 30cm long in sauropods
    • Various shapes
    • Up to 20 eggs/clutch
    • some contain fossilized embryos
  28. Dinosaur nests at Egg mountain, Montana
    • Circular to oval nexts and adult skeletons and hatchlings of hadrosaur Miasauria and hypsilophodontid Orodromeus, late Cretaceous.
    • Jack horner believes eggs covered by thin layer of vegetation and soil for protection and warmth becasue adults were too large to incubate eggs like birds.
    • Nest clusters strongly suggest parental care, but not positive proof.
  29. Maiasaura Hatchlings
    • head shape very different in adults and juveniles.
    • Large eyes of young ? attracted parental care.
    • Large snout developed later for feeding and ornamentation.
    • Developmental proportion shift common in artricial animals, including humans.
    • i.e., modes of feeding different in hatchlings nad adults: begging vs. foraging!
  30. Maiasaura nests
    • nests wouls have helped contain hatchlings and their food, preventing trampling and mixing of young between nests.
    • Unnecessary if eggs laid and abandoned. in contrast modern turtles hide eggs in the ground and camouflage site before abandoning.
  31. Maiasauria Herding and Migration
    • Herding and migration corrected; herds must migrate to maintain food supply and take advantage of seasonal food opportunities.
    • Part of the year is spent nesting in a fixed location until the young are developed enough to join the herd in its travels.
    • Behavior pattern of herding mammals and migratory bird flocks.
  32. Group behavior
    • Dinosaur trackways sometimes show many animals traveled parallel in the same direction.
    • Herding=suggested when large numbers of skeletons found together:
    • -psittachosaurus and protoceratops from china
    • -Centrosaurus from Alberta
    • -Coelophysis from New Mexico
    • -Large sauropods and allosaurus from Utah and Colorado
  33. Psittacosaurus mass death assemblage
    • six juveniles preserved together by a volcanic ash flow in northeastern china in early cretaceous.
    • Psittacosaurus has no horns or frills as did its descendents.
    • If this find is evidence for social behavior, then it shows that a complexity of social interaction preceded development of display structures.
    • Display structures would only be expected to develop in a community where there were courtship rituals and competition for mates.
  34. Evidence of Group Behavior
    • display structures
    • sexual dimorphism
    • changes in shape during growth
    • Mass death assemblages
    • Evidence for parental care
    • Trackways suggesting group travel
    • However, biologists consider herding more complex than mere group behavior, so herding inconclusive for dinosaurs.
  35. Attach and devense
    • some evidence that carnivorous dinosaurs hunted in packs, especially dromaeosaurs if they hunted large prey.
    • Three devense strategies available:
    • -large body size and whip-like tail (sauropods)
    • -Body armor/weaponry (stegosaurs, ankylosaurs, ceratopsians)
    • -Speed, camouflage, fleeing to water, large groups (hadrosaurs)
  36. Poikilothermy
    body temperature largely dependent on temperature of air or water in which animal lives.
  37. Homeothermy
    body temperature remains fairly constant in spite of different ambient temperatures.
  38. Ecothermy
    controlling body temperature through external means such as exercising or positioning body in sunlight
  39. Endothermy
    controlling body temperature by internal means such as burning of fat.
  40. Body temperature in living animals
    • endothermic animals burn energy to maintain a constant body temperature, but they have alow tolerance for temperature variation.
    • Ecothermic animals must allow their body temperature to fluctuate with the ambient temperature to some degree.
  41. Data for evaluation of endothermy
    • Posture and Gait (anatomy and trackways)
    • speed, levels of activity, and agility
    • Feeding adaptations
    • Bone microstructure
    • Blood pressure
    • Geographic distribution
    • Relation to known endotherms (birds)
    • Social behavior
    • Predator-prey ratios
    • Body size
    • Upright posture requires sustained activity and suggest endothermy. Sprawling posture makes resting easy for an animal with low metabolism.
    • dinosaur posture suggests high levels of activity compared to modern reptiles.
  42. dinosaur trackways
    • Ankylosaur and ceratopsian trackways suggest more erect forelimb postures than do studies of their bones and joints.
    • Forelimb trackway is wider than the hind limb trackway, however.
  43. Posture and activity levels
    • upright posture required for very large animals, but may not suggest activity levels comparable to mammals and birds.
    • Bipedality requires high activity level because of the need to balance on one foot at a time.
    • Therefore posture makes a stronger case for high activity in theropods and ornithopods than in sauropods and quadrupedal ornithischians.
    • Trackways also suggest that bipedal dinosaurs moved faster than quadrupedal forms.
  44. How fast could dinosaurs walk or run?
    • Dinosuar speed estimates vary because of different calcuation methods. Once recent extimate suggests that an average person might have been able to outrun an adult Tyrannosaurus. Two approaches are:
    • comparing to recorded speeds of modern animals of similar body size and build, and measureing distances between fossils footprints in a trackway and using these distances to calculate estimated speed. 
    • Walking speeds for medium-sized bipedal dinosaurs -4kph to 6kph; peak running speed 37kph to 88kph. Highest (88.6kph)=peak speed of current fastest land animals i.e. NA "antelope" but probably too high
  45. Bone histology
    bone microstructure reveals numerous haversian canals for blood vessels in modern mammals and birds as well as for most dinosaurs. These provide for rapid growth, mineral storage, and bone remodeling following injury or a change in conditions. Modern reptiles have few haversian canals and slow body growth.
  46. Blood pressure
    blood pressure measured in living animals can be extimated for dinosaurs using body dimensions.. Some dinosaurs had to have high blood pressure to force blood up a long neck. Such high blood pressure could also have sustained activity levels, but this can only be tested using other evidence.
  47. Blood pressure and heart structure
    living mammals have four-chambered heart structure that is more efficient for oxygenating the blood and proving high blood pressure than living reptiles. We can only guess at the structure of dinosaur hearts.
  48. Nasal turbinates
    living mammal endotherms use thin nasal turbinates to warm and moisten incoming air. Modern reptiles and dinosaurs lack turbinates. Birds have other ways to coserve water, so lack of turbinates may not demonstrate ectothermy.
  49. Geographic distribution
    • Modern ecotherm slimited to warmer climates, whereas some endotherms are adapted to even coldest conditions. Dinosauar fossils also found at extremem latitudes.
    • But climate warmer during Mesozoic, so a direct comparison impossible.
  50. Predator-prey ratios
    • endotherms need more fuel (food) than ectotherms: an endothermic population must be smaller compared to its food source.
    • If the food source is another animal of similar size, then a ratio of indiviual predator and prey animals can be calculated to assess predator metabolism.
    • Modern lions have ratios around 1% while crocodiles have ratios around 20%.
    • Carnivorous dinosaurs from Dinosaur Provicial Park have a ratio of 3-5%, similar to ratio found in fossils of saber-toothed cats.
    • This suggests endothermy in dinosaurs, right?
    • However, some recent studies have found highly variable predator-prey ratios in modern populations of endotherms and ectotherms, suggesting there is no good correlation.
    • We know far too little about dinosaur taphonomy to be certain that the ratio of fossils represents the ratio of living animals.
    • Therefore predator-prey ratios are poor evidence of endothermy in dinosaurs.
    • At best it could only tell us about the metabolism of the predators, not the prey species.
  51. Effect of body weight on Thermal Biology
    • Living reptiles over 3 kg have thermal conductance (rate of heat loss) equivalent to endothermic mammals.
    • Large dinosaurs in a subtropical climate would have had body temperature flucuations of 1-2 degrees C without internal heat production.
  52. Gigantothermy
    • Surface are-volume ratio=higher in small animals, so they lose heat quickly.
    • Giant animals have more difficulty collecting external heat or dissipating internal heat.
    • In most cases heat dissipation from normal activity=most serious problem in large dinosaurs, as in modern elephants.
    • Large dinosaurs benefitted from low metabolism, lack of insulation, and presence of heat dissipating structures with extra surface area.
    • Therefore endothermy in largest dinosaurs seems counterproductive and unlikely.
  53. Conclusions about thermo regulation
    • old view that all dinosaurs were as ectothermic as modern reptiles probably wrong.
    • Extremem view of Robert Bakker that all dinosaurs as endothermic as modern birds and mammals probably also wrong.
    • Various lines of evidence suggest diverse conclusions.
    • Good evidence for high metabolism and fair degree of endothermy in some dinosaurs.
    • Overly simplistic to think that all diverse dinosaurs had same metabolic levels and degree of endothermy.
  54. Flying Archosaurs (Pterosaurs)
    • Pterosaur means "winged lizard"
    • Pterosaurs 1st active fliers rather than gliders, but glided more than birds or bats.
    • 4th finger bones elongated to support wing membrane.
    • Two grades of pterosaurs:
    • -Pramphorhyncoids - long tails with a diamond shaped tip (late triassic-early jurassic)
    • -Pterodactyloids - more advanced, tail-less (late Jurassic and Cretaceous)
  55. Pterosaur characters
    • short body, long neck
    • Reduced, fused hip bones - new prepubis bone
    • Five long toes - divergent digit 5
    • Three short clawed fingers and elongate digit 4 to support the flight membrane
    • Pteroid bone in front of wrist to support a small anterior flight membrane
    • Tail stiffened with tendons (as in some dinosaurs)
  56. Pterosaur skulls
    • Skulls less modified in rhamphorhyncoids
    • Skulls more diverse in pterodactyloids
    • Head crests in some forms may have been for flight stability and/or attracting mates.
  57. Pteranodon - a pterodactylod (advanced pterosaur) with a head crest.
    • Quetzalcoatlus
    • largest flyer of all time.
    • 12-meter wingspan
    • pterodaustro
    • strained invertebrates for food.
  58. Pterosaurs and Flight
    • Pterosaurs now considered strong flyers.
    • Downstroke powered by pectoralis muscle.
    • Upstroke powered by supracoracoideus muscle.
    • Large wings=slow flying speeds and awkward landings, as with large soaring birds.
  59. Pterosaur endothermy
    • Many pterosaur fossils with good preservation show evidence of hiar in non-flight areas.
    • Some "hair" actually displaced fibers in wing membranes (actinofibrils).
  60. Turtles (Testudines or Chelonia)
    • Body covered in two-part shell:
    • -Carapace - top of plates attached to ten elongate trunk vertebrae and ribs.
    • -Plastron - bottom of plates attached to expanded shoulder girdle and gastralia
    • Triradiate shoulder and hip girdles, both internal rib cage.
    • Long flexible neck for retraction of head
    • Solid head lacking temporal openings
    • -Horny beak and loss of all teeth in most species.
    • -Deep embayment in quadrate supports eardrum
  61. Turtle characters
    • modern turtle showing modified girdles (a-c).
    • Primitive turtle retaining palatal teeth and some skull flexibility (d-f)
    • Plastron with openings for neck/legs (g)
    • Braincase fused to palate (h)
  62. Turtle groups
    • Proganochelydia (late triassic)
    • -Relatives of pareiasaurs and procolophonids
    • -Skull solidy built but not retractable
    • Australochelidae (late Triassic, early Jurassic)
    • -first fusion of braincase with palate
    • Partial enclosure of middle ear
    • Casichelydia (Jurassic to Recent)
    • -Advanced turtles
    • -Loss of lacrimal, tear duct, palatal teeth; single vomer; ear enclosed in pterygoid and opisthotic
  63. Advance turtles (casichelydia)
    • Pleurodira (Jurassic to Recent)
    • -Retract head sideways under edge of shell
    • -Pelvis fused to carapace and plastron
    • -Modern species are all freshwater and restricted to southern hemisphere.
    • Cryptodira (Jurassic to Recent)
    • -Radiated later in Jurassic than pleurodires
    • -Retract head into shell with vertical bend of neck
    • -Six main clades include terrestrial, freshwater, and marne forms, including the largest turtles
  64. Sea turtles
    • evolved during cretaceous
    • Archelon grew to 12 ft long
    • Fossil turtle Protostega gigas from Cretaceous Niobrara Chalk, Kansas
  65. Giant Cretaceous Sea Turtle
    openings between ribs in the carapace reduce weight and limbs=modified into paddles.
  66. Crocodylomorphs
    • Crocodilians=only surving crurotarsians - didn't arise until early Jurassic.
    • Late Triassic ancestors "uncrocodilian" with long slender limbs, possibly bipedal, and sharp slender teeth for eating small prey.
    • Elongate carpals (radiale and ulnare), posterior coracoids spine - open acetabulum=diagnostic crocodylomorph characters.
    • Quadrate alteration=diagnostic of early vs. later crocodylomorphs.
  67. Crocodylia
    • elongate snout with anterior nasal openings.
    • Secondary palate allows breating and eating at the same time.
    • Bondy scutes attached to each side of backbone by longitudinal muscles provide strong brace fora  variety of locomotor types.
  68. Crocodilian Classification
    • Protosuchia (early Jurassic)
    • -first true crocodilians
    • -Many adaptations for aquatic life
    • Mesosuchia (later Jurassic and Cretaceous)
    • -paraphyletic group of mid-grade corcodilians
    • Iusuchia (late Cretaceous to Recent)
    • -Full secondary palate with pterygoid included
    • -Three modern families:
    • --crocodiles - most like primitive forms
    • --Alligators - broad rounded snouts
    • --Gavials - more aquatic with long slender snouts
  69. Marine Crocodiles
    • Crocodiles evolved in Triassic as terrestrial animals.
    • Some adapted to marine environment by earliest Jurassic.
    • Marine species rare by Cretaceous.
    • Rapid swimmers.
  70. Sarcosuchus from Africa
    • Early cretaceous of Niger
    • Skull 1.6m long
    • body 12m long
    • bony scutes on back also correspondingly longer
    • probably preyed on dinosaurs that came to a river to drink
  71. Lepidosauria
    • Sphenodontia (triassic to recent)
    • -beaked jaws with acrodont teeth
    • -diapsid skull with rigid quadrate
    • -the surviving Sphenodonn is a livingn fossil
    • Squamata
    • -flat jaws with thecodont teeth (replacable)
    • -lose lower temproal bar, movable quadrate
    • -Lacertilla - lizards (triassic to recent)
    • -Serpentes - snakes (mid cretaceous to recent)
  72. Pleurosaurs
    aquatic late Jurassic sphenodonts
  73. Squmate Characters
    • kinetic joints in skull
    • Mesokinetic joints allow bending in middle of skull
    • Metakinetic joints occur between skull and braincase
    • Streptostylic joints make quadrate bones double-jointed, allowing the lowe rjaw to move forward and backward with respect to the skull
  74. Squmate classification
    • five main groups of lizards:
    • -iguania - primitive forms
    • --chameleons most specialized of group. Tongue protrusible for catching insects, tail prehensile (usable as grasping limb), limbs modified for grasping, skin can change color.
    • -Gekkos - suction cup toes for climbing
    • -Scincomorphs - sleek bodies with short limbs, peg like teeth
    • -Amphisbaenids - limbless burrowingn lizards
    • -Anguimorphs - monitor lizards and their Cretaceous marine relatives: aigialosurs, dolichosaurs, and mosasaurs
  75. Serpentes
    • Arose in early Cretaceous from unknown lizard ancestor.
    • Loss of limbs, great increase in number of vertebrae.
    • Boas retain vestiges of pelvis and femurs
    • increase in kinesis of skull to allow swallowing of prey much large than the head.
    • Poisonous snakes appear in late Eocene, some have fangs on jointed maxilla
  76. Mesozoic marine reptiles
    • nothosaurs
    • placodonts
    • plesiosaurs
    • icthyosaurs (fish lizards)
    • mosasaurs
    • crocodiles
    • sea turtles
    • triassic marine reptiles
    • Like modern seals, probably fed in the ocean but lounded on land. Laid eggs on land.
    • Placodont (left) 0 shellfhish eater
    • Nothosaurs (right) precursors of plesiosaurs
  77. Plesiosaurs
    • Evolved from Nothosaurs (triassic precursors)
    • Fish eaters - slender, curved teeth
    • Short, broad body attached to an extraordinarily long neck with a small head.
    • Others shorter neck and larger head.
    • Up to 40 ft long (12m)
    • Large paddle-like limbs of many bones.
    • long necked and short necked forms
  78. Icthyosaurs "fish-lizards"
    • most fish-like of mesozoic reptiles.
    • Resemble dolphins, but with upright rather than horizontal tails fins.
    • Top predators.
    • Large eyes to pursue prey.
    • Live young rather than eggs.
    • Became rare and disappeared during the Cretaceous.
  79. Mosasaurs
    • Cretaceous only.
    • Up to 50ft long (15m)
    • Probably top predators.
    • Attacked ammonoids, as evidenced by bite marks on ammonoid shells.

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