BIO 113 Dinosaurs
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Lab 9
Prehistoric Reptiles


  • This group includes mammals and the ancestors of mammals ("proto-mammals" or "mammal-like reptiles")
  • Synapsid skull with just one temporal foramen
  • Have two or more types of teeth

  • Most primitive and reptile-like of the Synapsids
  • Sprawling posture with legs held to side
  • Tail long
  • Some (but not all) had tall dorsal sail
  • Extinct by end of Permian Period (251 mya)
dimetrodon fossil
Dimetrodon fossil, Permian Period, North America (MAL1)

  • A paraphyletic group (some are more closely related to cynodonts and mammals)
  • Very diverse forms and lifestyles, inlcuding both herbivorous and carnivorous forms
  • Posture somewhat sprawling (less so than Pelycosaurs)
  • Varied teeth
  • Relatively short tail
dimetrodon fossil
Inostrancevia fossil, Permian Period, Russia (AMNH2)

  • A paraphyletic subgroup of Therapsids that are most closely related to mammals
  • Mostly relatively small and carnivorous
  • Complex teeth with multiple cusps (points)
  • May have been endothermic and with hair?
Skull replica of Thrinaxodon, a cynodont (Early Triassic; South Africa)

  • Diverse, with ≈5000 species alive today
  • Have hair (endothermy)
  • Mammary glands provide milk for young
  • Only 2 tooth generations (teeth not continuously replaced)
  • Upright posture with limbs held beneath body
  • During Mesozoic mammals were relatively small, inconspicuous
Palaeosinopa fossil
Palaeosinopa fossil, an otter-like mammal from a now-extinct group; Eocene Epoch (Green River Formation), WY (WDC3)
Mesozoic mammals


  • Primitive group of exinct reptiles
  • Anapsid skulls that lack temporal openings
  • Quadrupedal
  • Vary from lizard-like, to semi-aquatic to heavy, armored herbivores
  • Turtles were once thought to belong to this group
Captorhinus skull fossil
Captorhinus aguti skull, Permian Period, North America (MAL)


  • This group includes the modern lizards and snakes (squamates) plus the superficially lizard-like tuataras (only 2 living species)
  • All have a diapsid skull (though highly modified in lizards and especially snakes)
  • Although present throughout the Mesozoic Era, they were mostly small like modern species

  • Mosasaurs are a group of Cretaceous lizards related to modern monitor lizards
  • Mosasaurs became fully aquatic and very large
  • Note elongated body and tail, and moderate-sized flippers (compare to Pliosaurs)
  • Recent specimens indicate at least some (and perhaps all) mosasaurs had a forked tail fin
  • Note: The illustrated Megalainia is a closely related extinct monitor lizard, and the largest lizard known
Halisaurus fossil
Halisaurus fossil, late Cretaceous, North America (WDC)

Marine Reptiles: "Euryapsida"

  • All of the species grouped here have a semiaquatic (crocodile-like) or fully aquatic lifestyle
  • Fully aquatic lifestyles evolved several times independently in Mesozoic reptiles
  • Most have a euryapsid-type skull (single opening but in different location than synapsid skulls) that is derived from a diapsid skull
  • A few recent studies suggest that the species listed here may form a monophyletic clade: more data are needed.

  • The phylogenetic postion of this group has been debated and is uncertain
  • Dolphin-like appearance, with dorsal fin and vertical tail fluke
  • Long, slender snout (some were toothless)
  • Varied considerably in size
Stenopterygius fossil
Stenopterygius fossil, Jurassic Period, Germany (WDC)

  • Turtles
  • Their relationships have been long debated, but recent evidence suggests they may be related to other marine reptiles
  • Distinctive shell with upper and lower half (most primitive turtle only has lower half)
  • Shell derived from ribs, sternum, and dermal bone
  • Skull lacks teeth in all but most primitive species
  • Most are semiaquatic, but range from fully terrestrial to fully marine
Archelon fossil
Archelon ischyros skeleton, Late Cretaceous, North America (MAL)

  • Semiaquatic reptiles living near shores
  • Some were lizard-like in shape
  • Many had extensive turtle-like dermal armor
  • Triassic Period only

  • Semiaquatic reptiles with elongated necks and fairly small heads
  • Had webbed feet (instead of true flippers)
  • Intermediate between Placodonts and Plesiosaurs
  • Triassic Period only
Keichousaurus fossil
Keichousaurus hui fossil, Middle Triassic, China (UMS4)

  • Fully marine reptiles
  • Long flippers used for swimming
  • Body relatively short, broad, and stiff, with short tail (compare to Mosasaurs)
  • Body shapes range from long-necked and small headed to short-necked and large-headed (Pliosaurs)
Polycotylid fossil
Polycotylid plesiosaur skeleton, Cretaceous Period, Morocco (WDC)

Stem Archosaurs

  • Here we are combining various primitive members of the Archosauromorpha into a paraphyletic group
  • United by various small skeletal characteristics
  • Shapes and lifestyles are quite varied
  • Appeared in Permian, reached greatest diversity in Triassic Period
  • Main subgroups (don't need to know) represented in lab are:
    • Rhynchosauria: small, stocky herbivores with beak-like tooth plates (e.g. Hyperodapedon)
    • Choristodera: Crocodile-like predators (e.g. Champsosaurus, Hyphalosaurus)
    • Protorosauria: Varied, some lizard like, some long-necked and semi-aquatic (e.g. Tanystropheus), and some gliders (e.g. Sharovipteryx)
    • Proterosuchidae: Odd, long-nouted reptiles (e.g. Proterosuchus)
    • Euparkeria: a small, slender insect-eater with legs held beneath the body rather than sprawling; close to the ancestry of crocodiles, dinosaurs, pterosaurs
    • Phytosauria: Giant crocodile-like predators

Hyphalosaurus fossil Sharovipteryx fossil
Left: A juvenile Hyphalosaurus lingyuanensis (Choristodera), early Cretaceous, China; Right: Sharovipteryx mirabilis (Protorosauria) fossil replica, middle Triassic Period, Kyrgyzstan

  • Large, sprawling, semi-aquatic carnivores
  • Long, slender snout, with nostrils close to eyes
  • Convergently resemble modern crocodiles (and especially gavials)
Leptosuchus fossil
Leptosuchus skull, Triassic Period, North America (AMNH)


  • Like other archosaurs, have teeth set in sockets and antorbital and mandibular fenestrae
  • Distinguished by more flexible ankle joint
  • Surviving members include the crocodiles and alligators
  • Most (except crocodylomorphs) went extinct at end of Triassic Period

  • Heavily armored with bony plates covering back, spikes on sides
  • Relatively small head
  • Herbivorous (convergent with Ankylosaur dinosaurs)
Desmatosuchus fossil
Desmatosuchus Skeleton, Triassic Period, Chinle Formation, Arizona (PFNP6)

  • The dominant large, terrestrial predators during the Triassic
  • Large heads
  • Crocodile-like armor scutes on back
  • Legs held beneath body; hind legs longer than front (some bipedal?)
  • Recent studies suggest that this group is paraphyletic
Postosuchus fossil
Postosuchus Skeleton, Triassic Period, Chinle Formation, Arizona (PFNP)

  • Includes modern alligators and crocodiles
  • Generally had elongated snouts
  • Back covered in armored scutes
  • Semiaquatic forms (including all moderns species) had an elongated body, flattened tail, and sprawling limbs (secondarily derived)
  • Other forms were terrestrial, with long legs held beneath body
  • Some became fully aquatic with flippers instead of legs
Steneosaurus fossil
Steneosaurus fossil, early Jurassic Period, Europe (WDC)


  • Capable of true powered flight
  • Forelimbs modified into wings: membrane stretches from tip of greatly elongated finger
  • When not flying they were probably quadrupedal
  • Perhaps all had a hair-like covering

  • Paraphyletic grouping of various primitive forms
  • Long tail that ends in diamond-shaped flag
  • Small-sized
Rhamphorhynchus fossil
Rhamphorhynchus fossil, late Jurassic Period, Solnhofen Limestone, Germany (MAL)

  • The more derived clade of pterosaurs
  • Generally larger sized than Rhamphorhynchoids (but size variable)
  • Tail always short
  • Some were toothless
Tapejara fossil
Tapejara skeleton, Early Cretaceous, Brazil (WDC)

  1. MAL = Museum of Ancient Life, Lehi, UT
  2. AMNH = Arizona Museum of Natural History, Mesa
  3. WDC = Wyoming Dinosaur Center, Thermopolis
  4. UMS = Udo M Savalli, personal collection
  5. FMNH = Fernbank Museum of Natural History, GA
  6. PFNP = Petrified Forest National Park, AZ
This page last updated 14 October 2016 by Udo M. Savalli ()
Images and text Udo M. Savalli. All rights reserved.