BIO 113 — Dinosaurs

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Lab 8
Prehistoric Reptiles

See also Reptile Phylogeny Overview Page

See also Reptile Fossils Page

Synapsids

Pelycosauria 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, Permian Period, North America (MAL1)
View Larger Image Our Changing Views of Prehistoric Reptiles: Dimetrodon See lab manual Exercise 12 for more information View Larger Image

Therapsids 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	Inostrancevia skeleton, Permian Period, Russia (AZMNH)
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Cynodontia 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?	Chiniquodon kalanoro fossil, a cynodont; Middle Triassic; Madagascar (CFM)
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Mammalia 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, an otter-like mammal from a now-extinct group; Eocene Epoch (Green River Formation), WY (WDC)
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Stem Reptiles

The Earliest Reptiles The oldest known reptiles Generally lizard-like (though with front and hind legs more or less equal length) Have an anapsid skull (lack temporal openings) Includes the oldest known reptile (Hylonomus) from the Carboniferous Period	Labidosaurus hamatus skeleton, Permian Period, Texas (CFM)
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Parareptilia Have anapsid skulls that lack temporal openings Some are lizard-like quadrupeds (ancestral form) Others specialized into bipedal runners, semi-aquatic forms, and large armored herbivores Extinct (although turtles were once thought to belong to this group)	Bradysaurus baini skeleton, Permian Period, South Africa (CFM)
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Diapsida

"Avicephala" (Triassic Oddities) An obscure group of primitive diapsid reptiles Generally lizard-like body shapes (elongated body, long tail, limbs sprawl out to sides) Small, bird-like heads (large eyes, pointed snout) Arboreal: mostly lived in trees Many species had unusual adaptations, including wing-like structures for gliding, giant thumb claws, grasping tails ending in a "claw", broad, leaf-like tails, or a row of greatly elongated upright scales (?) along its back Known from Permian to Triassic Periods This group may be polyphyletic: relationships are being debated

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Thalattosauria (More Triassic Oddities) Relationships unknown besides being diapsids From Triassic only Semi-aquatic: broad, webbed feet, and long, flattened tail for swimming Snout distinctive, either long and pointed or with distinct down-turned tip

Skeleton model of the Thalattosaur Xinpusaurus kohi, Late Triassic Period, China

Lepidosaurs (except mosasaurs) This group includes the modern lizards and snakes (squamates), the superficially lizard-like tuataras (only 1 living species) and some extinct groups Generally lizard-like, with elongated bodies, usually long tails, and limbs that sprawl out to the sides Rhynchocephalians (tuataras and extinct relatives) are superficially lizard-like, but differ in their skull and other internal features Modern tuataras are similar to prehistoric species

Pleurosaurus fossil, a semi-aquatic rhynchocephalian (relative of tuataras) (WDC)

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Mosasaurs Part of the Lepidosaura, but very distinctive, so we will treat them separately Cretaceous lizards (Squamata) related to modern monitor lizards Most were fully aquatic and very large (the most primitive forms such as Dallasaurus were small & semi-aquatic) Note elongated body and tail (which distinguishes them from Pliosaurs) Moderate-sized flippers, with the front flippers larger than the back (compare to Thalattosuchian crocodiles) Recent specimens indicate at least some (and perhaps all) mosasaurs had a forked tail fin

Halisaurus fossil, late Cretaceous, North America (WDC)

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Our Changing Views of Prehistoric Reptiles: Mosasaurus See lab manual Exercise 12 for more information View Larger Image

Marine Reptiles: "Euryapsida"

Testudines 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 ischyros skeleton, Late Cretaceous, North America (MAL)
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Ichthyosauria The phylogenetic postion of this group has been debated and is uncertain Most with dolphin-like appearance, with a dorsal fin and vertical, crescent-shaped tail fluke Long, slender snout in most (some were toothless) Varied considerably in size Earliest ichthyosaurs had short snouts, lacked dorsal fin and lacked tail flukes (e.g. Cartorhynchus) Hupehsuchians were semi-aquatic ancestors of ichthyosaurs

Stenopterygius fossil, Jurassic Period, Germany (WDC)

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Placodontia Semiaquatic reptiles living near shores Some were lizard-like in shape Many had extensive turtle-like dermal armor Triassic Period only	Sinocyamodus? placodont fossil cast, Triassic, China (BHI)
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Nothosauria & Other Sauropterygians Semiaquatic reptiles with elongated necks and fairly small heads Had webbed feet (instead of true flippers) Nothosaurs Intermediate between Placodonts and Plesiosaurs Atopodentatus is a unique species likely related to the Placodonts Its wierd T-shaped head was filled with tiny teeth It probably grazed algae Triassic Period only	Neusticosaurus edwardsi fossil, Triassic, Switzerland (BHI)
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Plesiosauria Fully marine reptiles Long flippers used for swimming Body relatively short, broad, and stiff, with short tail (compare to Mosasaurs) Short tail may end in small fin (for stability, not power); orientation of fin uncertain Body shapes range from long-necked and small headed to short-necked and large-headed (Pliosaurs)	Polycotylid plesiosaur skeleton, Cretaceous Period, Morocco (WDC)
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Stem Archosaurs

Left: Hyphalosaurus lingyuanensis fossil (Choristodera), early Cretaceous, China (BHI); Right: Sharovipteryx mirabilis (Protorosauria) fossil cast, middle Triassic Period, Kyrgyzstan (AZMNH) Left: Left: Champsosaurus laramiensis skeleton (Choristodera), late Cretaceous, SD (DRC); Right: Icarosaurus siefkeri fossil cast (Allokotosauria?), late Triassic Period, NJ

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Pseudosuchia

Phytosauria Large, sprawling, semi-aquatic carnivores Long, slender snout, with nostrils close to eyes Convergently resemble modern crocodiles (and especially gavials)	Redondosaurus skeleton, Triassic Period, North America (NMMNH)
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Aetosauria Heavily armored with bony plates covering back, spikes on sides Relatively small head Herbivorous (convergent with Ankylosaur dinosaurs)	Desmatosuchus Skeleton, Triassic Period, Chinle Formation, Arizona (PFNP)
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Poposauroidea A monophyletic group of crocodile ancestors, generally related to the Rauisuchians Mostly moderately sized, with long necks and lacking osteoderms Diverse builds and lifestyles Some were toothless Some had sails Some were bipedal (convergent on theropod dinosaurs); others were quadrupeds

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"Rauisuchians" 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 Skeleton, Triassic Period, Chinle Formation, Arizona (PFNP)
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Notosuchia A group of stem crocs from the southern hemisphere Back covered in armored scutes Terrestrial, with legs held beneath body Diverse in appearance and lifestyle: includes herbivores, omnivores, carnivores Some were burrowers with armadillo-like bands of armor	Araripesuchus fossil, late Cretaceous Period, South America & Africa (FMNH)
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Neosuchia Includes modern alligators and crocodiles and extinct relatives Includes the modern Crocodylia Generally had elongated snouts Back covered in armored scutes Mostly semiaquatic (including all moderns species): elongated body, flattened tail, and sprawling limbs Paraphyletic: the fully aquatic Thalattosuchians are derived from within this group (see below)	Goniopholis skeleton, late Jurassic Period, North America & Eurasia (MAL)
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Thalattosuchia A clade of fully aquatic crocodylomorphs (crocodile relatives) Adapted to marine environment: limbs form paddle-like fins Hindfins larger than small front fins; toe structure usually visible Tail fluke present Lacked dermal armor of other Pseudosuchians	Macrospondylus (formerly Steneosaurus) fossil, early Jurassic Period, Europe (WDC)
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