ANCESTRY of the DINOSAURIA

AMNIOTA [Fig. 8.5]

• The amniotes include the mammal, reptiles, and birds
• Characteristics: Amniotes are derived with respect to amphibians in having the following:
  1. Amniote (cleidoic) egg 
     1. Amniotes are fully terrestrial and lay eggs on land . Amniotes not "tied to water" for purposes of reproduction.
     2. exclusively internal fertilization, No larval stages. No metamorphosis
  2. More effective jaw musculature
  3. Skeletal structure improved--greater strength and agility

an abbreviated AMNIOTE TAXONOMY

Amniota
        Synapsida (includes Mammalia)
        Reptilia
             Testudines (turtles may be diapsids)
             Diapsida
                  Archosauromorpha [Crocodylia, Dinosaurs including Aves]
                  Lepidosauromorpha [lizards and snakes]

Paul Olsen's web page has an excellent discussion of the origin of the amniotes and the early diversification of the archosaurs

TEMPORAL OPENINGS and AMNIOTE CLASSIFICATION [fig. 15.1]

Anapsids

dermal skull roof completely covered with bone;  this is primitive for the amniotes.  Turtles have traditionally been considered anapsids, but they are now conidered modified diapsids.

Synapsids

single opening low on the cheek, bordered by the jugal, squamosal, and postorbital; condition found in mammals

Diapsids

two pairs of temporal openings; one pair ventral to the postorbital and squamosal (as in the synapsids); the second pair dorsal to the postorbital and squamosal and lateral to the parietal

Euryapsids

derived from the diapsid condition by loss of the lower temporal opening; evolved independently in plesiosaurs (lower temporal bar lost) and ichthyosaurs (opening closed by enlargement of bones of the cheek region

CLASSIFICATION OF THE REPTILES

REPTILIA
     Testudines (turtles and tortoises) 
     Diapsida [see cladogram, Fig. 15.3]
           Lepidosauria (lizards and snakes)
           Archosauromorpha 
                Eupakaria [Fig. 15.6]
                Archosauria 
                     Crocodilia (alligators and crocodiles)
                     Ornithodira 
                          Pterosauria
                          Dinosauromorpha
                              Lagosuchus, etc.
                              DINOSAURIA
                                   Ornithischia
                                        Stegosauria and Ankylosauria
                                        Ornithopoda and Ceratopsia
                                   Saurischia
                                        Sauropoda
                                        Theropoda
                                             Aves (birds)

Infraclass ARCHOSAUROMORPHA

• premaxillae extend behind or below nares, excluding maxilla from contact with nares
• pineal foramen reduced

Superorder ARCHOSAURIFORMES

• antorbital fenestra [Fig. 7.2]
• mandibular fenestra [Fig. 7.2]
• fourth trochanter on femur
• flattened (instead of rounded) teeth set in sockets

Order Thecodontia

• This taxon has been used as a paraphyletic scrapbasket for primitive (Triassic) archosaurs; most of the taxa previously assigned to this order have been reassigned to groups more closely related to crocodiles (crocodylotarsi) or groups more closely related to birds (ornithodires). The taxa described below have a mesotarsal joint between proximal and distal tarsals, the primitive state also found in dinosaurs.
• The primitive Archosauriformes formerly placed in the Thecodontia are discussed by Alan Kazlev on the Palaios pages.

• small aquatic carnivores with unspecialized tarsus, short limbs, sprawling posture; Proterosuchus

• large-bodied, large-headed, short-necked, quadrupedal carnivores; Vjushkovia  

• small carnivore (0.5 m) with long hind limbs; ankle reduced and flexible; symmetrical foot with elongate third digit; quadrupedal and/or bipedal semi-erect gait; teeth set in separate sockets; Euparkeria [Fig. 15.6]

ARCHOSAURIA [Cladogram Fig. 15.8]

• absence of teeth on pterygoid, palatine and vomer
• The two major archosaur clades are distinguished by their ankle structure
  • they are the Crurotarsi which includes the crocodilians and the Ornithodira which includes the birds

CRUROTARSI

• hinge joint between astragalus and calcaneum (crocodiloid tarsus or crurotarsal joint)
• peg on astragalus; socket on calcaneum ('crocodile-normal') in Crocodylotarsi (Parasuchia and Suchia).
• 'crocodile reversed' in Ornithosuchidae

ORNITHOSUCHIDAE

• erect gait
• 'crocodile-reversed' ankle joint: peg on calcaneum; socket on astragalus 
• slender build with long hind limbs
• Ornithosuchus; Riojasuchus
• Benton considers the ornithosuchids to be the primitive sister taxon of the ornithodires. Sereno (1990) considers them the sister taxon of the Suchia (crocodilians), because he considers both crocodile-normal and reversed ankles variations of the same crurotarsal joint.

PARASUCHIA

Family Phytosauridae (Upper Triassic)

• Phytosaurs: crocodile-like; narrow jaws with conical teeth; elongate premaxilla; posteriorly displaced external nares on forehead; Parasuchus [Fig. 15.9]; Rutiodon

SUCHIA

Family Stagonolepididae (Upper Triassic)

• Aetosaurs - large armored herbivores; extensive dermal bone; small heads; small blunt teeth; Staganolepis [Fig. 15.12]

Family Rauisuchidae (middle and upper Triassic)

• terrestrial carnivores; the top carnivores of the middle and late Triassic
• quadrupedal; pillar erect gait;
• Saurosuchus, Postosuchus; Poposaurus

Superorder Crocodylimorpha

• elongate, rod shaped radiale and centrale; corocoid with backward pointing spine; pelvis with an open acetabulum; quadrate and quadratojugal displaced inward; cheek overhung by squamosal

Order Crocodylia (early Jurassic - Recent)

ORNITHODIRA

• Pterosaurs and dinosaurs have fourth trochanter of the femur an enlarged ridge located low down
• ankle joint simplified to a hinge-like arrangement with astragalus and calcaneum reduced and firmly attached to tibia and fibula ('advanced metatarsal' ankle)
• and middle three toes of pes elongate

Order Pterosauria

• flying archosaurs; limb bones with pneumatic foramina; plate-like sternum; reduced and fused hip bones; elongate metacarpal IV and digit IV; plate-like pelvis; digitigrade
• The Pterosaur Home Page from David Peters

Ramphorhynchoidea (U. Trias. - U. Jur.)

• long tails and long fifth toes; Eudimorphodon

Pterodactyloidea (U. Jur. - U. Cret.)

• short tails and short fifth toes; long wing metacarpal; includes Pteranodon  5-8 m wingspan; Quetzelcoatlus wing span of 11-12 m., weight of approximately 65 kg

DINOSAURIA

Previous classifications usually considered dinosaurs a polyphyletic group with independent origins from thecodont ancestors. Dinosauria are now considered monophyletic because of numerous autapomorphies including elongate vomers, three or more sacral vertebrae, scapulacoracoidal glenoid facing fully backwards, low deltopectoral crest running 1/3 to 1/2 way down humerus, three or fewer phalanges on fourth digit of manus, largely to fully open acetabulum, fully offset proximal head of femur with a distinct neck and ball, greatly reduced fibula, well-developed ascending process of astragalus; upright posture, digitigrade stance. See Peczkis, J, 1994, JVP 14:520-533 for estimates of body mass of dinosaurs.

THE DINOSAUROMORPHA

DINOSAUROMORPHA

Included taxa

• Lagosuchus (and related taxa) plus the Dinosauria

Characteristics

• cervical column with a strong sigmoid curve
• forelimb 50% or less than hind limb length
• astragalus with acute anteromedial corner (etc. Sereno, 1991, SVP Memoir 2)
• advanced mesotarsal hinge with joint between astragalus-calcaneum unit and rest of foot
• hind-limb carried close to axis of body
• capable of digitigrade bipedal posture, leaving 3-toed footprint.

Middle Triassic Dinosauromorphs: Lagosuchidae

Characteristics

• small lightly built; dinosaur-like tarsus with a mesotarsal hinge
• some forms may have been saltatorial

Included taxa

• Lagosuchus [Dingus & Rowe p. 162: "facultative biped, capable of bursts of speed on hind limbs"]
• Lagerpeton
• Pseudolagosuchus
• Marasuchus

• Why Lagosuchus is a Dinosauromorph, but not a Dinosaur

Reference

• Sereno, P. C. and Arcucci, A. B., 1994, Dinosaurian precursors from the Middle Triassic of Argentina: Marasuchus lilloensis, gen. nov. Journal of Vertebrate Paleontology, 14: 53-73.

DINOSAURIA

Included Taxa

• Herrerasauridae
• Saurischia
• Ornithishchia

Discussion

• Previous classifications usually considered dinosaurs a polyphyletic group with independent origins from thecodont ancestors. Dinosauria are now considered monophyletic because of numerous autapomorphies including 
  1. elongate vomers
  2. three or more sacral vertebrae
  3. scapulacoracoidal glenoid facing fully backwards
  4. low deltopectoral crest running 1/3 to 1/2 way down humerus
  5. three or fewer phalanges on fourth digit of manus
  6. grasping hand produced by a thumb that could bend inward - bending occurs at joint between metacarpal I and first thumb phalange [D & R p. 164]
  7. largely to fully open acetabulum [Dingus & Rowe p. 165]
  8. fully offset (inturned) proximal head of femur with a distinct neck and ball
  9.  greatly reduced fibula
  10. well-developed ascending process of astragalus
  11. upright posture [Dingus & Rowe p. 163]
  12. digitigrade stance.

CLASSIFICATION OF THE DINOSAURS

An overview of dinosaur classification from M. Alan Kazlev's Dinosaur Pages.

Everything you need to know about dinosaur classification in just 11 pages:  Paul C. Sereno.  1999.  The Evolution of Dinosaurs.  Science, 284:2137-2147.   This article is reprinted courtesy of Bill Tietjen, Bellarmine College

Dinosauria
      Herrerasauridae
      SAURISCHIA
            THEROPODA
                  Ceratosauria
                  Tetanurae
                        Carnosauria
                              Allosauridae
                        Coelurosauria
                              Tyrannosauridae
                              Ornithomimosauridae
                              Maniraptora
                                    Dromaeosauridae
                                    Aviale
                                          Archaeopterix
                                          Ornithurae
                                                Enantiornithes
                                                Aves
           Sauropodomorpha 
                  Prosauropoda
                  SAUROPODA
                        Vulcanodon, etc.
                         Neosauropoda
                              Diplodocidae
                              Macronaria
                                    Camarasaurus
                                    Titanosauriformes
                                           Brachiosauridae
                                           Titanosauridae

      ORNITHISCHIA
            Fabrosauridae [Lesothosaurus] 
            Genasauria
                  Thyreophora
                        unnamed group [Scelidosaurus, Scutellosaurus]
                        unnamed group
                              Ankylosauria
                                    Ankylosauridae
                                    Nodosauridae
                              Stegosauria
                                    Kentrosauridae
                                    Stegosauridae
                  Cerapoda
                        Marginocephalia
                              Pachycephalosauria
                              Ceratopsia
                                     Psittacosauridae
                                     Neoceratopsia
                                           Protoceratopsidae
                                           Ceratopsidae 
                        Ornithopoda
                              Heterodontosaurus
                              no name
                                    Hypsilophodontidae
                                    no name
                                          Iguanodontidae
                                          Hadrosauridae 

See Peczkis, J, 1994, JVP 14:520-533 for estimates of body mass of dinosaurs.

The oldest dinosaurs are Herrarasaurus and Eoraptor. 

•  Read a brief introduction to them from the Texas Memorial Museum
• or from the UCMP learn more about these first dinosaurs
• Read more about the Triassic World in lecture notes by Paul Olsen at Columbia
  University. 

HERRERASAURIDAE (upper Triassic)

Characteristics

• small, 2-4 meters long
• predatory, with grasping hands
• bipedal.
• Dingus & Rowe consider them primitive, generalized dinosaurs; Sereno and Novas (see references below) recognize them as primitive theropods.  Others have considered them as primitive dinosaurs, or even non-dinosaurs.  They lack the long neck, the elongated second digit, and contact between the ankle and the first metatarsal that are used to diagnose the Saurischia.  Fraser et al. consider Herrarasuarus, Eoraptor, and their new genus Agnostiphys to be Dinosauromorpha that are more derived than Marasuchus, but that lack the synapomorphies of the Dinosauria.

Included taxa

• Staurikosaurus (Brazil)
• Herrerasaurus (Argentina), about 3 to 4 m long
  • digital images from UT

Some references

• Fraser, N. C., K. Padian, G. M. Walkden, and A. L. Davis.  2002.  Basal dinosauriform remains from Britain and the diagnosis of the Dinosauria. Palaeontology, 45:79-95.
• Novas, F. E.  1993. New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropods: Herrerasauridae) from the Ischigualasto Formation (Upper Triassic) of Argentina. Journal of Vertebrate Paleontology, 13: 400-423.
• Sereno, P. C.  1993. The pectoral girdle and forelimb of the basal theropod Herrerasaurus ischigualastensis. Journal of Vertebrate Paleontology, 13:. 425-450.
• Sereno, P. C. and Novas, F. E.  1993.  The skull and neck of the basal theropod Herrerasaurus ischigualastensis. Journal of Vertebrate Paleontology, 13: 451-476.
• Read more about Herrerasaurus in Paul Olsen's dinosaur course notes
• T. M. Keesey in the Dinosauricon has numerous reconstructions.

SAURISCHIA

Characters

• distinctly asymmetrical manus with digit II the longest digit [Dingus & Rowe p. 179]
• elongate posterior cervical vertebrae giving a long neck
• antorbital fenestra well-developed
• large quadratojugal
• temporal musculature extends onto frontal
• triradiate saurischian pelvis (primitive) - illustrated below

ORNITHISCHIA

Characters

• herbivorous, with blunt or leaf-shaped teeth
• predentary bone in lower jaw [Dingus & Rowe p. 173]
• rostral tip of premaxilla toothless
• ossified tendons along at least part of spinal column
• reduced antorbital fenestra
• buccal emargination of the jaws
• at least 5 sacral vertebrae
• antorbital fenestra reduced or absent
• premaxillae expanded, form part of nares
• ornithischian (tetraradiate) pelvis with a prepubic process, process of pelvis points backward [Dingus & Rowe p. 172] to make room for the long digestive tract   (see below).

The Saurischian (on the left) and the Ornithischian (on the right) pelves.  Anterior is to the left.  Note the open acetabulum in both.  In the triradiate (primitive) saurischian pelvis, the pubis extends anteriorly.  In the tetraradiate ornithischian pelvis, note the anterior (to the left) extension of the ilium, and the backward (to the right) extension of the pubis below and adjacent to the ischium.  Illustrations from O. C. Marsh.  1896.  The dinosaurs of North America.  16th Annual Report, U.S. Geological Survey, pp. 133-244, 84 plates.

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