CMNH: Dinosauria & Other Extinct Clades

Dinosaurs arose and diversified in the Mesozoic era (220-65 million years ago).  Currently over 1000 species have been named and are very morphologically/anatomically diverse.  Dinosaurs belong to the clade Dinosauria which is within the clade Archosauromorpha.  Within Archosauromorpha, dinosaurs belong to the clade Ornithodira which would eventually become modern birds and is known to feature the following characteristics (among others):

• Supra-acetabular crest (Fig. 1)
• Enlarged 4th trochanter (Fig. 2)
• Antorbital fenestra (Fig. 3)
• Digitigrade stance (Fig. 4)

Fig. 1.  Supra-acetebular crest outlined with arrow pointing to the acetabulum which is mostly open for the head of the femur.

Fig. 2.  The enlarged fourth trochanter of the femur (outlined in red), which appears pendant like in this ornithichian dinosaur, Edmontosaurus sp.

Fig. 3.  Antorbital fenestra of saurichian dinosaur.

Fig. 4.  This saurichian dinosaur (Coelophysis) displays the typical digitigrade stance of standing on the tips of the toes.

Dinosauria is currently believed to be a monophyletic clade based on the following characters (among others):

Fig. 5. A cladogram showing the relationship between onithischian and saurischian dinosaur clades. 

• Elongate vomers that reach caudally to at least the level of the antorbital fenestra
• Three or more sacral vertebrae
• Scapulocoraoical glenoid fossa facing posteriorly (Fig. 6)
• Low deltopectoral crest that runs one-third or half the length of the shaft of the humerus
• Three or fewer phalanges on Digit IV of the manus
• Acetabulum completely or mostly open for the head of the femur (Fig. 1)
• Femur with a head and neck that is distinct from shaft
• Greatly reduced fibula (Fig. 7)
• Well-developed ascending process of astragalus (a bone of the ankle)

       Fig. 6. The posteriorly facing glenoid fosa of saurischian dinosaurs (shown in Tyrannosaurus rex, left, and Nannotyrannus sp., right)

**There is debate about whether the skeleton on the right, named Jane, is of a juvenile T. rex or of a separate genus, Nannotyrannus. If you are interested in reading more about this debate, visit: Craniofacial Ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria) 

    Fig. 7.  Two characteristics of ornithiscian dinosaurs (shown in Triceratops horridis):  a greatly reduced fibula (indicated by a red arrow) and a greatly reduced Digit V of the pes which also lacks phalanges (circled in red).

Within Dinosauria, there are two clades of non-avian dinosaurs which are differentiated on the basis of their pelvic girdle: 

     1.     Saurischia, or "lizard-hipped" dinosaurs, have a pubis that extends anteriorly                 and an  ischium that extends posteriorly. It is from this clade that modern birds are                  descendents.

Fig. 8

     2.     Ornithischia, or "bird-hipped" dinosaurs, have a pubis that extends posteriorly               parallel to the ischium.  

Fig. 9

Some other characteristics of Saurichians (among others) are listed below:

• Temporal musculature extending onto frontal
• Lateral overlap of quadratojugal onto jugal
• Elongate posterior cervical vertebrae
• Postzygapophyses set lateral to prezygapophyses
• Epipophyses present on postzygapophyses of anterior cervical vertebrae
• Presence of accessory intervertebral articulation on dorsal/trunk vertebrae
• Manus more than 45% length of humerus and radius combined (Fig. 10)
• 3-pronged pelvic girdle with pubis projecting anteriorly and ventrally (Fig. 8)

   

             Fig. 10.  The humerus (A), radius (B) and manus (C) of a saurichian dinosaur (Allosaurus fragilis).  Note how the manus is about half the length of the humerus and radius combined.

    Some other characteristics of Ornithischians (among others) include:

• Rostral tip of premaxilla toothless and roughened (Fig. 11)
• Horizontal and broadly arched palatal process of premaxilla
• Maxilla excluded from the margin of the external naris by a large lateral process of the premaxilla which meets the nasal
• Reduced antorbital fenestra (Fig. 11)
• Ventral margin of the antorbital fenestra that parallels the maxillary tooth row (Fig. 12)
• Palpebral in the orbit
• Prefrontal with a long caudal ramus that overlaps the frontal
• Subrectangular quadratojugal lying behind the infratemporal fenestra
• Elongate, massive quadrate (Fig. 11)
• Pre-dentary bone at the front of the mandible (Fig. 11)
• Dorsal border of the coronoid eminence formed by the dentary
• Mandibular condyle set below the tooth row (Fig. 13)
• Buccal emargination suggesting the possession of cheeks (Fig. 11)
• Cheek teeth with low triangular crowns with a well-developed cingulum beneath
• Crowns of check teeth with low and bulbous base
• Adjacent crowns of both maxillary and dentary teeth overlapping
• Recurvature absent in maxillary and dentary teeth
• Maximum tooth size near the middle of the maxillary and dentary tooth rows
• At least 5 sacral vertebrae
• Gastralia absent
• Ossified tendons at least above the sacral region
• Opisthopubic pelvis; pubis with small prepubic process (Fig. 9)
• Ilium with lateral swelling of the ischial tuberosity
• Iliac blade with a long and thin preacetabular process and a deep caudal process (Fig. 9)
• Pubis with an oburator notch, rather than a foreamen; obutator foramen formed between the pubis and ischium
• Distal pubic and ischial symphyses
• Pubic symphysis restricted to its dorsal end
• Ischial symphysis restricted to its distal end
• Pendant fourth trochanter on the femur 
• Finger-like lesser trochanter on the femur
• Digit V of the foot reduced to a small metatarsal with no phalanges (Fig. 7)
• Pubis typically directed posteriorly (Fig. 9)

          Fig. 11. Examples of Ornithiscian Dinosaurs. A. Parasaurolophus B. Corythosaurus C. Edmontosaurus  D. Diabloceratops.  All of these dinosaurs exhibit massive, elongate quadrate bones (red arrows), reduced antorbital fenestrae* (blue arrows), a predentary bone anterior to the dentary (green arrows), a toothless premaxilla (visible at the front of the rostrum), & indications of cheeks (shown by the yellow curves).  * Note that the antorbital fenestra is not visible in Corythosaurus (B.) 

     Fig. 12.  The ventral margin of the antorbital fenestra is parallel to the maxillary tooth row in this ornithischian dinosaur, Parasaurolophus sp.

     Fig. 13.  The mandibular condyle (blue line) is below the tooth row (red line) of an ornithischian dinosaur, Triceratops horridis.

      In this lab we also examined examples of other clades within Tetrapoda, including Ichthyosaurs (Fig. 15), Synapsids (Fig.16 & 17), Sphenodon (Fig. 18), Crocodylians (Fig. 19), Diatryma (Fig. 20) & Pterosaurs (Fig. 21). All of these clades, and their relationships to one another and to Dinosaurs, can be seen in the cladogram below (Fig. 14). 

Fig. 14. A cladogram of Tetrapoda. 

      

                     Fig. 15. Ichthyosaurus (artists reconstruction of an Ichthyosaur) and Stenopterigius (fossil Pleisiosaur), both fish-like Euryapsid reptiles  

               Fig. 16. Dimetrodon limbatus, an early synapsid. This organism has ONLY subtemporal fenestration.

Fig. 17. Three examples of early mammals, all of which have synapsid skull fenestration.
(From left to right: Irish Elk, Mastodon, & Saber-toothed cat) 

Fig. 18. The tuatara (Sphenodon punctatus) is an extant diapsid reptile, sister to Squamata in the clade Lepidosauromorpha.  

  Fig. 19. Phytosaurus (a member of Crocodylotarsi)  is a croc-like diapsid reptile with antorbital fenestration and dorsally-positioned orbits and nares. 

Fig. 20. Diatryma is a large, extinct, predatory bird that exhibits a typical bird-like pelvis with a posteriorly facing pubis. This bird is NOT a relative of the ornithischian dinosaurs, however, it is an ancestor to the modern Anseriformes (ducks & geese).   

Fig. 21. Pterosaurs are an extinct group of winged ornithodirans, NOT dinosaurs.

                                                                 --JT & KJ