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)
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Fig. 1.
Supra-acetebular crest outlined with arrow pointing to the acetabulum
which is mostly open for the head of the femur.
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Fig. 2. The enlarged
fourth trochanter of the femur (outlined in red), which appears pendant like in
this ornithichian dinosaur, Edmontosaurus
sp.
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Fig.
3. Antorbital fenestra of saurichian
dinosaur. |
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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):
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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)
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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) |
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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).
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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.
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Fig. 8 |
2. Ornithischia, or "bird-hipped" dinosaurs, have a pubis that extends posteriorly parallel to the ischium.
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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)
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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)
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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.)
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Fig. 12. The ventral
margin of the antorbital fenestra is parallel to the maxillary tooth row in
this ornithischian dinosaur, Parasaurolophus
sp.
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Fig. 13. The
mandibular condyle (blue line) is below the tooth row (red line) of an
ornithischian dinosaur, Triceratops
horridis.
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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).
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Fig. 14. A cladogram of Tetrapoda. |
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Fig. 15. Ichthyosaurus (artists reconstruction of an Ichthyosaur) and Stenopterigius (fossil Pleisiosaur), both fish-like Euryapsid reptiles |
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Fig. 16. Dimetrodon limbatus, an early synapsid. This organism has ONLY subtemporal fenestration. |
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Fig. 17. Three examples of early mammals, all of
which have synapsid skull fenestration. (From left to right: Irish Elk,
Mastodon, & Saber-toothed cat) |
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Fig. 18. The tuatara (Sphenodon punctatus) is an extant
diapsid reptile, sister to Squamata in the clade Lepidosauromorpha. |
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Fig. 19. Phytosaurus (a member of Crocodylotarsi) is a croc-like
diapsid reptile with antorbital fenestration and dorsally-positioned orbits and
nares.
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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). |
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Fig. 21. Pterosaurs are an extinct group of winged ornithodirans, NOT dinosaurs. |
--JT & KJ
Very nice. Closely mirrors what I took in a dinosaurs "bird course" at university.
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