A phylogenetic tree from the lab handout showing the evolutionary relationships between various groups of Saurischian and Ornithischian dinosaurs. |
Archosauromorpha
Archosauria
Crurotarsi
Crocodylia
Ornithodira
Pterosauria
Dinosauria
Ornithischia ("Bird-hipped" dinosaurs)
Saurischia
("Lizard-hipped" dinosaurs)
Carnosauria
Aves
(Birds)- Characteristics of Ornithodira include:
A.
A
supra-acetabular crest
B.
An
antorbital fenestra
C.
An
enlarged fourth trochanter (femur)
D.
A
digitigrade stance (standing on the tips of their toes)
- Synapomorphies that define the monophyletic group Dinosauria include:
1) Elongate vomers that reach caudally to at least the level of the
antorbital fenestra
(Source) |
2) Three or more sacral vertebrae
(Source) |
3) Scapulocoraoical glenoid fossa facing posteriorly
(Source) |
4) Low deltopectoral crest that runs one-third or half the length of
the shaft of the humerus
5) Three or fewer phalanges on the Digit IV of the manus
(Source) |
6) Acetabulum completely or mostly open for the head of the femur
7) Femur with a distinct head and neck that is distinct from shaft
8) Greatly reduced fibula
9) Well-developed ascending process of astragalus (a bone of the
ankle)
(Source 1, Source 2) |
- Members of Saurischia can be identified based on the following features:
1) Temporal musculature extending onto frontal
2) Lateral overlap of quadratojugal onto jugal
5) Epipophyses present on postzygapophyses of anterior cervical
vertebrae
Cervical vertebrae (epi = epipophyses, poz = postzygapophyses, prz = prezygapophyses; Source) |
6) Presence of accessory intervertebral articulation on dorsal/trunk
vertebrae
The Ornithischia are referred to as the “bird-hipped
dinosaurs” because the pubis of members of this group extends posteriorly and
is in contact with the ischium. Examples of Ornithischian
dinosaurs we saw at the museum are: Triceratops
horridis, Corythosaurus, Parasaurolophus, and Edmontosaurus.
- The defining features of Ornithischia include:
1) Rostral tip of premaxilla toothless and roughened
2) Horizontal
and broadly arched palatal process of premaxilla
3) Maxilla
excluded from the margin of the external naris by a large lateral process of
the premaxilla which meets the nasal
4) Reduced
antorbital fenestra
Note the reduced antorbital fenestra, the orientation of the antorbital fenestra relative to the maxillary tooth row, and the palpebral in the orbit (indicated by the arrow). |
5) Ventral
margin of the antorbital fenestra that parallels the maxillary tooth row
6) Palpebral
in the orbit
7) Prefrontal
with a long caudal ramus that overlaps the frontal
8) Subrectangular
quadratojugal lying behind the infratemporal fenestra
9) Elongate,
massive quadrate
10) Pre-dentary
bone at the front of the mandible
11) Dorsal
border of the coronoid eminence formed by the dentary
12) Mandibular
condyle set below the tooth row
13) Buccal
emargination of both upper and lower jaws, suggesting the possession of cheeks
14) Check
teeth with low triangular crowns with a well-developed cingulum beneath
(Source) |
15) Crowns
of check teeth with low and bulbous base
16) Adjacent
crowns of both maxillary and dentary teeth overlapping
17) Recurvature
absent in maxillary and dentary teeth
Note the representative teeth of a species belonging to Ornithischia (H; Source) |
18) Maximum
tooth size near the middle of the maxillary and dentary tooth rows
19) At
least 5 sacral vertebrae
20) Gastralia
absent
21) Ossified
tendons at least above the sacral region
22) Opisthopubic
pelvis; pubis with small prepubic process (on some specimens, this process was
quite large)
23) Ilium with lateral swelling of the ischial tuberosity
24) Iliac
blade with a long and thin preacetabular process and a deep caudal process
25) Pubis
with an oburator notch, rather than a foreamen; obutator foramen formed between
the pubis and ischium
26) Distal
pubic and ischial symphyses
27) Pubic
symphysis restricted to its dorsal end
28) Ischial
symphysis to its distal end
29) Pendant
fourth trochanter on the femur
30) Finger-like
lesser trochanter on the femur
32) Pubis
typically directed posteriorly
Baby T. rex or Nannotyrannus? The debate rages on. |
The
first specimen examined was a fossil of a dinosaur that is the subject of a great debate. The
question is whether this specimen is a young Tyrannosaurus rex or an adult Nannotyrannus. Although we were unable to reach a consensus based on the scant clues available to differentiate the taxa, the specimen is definitely a
member of Saurischia. The skeleton exhibited a pubis that projected anterioventrally
and the manus exceeded 45% the length of the humerus and radius combined.
Mammals belong to Synapsida, a group of amniotes with a singular lower temporal fenestra (subtemporal fenestra) |
There were a few mammalian skeletons on display that
exhibited the synapsid skull condition, which is a single, subtemporal fenestra.
There were also distinct differences between the pelvic girdles of early
mammals and early reptiles, indicating that these groups likely had vastly different styles of locomotion. Synapsida falls within Amniota and gave rise to
modern mammals.
Diatryma,
a powerful avian predator, displayed the typical bird pelvic condition. Although
Aves belongs to Saurischia, the pelvis appears similar to members of
Ornithischia, as the pubis is displaced posteriorly.
All of these photos highlight the inturned head of the femur. The center photo shows the pelvic girdle of a T. Rex and the photo on the right is an H. delfsi specimen. |
The
limbs and girdles of the large saurapod, Haplocanthosaurus
delfsi, and T. Rex revealed a unique feature shared among dinosaurs, but different from
other amniotes alive during the Mesozoic (for example, the members of Synapsida that gave rise to modern mammals).
It is obvious that a dinosaur’s limbs hold the body well above the
substrate, but the shift in orientation of the limbs is more subtle. This change in
orientation is achieved by the medial inturning of the femur, which contacts the
laterally-facing acetabulum of the pelvic girdle.
Coelophysis was an interesting dinosaur on
display. When trying to classify the specimen into either Saurischia or
Ornithischia, the pelvic girdle could offer no definitive answer. However, it
was clear that this animal did possess a classic diaspsid skull and an
antoribital fenestra.
Representatives of Ichthyosauria. The top photo shows a fossilized icthyosaurian birth! (Source) |
Phytosaur fossils showing the diapsid skull condition |
The museum also had a variety of non-dinosaur fossils on
display, including the early eurapsids Ichthyosaurus
and Stenopterygius, as well as Phytosaurus, an early reptile that
appears morphologically similar to the Crocodylotarsi. Euryapsid reptiles have
a single supratemporal fenestra in their skull. Ichythosaurus and Stenopterygius
exemplify the fusiform marine predators that comprise Ichthyosauria, the
“fish-like reptile” group. The fossils of Phytosaurus
exhibited the diapsid skull condition, in which a skull has both subtemporal
and supratemporal fenestrae.
Note the subtemporal fenestra inferior to the orbit |
Dimetrodon limbatus was not a dyapsid reptile, but an
early synapsid. This is evidenced by the possession of a single, subtemporal
fenestra.
The tuatara (Sphenodon punctatus) is a member of Spenodontida, a group
in which there is thought to be only two extant species (depending on who you
ask). The tuatara exhibits a classic
diapsid skull, copulates via “cloacal kissing,” has two pairs of conchae (olfactory structures), and a
chisel-like beak. This species is thought to only inhabit parts of New Zealand.
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