Laboratory #4: Dinosauria: Non-Crocodylian Archosauromorphs by Alex and Julie (7 February 2014)

A phylogenetic tree from the lab handout showing the evolutionary relationships between various groups of Saurischian and Ornithischian dinosaurs.

Today’s lab was a field trip to the Cleveland Museum of Natural History to examine fossils of dinosaurs and other members of extinct reptilian clades. A radiation of dinosaur diversity occurred during the Mesozoic Era (220-65 million years ago), and to date, about 1,000 species have been described. As shown in the hierarchical classification below, dinosaurs fall within the Archosauromorpha clade of Reptilia (which also includes Crocodylia and possibly Testudines), specifically the Ornithodira (which also includes the SEPARATE lineage of Pterosauria). 

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

 

(CW from upper left: Source 1, Source 2, Source 3)

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)

There are two main groups that compose Dinosauria: Saurischia and Ornithischia. Members of the clade Saurischia are referred to as the "lizard-hipped" dinosaurs. The term "lizard-hipped" refers to the condition of the pubis, which extends anterioventrally. Examples of Saurischian dinosaurs we saw at the museum are: Nannotyrannus, Tyrannosaurus rex, Haplocanthosaurus delfsi, Allosaurus fragilis, and perhaps Coelophysis.

• Members of Saurischia can be identified based on the following features: 

1) Temporal musculature extending onto frontal

2) Lateral overlap of quadratojugal onto jugal

3) Elongate posterior cervicals

4) Postzygapophyses set lateral to prezygapophyses

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

7) Manus more than 45% length of humerus and radius combined

8) 3-pronged pelvic girdle with pubis projecting anteriorly and ventrally

 

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

This specimen of Corythosaurus shows the diapsid skull condition and a reduced antorbital fenestra. Note the elongated quadrate (articulates with the lower jaw; indicated by the arrow), the "rough" pre-dentary bone in front of the mandible, and the mandibular condyle that is inferior to the tooth row.

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

31) Digit V of the foot reduced to a small metatarsal with no phalanges

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 exhibits the diapsid skull condition and a prominent antorbital fenestra.

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.