The evolution of Thyreophora

Thyreophora

We can be reasonably confident that the evolution of thyreophorans embodied increasing size and a return to quadrupedality, because the cladograms show that the quadrupedal eury-podan clades were all derived relative to these primitive forms. The primitive thyreophorans suggest an evolutionary sequence from gracile, small, bipedal creatures like Scutellosaurus to larger, quadrupedal dinosaurs like Scelidosaurus (see Figures II.4 and II.5).

Eurypoda

It is not difficult to imagine the evolution of an ankylosaur from an armored, primitive quadruped like Scelidosaurus. The cladogram suggests that the basal eurypodan must have looked something like Scelidosaurus, and the step to a larger, more powerful, more heavily armored primitive ankylosaur or stegosaur is easy to conceive.

Stegosauria

Stegosauria is a monophyletic clade of ornithischian dinosaurs, diagnosed on the basis of a number of important features shown in Figure 5.22. The ancestral stegosaur must have been an animal with spine-shaped osteoderms and fore- and hindlimbs of not too dissimilar lengths. Within Stegosauria, the basal split is between Huayangosaurus on the one hand and remaining species on the other (Figure 5.23). This divergence took place sometime before the latter half of the Middle Jurassic. Huayangosaurus itself has a number of uniquely derived features shared by a more inclusive group of stegosaurs, Stegosauridae (Figure 5.23)

Within Stegosauridae, Dacentrurus represents the most basal form. The remainder of Stegosauridae includes Stegosaurus, Wuerhosaurus, Kentrosaurus, and Tuojiangosaurus. The evolution of this group was evidently characterized by an increase in the difference in length between the fore- and hindlimbs (Figure 5.23).

Finally, there is Stegosaurus itself, the best-known, most common stegosaur. Stegosaurus must have evolved its distinctive plates from the spiny, conical osteoderms present in its ancestry. Plates, however, are only known in Stegosaurus, and their evolution occurred sometime during the Middle or early Late Jurassic.

Ankylosauria

Reflecting the importance of heavy armor to ankylosaurs and the ease of its preservation, it is not surprising that armor and/or its support comprise the majority of derived features uniting

Ankylosaur Temporal Fenestra
Thyreophora

Ornithischia

(b) 10 cm

Figure 5.22. Cladogram emphasizing the monophyly of Eurypoda and Stegosauria. Derived characters include: at 1 (Eurypoda), bones that fuse to the margins of the eye sockets, loss of a notch between the quadrate (see Figure 4.6) and the back of the skull, and enlargement of the anterior part of the ilium; at 2 (Stegosauria), back vertebrae with very tall neural arches and highly angled transverse processes, loss of ossified tendons down the back and tail, a broad and plate-like acromial process, large and block-like wrist bones, elongation of the prepubic process, loss of the first pedal digit, and loss of one of the phalanges of the second pedal digit, and a great number of features relating to the development of osteoderms, and formation of long spines on plates from the shoulder toward the tip of the tail. (b) The front and left lateral view of one of the back vertebrae of Stegosaurus. Note the great height of the neural arch; a diagnostic stegosaurian character at 2.

Figure 5.22. Cladogram emphasizing the monophyly of Eurypoda and Stegosauria. Derived characters include: at 1 (Eurypoda), bones that fuse to the margins of the eye sockets, loss of a notch between the quadrate (see Figure 4.6) and the back of the skull, and enlargement of the anterior part of the ilium; at 2 (Stegosauria), back vertebrae with very tall neural arches and highly angled transverse processes, loss of ossified tendons down the back and tail, a broad and plate-like acromial process, large and block-like wrist bones, elongation of the prepubic process, loss of the first pedal digit, and loss of one of the phalanges of the second pedal digit, and a great number of features relating to the development of osteoderms, and formation of long spines on plates from the shoulder toward the tip of the tail. (b) The front and left lateral view of one of the back vertebrae of Stegosaurus. Note the great height of the neural arch; a diagnostic stegosaurian character at 2.

Stegosauria

Stegosauridae

Stegosauria

Figure 5.23. Cladogram of Stegosauria, with Ankylosauria and Scelidosaurus as successively more distant relatives. Derived characters include: at 1. large antitrochanter, long prepubic process, long femur, absence of lateral rows of osteoderms on the trunk; at 2, widening of the lower end of the humerus, an increase in femoral length, and an increase in the height of the neural arch of the back and tail vertebrae. Relationships of genera of the ultimate node on the cladogram remain uncertain.

Eurypoda the clade Ankylosauria (Figure 5.24). Ankylosaur evolution followed two principal pathways since the origin of the group sometime in the Jurassic: Ankylosauridae and Nodosauridae.

Primitively in all ankylosaurs, the beak was scoop-shaped but relatively narrow (although slightly broader than in stegosaurs) and remained so in the nodosaurids and in the

Thyreophora

Figure 5.24. Cladogram of Eurypoda, emphasizing the monophyly of Anky-losauria. Derived characters include: at 1, closure of antorbital and upper temporal fenestrae, ossification and fusion of keeled plate onto side of lower jaw, fusion of first tail vertebrae to sacral vertebrae and ilium, rotation of ilium to form flaring blades, closure of hip joint, development of dorsal shield of symmetrically placed bony plates and spines.

Thyreophora

Ornithischia ankylosaurid Shamosaurus. In all other ankylosaurids, by contrast, the beak became very broad, which matched the general broadening of the animal and the development of tail-clubs.

Ankylosauridae. Ankylosaurids share a suite of derived features. Figure 5.25 highlights the relationships among ankylosaur genera, as well as some of the key characters supporting these relationships.

Eurypoda

Ankylosauria

Figure 5.25. Cladogram of Ankylosauridae, with its two closest relatives, Nodosauridae and Stegosauria. Derived characters include: at 1, pyramidal squamosal boss, shortening of premaxil-lary palate, presence of premaxillary notch, rostrolaterally directed mandibular ramus of pterygoid, tail-club; at 2, anterodorsal and posteroventral arching of palate, vertical nasal septum, rugose and crested basal tubera; at 3, caudal end of postaxial cervical vertebrae dorsal to cranial end, fusion of sternals.

Eurypoda

Ankylosauria

Figure 5.25. Cladogram of Ankylosauridae, with its two closest relatives, Nodosauridae and Stegosauria. Derived characters include: at 1, pyramidal squamosal boss, shortening of premaxil-lary palate, presence of premaxillary notch, rostrolaterally directed mandibular ramus of pterygoid, tail-club; at 2, anterodorsal and posteroventral arching of palate, vertical nasal septum, rugose and crested basal tubera; at 3, caudal end of postaxial cervical vertebrae dorsal to cranial end, fusion of sternals.

Figure 5.26. Cladogram of Nodosau-ridae, with its two closest relatives, Ankylosauridae and Stegosauria. Derived characters include: at 1, knob-like acromion on scapula, occipital condyle derived from but one bone (the basioc-cipital, large knob (supraorbital boss) above the eye; at 2, premaxillary teeth lacking, distinct rostro-dorsal feature of secondary palate.

Figure 5.26. Cladogram of Nodosau-ridae, with its two closest relatives, Ankylosauridae and Stegosauria. Derived characters include: at 1, knob-like acromion on scapula, occipital condyle derived from but one bone (the basioc-cipital, large knob (supraorbital boss) above the eye; at 2, premaxillary teeth lacking, distinct rostro-dorsal feature of secondary palate.

Eurypoda

Ankylosauria

Nodosauridae. Turning to the other great clade of ankylosaurs, nodosaurids share a number of derived features, as shown in Figure 5.26, particularly the well-developed acromial process. This musculature, and the parascapular spines that accompanied it, may have played a role in their defensive behavior. Nodosaurids changed little during their tenure on Earth; however, various diagnostic characters allow us to learn something of their relationships (Figure 5.26).

0 0

Responses

  • nicholas hazard
    What is the origin of the Thyreophorians?
    2 years ago

Post a comment