Definition and Unique Characteristics of Thyreophora

Thyreophora is a stem-based clade within Ornithischia and Genasauria, and a sister clade to Cerapoda within Genasauria. Thyreophora contains two stem-based clades

Ankylosauridae Nodosauridae

Stegosauria

Scelidosaurus

Emausaurus

Scutellosaurus

Scutellosaurus

FIGURE 12.1 Cladogram showing interrelationships between basal thyreophorans (Scelidosaurus, Scutellosaurus, and Emausaurus) and other clades within Thyreophora, particularly Ankylosauria and Stegosauria.

of importance (Fig. 12.1): Ankylosauria, which includes Nodosauridae (nodosaurids) and Ankylosauridae (ankylosaurids); and Stegosauria (stegosaurs). Members of Thyreophora have been known since near the beginning of dinosaur studies. The nodosaurid Hylaeosaurus was named by Gideon Mantell in 1833, followed several decades later by T. H. Huxley's naming of the nodosaurid Polacanthus and ankylosaurid Acanthopholis (Chapter 3). Richard Owen also described the basal thyreophoran Scelidosaurus from a well-preserved specimen in 1861. By comparison, stegosaurs were among the later discoveries. Public knowledge of their existence commenced with the naming of Stegosaurus in 1877 by O. C. Marsh. A few years before the first description of Stegosaurus, fragments of the European stegosaurs Lexovisaurus and Dacentrurus had been studied but were not allied with a stegosaur origin until early in the twentieth century.

For the purposes of this discussion, the following two criteria, especially the first, are key features for thyreophorans:

1 dermal armor, typically as scale-like scutes, in rows parallel to the midline of the body on both lateral and dorsal surfaces of the torso; and

2 well-developed postorbital process associated with the jugal and a palpe-bral (supraorbital bone).

Although the most recognizable trait of a thyreophoran is its dermal armor, this feature is not diagnostic by itself. In fact, an assumption that dermal armor was a foolproof identifier of a former thyreophoran presence led to an interpretation that they lived in South America because isolated osteoderms had been found in some Cretaceous deposits there. Much later, these structures were instead found in association with titanosaurids (Chapter 10). As a result, thyreophoran identifications are now applied more cautiously, and they are still poorly known in South

Eurypoda

Ankylosauria

Eurypoda

As with most clades, long, detailed lists of derived characters and cladograms are used to define Thyreophora. However, this clade can be described more simply by contrasting it with most other dinosaur clades.

America. Another qualifier is that not all thyreophorans were equally armored. Ankylosaurs had hundreds of scutes over a majority of the surface areas of their bodies, whereas stegosaurs only had localized patches and plates. Thus, an initial hypothesis that dinosaur remains are thyreophoran can be made when scutes or any other osteoderms are found in continental Jurassic or Cretaceous deposits, but must be followed by more stringent identifications and analyses.

Another general distinguishing characteristic of thyreophorans is a squat stature caused by short legs in relation to the rest of the body. These legs also have fore limbs about half the length of the hind limbs (a ratio of about 35 : 65). Nearly all thyreophorans were obligate quadrupeds because of these proportions. One basal thyreophoran, Scutellosaurus from the Early Jurassic of the southwestern USA, was possibly bipedal because of its relatively shorter fore limbs. However, other features, such as its large number of osteoderms, a long torso and fore limbs, and big manus, argue more for quadrupedalism. This mixture of features may mean that it was descended from bipedal ancestors and represents a transition between the two types of locomotion (Chapter 6). Secondary quadrupedalism is typical for quadrupedal dinosaurs in general, but was certainly a necessary adaptation for ankylosaurs in particular, once they developed more extensive body armor.

Thyreophorans were ornithischians but their deeply inset cheek teeth also help to unite them with Genasauria. Thyreophoran teeth represented variations on a main theme: they began as laterally compressed teeth crowned by apical denticles, which was also typical of basal ornithischians (Chapter 11). They then became less developed in later thyreophorans. Ankylosaurs and stegosaurs both had cheek teeth but they were more medial (inward). The result was broad shelves on their den-taries and maxillas, which gave them space for large cheeks.

Ankylosaurs and stegosaurs also had beaks in the anterior portions of their skulls with a horny covering called a rhampotheca. In all but one genera of stegosaur, the Middle Jurassic Huayangosaurus of China, the premaxillary teeth are missing. Likewise, ankylosaurids and all but the most primitive nodosaurids, such as the Early Cretaceous Silvisaurus of Kansas, have no premaxillary teeth. This implies that their beaks were the main tools used for cropping plants. Cheek teeth in stegosaurs had triangular profiles and broad crowns but long roots. The crowns were broadest at their bases, forming a cignulum (small shelf), and were distinguished by numerous vertically oriented ridges. Ankylosaur cheek teeth had a comparable morphology but were more poorly-developed versions of stegosaur teeth. Overall, thyreophoran teeth reflect their common ancestry, although some changes through time are evident in their lineages. These differences are attributable to evolutionary trends in response to available foodstuff in Mesozoic terrestrial ecosystems.

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Responses

  • ronald
    What is the appearance of a lexovisaurus?
    1 year ago
  • Albertina
    What charactersitc is common to both nodosaurids and ankylosuarids?
    9 months ago
  • BENILDE
    Which of the following is not characteristic of thyreophorans?
    6 months ago

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