Description
Thyreophorans include well-known
suborders such as the
Ankylosauria and
Stegosauria as well as
lesser-known groups. Among the Ankylosauria, the two main groups are the
Ankylosaurids and
Nodosaurids. In both
groups, the forelimbs were much shorter than the hindlimbs, and this was
particularly exaggerated in stegosaurs. The
clade has been defined
as the group consisting of all species more closely related to
Ankylosaurus than to
Triceratops.
Thyreophora is the
sister group of the
Cerapoda within the
Genasauria.
Ankylosaurids
are noted by the presence of a large tail club composed of distended vertebrae that have
fused into a single mass. They were heavy-set and heavily armored from head
to tail in bony armor, even down to minor features such as the eyelids.
Spikes and nodules, often of
horn, were set into the
armor. The head was flat, stocky, with little or no "neck", roughly
shovel-shaped and characterized by two spikes on either side of the head
approximately where the ears and cheeks were.
Euoplocephalus tutus
is perhaps the best-known ankylosaurid.
Nodosaurids,
the other family in the Ankylosauria, may actually include the ancestors of
the ankylosaurids. They lived during the middle Jurassic (approx 170 mya) on
up through the late Cretaceous (65 mya) and, while armored as the
ankylosaurids, did not have a tail club. Instead, the bony bumps and spikes
that covered the rest of their body continued out to the tail and/or were
augmented with sharp spines. Two examples of nodosaurs are
Sauropelta and
Edmontonia, the latter
most notable for its formidable forward-pointing shoulder spikes.
The
Stegosauria suborder comprises the Stegosauridae and Huayangosauridae. These
dinosaurs lived mostly from the Middle to Late Jurassic, although some
fossils have been found in the Early Cretaceous. Stegosaurs had very small
heads with simple, leaf-like teeth. Stegosaurs possessed rows of plates
and/or spikes running down the dorsal midline and elongated dorsal vertebra.
It has been suggested that stegosaur plates functioned in control of body
temperature (thermoregulation)
and/or were used as a display to identify members of a species, as well as
to attract mates and intimidate rivals. Well known stegosaurs are
Stegosaurus and
Kentrosaurus.
Taxonomy
While ranked taxonomy has largely fallen out of favor
among dinosaur paleontologists, a few 21st century publications have
retained the use of ranks, though sources have differed on what its rank
should be. Most have listed Thyreophora as an unranked taxon containing the
traditional suborders Stegosauria and Ankylosauria, though Thyreophora is
also sometimes classified as a suborder, with Ankylosauria and Stegosauria
as infraorders.
