Thyreophora

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Thyreophorans Temporal range: Ma PreꞒ Ꞓ O S D C P T J K Pg N
Skeletal mount of Gastonia burgei, BYU Museum of Paleontology
Skeletal mount of Stegosaurus stenops, Natural History Museum , London
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	†Ornithischia
Clade:	†Genasauria
Clade:	†Thyreophora Nopcsa, 1915
Subgroups
†Bienosaurus †Emausaurus †Jakapil?[1] †Laquintasaura? †Lesothosaurus? †Lusitanosaurus †Scelidosaurus †Scutellosaurus †Tatisaurus †Yuxisaurus †Eurypoda Sereno, 1986 †Ankylosauria †Stegosauria
Thyreophorans distribution map

Thyreophora ("shield bearers", often known simply as "armored dinosaurs") is a group of armored ornithischian dinosaurs that lived from the Early Jurassic until the end of the Cretaceous.

Thyreophorans are characterized by the presence of body armor lined up in longitudinal rows along the body. Primitive forms had simple, low, keeled scutes or osteoderms, whereas more derived forms developed more elaborate structures including spikes and plates. Most thyreophorans were herbivorous and had relatively small brains for their body size.

Thyreophora includes various subgroups, including the clades Ankylosauria and Stegosauria. In both clades, the forelimbs were much shorter than the hindlimbs, particularly in stegosaurs. Thyreophora has been defined as the group consisting of all species more closely related to Ankylosaurus and Stegosaurus than to Iguanodon and Triceratops. It is the sister group of Cerapoda within Genasauria.^[2]

Groups of thyreophorans

Basal thyreophorans

Basal thyreophorans form a grade leading to Ankylosauria and Stegosauria,^[3][4] or are instead sister to Ankylosauria with Stegosauria being more basal than either of them.^[5]

These were small-to-medium size dinosaurs with small, primitive plates. Some of them are thought to have walked bipedally. The majority of these, such as Scelidosaurus, Scutellosaurus, Emausaurus, and Yuxisaurus are known from the Northern Hemisphere, in North America, Europe and China.

Ankylosauria

Among the Ankylosauria, the two main groups are the

Ankylosaurids are one of the two families of Euankylosauria. They 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.

Nodosauridae

Nodosaurids, the other family in the Euankylosauria, may actually include the ancestors of the ankylosaurids. They lived during the middle Jurassic (approx 170 mya) on up through the late Cretaceous (66 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.

Parankylosauria

The Parankylosauria are a far more basal group of ankylosaurs recognized as a distinct group in 2021. They may have diverged from euankylosaurs during the mid-Jurassic. Unlike the euankylosaurians, these had a Gondwanan distribution, being known from southern South America, Australia, and Antarctica. They retain more basal traits such as longer and more slender limbs, but the most distinctive trait are their tail weapons or macuahuitls (named after the weapon of the same name), which consist of a flat array of osteoderms that form a fan-like structure on the underside of the tail. This structure is similar to but distinct from the thagomizers of stegosaurians and the tail clubs of ankylosaurids. Macahuitls are completely known from Stegouros and possibly from fragmentary remains in Antarctopelta.^[6]

Stegosauria

The clade Stegosauria comprises Stegosauridae and Huayangosauridae. These dinosaurs lived mostly from the Middle to Late Jurassic, although some fossils have been found in the 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.

Classification

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.

Phylogeny

Thyreophora was first named by

In 2021, an international group of researchers led by Daniel Madzia registered almost all of the most commonly used ornithischian clades under the International Code of Phylogenetic Nomenclature, with the intent of standardizing their definitions. According to Madzia et al., Thyreophora defined as the largest clade containing Ankylosaurus magniventris and Stegosaurus stenops but not Iguanodon bernissartensis and Triceratops horridus.^[2]

The following

Thyreophora	Scutellosaurus Emausaurus Scelidosaurus Eurypoda Stegosauria Huayangosaurus Stegosauridae Ankylosauria Parankylosauria Euankylosauria Ankylosauridae Nodosauridae

In 2020, as part of his monograph on

Thyreophora	Stegosauria   Scutellosaurus Emausaurus      Scelidosaurus Ankylosauria

See also

• thyreophoroi, Greek soldiers bearing a thyreos shield
• List of thyreophoran type specimens

References

1. doi:10.1080/14772019.2023.2205433
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2. ^
   PMID 34966571
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3. S2CID 86002282.{{cite journal}}: CS1 maint: multiple names: authors list (link
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4. ^
   PMID 35953515
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5. ^
   ISSN 0024-4082
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6. ^
   S2CID 244799975
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7. ^ Nopcsa, Ferenc (1915). "Die dinosaurier der Siebenbürgischen landesteile Ungarns" (PDF). Mitteilungen aus dem Jahrbuche der KGL. 23: 1–24.
8. ^ Sereno, Paul (1986). "Phylogeny of the bird-hipped dinosaurs (order Ornithischia)". National Geographic Research. 2 (2): 234–256.
9. doi:10.1127/njgpa/210/1998/41
   .