MAJOR GROUPS OF THYREOPHORANS
Thyreophora are united by various skeletal attributes, the most obvious of which is:
The derived thyreophorans are the plated Stegosauria (shingled lizards) and the heavily armored Ankylosauria (fused lizards). Both clades are present by the Middle Jurassic. There are several Early Jurassic taxa which fall outside either of the two advanced clades.
Lesothosaurus of the Early Jurassic of southern Africa may be the oldest and most primitive known thyreophoran, based on several recent new analyses. If so, it is united with the later thyreophorans on various skeletal traits that are outside the scope of this course. Importantly, it would be the only known unarmored thyreophoran. (But we would expect the oldest ones to have been unarmored.)
The oldest and most primitive definite (i.e., armored) thyreophoran is Scutellosaurus of the Early Jurassic of western North America. It was a 1.5 m long biped (possibly facultative biped) not very dissimilar to other primitive ornithischians like Lesothosaurus or Hexinlusaurus: small herbivores with small hands. The primary distinction of Scutellosaurus is the presence of a great many small osteoderms over the body. These would protect against small-bodied predators, but might not help against the new larger theropods that had begun to appear in the Early Jurassic.
In response, thyreophorans evolved larger size and heavier armor, as seen in Emausaurus and Scelidosaurus (new specimen shown here), both of Europe. The larger body size (3-4 m long) and proportionately larger osteoderms may have been more effective defense against attakcing predators, but forced them onto all fours (at least for Scelidosaurus: in other words, they were obligate quadrupeds.
(Note that the hypothesis shown here is that Scutellosaurus, Emausaurus, and Scelidosaurus were progressive closer to the Stegosauria-Ankylosauria clade (Eurypoda). However, some paleontologists have considered Emausaurus to be a primitive stegosaur, and others that Scelidosaurus was the oldest and most primitive ankylosaur. However, eurypods share a number of transformation not found in these Early Jurassic taxa: these include:
From a Scelidosaurus-like ancestor, the stegosaurs evolved armor that was less covering and more concentrated. While they had some small osteoderms in their skin (particularly around the neck and the hips), most of their armor was specialized as:
In general, the stegosaur armament suggests active defense: the dinosaur probably turned in response to attacking predators, trying to keep the tail towards the theropod so that it could use its thagomizer. Damaged thagomizer spikes and theropod bones with thagomizer-generated puncture wounds confirms their use in defense.
Early stegosaurs were only about 2.5-3 m long, but the most derived forms ranged up to 9 m or more. Their narrow snouts suggests that they were rather picky eaters (that is, instead of munching a lot of plants at once, they were selective as to which ones they chomped.) Although they were obligate quadrupeds in terms of locomotion,
Primitive stegosaurs include Middle Jurassic Huayangosaurus and Late Jurassic Chungkingosaurus, Gigantspinosaurus and Tuojiangosaurus (all from China). More derived stegosaurs form the clade Stegosauridae, and include Late Jurassic Dacentrurus and long-necked Miragaia of Europe, Kentrosaurus of eastern Africa, and the Stegosaurinae.
Stegosaurids had dorsoventrally stretched neural arches and disproportionately short forelimbs, giving them an odd profile.
The most derived stegosaurids (Stegosaurinae) lacked shoulder spines (also missing in Tuojiangosaurus), had only plates rather than spikes along the back (again, shared with Tuojiangosaurus), and had alternating rather than parallel plates. This advanced group includes the two Late Jurassic western North American genera (Hesperosaurus and famous Stegosaurus [also known from Portugal]), and Early Cretaceous Wuerhosaurus of China, the last of the stegosaurs. (Some stegosaur fragments are known from Europe about the same age as Wuerhosaurus, but are not distinctive enough to place within the stegosaur phylogeny).
Stegosaurs are relatively common in Middle and Late Jurassic formations (especially so in China), are present but rare in some European assemblages, and vanish before the end of the Early Cretaceous. Claims of later stegosaurs have so far turned out to be either mis-dated or misidentified.
While the stegosaurs evolved active defense, the ankylosaurs (at least at first) seem to have been selected for passive defense: the ability to stay put and absorb damage from an attack. They were even more extensively armored than Scelidosaurus, and were characterized by:
Ankylosaur systematics remains contentious. There does seem to be a well-supported clade comprised of Early Cretaceous Sauropelta and Late Cretaceous Edmontonia and Panoplosaurus (all of North America): the Nodosauridae. There is also the well-supported clade Ankylosauridae discussed below. There are a number of other ankylosaurs, however, which are considered by some to be either primitive nodosaurids and ankylosaurids; and/or to lie outside a nodosaurid-ankylosaurid clade; and/or to form their own clade Polacanthidae. The latest study places such forms as either basal nodosaurids or basal ankylosaurids. However, the oldest ankylosaur (Middle-Late Jurassic Tianchisaurus of China) was not included in this study and may represent an ankylosaur outside the Nodosauridae-Ankylosauridae clade.
Some primitive ankylosaurs, such as the primitive Early Cretaceous ankylosaurids Minmi of Australia and Liaoningosaurus of China, were fairly small at 2-3 m long.
Gargoyleosaurus of the Late Jurassic of North America and Early Cretaceous Gastonia of North America, Polacanthus and Hylaeosaurus of Europe share flattened laterally-directed osteoderms and a sacral shield of fused osteoderms over the pelvis. These were once thought to form a clade Polacanthidae, but the latest study shows that these 3-5 m long North American and European forms were primitive nodosaurids.
Advanced nodosaurids are united by a large muscle attachment projection on the scapula and often have a tall laterally-directed shoulder spine. In addition to the North American taxa listed above, nodosaurids include many additional Cretaceous North American taxa such as Animantarx, some recently described taxa from the Late Cretaceous of Asia, and Struthiosaurus of Late Cretaceous Europe. They range from about 3-7 m in length. They are known (as shown above) from Europe, North America, and Asia; there are possible nodosaurid bones from other regions (including Antarctopelta from Antarctica), but these may belong to more primitive ankylosaurs.
The most specialized clade of ankylosaurs is Ankylosauridae. This clade is transformed relative to other armored dinosaurs in having:
Advanced ankylosaurids are only known from the Late Cretaceous of Asia and western North America at present; similar distributions are known for advanced pachycephalosaurs, ceratopsids, and tyrannosaurids.
EVOLUTIONARY PATTERNS IN THYREOPHORA
Probably the most conspicuous aspect of their evolution. Thyreophorans begin with a set of small scutes; develop larger scutes at the expense of bipedalism (and speed); than split between specialists in active (Stegosauria) vs. passive (Ankylosauria) defenses. Within the ankylosaurs, though, Ankylosauridae independantly evolves an active defensive tail weapon.
Stegosaurs and ankylosaurs, as sister taxa, have their origins at the same time. However, stegosaurs flourish first (in the Middle and Late Jurassic), only to peter out during the Early Cretaceous and disappear before its end. Ankylosaurs are typically rare in the Jurassic (although at one location Gargoyleosaurus is very common), but become abundant in the Cretaceous.
Narrow-snouted basal thyreophorans and stegosaurs differ from broader-snouted ankylosaurs. The latter have a greatly reduced predentary bone, which may have allowed for more complex motion of the dentaries (for additional munching power) while feeding.
Thyreophorans are only rarely found in mass death assemblages, and so (unlike some sauropods, ornithopods, and marginocephalians) probably did not live in large groups.
The spikes, plates, and osteoderms of thyreophorans almost certainly had a defensive function. But many are very broad (particularly stegosaur plates), and the patterns tend to be specific to each species. They may have served an additional function: as visual displays of species recognition. There may also have been a sexual display function to them, but at present it is uncertain if the variation we see in some thyreophorans is from sexual dimorphism or if it is from multiple species of the same genus living together.
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