Thyreophora

Introduction

The predominately quadrupedal and armored Thyreophora dinosaurs were a diverse group of herbivores that first appeared during the Early Jurassic period some 200 million years ago. They are considered the most primitive of ornithischian dinosaurs and showed a wide range in size.

Thyreophorans were characterized by rows of bony body armor in the form of dermal ossifications–spines, scutes–along their backs. Specifically, this group is defined as all dinosaurs more closely related to the plated stegosaurs and armored ankylosaurs than to Cerapods and included famous species such as Stegosaurus, Kentosaurus, Ankylosaurus and Euplocephalus, as well as their more basal relatives Scutellosaurus and Scelidosaurus.

Fast Facts

Pronunciation: The term Thyreophora (THIHR-ee-OF-o-ra) was first introduced by Baron Franz von Nopsca in 1915 and means “shield bearers.”

Time Period: Early Jurassic (201 million years ago) to Late Cretaceous (66 million years ago)

Size: 1.2 to 10.7 m (4–35 ft) in length

Weight: 10 kg to 6 ton (22–13,300 lb)

Diet: Herbivorous

Location: World-wide

Lifespan: Up to 30 years of age or more

Homologous Traits

All dinosaurs are defined as either bird-hipped ornithischians or lizard-hipped saurischians. Thyreophora is a subgroup of the Ornithischia. They share all those characteristics that define dinosaurs, such as upright posture, modified fourth and fifth digits on the hands, three-toed feet, specialized crests on the humerus and tibia bones, and a femur with a ball-like head at one end, as well as those traits that define the bird-hipped Ornithischia such as a backwards pointing pubis and ischium, an unpaired predentary bone at the lower jaw, a toothless snout tip, a narrow palpebral “eyelid” bone, leaf-shaped cheek teeth, at least five sacral vertebrae, and hardened tendons above the pelvis area to stiffen the backbone.

Thyreophora includes Stegosauria (which is comprised of Huayangosaurus and Stegosauridae) and Ankylosauria (which is comprised of Ankylosauridae and Nodosauridae). Thyreophora are armored dinosaurs characterized by the rows of dermal ossifications, spines or plates that ran along their arched backs, as well as having hind limbs longer than their fore limbs.

These groups all possess common derived traits (synapomorphies), but there are a number of anatomical characteristics that define each group of thyreophorans. Stegosaurids are characterized by the highly specialized scutes that formed the famous and species defining dermal plates, shoulder spines and paired tail spikes commonly seen on the stegosaurs. Ankylosaurs possessed extensive areas of external bone fused into plates across their backs and sides, as well as odd spikes and fused scutes around their necks and at the back of their large heads. Most also had small horns on their skulls and clubs at the end of their tails.

Evolutionary Divergences

Thyreophorans are often considered to be among the more primitive of ornithischian dinosaurs. The very earliest (basal) thyreophorans were small and bipedal, but later forms were much larger in size, were quadrupedal, and possessed heavy armor. Strong evidence points to the evolution of Thyreophora as one of increasing size and a return to quadrupedality.

Researchers believe that before the end of the Triassic, and certainly by the Early Jurassic, the ornithischian group had split into the armored Thyreophora and unarmored Cerapoda, which include ornithopods and marginocephalians (the ceratopsians and pachycephalosaurs). This phylogenetic division is only based on a small number of characteristics, which suggests that this evolutionary divergence occurred very quickly.

Currently, the earliest known thyreophoran is also one of the smallest. Scutellosaurus only grew to about 1 meter (3.3 ft) long and roamed the North American landscape during the Early Jurassic almost 200 million years ago. The second oldest thyreophorans is Scelidosaurus, a 4 meter (13 ft) quadruped found in Western Europe during the Early Jurassic period some 195 million years ago. Like all Thyreophora, these two species were armored. As with most ancient creatures, however, classifying dinosaurs and understanding evolutionary relationships remains very difficult. The paucity in the fossil record means that paleontologists are often reevaluating classifications as new fossils are found and as better technology is developed. One case in point is that Scelidosaurus was only unearthed in 1980, and yet understanding its phylogenic position has been instrumental in clarifying basal thyreophoran relationships and the evolution of this group.

With the exception of Scutellosaurus and Scelidosaurus, the two oldest known Thyreophora species, all other more derived thyreophorans fall into one of two groups: the Stegosauria and the Ankylosauria. Stegosaurids first appeared in the Middle Jurassic and are characterized by the rows of specialized dermal osteoderms. Ankylosaurians were closely related to the stegosaurs and first appeared in China during the Late Jurassic period and survived until the end of the Cretaceous.

Creatures in This Group

Ornithischia are one of the two orders of Dinosauria, predominately classified by the arrangement of their hips. Basal ornithischian dinosaurs split to form a single Jurassic species known as Lesothosaurus and the Genasauria clade. Lesothosaurus is considered the most basal ornithischian dinosaur, with all other species falling within the Genasauria clade. This clade is separated from Lesothosaurus based on their deep-set rows of teeth, different shaped mandibles, and a reduction in their mandible opening (foramen). Genasauria is currently divided into the Thyreophora (“shield bearer”) and Cerapoda (“horn-foot”).

Thyreophora is a holophyletic clade that includes the stegosaurs, ankylosaurs, and their basal ancestors. These two groups are distinguished from each other by a number of key evolutionary divergences. Stegosauria include those ornithischians more closely related to Stegosaurus stenops than to Ankylosaurus magniventris. They first appeared in the Middle Jurassic and are characterized by the rows of specialized dermal plates, shoulder spines, and tail spikes. These species were particularly successful during the Late Jurassic in terms of diversity and distribution, although some few species lasted until the end of the Early Cretaceous about 140 million years ago. Stegosauria include such species as the Late Jurassic Tuojiangosaurus from Asia, Kentrosaurus from Africa, Huayangosaurus from China, and Stegosaurus from America.

Ankylosauria include those ornithischians more closely related to Ankylosaurus magniventris than to Stegosaurus stenops. They first appeared in the Middle Jurassic, although they were quite rare until the Cretaceous. These quadrupeds were much more heavily armored than the basal thyreophorans and possessed extensive areas of external bone fused into plates across their backs and sides, as well as odd spikes and fused scutes around their necks and at the back of their large heads. Before the end of the Jurassic, the ankylosaurs had split into the distinctive nodosaurids and ankylosaurids. These species diversified and mostly existed during the Cretaceous, and they suffered the same extinction fate as all non-avian dinosaurs at the end of the Late Cretaceous. Species from this group include the west American Ankylosaurus and Euoplocephalus from the Late Cretaceous and the Chinese Pinacosaurus, also from the Late Cretaceous.

Ecology

Generally speaking, herbivorous dinosaurs were less intelligent than predators. Thyreophorans were not the most intelligent dinosaurs, with current body to brain ratio measures, known as the encephalization quotient, placing the carnivorous theropods at the top. While ankylosaurs and stegosaurs were less intelligent than the other ornithischian dinosaurs (ceratopsians and ornithopods) all were more intelligent than the sauropods.

To avoid or defend against the more intelligent carnivores, Thyreophora species developed highly successful defensive armor as a survival strategy. Although there has been paleontological debate regarding whether the function of the dermal osteoderms (special bones embedded in the skin) was behavioral or physiological, most researchers agree that the plates and clubbed tails were used as defensive structures to ward off predator attack and possibly protect their young. The particularly low-slung bodies of the ankylosaurs are also thought to be a defensive technique allowing the animal to crouch when attacked.

Like all dinosaurs, thyreophorans were oviparous and laid several eggs in a clutch. Although fossilized nesting sites are rare, some have been found for both stegosaurs and ankylosaurs, which suggests possible post-hatching parental care. This is also supported by the likelihood that Thyreophora species formed and lived in mixed-age herds.

Thyreophorans were herbivorous and displayed a generalized feeding pattern that involved cropping and plucking of foliage with beak-like mouths and grinding with their rear cheek-teeth. The low-to-ground position of most thyreophoran heads indicates that feeding was restricted to low-lying gymnosperms, such as ferns, horsetails, cycads, and mosses, as well as the later evolving shrubby angiosperms.

Paleontology News

Although the term Thyreophora (shield bearers) was first introduced by Baron Franz von Nopsca in 1915, species of armored dinosaurs were actually named much earlier, including the ankylosaurian Hylaeosaurus named in 1833 by Gideon Mantell and Stegosaurus named in 1877 by Othniel Charles Marsh during the infamous Bone Wars.

When Thyreophora was first proposed in 1915, it included Ceratopsia as well as Scutellosaurus, Scelidosaurus, Stegosauria, and Ankylosauria. However, the Thyreophora clade was redefined in 1986 by Paul Sereno, who placed Ceratopsia as a distinct sister clade.

Despite this long history of discovery and nomenclature, thyreophoran species are among the least studied dinosaurs due to their relatively poor fossil record in comparison to other Ornithischia dinosaurs such as the ceratopsians and hadrosaurs. However, recent fossil finds of new Thyreophora species in China and the United States has rekindled interest in this group, as has the discovery and appreciation of their significant diversity in morphology and ecology. One of the most recent discoveries was a member of Stegosaurus that resided in Europe, which is significant because until this fossil find, Stegosaurus was only thought to have existed in the North American region. Such discoveries support the idea of intermittent fauna exchange between landmasses during the Jurassic Period.

Bibliography

Books

Carpenter, Kenneth. Armored Dinosaurs. Bloomington: Indiana University Press, 2001.

  • A thorough examination of the Thyreophoran dinosaurs, with contributions of various experts within the field and focusing on classification, ecology and evolutionary relationships.

Fastovsky, David E., and David B. Weishampel. Dinosaurs: A Concise Natural History. New York: Cambridge University Press, 2009.

  • A book aimed at non-specialists and those with little understanding or background in dinosaurs, it focuses on many aspects of the natural sciences and how they relate to dinosaur biology, evolution, life history, and classification.

—. Evolution and Extinction of the Dinosaurs. New York: Cambridge University Press, 2005.

  • This is a very well illustrated text, covering complex scientific processes and dinosaur species in an informative and comprehensible way.

Lucas, Spencer George. Dinosaurs: The Textbook. New York: McGraw-Hill, 2004.

  • This book gives students a great précis regarding important aspects of dinosaur discovery, behavior, biology, and classification.

Martin, Anthony J. Introduction to the Study of Dinosaurs. Malden, MA: Blackwell, 2006.

  • Introduction to the Study of Dinosaurs is exactly as the title claims, a comprehensive and up-to-date introduction to dinosaurs, providing scientific based chapters on the major dinosaur clades, as well as anatomical, physiological and behavioral information.

Parker, Steve. Dinosaurus: The Complete Guide to Dinosaurs. Richmond hill, on: Firefly Books, 2009.

  • This text provides useful information on more than 500 dinosaurs, such as discovery and location and characteristics, such as anatomy and diet.

Weishampel, David B., Peter Dodson, and Halszka Osmólska. Dinosauria. Berkeley: University of California Press, 2007.

  • This book is an in-depth text providing resources and scientific papers on dinosaurs, especially looking at the saurichia and ornithisia orders in regards to their evolution, distribution, and ecology.

Journals

Brusatte, Stephen L., et al. “The Origin and Early Radiation of Dinosaurs.” Earth-Science Reviews 101.1–2 (2010): 68–100.

Langer, Max C., Martin D. Ezcurra, Jonathas S. Bittencourt, and Fernando E. Novas. “The Origin and Early Evolution of Dinosaurs.” Biological Review 85 (2010): 55–110.

Main, Russell P., Armand de Ricqlès, John R. Horner, and Kevin Padian. “The Evolution and Function of Thyreophorans Dinosaur Scutes: Implications for Plate Function in Stegosaurs.” Paleobiology 31.2 (2005): 291–314.

Nesbitt Sterling J. “The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades.” Bulletin of the American Museum of Natural History 352 (2011): 1–292.

Additional Works Used

Butler, Richard J., Paul Upchurch, and David B. Norman. “The Phylogeny of the Ornithischian Dinosaurs.” Journal of Systematic Palaeontology 6.1 (2008): 1–40.

Hayashi, Shoji, Kenneth Carpenter, and Daisuke Suzuki. “Different Growth Patterns between the Skeleton and Osteoderms of Stegosaurus (Ornithischia: Thyreophora).” Journal of Vertebrate Paleontology 29.1 (2009): 123–131.

Hopson, James A. “Relative Brain Size and Behavior in Archosaurian Reptiles.” Annual Review of Ecology and Systematics 8 (1977): 429–48.

Jerison, H. J. “Brain Evolution and Dinosaur Brains.” American Naturalist 103 (1969): 575–88.

Maidment, Susannah C. R., David B. Norman, Paul M. Barrett, and Paul Upchurch. “Systematics and Phylogeny of Stegosauria (Dinosauria: Ornithischia).” Journal of Systematic Palaeontology 6.4 (2008): 367–407.

Norell, Mark, et al. Discovering Dinosaurs: Evolution, Extinction, and the Lessons of Prehistory. Berkeley: University of California Press, 1995.

“Ornithischia” and “Thyreophora.” Wikipedia. Web. June 2011. <http://en.wikipedia.org.>

Sereno, P. C. “Basal Archosaurs: Phylogenetic Relationships and Functional Implications.” Journal of Vertebrate Paleontology 11.S4 (1991): 1–53.

University of Maryland. Web. June 2011. <http://www.geol.umd.edu>.