Characters and Anatomy
Two familiar stegosaurs: Stegosaurus (Carnegie Museum of Natural History, Pittsburgh, PA) and Kentrosaurus (Museum fur Naturkunde, Berlin; photo courtesy S.C.R. Maidment)
Stegosaurs
were quadrupedal and herbivorous dinosaurs, and their general body plan
reflects this mode of life. Reaching lengths of approximately
nine meters, stegosaurs are characterized by short, massive, and
columnar limbs, which terminate in short hands and feet capped by
hooflike unguals. This arrangement benefited a slow-moving
quadrupedal lifestyle, and indicates that most stegosaurs were strictly
quadrupedal. The skull of stegosaurs is small, lightly-built, and
proportionally long and narrow. The front of the skull in most
stegosaurs is toothless, and likely supported a keratinous beak used
for cropping vegetation. Farther back, the tooted portion of the
skull houses numerous small, leaf-shaped teeth ideal for shearing plant
matter.
In addition to this general body plan, stegosaurs are characterized by an assortment of unique, derived characters (synapomorphies), including:
1) Parasagittal Rows of Plates and Spines
Perhaps the most unique and recognizable feature of stegosaurs are their plates, scutes, and spines, which are located along the back and tail (see below). These structures occur in two rows, which parallel the vertebral column (parasagittal). However, the exact form of plates, scutes, and spines varies greatly among different stegosaur genera. They are thin and plate-like in several stegosaurs, including the best-known and most familiar genus, Stegosaurus (top photo on right, Yale Peabody Museum), as well as Hesperosaurus and Wuerhosaurus. In contrast, they are more robust and spine-like in the Chinese Huayangosaurus and Tuojiangosaurus, and the African Kentrosaurus. The surface of these plates is invariably rugose and marked by dense vascular grooves, suggesting they were covered by keratin, tight-fitting skin, or some other form of soft tissue. Histological studies suggest these plates were too weak to be used as defensive armor, but they may have been ideally suited for sexual display.
Many stegosaurs also possess some form of tail spikes or spines, which may have been a defensive weapon against predators. Stegosaurs
(bottom photo on right, Yale Peabody Museum) is characterized by
especially robust and elongate tail spines. This specimen is
reconstructed showing four pairs of spikes, but recent studies suggest
only two pairs may have been present. Five pairs are known in Kentrosaurus, while Huayangosaurus and Tuojiangosaurus possessed two pairs.
Additionally, several other ossifications are known in some stegosaurs, including parascapular spines near the shoulder girdles (absent in Stegosaurus) and a series of small scutes in the neck region.
2) Elongated Dorsal Neural Spines and Upturned Transverse Processes
The dorsal vertebrae (those of the back region) of stegosaurs are unique among dinosaurs in two distinctive features. First, the neural spine (the portion above the neural canal) is extremely elongated, and at least 1.5 times as deep as the centrum (the main body of the vertebra below the neural canal). In most stegosaurs, including Stegosaurus (photo on right, courtesy of S.C.R. Maidment and modified from Ostrom and and McIntosh 1966), the neural spine looks as if it's been stretched like a block of taffy. An entire dorsal vertebral series of such stretched vertebrae gives the back of stegosaurs a distinctive profile, but the function of this unique morphology is unclear. Second, the transverse processes of the dorsal vertebrae are strongly upturned, often at an angle of 50 degrees or greater relative to the horizontal. The transverse process marks the attachment site for one of the heads of the dorsal rib. The other attachment site, called the parapophysis, is located at the base of the transverse process. In general, both attachment sites are located far dorsally (high up) on the stretched neural spine. Thus, the dorsal ribs would have articulated far dorsally, resulting in a deep and wide-barreled ribcage that would have accompanied the large gut in these herbivores.
3) Hand with Distinctive Carpal Morphology
The carpals are a series of wrist bones that bridge the forearm (ulna and radius) and the palm of the hand (metacarpals). In stegosaurs there are only two large, block-shaped carpals: the ulnare and radiale. A third large carpal called the intermedium is present in many other dinosaurs, but this element is fused to the ulnare in stegosaurs. Additionally, the ulnare and radiale are stout, robust, and block-shaped, as can be seen in this image of a Stegosaurus hand (American Museum of Natural History, New York; photo courtesy of S.C.R. Maidment). A series of smaller carpals that bridge the ulnare/radiale and the metacarpals are present in many dinosaurs, but these "distal carpals" are unknown in stegosaurs, and if present were likely cartilaginous in life.
In addition to this general body plan, stegosaurs are characterized by an assortment of unique, derived characters (synapomorphies), including:
1) Parasagittal Rows of Plates and SpinesPerhaps the most unique and recognizable feature of stegosaurs are their plates, scutes, and spines, which are located along the back and tail (see below). These structures occur in two rows, which parallel the vertebral column (parasagittal). However, the exact form of plates, scutes, and spines varies greatly among different stegosaur genera. They are thin and plate-like in several stegosaurs, including the best-known and most familiar genus, Stegosaurus (top photo on right, Yale Peabody Museum), as well as Hesperosaurus and Wuerhosaurus. In contrast, they are more robust and spine-like in the Chinese Huayangosaurus and Tuojiangosaurus, and the African Kentrosaurus. The surface of these plates is invariably rugose and marked by dense vascular grooves, suggesting they were covered by keratin, tight-fitting skin, or some other form of soft tissue. Histological studies suggest these plates were too weak to be used as defensive armor, but they may have been ideally suited for sexual display.
Many stegosaurs also possess some form of tail spikes or spines, which may have been a defensive weapon against predators. Stegosaurs
(bottom photo on right, Yale Peabody Museum) is characterized by
especially robust and elongate tail spines. This specimen is
reconstructed showing four pairs of spikes, but recent studies suggest
only two pairs may have been present. Five pairs are known in Kentrosaurus, while Huayangosaurus and Tuojiangosaurus possessed two pairs.Additionally, several other ossifications are known in some stegosaurs, including parascapular spines near the shoulder girdles (absent in Stegosaurus) and a series of small scutes in the neck region.
2) Elongated Dorsal Neural Spines and Upturned Transverse ProcessesThe dorsal vertebrae (those of the back region) of stegosaurs are unique among dinosaurs in two distinctive features. First, the neural spine (the portion above the neural canal) is extremely elongated, and at least 1.5 times as deep as the centrum (the main body of the vertebra below the neural canal). In most stegosaurs, including Stegosaurus (photo on right, courtesy of S.C.R. Maidment and modified from Ostrom and and McIntosh 1966), the neural spine looks as if it's been stretched like a block of taffy. An entire dorsal vertebral series of such stretched vertebrae gives the back of stegosaurs a distinctive profile, but the function of this unique morphology is unclear. Second, the transverse processes of the dorsal vertebrae are strongly upturned, often at an angle of 50 degrees or greater relative to the horizontal. The transverse process marks the attachment site for one of the heads of the dorsal rib. The other attachment site, called the parapophysis, is located at the base of the transverse process. In general, both attachment sites are located far dorsally (high up) on the stretched neural spine. Thus, the dorsal ribs would have articulated far dorsally, resulting in a deep and wide-barreled ribcage that would have accompanied the large gut in these herbivores.
3) Hand with Distinctive Carpal Morphology
The carpals are a series of wrist bones that bridge the forearm (ulna and radius) and the palm of the hand (metacarpals). In stegosaurs there are only two large, block-shaped carpals: the ulnare and radiale. A third large carpal called the intermedium is present in many other dinosaurs, but this element is fused to the ulnare in stegosaurs. Additionally, the ulnare and radiale are stout, robust, and block-shaped, as can be seen in this image of a Stegosaurus hand (American Museum of Natural History, New York; photo courtesy of S.C.R. Maidment). A series of smaller carpals that bridge the ulnare/radiale and the metacarpals are present in many dinosaurs, but these "distal carpals" are unknown in stegosaurs, and if present were likely cartilaginous in life.
Author: Stephen Brusatte
Last updated: 01/11/2006
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