Athena Review Vol. 5, no. 1
Records of Life: Fossils as Original Sources
30. Ornithischians 1: Stegosaurs and Ankylosaurians
Ornithischians ("bird-hipped") dinosaurs are one of the two major
orders of dinosaurs, the other being saurischians ("lizard hipped"),
as described in prior sections (Seeley (1888; Romer
1966). The ornithischian or bird-like pelvis, which evolved
from the original reptile pelvis, is arranged with the pubis
rotated backward, parallel with the ischium, often also with a
forward-pointing process, giving a four-pronged structure
(fig.1). Fig.1: Ornithischian or "bird-hipped" pelvis structure.
The order Ornithischia includes four main dinosaur groups or suborders: Stegosauria, Ankylosauria, Ceratopsia, and Ornithopoda.
Stegosauria
Stegasaurs are grouped with the Ankylosauria in the armored dinosaur clade Thyreophora. The type species of Stegasauria is Stegasaur stenops (fig.2).
Stegosaurus
Stegosaurus ("roofed lizard"), were plant-eating ornithischian dinosaurs from the Late Jurassic period (155-150 mya). They are found in Kimmeridgian to early Tithonian strata in the Morrison Formation of the western US and in Portugal. Over 80 Stegosaurus fossils have been found in the Morrison Formation, divided into three species: S. ungulatus (the largest of the three), S. stenops, and S. sulcatus. Stegosaurus remains were first identified by Marsh (1877) from remains recovered north of Morrison, Colorado, at Dinosaur Ridge National Landmark. These first bones became the holotype of Stegosaurus armatus (now synonymous with S. stenops). Marsh initially believed the plates lay overlapped over the animal's back, providing the basis for its scientific name, "roofed lizard."
In 1879 Marsh named a second species, Stegosaurus ungulatus, "hoofed roof lizard", from remains recovered at Como Bluff, Wyoming.Fig.2: Skeleton of Stegosaurus stenops (Natural History Museum, London).
The first known skeletons were fragmentary, and many years of fossil collection were necessary to understand the true appearance of Stegosaurs, notable for the plates aligned vertically along their backs (Marsh 1891; Gilmore 1914). They were large, herbivorous quadrupeds who grew up to 9 m (29.5 ft) in length, with rounded backs, short forelimbs and long hind limbs (fig.2). The hind feet each had three short toes, while each forefoot had five toes; only the inner two toes had a blunt hoof.
As the forelimbs were shorter than the hind limbs, this resulted in an unusual posture. The tail appears to have been held well clear of the ground, while the head of Stegosaurus was positioned relatively low down, probably no higher than 1 meter above the ground (Carpenter 1998). The skull's low position suggests that Stegosaurus may have been a browser of low-growing vegetation. Their spiked tails were most likely used for defense against large predators such as Allosaurus (Carpenter et al 2005), while the dermal plates along their back may have been used primarily for display, and secondarily for body temperature functions (Farlow et al. 1976). Between 17 and 22 dermal plates grew out of the skin rather than the skeleton. These were bony-cored scales or osteoderms, such as today occur in crocodiles and lizards. The largest plates measured up to 60 x 60 cm, and occurred over the hips (fig.2). Differences in the shapes and sizes of the plates of a related Stegasaurid named Hesperosaurus suggested that they were sexually dimorphic, with wide plates belonging to males and taller plates belonging to females (Saitta 2015).
The relatively small skull (fig.3) had a fenestra or opening between the nose and eye. This is common trait of diapsids and occurred in most archosaurs, including modern birds, though it was lost in extant crocodylians. The absence of front teeth suggests their function was replaced by a horny beak. The Stegasaur's cheek teeth were small, triangular, and flat. Wear facets indicate a grinding of plant food such as grasses containing silica particles. The braincase of Stegosaurus was small, based on endocasts such as taken by Marsh in the 1880s, which showed it had a small brain compared with other dinosaur endocasts then known, although now known to be a more normal condition for large herbivores including Sauropsids as well as Stegasaurids (cf. Bakker 1986).
Fig.3: Skull of Stegosaurus stenops (Utah Museum of Natural History, cast).
Ankylosauria
Ankylosauria, first named by Osborn (1923), are a group of armored, herbivorous dinosaurs. They first appeared in the Kimmeridgian stage of the Late Jurassic (156-152 mya), and persisted for about 90 million years until the end of the Cretaceous (65 mya). They had a global distribution, including Antarctica whose first known dinosaur was the ankylosaurian Antarctopelta, from Ross Island (Gasparini et al., 1996).
Ankylosaurs
are quadrupedal, with a
wide, low body covered with armored scales. The facets for the ribs are
often angled upwards so that the ribs produce a wide, barrel-like
body. Their body armor typically
consists of oval to rectangular plates, having a keel or ridge
extending their length. Gaps between the plates and undersides of the
body, as well as the legs, are encased in smaller scales (Carpenter). Late Jurassic ankylosaurs
Ankylosaurs from the Late Jurassic Morrison Formation of Colorado and Wyoming include Gargoyleosaurus (figs.4A,5, and 6) and Mymoorapelta from the primitive ankylosaur family Polacanthidae (Kirkland and Carpenter, 1994). This family was first identified from the Lower Cretaceous of Europe in the armored herbivore Hyleosaurus armatus by Gideon Mantell (1833; see also Chapter 32 of this report).
Fig.4: A) Gargoyleosaurus; B) Sauropelta; C) Euoplocephalus (after Carpenter).
Gargoyleosaurus ("gargoyle lizard") is one of the earliest ankylosaurs known from largely complete fossil remains. Its skull is 29 cm ( 1 foot) long, and its total body length is an estimated 3 -4 me
ters (figs.4A and 6). The holotype
was discovered at the Bone Cabin Quarry West site of the Morrison
Formation, in Albany County, Wyoming, in exposures of the Upper
Jurassic Kimmeridgian to Tithonian stages (154-150 mya),
considered as stratigraphic zone 2 of the Formation.Fig.5: Skull of the Late Jurassic ankylosaur Gargoyleosaurus (afterCarpenter et al. 1998, fig.1)
The type species, G. parkpinorum, described by Carpenter et al. (1998), is now in the collections of the Denver Museum of Nature and Science. The holotype consists of most of the skull and a partial postcranial skeleton (fig.6). Two other partial skeletons are known.
The skull of Gargoyleosaurus (fig.5) displays cranial sculpturing,and has a narrow rostrum. There are seven conical teeth in each premaxilla, which contrrasts with later Ankylosaurs, which lacked teeth in the premaxilla. Regarding bodily armor, it has two sets of ossified cervical plates and a number of elongate conical spines (fig.6).
Fig.6: Skeleton of the Late Jurassic ankylosaur Gargoyleosaurus (Denver Museum of Nature & Science).
Cretaceous ankylosaurs
Ankylosaur fossils are most abundant in the Cretaceous period (127-65 mya). The Early Cretaceous saw two major ankylosaur families, the Nodosauridae and Ankylosauridae. These showed differences in body size and in the type of defensive spikes or clubs that grew on their backs and tails, as well as differences in their subsistence habits and favored terrains (Carpenter 2001). The skull of nodosaurids (fig.7) is significantly longer than wide, whereas the skulls of polacanthids and ankylosaurids (fig.8) are nearly as wide, or wider than long. In nodosaurids, two of the five pairs of skull openings (fenestrae) in most dinosaurs have fused shut, including the antorbital fenestrae in front of the orbits, and the supratemporal fenestrae on the skull roof. In ankylosaurids, a third pair of openings behind the orbits are also closed (Carpenter). As fig.4. indicates, the Cretaceous ankylosaurs are significantly larger than the earlier, Late Jurassic taxa such as Gargoyleosaurus.
Nodosauridae
Nodosauridae were dominant in the Early and Middle Cretaceous (125-83.5 mya), including the genera Gastonia, Nodosaurus, Edmontonia, and Sauropelta (fig.4B). In the nodosaurids, who had very muscular shoulders, the armor plates are supplemented with long spines or spikes projecting from the neck and shoulders used for self-defense against predators. Nodosaurids had smaller, narrow beaks than the ankylosaurids, suggesting that nodosaurids were browsers, selectively cropping plant parts or particular types of vegetation; in contrast, ankylosaurids were grazers, cropping all low vegetation (Carpenter 2001). Many nodosaurid finds have been recovered from secondary marine deposits, when bodies were carried to sea from coastal plain habitats. The absence of ankylosaurids from marine deposits suggests they lived in inland or upland habitats.
Gastonia (fig.7) was a medium-sized herbivorous nodosaurid dinosaur from the Early Cretaceous of North America, around 125 million years ago. The type specimen of Gastonia burgei was discovered in a bonebed of the lower Cedar Mountain Formation in Grand County, Utah. Gastonia is among the most common dinosaur fossils in the Cedar Mountain Formation, with more complete mate
rial known than for any other basal ankylosaur (Kirkland and Madsen 2007).Gastonia had a body length of 5-6 meters. It had a flat and broad rump and a moderately long tail which lacked a tail club. The skull (fig.7) is somewhat elongated, about 30 cm in length.. The notch in the toothless upper beak is one of the distinguishing features of Gastonia. The tooth rows of the maxillae are rather straight, each consisting of fifteen to sixteen small teeth. There is no armour on the snout. The squamosal horns at the rear skull corners, and jugal horns at the cheeks, are small.
Fig.7: Gastonia skull (Indianapolis Children's Mus.)
Gastonia, like other ankylosurids, was protected from carnivore predators by osteoderms of various forms. The sides of the thorax was covered by about five pairs of large flat triangular spikes which gradually decline in length in the rear. The hip region was covered by a large pelvic shield consisting of fused osteoderms. These were patterned as rosettes with a larger plate in the middle, surrounded by at least two rings of smaller plates.
Ankylosauridae
During the Late Cretaceous, ankylosaurids increased in diversity an
d worldwide abundance, including Ankylosaurus, Euoplocephalus (fig.4C), and the very similar Scolosaurus (fig.9) from North America, as well as numerous taxa from Mongolia including Tarchia (fig.8). Ankylosaurids
had much longer and wider bodies than earlier ankylosaurs. The ankylosaurid skull, in addition to its beak-like predentary
bones, has prominent jugal and postorbital bones that resemble horns, which are more prominent than in nodosauridae skulls
(fig.7). Fig.8: Skull of Tarchia teresae, an ankylosaur from the Upper Cretaceous of Mongolia (PIN 3142/250l; photo Ghedoghedo; labels after Carpenter).
The large clubs at the end of their tails may have been used in self-defense against predators and/or in sexual selection. The clubs were made of several plates of bone permeated with soft tissue, allowing them to absorb thousands of pounds of force. Their larger and broader beaks indicate that ankylosaurs were generalists in their diet.
.
Scolosaurus
("pointed stake lizard") was an ankylosaur from the Late
Cretaceous Campanian stage (76.5 mya) in Alberta. Two
species are known, S. cutleri and S. thronus (fig.9). The type
fossil for S. cutleri was
a nearly complete skeleton, with unusual preservation of
osteoderms and skin impression. This was discovered by William E.
Cutler in 1914 at the bottom of the Dinosaur Park Formation in
Alberta in fine-grained sandstone and claystone Fig.9.: Skeleton of the Late Cretaceous ankylosaur Scolosaurus thronus (Royal Tyrell Museum)
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Glossary