Sauropelta


Sauropelta

Taxobox



image_width = 220px
name = "Sauropelta"
fossil_range = Early Cretaceous
regnum = Animalia
phylum = Chordata
classis = Sauropsida
superordo = Dinosauria
ordo = Ornithischia
subordo = Thyreophora
infraordo = Ankylosauria
familia = Nodosauridae
genus = "Sauropelta"
genus_authority = Ostrom, 1970
subdivision_ranks = Species
subdivision =
*"S. edwardsorum" Ostrom, 1970 (type)

"Sauropelta" (pronEng|ˌsɔroʊˈpɛltə or SAWR-o-"PEL"-ta; meaning 'lizard shield') is a genus of nodosaurid dinosaur that existed in the Early Cretaceous Period of North America. One species ("S. edwardsorum") has been named although others may have existed. Anatomically, "Sauropelta" is one of the most well-understood nodosaurids, with fossilized remains recovered in the U.S. states of Wyoming, Montana, and possibly Utah. It is also the earliest known genus of nodosaurid; most of its remains are found in the Cloverly Formation, which dates to about 115 to 110 Ma (million years ago).

It was a medium-sized nodosaurid, measuring about 5 meters (16.5 ft) long. "Sauropelta" had a distinctively long tail which made up about half of its body length. Although its body was smaller than a modern black rhinoceros, "Sauropelta" was about the same mass, weighing in at about 1500 kilograms (3300 lb). The extra weight was largely due to its extensive bony body armor, including the characteristically large spines projecting from its neck.

Description

"Sauropelta" was a heavily-built quadrupedal herbivore with a body length of approximately 5 meters (16.5 ft).cite journal |last=Carpenter |first=Kenneth. |authorlink=Kenneth Carpenter |year=1984 |title=Skeletal reconstruction and life restoration of "Sauropelta" (Ankylosauria: Nodosauridae) from the Cretaceous of North America |journal=Canadian Journal of Earth Sciences |volume=21 |pages=1491–1498] The skull was triangular when viewed from above, with the rear end wider than the tapering snout. One skull measured 35 centimeters (13.75 in) in width at its widest point, behind the eyes.cite book |last=Carpenter |first=Kenneth |authorlink=Kenneth Carpenter |coauthors=& Kirkland, James I. |year=1998 |chapter=Review of Lower and Middle Cretaceous Ankylosaurs from North America|editor= Lucas, Spencer G.; Kirkland, James I; & Estep, J.W. (eds.). |title=Lower and Middle Cretaceous Ecosystems |series="New Mexico Museum of Natural History and Science Bulletin" 14 |pages=249-270] Unlike some other nodosaurids, the roof of the skull was characteristically flat, not domed. The roof of the skull was very thick and covered in flat, bony plates that are so tightly fused that there appear to be no sutures (boundaries) like the ones seen in "Panoplosaurus", "Pawpawsaurus", "Silvisaurus", and many other ankylosaurs. This could also be an artifact of preservation or preparation. As in other ankylosaurs, thick triangular scutes projected from the postorbital bone, above and behind the eyes, as well as the jugal bone, below and behind the eyes. More typically for nodosaurids, leaf-shaped teeth lined both upper and lower jaws, used for cutting plant material. The front end of the skull is unknown, but there would have been a sharp bony ridge (tomium) at the end of both upper and lower jaws, as seen in other ankylosaurs. This ridge probably would have supported a keratinous beak.cite book |last=Vickaryous |first=Matthew K. |coauthors=Maryanska, Teresa; & Weishampel, David B. |year=2004 |chapter=Ankylosauria |editor=Weishampel, David B.; Dodson, Peter; & Osmólska, Halszka (eds.). |title=The Dinosauria |edition=Second Edition |location = Berkeley |publisher = University of California Press |pages=363-392] The tail of "Sauropelta" was characteristically long and made up nearly half of the body length. One skeleton preserved forty caudal (tail) vertebrae, although some were missing, suggesting that the true number of caudal vertebrae may have exceeded fifty. Ossified tendons stiffened the tail along its length. Like other ankylosaurs, "Sauropelta" had a wide body, with a very broad pelvis and ribcage. The forelimbs were shorter than the hindlimbs, which resulted in an arched back, with the highest point over the hips. Its feet, limbs, shoulders, and pelvis were all very stoutly constructed and reinforced to support a great deal of weight. American paleontologist Ken Carpenter estimated the mass of "S. edwardsorum" at 1500 kilograms (3300 lb). Like other nodosaurids, "Sauropelta" was covered in armor formed from bony masses embedded in the skin (osteoderms). The discovery of a skeleton with the body armor preserved "in situ" allowed Carpenter and other scientists to accurately describe this protection. Two parallel rows of domed scutes ran down the top of the neck, along the anteroposterior axis (front to back). On the upper surface of the back and tail, the skin was covered in small, bony nodules ("ossicles"), which separated larger conical scutes arranged in parallel rows along the mediolateral axis (side to side). Over the hips, the ossicles and larger domed plates were interlocked very tightly to form a structure called a sacral shield. This shield is also found in ankylosaurs like "Polacanthus" and "Antarctopelta".cite journal |last=Salgado |first=Leonardo |coauthors=& Gasparini, Zulma. |year=2006 |title=Reappraisal of an ankylosaurian dinosaur from the Upper Cretaceous of James Ross Island (Antarctica). |journal=Geodiversitas |volume=28 |issue=1 |pages=119–135] Dramatically large, pointed spines lined the sides of the neck, increasing in size towards the shoulders, and then decreasing in size again along the side of the body until they stopped just before the hips. Behind the hips, flat triangular plates lined the tail on both sides, pointing laterally (outwards) and decreasing in size towards the end of the tail. Carpenter originally described the cervical (neck) spines and caudal plates as belonging to a single row on each side, although more recently he and Jim Kirkland reconstructed them in two parallel rows on each side, one above the other. The upper row of cervical spines pointed backwards and upwards (posterodorsally), while the lower row pointed backwards and outwards (posterolaterally). The bases of each pair of cervical spines and each pair of caudal plates were fused together, greatly restricting mobility in both the neck and upper tail.

Classification and systematics

Since John Ostrom first described "Sauropelta" in 1970, it has been recognized as a member of the family Nodosauridae.cite journal |last=Ostrom |first=John H. |authorlink=John Ostrom |year=1970 |title=Stratigraphy and paleontology of the Cloverly Formation (Lower Cretaceous) of the Bighorn Basin area, Wyoming and Montana |journal=Bulletin of the Peabody Museum of Natural History |volume=35 |pages=1–234] The nodosaurids, along with the family Ankylosauridae, belong within the infraorder Ankylosauria. Nodosaurids are characterized by certain features of the skull, including the mandible (lower jaw), which curves downwards at the end. Overall, nodosaurids had narrower snouts than the ankylosaurids, and also lacked the heavy ankylosaurid tail clubs.cite book |last= Carpenter |first=Kenneth. |authorlink=Kenneth Carpenter |year=1997 |chapter=Ankylosauria |editor = Currie, Philip J.; & Padian, Kevin (eds.). |title=The Encyclopedia of Dinosaurs |publisher = University of California Press |location = Berkeley |pages=16-17] Nodosaurids, like ankylosaurids, are found in North America, Asia and Europe.

While the systematics (evolutionary relationships) of nodosaurids have not been firmly established, the genera "Sauropelta", "Silvisaurus" and "Pawpawsaurus" are sometimes considered to be basal to geologically younger nodosaurids like "Panoplosaurus", "Edmontonia" and "Animantarx".cite journal |last=Hill |first=Robert V. |coauthors=Witmer, Lawrence M.; & Norell, Mark A. |year=2003 |title=A new specimen of "Pinacosaurus grangeri" (Dinosauria: Ornithischia) from the Late Cretaceous of Mongolia: Ontogeny and Phylogeny of Ankylosaurs. |journal=American Museum Novitates |volume=3395 |pages=1–29 |doi=10.1206/0003-0082(2003)395<0001:ANSOPG>2.0.CO;2 |doilabel=10.1206/0003-0082(2003)3950001:ANSOPG2.0.CO;2] In a 2001 analysis, Carpenter included the former three genera in a sister clade to a group containing the latter three, although he found that "Panoplosaurus" could belong to either clade, depending which taxa and characters were chosen.cite book |last= Carpenter |first=Kenneth. |authorlink=Kenneth Carpenter |year=2001 |chapter=Phylogenetic analysis of the Ankylosauria |editor = Carpenter, Kenneth (ed.). |title=The Armored Dinosaurs |pages=455-483 |publisher=Indiana University Press |location=Bloomington]

Discovery and naming

In the early 1930s, famed dinosaur hunter and paleontologist Barnum Brown collected the holotype specimen of "Sauropelta" (AMNH 3032, a partial skeleton) from the Cloverly Formation in Big Horn County, Montana. The locality is inside the Crow Indian Reservation. Brown also discovered two other specimens (AMNH 3035 and 3036). The latter is one of the best-preserved nodosaurid skeletons known to science, includes a large amount of "in situ" armor, and is on display in the American Museum of Natural History in New York City. AMNH 3035 preserves the cervical armor and most of a skull, missing only the end of the snout. Expeditions in the 1960s led by the equally renowned John Ostrom of Yale University's Peabody Museum of Natural History recovered additional incomplete specimens from the Cloverly. In 1970, Ostrom coined the genus "Sauropelta" to include remains discovered by both expeditions. Derived from the Greek "σαυρος"/"sauros" ('lizard') and "πελτε"/"pelte" ('shield'), this name is a reference to its bony armor. Although Ostrom originally named the species "S. edwardsi", nomenclaturist George Olshevsky corrected the spelling to "S. edwardsorum" in 1991 to conform to Latin grammar rules.cite book |author=Olshevsky, George. |authorlink=George Olshevsky |year=1991 |title=A Revision of the Parainfraclass Archosauria Cope, 1869, Excluding the Advanced Crocodylia. |series=Mesozoic Meanderings No. 2 |publisher=Publications Requiring Research |location=San Diego |pages=196pp]

Despite the naming of "Sauropelta" two years earlier, confusion arose in 1972 when the name "Peltosaurus" was inadvertently published as the caption of a photograph of AMNH 3036.cite book |last=Glut |first=Donald F. |authorlink=Donald F. Glut |year=1972 |title=The Dinosaur Dictionary |publisher=Citadel Press |location=Secaucus |pages=217pp] Although Brown never published a name or description for the remains which are now known as "Sauropelta edwardsorum", "Peltosaurus" was the name he informally used in lectures and museum exhibits. However, the name "Peltosaurus" was preoccupied by a genus of North American lizard from an extinct branch of the modern family Anguidae (the alligator lizards and the legless glass lizards) and is no longer used to refer to the dinosaur.cite book |last=Chure |first=Daniel J. |coauthors=& McIntosh, John S. |year=1989 |title=A Bibliography of the Dinosauria (Exclusive of the Aves), 1677-1986 |series="Paleontology Series" 1 |publisher=Museum of Western Colorado |location=Grand Junction |pages=226pp]

In 1999, Carpenter and colleagues described material of a large nodosaurid from Utah, discovered in a member of the Cedar Mountain Formation called the Poison Strip Sandstone, which is contemporaneous with the Cloverly Formation. He originally referred it to "Sauropelta" as a possible new species, but it was never named.cite book |last=Carpenter |first=Kenneth |authorlink=Kenneth Carpenter |coauthors=Kirkland, James I.; Burge, Donald; & Bird, John. |year=1999 |chapter=Ankylosaurs (Dinosauria: Ornithischia) of the Cedar Mountain Formation, Utah, and their stratigraphic distribution |editor = Gillette, David (ed.). |title=Vertebrate Paleontology of Utah. |series="Utah Geological Survey Miscellaneous Publication" 99-1 |pages=244-251] In more recent publications, Carpenter no longer refers the Poison Strip animal to "Sauropelta", only to the family Nodosauridae.cite book |last=Carpenter |first=Kenneth. |authorlink=Kenneth Carpenter |year=2006 |chapter=Assessing dinosaur faunal turnover in the Cedar Mountain Formation (Lower Cretaceous) of Eastern Utah, USA |editor=Barrett, Paul M.; & Evans, S.E (eds.). |title=Ninth International Symposium on Mesozoic Terrestrial Ecosystems and Biota |publisher = Natural History Museum |location = London |pages=21-25]

Other recent, but undescribed, discoveries include a complete skull from the Cloverly of Montanacite journal |last=Parsons |first=William L. |coauthors=& Parsons, Kristen M. |year=2001 |title=Description of a new skull of "Sauropelta" cf. "S. edwardsi" Ostrom, 1970 (Ornithischia: Ankylosauria) |journal=Journal of Vertebrate Paleontology |volume=21 |issue=Supplement to 3 - Abstracts of Papers, 61st Annual Meeting of the Society of Vertebrate Paleontology |pages=87A ] and a huge fragmentary skeleton from the Cedar Mountain Formation of Utah.cite journal |last=Warren |first=David |coauthors=& Carpenter, Kenneth. |year=2004 |title=A large nodosaurid ankylosaur from the Cedar Mountain Formation of Utah |journal=Journal of Vertebrate Paleontology |volume=24 |issue=Supplement to 3 - Abstracts of Papers, 64th Annual Meeting of the Society of Vertebrate Paleontology |pages=126A ] These discoveries have been published only as abstracts for the annual Society of Vertebrate Paleontology conference, and may or may not prove to belong to "S. edwardsorum" or even "Sauropelta" when formally published.

Paleoecology

"Sauropelta" is the earliest known nodosaurid genus. All specimens of "S. edwardsorum" were recovered from the middle section of the Cloverly Formation in Wyoming and Montana, which dates to the late Aptian through early Albian stages of the Early Cretaceous, or about 115 to 110 Ma (million years ago).cite journal |last=Kirkland |first=James I. |authorlink=James Kirkland (paleontologist) |coauthors=Britt, Brooks; Burge, Donald L.; Carpenter, Kenneth; Cifelli, Richard; DeCourten, Frank; Eaton, Jeffrey; Hasiotis, Steven; & Lawton, Timothy. |year=1997 |title=Lower to Middle Cretaceous Dinosaur faunas of the central Colorado Plateau: a key to understanding 35 million years of tectonics, sedimentology, evolution, and biogeography |journal=Brigham Young University Geology Studies |volume=42 |issue=II |pages=69–103] "Sauropelta" lived in wide floodplains around rivers that drained into the shallow inland sea to the north and east, carrying sediment eroded from the low mountains to the west. Periodic flooding of these rivers covered the surrounding plains with new muddy sediments, creating the Cloverly Formation and burying the remains of many animals, some of which would be fossilized. At the end of Cloverly times, the shallow sea would expand to cover the entire region and would eventually split North America completely in half, forming the Western Interior Seaway.cite book |last=Maxwell |first=W. Desmond. |year=1997 |chapter=Cloverly Formation |editor = Currie, Philip J.; & Padian, Kevin (eds.). |title=The Encyclopedia of Dinosaurs |publisher=Academic Press |location=San Diego |pages=128-129] Abundant fossil remains of coniferous trees suggest that these plains were covered in forests. Grasses would not evolve until later in the Cretaceous, so "Sauropelta" and other Early Cretaceous dinosaurian herbivores browsed from a variety of conifers and cycads.cite journal |last=Prasad |first=Vandana |coauthors=Strömberg, Caroline A.E.; Alimohammadian, Habib; & Sahni, Ashok. |year=2005 |title=Dinosaur coprolites and the early evolution of grasses and grazers |journal=Science |volume=310 |pages=1177–1180 |doi=10.1126/science.1118806] Nodosaurids like "Sauropelta" had narrow snouts, an adaptation seen today in animals that are selective browsers as opposed to the wide muzzles of grazers.

While "Sauropelta" was an important part of the Cloverly herbivore guild, the most abundant herbivorous dinosaur of the time was the large ornithopod "Tenontosaurus". The smaller ornithopod "Zephyrosaurus", rare titanosaur sauropods, and an unknown type of ornithomimosaur also lived alongside "Sauropelta". The dromaeosaurid theropod "Deinonychus" fed upon some of these herbivores, and the sheer number of "Deinonychus" teeth scattered throughout the formation are a testament to its abundance. "Microvenator", a small basal oviraptorosaur, hunted smaller prey, while the apex predators of the Cloverly were large allosauroid theropods. These beasts are known only from fragments, but may be related to the contemporaneous "Acrocanthosaurus" of Texas and Oklahoma to the south. Lungfish, triconodont mammals and several species of turtles lived in the Cloverly, while crocodilians prowled the rivers, lakes and swamps, providing evidence of a year-round warm climate. The Late Jurassic fauna dominated by allosauroids, stegosaurs and many varieties of huge sauropods gave way by Cloverly times to an Early Cretaceous fauna in which dromaeosaurs, ornithopods, and nodosaurs like "Sauropelta" were predominant. After the Cloverly ended, a large wave of Asian animals, including tyrannosaurids, ceratopsians and ankylosaurids would disperse into western North America, forming the mixed fauna seen throughout the Late Cretaceous.

References

External links

*Entries in [http://paleo.amnh.org/fossil/seek.html Online Collections Database] at the American Museum of Natural History:
** [http://paleo.amnh.org/fossil/show.html?cat_num=FR%203032 AMNH 3032] (pictures of assorted bones included)
** [http://paleo.amnh.org/fossil/show.html?cat_num=FR%203035 AMNH 3035] (pictures of skull and cervical armor included)
** [http://paleo.amnh.org/fossil/show.html?cat_num=FR%203036 AMNH 3036]
*Entry for [http://tolweb.org/Nodosauridae/15772 Nodosauridae] on [http://tolweb.org/tree/phylogeny.html Tree of Life] , including modern restoration of "Sauropelta" armor


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