Stygimoloch
Encyclopedia
Stygimoloch is a putative genus
of pachycephalosaurid
dinosaur
from the end of the Cretaceous
period, roughly 65 million years ago. It is currently known from the Hell Creek Formation
, Ferris Formation
, and Lance Formation
of the Western Interior (United States
), where it lived alongside Tyrannosaurus
and Triceratops
. It may represent a sub-adult form of Pachycephalosaurus
.
The type species
, S. spinifer, was described by British vertebrate paleontologist
Peter Galton
and German paleontologist Hans-Dieter Sues
of the National Museum of Natural History
in 1983.
It is a relatively large pachycephalosaur, with the skull being about 46 centimeters long (18 in). Among North American pachycephalosaurs, only Pachycephalosaurus
is larger. Unlike other pachycephalosaurs, the domed skull is relatively small, slightly flattened from side to side, and pear-shaped; even when isolated this unusual dome can easily be distinguished from the broader, larger domes of Pachycephalosaurus.
While the dome is reduced in size, the ornamentation over the skull is more elaborate than in any other pachycephalosaur. Short, conical hornlets covered the nose, and the back corners of the skull bore an enormous pair of massive, backward-pointing spikes, up to 5 centimeters in diameter (2 in) and 15 centimeters long (6 in); these are surrounded by two or three smaller spikes. The function of this unusual ornamentation is unknown. Even if other pachycephalosaurs did butt heads (which is a subject of continuing debate), the small dome of Stygimoloch suggests that this behavior was not as important. Instead, the skull ornament might have functioned for display, may have been used for self-defense, or perhaps were locked together and used in shoving matches, like the horns of deer. More likely, however, is that the squamosal horns were used to inflict pain during flank-butting.
may actually be an individual of Stygimoloch or Pachycephalosaurus in which the dome and horns are not well-developed, either because the animal was a juvenile or a female. This consideration was supported at the 2007 annual meeting of the Society of Vertebrate Paleontology. Jack Horner of Montana State University
presented evidence, from analysis of the skull of the single existing Dracorex specimen, that this dinosaur may well be a juvenile form of Stygimoloch. In addition, he presented data that indicates that both Stygimoloch and Dracorex may be juvenile forms of Pachycephalosaurus. Horner and M.B. Goodwin published their findings in 2009, showing that the spike/node and skull dome bones of all three 'species' exhibit extreme plasticity, and that both Dracorex and Stygimoloch are known only from juvenile specimens while Pachycephalosaurus is known only from adult specimens. These observations, in addition to the fact that all three forms lived in the same time and place, lead them to conclude that Dracorex and Stygimoloch were simply juvenile Pachycephalosaurus, which lost spikes and grew domes as they aged. A 2010 study by Nick Longrich and colleagues also supported the hypothesis that all flat-skulled pachycephalosaurs were juveniles, suggesting that flat-skulled forms like Goyocephale and Homalocephale represent juveniles of dome-skulled adults.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of pachycephalosaurid
Pachycephalosauria
Pachycephalosauria is a clade of ornithischian dinosaurs. Well-known genera include Pachycephalosaurus, Stegoceras, Stygimoloch, and Dracorex. Most lived during the Late Cretaceous Period, in what is now North America and Asia. They were all bipedal, herbivorous/omnivorous animals with thick skulls...
dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
from the end of the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
period, roughly 65 million years ago. It is currently known from the Hell Creek Formation
Hell Creek Formation
The Hell Creek Formation is an intensely-studied division of Upper Cretaceous to lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana...
, Ferris Formation
Ferris Formation
The Ferris Formation is a Late Cretaceous to Paleocene , fluvial-deltaic geological formation in southern Wyoming. It contains a variety of trace and body fossils, preserved in sandy fluvial channel deposits and overbank units...
, and Lance Formation
Lance Formation
The Lance Formation is a division of Late Cretaceous rocks in the western United States. Named after Lance Creek, Wyoming, the microvertebrate fossils and dinosaurs represent important components of the latest Mesozoic vertebrate faunas...
of the Western Interior (United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
), where it lived alongside Tyrannosaurus
Tyrannosaurus
Tyrannosaurus meaning "tyrant," and sauros meaning "lizard") is a genus of coelurosaurian theropod dinosaur. The species Tyrannosaurus rex , commonly abbreviated to T. rex, is a fixture in popular culture. It lived throughout what is now western North America, with a much wider range than other...
and Triceratops
Triceratops
Triceratops is a genus of herbivorous ceratopsid dinosaur which lived during the late Maastrichtian stage of the Late Cretaceous Period, around 68 to 65 million years ago in what is now North America. It was one of the last dinosaur genera to appear before the great Cretaceous–Paleogene...
. It may represent a sub-adult form of Pachycephalosaurus
Pachycephalosaurus
Pachycephalosaurus is a genus of pachycephalosaurid dinosaur. It lived during the Late Cretaceous Period of what is now North America. Remains have been excavated in Montana, South Dakota, and Wyoming. It was an herbivorous or omnivorous creature which is only known from a single skull and a few...
.
The type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
, S. spinifer, was described by British vertebrate paleontologist
Vertebrate paleontology
Vertebrate paleontology is a large subfield to paleontology seeking to discover the behavior, reproduction and appearance of extinct animals with vertebrae or a notochord, through the study of their fossilized remains...
Peter Galton
Peter Galton
Peter M. Galton is a British vertebrate paleontologist working in America, who has to date written or co-written about a hundred papers in scientific journals or chapters in paleontology textbooks, especially on ornithischian and prosauropod dinosaurs.With Robert Bakker in a joint article...
and German paleontologist Hans-Dieter Sues
Hans-Dieter Sues
Hans-Dieter Sues is a German-born paleontologist who is Senior Scientist and Curator of Vertebrate Paleontology at the National Museum of Natural History of the Smithsonian Institution in Washington, DC. He received his education at Johannes Gutenberg-Universität Mainz , University of Alberta, and...
of the National Museum of Natural History
National Museum of Natural History
The National Museum of Natural History is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. Admission is free and the museum is open 364 days a year....
in 1983.
Description
Pachycephalosaurus
Pachycephalosaurus is a genus of pachycephalosaurid dinosaur. It lived during the Late Cretaceous Period of what is now North America. Remains have been excavated in Montana, South Dakota, and Wyoming. It was an herbivorous or omnivorous creature which is only known from a single skull and a few...
is larger. Unlike other pachycephalosaurs, the domed skull is relatively small, slightly flattened from side to side, and pear-shaped; even when isolated this unusual dome can easily be distinguished from the broader, larger domes of Pachycephalosaurus.
While the dome is reduced in size, the ornamentation over the skull is more elaborate than in any other pachycephalosaur. Short, conical hornlets covered the nose, and the back corners of the skull bore an enormous pair of massive, backward-pointing spikes, up to 5 centimeters in diameter (2 in) and 15 centimeters long (6 in); these are surrounded by two or three smaller spikes. The function of this unusual ornamentation is unknown. Even if other pachycephalosaurs did butt heads (which is a subject of continuing debate), the small dome of Stygimoloch suggests that this behavior was not as important. Instead, the skull ornament might have functioned for display, may have been used for self-defense, or perhaps were locked together and used in shoving matches, like the horns of deer. More likely, however, is that the squamosal horns were used to inflict pain during flank-butting.
Classification
The pachycephalosaur DracorexDracorex
Dracorex is a dinosaur genus of the family Pachycephalosauridae, from the Late Cretaceous of North America.The type species is Dracorex hogwartsia, meaning "dragon king of Hogwarts". It is known from one nearly complete skull , as well as four cervical vertebrae including the atlas, third, ninth...
may actually be an individual of Stygimoloch or Pachycephalosaurus in which the dome and horns are not well-developed, either because the animal was a juvenile or a female. This consideration was supported at the 2007 annual meeting of the Society of Vertebrate Paleontology. Jack Horner of Montana State University
Montana State University - Bozeman
Montana State University – Bozeman is a public university located in Bozeman, Montana. It is the state's land-grant university and primary campus in the Montana State University System, which is part of the Montana University System...
presented evidence, from analysis of the skull of the single existing Dracorex specimen, that this dinosaur may well be a juvenile form of Stygimoloch. In addition, he presented data that indicates that both Stygimoloch and Dracorex may be juvenile forms of Pachycephalosaurus. Horner and M.B. Goodwin published their findings in 2009, showing that the spike/node and skull dome bones of all three 'species' exhibit extreme plasticity, and that both Dracorex and Stygimoloch are known only from juvenile specimens while Pachycephalosaurus is known only from adult specimens. These observations, in addition to the fact that all three forms lived in the same time and place, lead them to conclude that Dracorex and Stygimoloch were simply juvenile Pachycephalosaurus, which lost spikes and grew domes as they aged. A 2010 study by Nick Longrich and colleagues also supported the hypothesis that all flat-skulled pachycephalosaurs were juveniles, suggesting that flat-skulled forms like Goyocephale and Homalocephale represent juveniles of dome-skulled adults.
External links
- TEDx talk by Jack Horner on shape-shifting dinosaur skulls and dinosaur misclassification.