Pachycephalosauria
| Pachycephalosauria | ||||||||||||
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Live reconstruction of Dracorex |
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| Temporal occurrence | ||||||||||||
| Middle Jurassic to Upper Cretaceous | ||||||||||||
| 166.1 to 66 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Pachycephalosauria | ||||||||||||
| Maryańska & Osmólska, 1974 |
The Pachycephalosauria (German sometimes "Dickkopfsaurier" ) are a taxon (a systematic group) of bird pelvic dinosaurs (Ornithischia). They were two-legged, presumably herbivorous dinosaurs , which were characterized by a noticeably thickened skull roof . Fossil finds of these dinosaurs come from the middle Jurassic as well as from the entire Cretaceous period of North America and Asia.
features
General
Only poorly preserved fossils of many Pachycephalosauria have survived, often only parts of the skull roof. The entire skull or the trunk and limb skeleton is known only from a few genera, so that much information about this group remains vague. The Pachycephalosauria were rather small, bipedal (moving on two legs) dinosaurs. Size information is often speculative - the smallest representatives such as Micropachycephalosaurus are likely to have been less than 1 meter long, while the largest genera such as Pachycephalosaurus reached 4.5 meters in length.
Skull and teeth
The most noticeable feature in the structure of the skull was the thickened cranial roof, which was formed from the frontal bone and the parietal bone . There were two basic patterns: In the flat-headed representatives, these bones were separated by sutures (skull sutures) and the top of the skull was thickened but not bulged. In the other pachycephalosaurs, these two bones had grown together to form a frontoparietal and arched like a dome. The fenestra supratemporalis (the upper cranial window of the temporal region ) is pronounced in the flat-headed species, but very small or closed in the dome-headed species. Under the thickened roof of the skull there was a row of bony cusps on the side and back of the scaly bone (Os squamosum), which in some representatives such as Stygimoloch and Dracorex were elongated like horns . Other regions of the skull such as the postorbital (behind the eye) or the nasal bone could also have bony cusps. The eye socket (orbit) is reinforced by two overlying bones ( supraorbital ).
Pachycephalosauria had different types of teeth ( heterodontics ). On the premaxillary (the foremost part of the upper jaw) there were several pin-like, slightly curved teeth, the rearmost of which was enlarged like a canine . Behind it there was a gap called a diastema . The teeth of the maxilla (upper jaw) were relatively small and had triangular, slightly jagged crowns. The lower jaw is only known from a few genera. The lower jaw teeth mostly resembled those of the maxilla, but in some genera such as Goyocephale the foremost was significantly enlarged. The predentale - the foremost part of the lower jaw - has not yet been found, but traces on the lower jaw suggest that the pachycephalosauria, like all bird hippopotamuses, probably had a hornbill.
Trunk skeleton and limbs
So far there have only been finds of the trunk skeleton or limbs of a few genera, many areas in the body structure of the Pachycephalosauria - such as the number and structure of the cervical vertebrae or the shape of the hand - are not known. The trunk was relatively stocky, there are abnormalities in the structure of the spine : the vertebrae were reinforced by interlocking connections, the sacral vertebrae that had grown together to form the sacrum had long ribs (sacral ribs). The anterior caudal vertebrae were also equipped with ribs, the posterior caudal vertebrae were stiffened by chevron bones (V-shaped appendages on the underside) and ossified tendons .
The shoulder blade was slender and elongated, the front limbs were very short and only reached about a quarter of the length of the hind limbs. As mentioned above, the construction of the hand is not known. The pool was very wide and, above all characterized in that the pubic bone almost no part in the acetabulum has (acetabulum). The hind legs were long with the lower legs slightly longer than the thighs. The foot is believed to have had three functional toes (toes II, III, and IV), with the middle being the largest. The first toe was greatly reduced, the fifth was completely missing. The toes ended in conical, but not curved, hoof-like claws.
Paleobiology
distribution and habitat
Finds that can be clearly assigned to the pachycephalosaurs are only available from East Asia and North America , dubious finds also from Europe , so that these dinosaurs were limited to the continents of the northern hemisphere according to current knowledge. The finds from North America are almost exclusively limited to skulls, which, moreover, often show signs of washing away in rivers after the animals have died. This may lead to the conclusion that these animals lived far away from habitats suitable for fossilization - for example in the mountains. Asian finds, on the other hand, are often better preserved; they suggest other habitats, often located near lakes or rivers. However, these interpretations are very uncertain and could be refuted by new discoveries.
Getting around and eating
Pachycephalosauria moved on their hind legs. The spine was kept almost horizontal, the heavy, stiff tail served as a counterweight to the front part of the body. Because the lower legs were longer than the thighs, these animals were probably able to reach high speeds.
The structure of the teeth suggests a predominantly plant-based diet, although the function of the different tooth types has not been precisely clarified. Possibly the front, pin-like teeth were used to ingest food and the back teeth were used to chew. The broad trunk shows signs of a voluminous digestive tract, in which further digestion of the food took place. Their physique suggests that they ate plants close to the ground, such as leaves, stems, seeds and fruits, possibly supplemented by insects, other small animals or eggs.
Function of the skull roof
The function of the thickened roof of the skull is probably the most discussed question in the pachycephalosaurs paleobiology. For the first time in the 1950s, almost simultaneously, the paleontologists E. Colbert and L. Davitashvili put forward the theory that it could have been used to butt the head. Various physical characteristics are also used to support this theory. The interlocking vertebrae could have served to absorb the shocks, and the structure of the pelvis is also interpreted in this direction. However, no cervical vertebrae are known of any Pachycephalosauria , which could probably provide clues about the way of life of these animals and - if the skulls were used to strike - may also have been specially shaped. Since a sexual dimorphism with regard to the shape of the skull is recognizable in Stegoceras , the assumption was made that this structure was used for interaction with conspecifics and not for defense against predators, which is still considered the most plausible today.
Probably the most popular point of view is that of these dinosaurs in head fights. According to this, they competed against each other with their skulls lowered, in a similar way to today's horn-carriers , such as bighorn sheep . This perspective was worked out in detail by Peter Galton. Other theories contradict this, suggesting that these animals would not have rammed the opponent's head, but tried to hit him on the flank. First formulated by Hans-Dieter Sues in 1978, this point of view was revisited by Kenneth Carpenter in 1997. He justifies this, among other things, with the fact that with the dome-headed pachycephalosaurs - especially with Stygimoloch with the pointed top of the skull - the impact surface would have been much too small. However, he thinks it is conceivable that the flat-headed representatives would have rammed or at least pushed duels with the skulls. Other researchers such as Ralph E. Chapman, however, also consider the skull tops of the dome-headed pachycephalosaurs to be suitable for ramming their heads.
Another theory is that the skulls of these dinosaurs were not suitable for ramming at all. A study by Mark Goodwin and John Horner came to the conclusion that the skulls of these animals were permeated with fine blood vessels and therefore no impacts, regardless of whether on the head or the flank of the opponent, were possible. As a result, the cranial tops and thorn-like occiput spines of this genus would only have been used for display or identification, but not for direct combat.
Teresa Maryańska et al. think it is conceivable that different methods were used in different genera of pachycephalosaurs. Depending on the shape of the head, rams could have been made to the flanks, rams to the head or just visual purposes. Ultimately, the function of the thickened skull roof cannot currently be answered.
Systematics and history of development
External system
The external systematics of the Pachycephalosauria was controversial for a long time. In the past they were sometimes thought to be closely related to the ornithopods or stegosauria . Today the Ceratopsia are regarded as their sister group, together they form the taxon Marginocephalia . Marginocephalia and Ornithopoda are summarized as Cerapoda , which in turn are incorporated into the bird pelvic dinosaur (Ornithischia). A simplified cladogram that shows only the most important taxa looks like this:
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Development history
Undoubted fossil finds of the Pachycephalosauria come only from the Upper Cretaceous ( Santonian to Maastrichtian ), from a period from 85 to 66 million years ago, and only from East Asia and North America . However, there are several poorly preserved finds that suggest an earlier and further occurrence of these animals: Ferganocephale from the Middle Jurassic from Central Asia and Yaverlandia and Stenopelix from the Lower Cretaceous Europe. However, these finds are very fragmentary - for example, only teeth were found from Ferganocephale - and do not allow an unequivocal systematic classification. Some authors like Robert Sullivan (2006) therefore do not count these three genera among the pachycephalosaurs.
Internal system
The internal systematics of the Pachycephalosauria is controversial, which is partly due to the fact that of many genera only the top of the skull or other parts of the skull are known. Traditionally, they were divided into two subgroups: The Homalocephalidae were characterized by a flat cranial roof and large fenestra supratemporalis (upper cranial window in the temporal region ), while the Pachycephalosauridae had a dome-shaped, arched cranial roof and small or closed fenestra supratemporalis. The Homalocephalidae were seen as the more primitive and the Pachycephalosauridae as a more developed group. However, the Homalocephalidae had long been known as paraphyletic , as the Pachycephalosauridae developed from them, so they do not form a natural group. With the discovery of Dracorex , these perspectives have wavered. This genus shows some more developed features and is believed to be closely related to the domed-headed species Pachycephalosaurus and Stygimoloch , but has a flat skull. This led Robert Sullivan to theorize that the flat skull is a derived feature. That is why he rejects the older classifications and summarizes all representatives as Pachycephalosauridae.
The system used here presents a more traditional approach. It largely follows T. Maryańska et al. (2004), but includes genera described since then:
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Pachycephalosauria
- Ferganocephale (controversial)
- Yaverlandia (controversial)
- Stenopelix (controversial)
- Wannanosaurus
- Goyocephala
- Goyocephale
- Homalocephaloidea
- Pachycephalosauria incertae sedis
literature
- Teresa Maryańska , Ralph E. Chapman, David B. Weishampel : Pachycephalosauria. In: David B. Weishampel, Peter Dodson , Halszka Osmólska (eds.): The Dinosauria . 2nd edition. University of California Press, Berkeley CA et al. a. 2004, ISBN 0-520-24209-2 , pp. 464-477.
- David E. Fastovsky , David B. Weishampel: The Evolution and Extinction of the Dinosaurs. 2nd edition. Cambridge University Press, Cambridge u. a. 2005, ISBN 0-521-81172-4 .
- Robert M. Sullivan: A taxonomic review of the Pachycephalosauridae (Dinosauria: Ornithischia). In: Spencer G. Lucas, Robert M. Sullivan (Eds.): Late Cretaceous Vertebrates from the Western Interior (= New Mexico Museum of Natural History and Science. Bulletin. 35, ISSN 1524-4156 ). New Mexico Museum of Natural History and Science, Albuquerque NM 2006, pp. 347–365, digital version (PDF; 4.79 MB) .
Web links
Individual evidence
- ↑ a b c d Pachycephalosaurian Biostratigraphy, Paleoecology and Paleobiology. In: Maryańska et al. 2004, pp. 475-477.
- ↑ Thomas R. Holtz Jr .: Dinosaurs. The most complete, up-to-date Encyclopedia for Dinosaur Lovers of all ages. Random House, New York NY 2007, ISBN 978-0-375-82419-7 , p. 272.
- ↑ Ralph E. Chapman, Peter M. Galton , J. John Sepkoski , William P. Wall: A Morphometric Study of the Cranium of the Pachycephalosaurid Dinosaur Stegoceras. In: Journal of Paleontology. Vol. 55, No. 3, 1981, ISSN 0022-3360 , pp. 608-618, abstract .
- ↑ Peter M. Galton: Pachycephalosaurids - dinosaurian battering rams. In: Discovery. The Magazine of the Yale Peabody Museum of Natural History. Vol. 6, 1970, ISSN 0012-3625 , pp. 23-32.
- ↑ Hans-Dieter Sues : Functional morphology of the dome in pachycephalosaurid dinosaurs. In: New Yearbook for Geology and Paleontology. Monthly books. 1978, ISSN 0028-3630 , pp. 459-472.
- ↑ Kenneth Carpenter : Agonistic behavior in pachycephalosaurs (Ornithischia: Dinosauria): a new look at head-butting behavior. In: Contributions to Geology. Vol. 32, No. 1, 1997, ISSN 0010-7980 , pp. 19-25, digitized version (PDF; 984.18 kB) ( Memento of the original from March 3, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. .
- ^ Mark B. Goodwin, John R. Horner : Cranial histology of pachycephalosaurs (Ornithischia: Marginocephalia) reveals transitory structures inconsistent with head-butting behavior. In: Paleobiology. Vol. 30, No. 2, 2005, ISSN 0094-8373 , pp. 253-267, doi : 10.1666 / 0094-8373 (2004) 030 <0253: CHOPOM> 2.0.CO; 2 .
- ↑ after Fastovsky & Weishampel 2005.
- ↑ David C. Evans, Ryan K. Schott, Derek W. Larson, Caleb M. Brown, Michael J. Ryan: The oldest North American pachycephalosaurid and the hidden diversity of small-bodied ornithischian dinosaurs. In: Nature Communications. Vol. 4, Article 1828, 2013, ISSN 2041-1723 , doi : 10.1038 / ncomms2749 .
- ↑ Mahito Watabe, Khishigjaw Tsogtbaatar, Robert M. Sullivan: A new pachycephalosaurid from the Baynshire Formation (Cenomanian-late Santonian), Gobi Desert, Mongolia. In: Robert M. Sullivan, Spencer G. Lucas, Justin A. Spielmann (eds.): Fossil Record 3 (= New Mexico Museum of Natural History and Science. Bulletin. 53). New Mexico Museum of Natural History and Science, Albuquerque NM 2011, pp. 489–497, digitized version (PDF; 3.96 MB) .
- ↑ Nicholas R. Longrich, Julia Sankey, Darren Tanke: Texacephale langstoni, a new genus of pachycephalosaurid (Dinosauria: Ornithischia) from the upper Campanian Aguja Formation, southern Texas, USA. In: Cretaceous Research. Vol. 31, No. 2, 2010, ISSN 0195-6671 , pp. 274-284, doi : 10.1016 / j.cretres.2009.12.002 .
