Styracosaurus ( / s t ɪ ˌ r æ k ə s ɔr ə s / STI RAK -ə- SOR -əs ; significa "lagarto claveteado" del griego antiguo styrax / στύραξ "pico en la culata de un Spear eje" y sauros / σαῦρος 'lagarto') [1] es un género de herbívoros ceratopsian dinosaurio del Cretácico Período ( Campanian etapa), hace unos 75,5 a 75 millones de años . Tenía de cuatro a seis púas parietales largas que se extendían desde el volante de su cuello , un cuerno yugal más pequeño en cada una de sus mejillas y un solo cuerno que sobresalía de su nariz, que puede haber tenido hasta 60 centímetros (2 pies) de largo y 15 centímetros ( 6 pulgadas) de ancho. La función o funciones de los cuernos y volantes se ha debatido durante muchos años.
Styracosaurus | |
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Esqueleto holotipo , Museo Canadiense de la Naturaleza | |
clasificación cientifica | |
Reino: | Animalia |
Filo: | Chordata |
Clade : | Dinosaurio |
Pedido: | † Ornitischia |
Familia: | † Ceratopsidae |
Subfamilia: | † Centrosaurinae |
Clade : | † Eucentrosaura |
Tribu: | † Centrosaurini |
Género: | † Styracosaurus Lambe , 1913 |
Especie tipo | |
† Styracosaurus albertensis Lambe, 1913 | |
Otras especies | |
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Sinónimos | |
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Styracosaurus era un dinosaurio relativamente grande, que alcanzaba una longitud de 5,5 metros (18 pies) y pesaba casi 3 toneladas . Tenía unos 1,8 metros (5,9 pies) de altura. Styracosaurus poseía cuatro patas cortas y un cuerpo voluminoso. Su cola era bastante corta. El cráneo tenía un pico y dientes de las mejillas dispuestas en baterías dentales continuas , lo que sugiere que el animal cortó plantas en rodajas. Como otros ceratopsianos, este dinosaurio pudo haber sido un animal de manada , viajando en grandes grupos, como lo sugieren los lechos de huesos .
Nombrado por Lawrence Lambe en 1913, Styracosaurus es miembro de Centrosaurinae . Una especie , S. albertensis , está actualmente asignada a Styracosaurus . Otra especie, S. ovatus , nombrada en 1930 por Charles Gilmore fue reasignada a un nuevo género, Rubeosaurus , por Andrew McDonald y Jack Horner en 2010, [2] pero recientemente se ha considerado una especie de Styracosaurus (o incluso un espécimen de S. albertensis ) [3] nuevamente.
Descubrimientos y especies
Los primeros restos fósiles de Styracosaurus fueron recolectados en Alberta , Canadá por CM Sternberg (de un área ahora conocida como Dinosaur Provincial Park , en una formación ahora llamada Dinosaur Park Formation ) y nombrada por Lawrence Lambe en 1913. Esta cantera fue revisada en 1935 por un equipo del Museo Real de Ontario que encontró la mandíbula inferior que faltaba y la mayor parte del esqueleto. Estos fósiles indican que S. albertensis medía alrededor de 5,5 a 5,8 metros (18 a 19 pies) de largo y tenía alrededor de 1,65 metros (5,4 pies) de altura en las caderas. [4] Una característica inusual de este primer cráneo es que la punta del volante más pequeña del lado izquierdo está parcialmente solapada en su base por la siguiente punta. Parece que el volante sufrió una rotura en este punto de la vida y se acortó unos 6 centímetros (2,4 pulgadas). Se desconoce la forma normal de esta zona porque no se recuperó la zona correspondiente del lado derecho del volante. [5]
Barnum Brown y su equipo, que trabajaban para el Museo Americano de Historia Natural en Nueva York, recolectaron un esqueleto articulado casi completo con un cráneo parcial en 1915. Estos fósiles también se encontraron en la Formación Dinosaur Park, cerca de Steveville , Alberta. Brown y Erich Maren Schlaikjer compararon los hallazgos y, aunque admitieron que ambos especímenes eran de la misma localidad general y formación geológica, consideraron que el espécimen era lo suficientemente distinto del holotipo como para justificar la erección de una nueva especie, y describieron los fósiles como Styracosaurus Parksi. , nombrado en honor a William Parks . [6] Entre las diferencias entre los especímenes citados por Brown y Schlaikjer estaban un pómulo bastante diferente al de S. albertensis y vértebras de la cola más pequeñas . S. Parksi también tenía una mandíbula más robusta, un dentario más corto y el volante difería en forma de la de la especie tipo. [6] Sin embargo, gran parte del cráneo consistió en una reconstrucción con yeso, y el documento original de 1937 no ilustraba los huesos del cráneo reales. [4] Ahora se acepta como un espécimen de S. albertensis . [5] [7]
En el verano de 2006, Darren Tanke del Museo de Paleontología Royal Tyrrell en Drumheller , Alberta, reubicó el sitio perdido de S. Parksi . [5] En la cantera se encontraron fragmentos del cráneo, evidentemente abandonados por la tripulación de 1915. Estos fueron recolectados y se espera que se encuentren más piezas, tal vez suficientes para justificar una redescripción del cráneo y probar si S. albertensis y S. Parksi son iguales. El Museo Tyrrell también ha recogido varios cráneos de Styracosaurus parciales . [8] También se ha explorado al menos un lecho óseo confirmado (lecho óseo 42) en el Parque Provincial de Dinosaurios (otros lechos óseos de Styracosaurus propuestos en su lugar tienen fósiles de una mezcla de animales y restos ceratopsianos no diagnósticos). Se sabe que Bonebed 42 contiene numerosas piezas de cráneos, como cuernos, mandíbulas y volantes. [5]
Una tercera especie, S. ovatus , de la Formación Two Medicine de Montana , fue descrita por Gilmore en 1930. El material fósil es limitado, siendo la mejor una porción del hueso parietal del volante, pero una característica inusual es que el el par de picos más cercanos a la línea media convergen hacia la línea media, en lugar de alejarse de ella como en S. albertensis . También puede haber solo dos juegos de picos a cada lado del volante, en lugar de tres. Las espigas son mucho más cortas que en S. albertensis , con las más largas de solo 295 milímetros (11,6 pulgadas) de largo. [9] Una revisión de 2010 de restos de cráneos de estirracosaurio por Ryan, Holmes y Russell encontró que era una especie distinta, [5] y en 2010 McDonald y Horner lo colocaron en su propio género, Rubeosaurus . [10] Sin embargo, todavía puede pertenecer a Styracosaurus .
Holmes et al. (2020) argued further that the proposed diagnostic characters of Styracosaurus ovatus/Rubeosaurus fall within the range of asymmetry and individual variation found in Styracosaurus albertensis. The authors considered R. ovatus to be a junior synonym of Styracosaurus albertensis.[3] Another 2020 study describing a juvenile specimen of Styracosaurus further cast doubt on the usefulness of frill spike arrangement for classification. In that study, authors Caleb Brown, Robert Holmes, and Phillip Currie, concluded that the features used to differentiate S. ovatus were likely within the range of variation for the species S. albertensis. They pointed out that several specimens that are otherwise consistent with S. albertensis have been found with inward angled midline frill spikes, though not the same degree as S. ovatus. Given its slightly higher stratigraphic position and more angled spikes, they suggested that it may just be an "extreme morph" of S. albertensis.[11]
Several other species which were assigned to Styracosaurus have since been assigned to other genera. S. sphenocerus, described by Edward Drinker Cope in 1890 as a species of Monoclonius and based on a nasal bone with a broken Styracosaurus-like straight nose horn, was attributed to Styracosaurus in 1915.[12] "S. makeli", mentioned informally by amateur paleontologists Stephen and Sylvia Czerkas in 1990 in a caption to an illustration, is an early name for Einiosaurus.[13] "S. borealis" is an early informal name for S. parksi.[14]
A second specimen, MOR 492, composed of a partial skull including a partial left premaxilla, co-ossified left and right nasals with horncore, partial left postorbital with horncore, and a nearly complete right parietal with two spikes, was discovered in 1986 and referred to R. ovatus in 2010.[2] A third, subadult specimen with very short frill spikes (USNM 14765) was referred in 2011.[15] However, subsequent studies suggested these specimens belonged to a distinct genus and species, Stellasaurus ancellae.[16]
Descripción
Individuals of the genus Styracosaurus were approximately 5.5 meters (18 ft) long as adults and weighed around 2.7 tonnes.[17] The skull was massive, with a large nostril, a tall straight nose horn, and a parietosquamosal frill (a neck frill) crowned with at least four large spikes. Each of the four longest frill spines was comparable in length to the nose horn, at 50 to 55 centimeters (20 to 22 inches) long.[4] The nasal horn was estimated by Lambe at 57 centimeters (22 inches) long in the type specimen,[18] but the tip had not been preserved. Based on other nasal horn cores from Styracosaurus and Centrosaurus, this horn may have come to a more rounded point at around half of that length.[5]
Aside from the large nasal horn and four long frill spikes, the cranial ornamentation was variable. Some individuals had small hook-like projections and knobs at the posterior margin of the frill, similar to but smaller than those in Centrosaurus. Others had less prominent tabs. Some, like the type individual, had a third pair of long frill spikes. Others had much smaller projections, and small points are found on the side margins of some but not all specimens. Modest pyramid-shaped brow horns were present in subadults, but were replaced by pits in adults.[5] Like most ceratopsids, Styracosaurus had large fenestrae (skull openings) in its frill. The front of the mouth had a toothless beak.
The bulky body of Styracosaurus resembled that of a rhinoceros. It had powerful shoulders which may have been useful in intraspecies combat. Styracosaurus had a relatively short tail. Each toe bore a hooflike ungual which was sheathed in horn.[17]
Various limb positions have been proposed for Styracosaurus and ceratopsids in general, including forelegs which were held underneath the body, or, alternatively, held in a sprawling position. The most recent work has put forward an intermediate crouched position as most likely.[19]
Clasificación
Styracosaurus is a member of the Centrosaurinae. Other members of the clade include Centrosaurus (from which the group takes its name),[20][21] Pachyrhinosaurus,[20][22] Avaceratops,[20] Einiosaurus,[22][23] Albertaceratops,[23] Achelousaurus,[22] Brachyceratops,[7] and Monoclonius,[20] although these last two are dubious. Because of the variation between species and even individual specimens of centrosaurines, there has been much debate over which genera and species are valid, particularly whether Centrosaurus and/or Monoclonius are valid genera, undiagnosable, or possibly members of the opposite sex. In 1996, Peter Dodson found enough variation between Centrosaurus, Styracosaurus, and Monoclonius to warrant separate genera, and that Styracosaurus resembled Centrosaurus more closely than either resembled Monoclonius. Dodson also believed one species of Monoclonius, M. nasicornis, may actually have been a female Styracosaurus.[24] However, most other researchers have not accepted Monoclonius nasicornis as a female Styracosaurus, instead regarding it as a synonym of Centrosaurus apertus.[5][25] While sexual dimorphism has been proposed for an earlier ceratopsian, Protoceratops,[26] there is no firm evidence for sexual dimorphism in any ceratopsid.[27][28][29]
The cladogram depicted below represents a phylogenetic analysis by Chiba et al. (2017):[30]
Centrosaurinae |
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Origins and evolution
The evolutionary origins of Styracosaurus were not understood for many years because fossil evidence for early ceratopsians was sparse. The discovery of Protoceratops, in 1922, shed light on early ceratopsid relationships,[31] but several decades passed before additional finds filled in more of the blanks. Fresh discoveries in the late 1990s and 2000s, including Zuniceratops, the earliest known ceratopsian with brow horns, and Yinlong, the first-known Jurassic ceratopsian, indicate what the ancestors of Styracosaurus may have looked like. These new discoveries have been important in illuminating the origins of horned dinosaurs in general, and suggest that the group originated during the Jurassic in Asia, with the appearance of true horned ceratopsians occurring by the beginning of the late Cretaceous in North America.[7]
Goodwin and colleagues proposed in 1992 that Styracosaurus was part of the lineage leading to Einiosaurus, Achelousaurus and Pachyrhinosaurus. This was based on a series of fossil skulls from the Two Medicine Formation of Montana.[32] The position of Styracosaurus in this lineage is now equivocal, as the remains that were thought to represent Styracosaurus have been transferred to the genus Rubeosaurus.[10]
Styracosaurus is known from a higher position in the formation (relating specifically to its own genus) than the closely related Centrosaurus, suggesting that Styracosaurus displaced Centrosaurus as the environment changed over time and/or dimension.[25] It has been suggested that Styracosaurus albertensis is a direct descendant of Centrosaurus (C. apertus or C. nasicornis), and that it in turn evolved directly into the slightly later species Rubeosaurus ovatus. Subtle changes can be traced in the arrangement of the horns through this lineage, leading from Rubeosaurus to Einiosaurus, to Achelousaurus and Pachyrhinosaurus. However, the lineage may not be a simple, straight line, as a pachyrhinosaur-like species has been reported from the same time and place as Styracosaurus albertensis.[2]
In 2020, during the description of Stellasaurus, Wilson et al. found Styracosaurus (including S. ovatus) to be the earliest member of a single evolutionary lineage that eventually developed into Stellasaurus, Achelousaurus, and Pachyrhinosaurus.[15]
Paleobiología
Styracosaurus and other horned dinosaurs are often depicted in popular culture as herd animals. A bonebed composed of Styracosaurus remains is known from the Dinosaur Park Formation of Alberta, about halfway up the formation. This bonebed is associated with different types of river deposits.[8][33] The mass deaths may have been a result of otherwise non-herding animals congregating around a waterhole in a period of drought, with evidence suggesting the environment may have been seasonal and semiarid.[34]
Paleontologists Gregory Paul and Per Christiansen proposed that large ceratopsians such as Styracosaurus were able to run faster than an elephant, based on possible ceratopsian trackways which did not exhibit signs of sprawling forelimbs.[35]
Dentition and diet
Styracosaurs were herbivorous dinosaurs; they probably fed mostly on low growth because of the position of the head. They may, however, have been able to knock down taller plants with their horns, beak, and bulk.[7][36] The jaws were tipped with a deep, narrow beak, believed to have been better at grasping and plucking than biting.[37]
Ceratopsid teeth, including those of Styracosaurus, were arranged in groups called batteries. Older teeth on top were continually replaced by the teeth underneath them. Unlike hadrosaurids, which also had dental batteries, ceratopsid teeth sliced but did not grind.[7] Some scientists have suggested that ceratopsids like Styracosaurus ate palms and cycads,[38] while others have suggested ferns.[39] Dodson has proposed that Late Cretaceous ceratopsians may have knocked down angiosperm trees and then sheared off leaves and twigs.[40]
Horns and frill
The large nasal horns and frills of Styracosaurus are among the most distinctive facial adornments of all dinosaurs. Their function has been the subject of debate since the first horned dinosaurs were discovered.
Early in the 20th century, paleontologist R. S. Lull proposed that the frills of ceratopsian dinosaurs acted as anchor points for their jaw muscles.[41] He later noted that for Styracosaurus, the spikes would have given it a formidable appearance.[42] In 1996, Dodson supported the idea of muscle attachments in part and created detailed diagrams of possible muscle attachments in the frills of Styracosaurus and Chasmosaurus, but did not subscribe to the idea that they completely filled in the fenestrae.[43] C. A. Forster, however, found no evidence of large muscle attachments on the frill bones.[27]
It was long believed that ceratopsians like Styracosaurus used their frills and horns in defence against the large predatory dinosaurs of the time. Although pitting, holes, lesions, and other damage on ceratopsid skulls are often attributed to horn damage in combat, a 2006 study found no evidence for horn thrust injuries causing these forms of damage (for example, there is no evidence of infection or healing). Instead, non-pathological bone resorption, or unknown bone diseases, are suggested as causes.[44]
However, a newer study compared incidence rates of skull lesions in Triceratops and Centrosaurus and showed that these were consistent with Triceratops using its horns in combat and the frill being adapted as a protective structure, while lower pathology rates in Centrosaurus may indicate visual rather than physical use of cranial ornamentation, or a form of combat focused on the body rather than the head;[45] as Centrosaurus was more closely related to Styracosaurus and both genera had long nasal horns, the results for this genus would be more applicable for Styracosaurus. The researchers also concluded that the damage found on the specimens in the study was often too localized to be caused by bone disease.[46]
The large frill on Styracosaurus and related genera also may have helped to increase body area to regulate body temperature,[47] like the ears of the modern elephant. A similar theory has been proposed regarding the plates of Stegosaurus,[48] although this use alone would not account for the bizarre and extravagant variation seen in different members of the Ceratopsidae.[7] This observation is highly suggestive of what is now believed to be the primary function, display.
The theory of frill use in sexual display was first proposed in 1961 by Davitashvili. This theory has gained increasing acceptance.[27][49] Evidence that visual display was important, either in courtship or in other social behavior, can be seen in the fact that horned dinosaurs differ markedly in their adornments, making each species highly distinctive. Also, modern living creatures with such displays of horns and adornments use them in similar behavior.[50]
The use of the exaggerated structures in dinosaurs as species identification has been questioned, as no such function exists in vast majority of modern species of tetrapods (terrestrial vertebrates).[51]
A skull discovered in 2015 from a Styracosaurus indicates that individual variation was likely commonplace in the genus. The asymmetrical nature of the horns in the specimen has been compared to deer, which often have asymmetrical antlers in various individuals. The study carried out may also indicate that the genus Rubeosaurus may be synonymous with Styracosaurus as a result.[3]
Paleoecología
Styracosaurus is known from the Dinosaur Park Formation, and was a member of a diverse and well-documented fauna of prehistoric animals that included horned relatives such as Centrosaurus and Chasmosaurus, duckbills such as Prosaurolophus, Lambeosaurus, Gryposaurus, Corythosaurus, and Parasaurolophus, tyrannosaurids Gorgosaurus, Daspletosaurus, and armored Edmontonia and Euoplocephalus.[52]
The Dinosaur Park Formation is interpreted as a low-relief setting of rivers and floodplains that became more swampy and influenced by marine conditions over time as the Western Interior Seaway transgressed westward.[53] The climate was warmer than present-day Alberta, without frost, but with wetter and drier seasons. Conifers were apparently the dominant canopy plants, with an understory of ferns, tree ferns, and angiosperms.[54]
Ver también
- Timeline of ceratopsian research
Referencias
- ^ Liddell & Scott (1980). Greek-English Lexicon, Abridged Edition. Oxford University Press, Oxford, UK. ISBN 978-0-19-910207-5.
- ^ a b c Andrew T. McDonald & John R. Horner, (2010). "New Material of "Styracosaurus" ovatus from the Two Medicine Formation of Montana". Pages 156–168 in: Michael J. Ryan, Brenda J. Chinnery-Allgeier, and David A. Eberth (eds), New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium, Indiana University Press, Bloomington and Indianapolis, IN.
- ^ a b c Holmes, R.B.; Persons, W.S.; Singh Rupal, B.; Jawad Qureshi, A.; Currie, P.J. (2020). "Morphological variation and asymmetrical development in the skull of Styracosaurus albertensis". Cretaceous Research. 107: 104308. doi:10.1016/j.cretres.2019.104308.
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- ^ Although this article mentioned two bonebeds, including BB 156, the recent review by Ryan et al. only accepted BB 42.
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