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Brachiosaurus

Saturday, November 19, 2016

Brachiosaurus by damir-g-martin on DeviantArt

Brachiosaurus is a genus of sauropod dinosaur from the Jurassic Morrison Formation of North America. It was first described by Elmer S. Riggs in 1903 from fossils found in the Grand River Canyon (now Colorado River) of western Colorado, in the United States. Riggs named the dinosaur Brachiosaurus altithorax, declaring it “the largest known dinosaur”. Brachiosaurushad a disproportionately long neck, small skull, and large overall size, all of which are typical for sauropods. However, the proportions of Brachiosaurus are unlike most sauropods: the forelimbs were longer than the hindlimbs, which resulted in a steeply inclined trunk, and its tail was shorter in proportion to its neck than other sauropods of the Jurassic.

Brachiosaurus is the namesake genus of the family Brachiosauridae, which includes a handful of other similar sauropods. Much of what is known by laypeople about Brachiosaurus is in fact based on Giraffatitan brancai, a species of brachiosaurid dinosaur from the Tendaguru Formation of Tanzania that was originally described by German paleontologist Werner Janensch as a species of Brachiosaurus. Recent research shows that the differences between the type species of Brachiosaurusand the Tendaguru material are significant enough that the African material should be placed in a separate genus. Several other potential species of Brachiosaurus have been described from Africa and Europe, but none of them are thought to belong to Brachiosaurus at this time.

Brachiosaurus is one of the rarer sauropods of the Morrison Formation. The type specimen of B. altithorax is still the most complete specimen, and only a relative handful of other specimens are thought to belong to the genus. It is regarded as a high browser, probably cropping or nipping vegetation as high as possibly 9 metres (30 ft) off of the ground. Unlike other sauropods, and its depiction in the film Jurassic Park, it was unsuited for rearing on its hindlimbs. It has been used as an example of a dinosaur that was most likely ectothermic because of its large size and the corresponding need for forage, but more recent research finds it to have been warm-blooded.

Bronze Brachiosaurus mount outside of the Field Museum of Natural History, Chicago, IL.

 

Species

 

Brachiosaurus altithorax

FMNH P 25107, the holotype of both the genus Brachiosaurus and the species B. altithorax, consists of the right humerus (upper arm bone), the right femur (thigh bone), the right ilium (a hip bone), the right coracoid (a shoulder bone), the sacrum (fused vertebrae of the hip), the last seven thoracic (trunk) and two caudal (tail) vertebrae, and a number of ribs. Riggs described the coracoid as from the left side of the body, but restudy has shown it to be a right coracoid.

In 1969, during a study by Kingham, Brachiosaurus altithorax, along with species now assigned to other genera, were moved from the genus. Kingham found “B.” atalaiensis, “B.” brancai, and B. altithorax were referable to Astrodon creating many new species of Astrodon apart from the type. Kingham’s views of brachiosaurid taxonomy have not been accepted by many authors.

Brachiosaurus altithorax hi-fi skeletal by paleo king

Other assigned species

  • “B.” atalaiensis: Originally described by de Lapparent and Zbyszewski, this material’s reference to Brachiosaurus was doubted by Upchurch, Barret and Dodson, who listed it as an unnamed brachiosaurid, and placed in its own genus Lusotitan by Antunes and Mateus. De Lapparent and Zbyszewski described a series of remains but did not designate a type specimen. Antunes and Mateus selected a partial postcranial skeleton (MIGM 4978, 4798, 4801–4810, 4938, 4944, 4950, 4952, 4958, 4964–4966, 4981–4982, 4985, 8807, 8793–87934) as a lectotype; this specimen includes 28 vertebrae, chevrons, ribs, a possible shoulder blade, humeri, forearm bones, partial left pelvis, lower leg bones, and part of the right ankle. The low neural spines, the prominent deltopectoral crest of the humerus (a muscle attachment site on the upper arm bone), the elongated humerus (very long and slender), and the long axis of the ilium tilted upward indicate that Lusotitan is a brachiosaurid.
  • “B.” brancai: Janensch based his description on “Skelett S” (skeleton S) from Tendaguru, but later realized that it comprised two partial individuals: S I and S II. He at first did not designate them as a syntype series, but in 1935 made S I (MB.R.2180) the lectotype. Taylor in 2009, unaware of this action, proposed the larger and more complete S II (MB.R.2181) as the lectotype. It includes, among other bones, several dorsal (trunk) vertebrae, the left scapula, both coracoids, both sternals (breastbones), both humeri, both ulna and radii (lower arm bones), a right hand, a partial left hand, both pubes (a hip bone) and the right femur, tibia and fibula (shank bones). Later Taylor realised that Janensch had in 1935 designated the smaller skeleton S I as the lectotype. A re-assessment of the relation between the African and American brachiosaur material indicates that a separate generic name is warranted for the Tendaguru material, so “B.” brancai has been moved to its own genus: Giraffatitan.
  • “B.” fraasi: erected by Janensch in 1914, but later synonymized with “B.” brancai; this material now belongs to Giraffatitan.
  • “B.” nougaredi: This species is known from fragmentary remains discovered in eastern Algeria, in the Sahara Desert. The present type material consists of a sacrum and some of the left metacarpals and phalanges. Found at the discovery site but not collected were partial bones of the left forearm, wrist bones, a right shin bone, and fragments that may have come from metatarsals. de Lapparent, who described and named the material in 1960, reported the discovery locality as being in the Late Jurassic–age Taouratine Series (he assigned the rocks this age in part because of the presumed presence of Brachiosaurus). This material was found disjointed over an area of several hundred meters, and probably does not represent a single species.

Classification

Brachiosaurus is the namesake genus of Brachiosauridae. Over the years, a number of sauropods have been assigned to Brachiosauridae, such as AstrodonBothriospondylusDinodocusPelorosaurusPleurocoelus, and Ultrasaurus, but most of these are currently regarded as dubious or of uncertain placement. A phylogenetic analysis of sauropods published in the description of Abydosaurus found that genus to form a clade with Brachiosaurus and Giraffatitan (included in Brachiosaurus). A more recent analysis focused on possible Asian brachiosaurid material found a clade including AbydosaurusBrachiosaurusCedarosaurusGiraffatitan, and Paluxysaurus, but not Qiaowanlong, the putative Asian brachiosaurid. Related genera include Lusotitan and Sauroposeidon. Brachiosauridae is situated at the base of Titanosauriformes, a group of sauropods that also includes the titanosaurs.

Giraffatitan skeleton in Berlin, formerly referred to as Brachiosaurus. © Raimond Spekking / CC BY-SA 4.0 (via Wikimedia Commons)

According to the revised diagnosis by Taylor, Brachiosaurus altithorax is diagnosed by a plethora of characters, many to be found on the dorsal (back) vertebrae. Among the characters placing it in the family Brachiosauridae are a ratio of humerus length to femur length of at least 0.9 (i.e. the upper arm bone is at least nearly as long as the thigh bone), and a very flattened femur shaft (ratio ≥1.85).

Camarasaurus

Saturday, November 19, 2016

Camarasaurus

Camarasaurus was a genus of quadrupedal, herbivorous dinosaurs. It was the most common of the giant sauropods to be found in North America. Its fossil remains have been found in the Morrison Formation of Colorado and Utah, dating to the Late Jurassic epoch (Kimmeridgian to Tithonian stages), between 155 and 145 million years ago.

Camarasaurus presented a distinctive cranial profile of a blunt snout and an arched skull that was remarkably square. It likely travelled in herds, or at least in family groups.

The name means “chambered lizard”, referring to the hollow chambers in its vertebrae (Greek καμαρα/kamara meaning “vaulted chamber”, or anything with an arched cover, and σαυρος/sauros meaning “lizard”).

When the noted American paleontologist Edward Drinker Cope described Camarasaurus in 1877, he was obviously impressed by the hollow, box-like vertebrae in the neck. This feature made the neck much lighter and easier for the animal to carry, and it is this characteristic that gave the animal its name: “Chambered lizard.”

Camarasaurus was a stout, compact sauropod with a relatively short neck and short tail. The front legs were slightly shorter than the back legs. The head can be described as a bubble of air encased by thin struts of bone. Huge holes for the nostrils, eye sockets, and other skull cavities made the skull as light as possible but strong enough to withstand the bite forces from the doglike snout.

Camarasaurus supremus (#1) bones being exvataed by Oramel Lucas for Edward Drinker Cope, c. 1877.

The teeth were stumpy but strong – much more robust than the peglke teeth of other sauropods – and the jaws had a powerful bite. Camarasaurus was probably capable of dealing with a wider variety of tough plants, thereby giving it an advantage in mixed forests.

Camarasaurus is the most common sauropod in North America. A number of complete skeletons have been recovered, as well as numerous partial skeletons and isolated bones. Several specimens can be seen in the rocks of the dinosaur National Monument in Utah. As a result of these fossil finds, we now know more about Camarasaurus than we know about any other of the sauropod dinosaurs.

Fossil specimen in the Natural History Museum of Utah, Salt Lake City, Utah, USA. Photography was permitted in the museum without restriction. Author: Daderot

 

Carcharodontosaurus

Saturday, November 19, 2016

Carcharodontosaurus

Carcharodontosaurus is a genus of carnivorous carcharodontosaurid dinosaurs that existed between 100 and 94 million years ago, during the Cenomanian stages of the mid-Cretaceous Period. It is currently known to include two species, C.saharicusand C.iguidensis, which were among the larger theropods, as large as or slightly bigger than Tyrannosaurus and possibly slightly larger than Giganotosaurus, but not quite as large as Spinosaurus.

The genus Carcharodontosaurus is named after the shark genus Carcharodon (itself named from the Greek καρχαρο (karcharo) meaning “jagged” or “sharp” and οδοντο (odonto) meaning “teeth”)), and σαυρος (sauros), meaning “lizard”.

Description

Carcharodontosaurus includes some of the longest and heaviest known carnivorous dinosaurs, with various scientists proposing length estimates for the species C. saharicus ranging between 12 and 13.3 m (39 and 44 ft) and weight estimates between 6.2 and 15.1 metric tons.

Carcharodontosaurus were carnivores, with enormous jaws and long, serrated teeth up to eight inches long. A skull length of about 1.6 meters (5.2 ft) has been restored for C. saharicus, and the skull of C. iguidensis is reported to have been about the same size. Currently, the largest known theropod skull belongs to another huge carcharodontosaurid dinosaur, the closely related Giganotosaurus (with skull length estimates up to 1.95 m) (6.4 ft). Gregory S. Paul estimates Carcharodontosaurus iguidensis at 10 m (33 ft) and 4 t (4.4 short tons).

Brain and inner ear

In 2001, Hans C. E. Larsson published a description of the inner ear and endocranium of Carcharodontosaurus saharicus. Starting from the portion of the brain closest to the tip of the animal’s snout is the forebrain, which is followed by the midbrain. The midbrain is angled downwards at a 45 degree angle and towards the rear of the animal. This is followed by the hind brain, which is roughly parallel to the forebrain and forms a roughly 40 degree angle with the midbrain. Overall, the brain of C. saharicus would have been similar to that of a related dinosaur, Allosaurus fragilis. Larsson found that the ratio of the cerebrum to the volume of the brain overall in Carcharodontosaurus was typical for a non-avian reptile. Carcharodontosaurus also had a large optic nerve.

Carcharodontosaurus skull cast, Science Museum of Minnesota. Photo by Matthew Deery

The three semicircular canals of the inner ear of Carcharodontosaurus saharicus, when viewed from the side, had a subtriangular outline. This subtriangular inner ear configuration is present in Allosaurus, lizards, turtles, but not in birds. The semi-“circular” canals themselves were actually very linear, which explains the pointed silhouette. In life, the floccular lobe of the brain would have projected into the area surrounded by the semicircular canals, just like in other non-avian theropods, birds, and pterosaurs.

Discovery and naming

In 1924, two teeth were found in the Continental intercalaire of Algeria, showing what were at the time unique characteristics, this teeth were described by Depéret and Savornin (1925) as representing a new taxon, which they named Megalosaurus saharicus and later referred by the same authors to the subgenus Dryptosaurus. Some years later, paleontologist Ernst Stromer described the remains of a partial skull and skeleton from Cenomanian aged rocks in the Bahariya Formation of Egypt (Stromer, 1931), originally excavated in 1914, the remains consisted on a partial skull, teeth, vertebrae, claw bones and assorted hip and leg bones. The teeth in this new finding matched the characteristic of those described by Depéret and Savornin which lead Stromer to conserve the species name saharicus but found necessary to erect a new genus for this species, Carcharodontosaurus, for their strong resemblance to the teeth of Carcharodon (Great white shark).

The fossils described by Stromer were destroyed in 1944 during World War II but a new more complete skull was found in the Kem Kem Formation of Morocco during an expedition led by paleontologist Paul Sereno in 1995, not too far from the Algerian border and the locality where the teeth described by Depéret and Savornin (1925) were found, the teeth found with this new skull matched those described by Depéret and Savornin (1925) and Stromer (1931), the rest of the skull also matched that described by Stromer. This new skull was designated as the neotype by Brusatte and Sereno (2007) who also described a second species of CarcharodontosaurusC. iguidensis from the Echkar Formation of Niger, differing from C. saharicus in aspects of the maxilla and braincase.

Classification

The following cladogram after Apesteguía et al., 2016, shows the placement of Carcharodontosaurus within Carcharodontosauridae.

Cladogram after Apesteguía et al., 2016

 

Paleobiology

Hunting

A study by Donald Henderson, the curator of dinosaurs at the Royal Tyrrell Museum suggests that Carcharodontosaurus was able to physically lift animals weighing a maximum of 424 kg (935 lb) in its jaws based on a study of the strength of its jaws, neck, and its center of mass.

Pathology

SGM-Din 1, a Carcharodontosaurus saharicus skull, has a circular puncture wound in the nasal and “an abnormal projection of bone on the antorbital rim”.

Carnotaurus

Saturday, November 19, 2016

Illustration of Carnotaurus by Fred Wierum

Carnotaurus is a genus of large theropod dinosaur that lived in South America during the Late Cretaceous period, between about 72 and 69.9 million years ago. The only species is Carnotaurus sastrei. Known from a single well-preserved skeleton, it is one of the best-understood theropods from the Southern Hemisphere. The skeleton, found in 1984, was uncovered in the Chubut Province of Argentina from rocks of the La Colonia Formation. Derived from the Latin carno [carnis] (“flesh”) and taurus (“bull”), the name Carnotaurus means “meat-eating bull”, an allusion to the animal’s bull-like horns. Carnotaurus is a derived member of the Abelisauridae, a group of large theropods that occupied the large predatorial niche in the southern Landmasses of Gondwana during the late Cretaceous. The phylogenetic relations of Carnotaurus are uncertain; it might have been closer to either Majungasaurus or Aucasaurus.

Carnotaurus was a lightly built, bipedal predator, measuring 8 to 9 m (26.2 to 29.5 ft) in length and weighing at least 1.35 metric tons (1.33 long tons; 1.49 short tons). As a theropod, Carnotaurus was highly specialized and distinctive. It had thick horns above the eyes, a feature unseen in all other carnivorous dinosaurs, and a very deep skull sitting on a muscular neck. Carnotaurus was further characterized by small, vestigial forelimbs and long and slender hindlimbs. The skeleton is preserved with extensive skin impressions, showing a mosaic of small, non-overlapping scales approximately 5 mm in diameter. The mosaic was interrupted by large bumps that lined the sides of the animal, and there are no hints of feathers.

The distinctive horns and the muscular neck may have been used in fighting conspecifics. According to separate studies, rivaling individuals may have combated each other with quick head blows, by slow pushes with the upper sides of their skulls, or by ramming each other head-on, using their horns as shock absorbers. The feeding habits of Carnotaurus remain unclear: some studies suggest the animal was able to hunt down very large prey such as sauropods, while other studies find it preyed mainly on relatively small animals. Carnotaurus was well adapted for running and was possibly one of the fastest large theropods.

Description

Carnotaurus was a large but lightly built predator. The only known individual was about 8–9 m (26.2–29.5 ft) in length, making Carnotaurus one of the largest abelisaurids. While Ekrixinatosaurus and possibly Abelisaurus, highly incomplete, would have been similar or larger in size, a 2016 study found that only Pycnonemosaurus, at 8.9 m (29.2 ft), was longer than Carnotaurus, which was estimated at 7.8 m (25.6 ft). Its mass is estimated to have been 1,350 kg (1.33 long tons; 1.49 short tons) 1,500 kg (1.5 long tons; 1.7 short tons) and 2,100 kg (2.1 long tons; 2.3 short tons) in separate studies that used different estimation methods. Carnotaurus was a highly specialized theropod, as seen especially in characteristics of the skull, the vertebrae and the forelimbs. The pelvis and hindlimbs, on the other hand, remained relatively conservative, resembling those of the more basal Ceratosaurus. Both the pelvis and hindlimb bones were long and slender. The left thigh bone of the individual measures 103 cm in length, but shows an average diameter of only 11 cm.

Mounted skeletal cast at Chlupáč Museum in Prague

 

Classification

Carnotaurus is one of the best-understood genera of the Abelisauridae, a family of large theropods restricted to the ancient southern supercontinent Gondwana. Abelisaurids were the dominant predators in the Late Cretaceous of Gondwana, replacing the carcharodontosaurids and occupying the ecological niche filled by the tyrannosaurids in the northern continents. Several notable traits that evolved within this family, including shortening of the skull and arms as well as peculiarities in the cervical and caudal vertebrae, were more pronounced in Carnotaurus than in any other abelisaurid.

Though relationships within the Abelisauridae are debated, Carnotaurus is consistently shown to be one of the most derived members of the family by cladistical analyses. Its nearest relative might have been either Aucasaurus or Majungasaurus; this ambiguity is largely due to the incompleteness of the Aucasaurus skull material. A recent review suggests that Carnotauruswas not closely related with either Aucasaurus or Majungasaurus, and instead proposed Ilokelesia as its sister taxon.

Carnotaurus is eponymous for two subgroups of the Abelisauridae: the Carnotaurinae and the Carnotaurini. Paleontologists do not universally accept these groups. The Carnotaurinae was defined to include all derived abelisaurids with the exclusion of Abelisaurus, which is considered a basal member in most studies. However, a 2008 review suggested that Abelisaurus was a derived abelisaurid instead. Carnotaurini was proposed to name the clade formed by Carnotaurus and Aucasaurus; only those paleontologists who consider Aucasaurus as the nearest relative of Carnotaurus use this group.

Abelisaurid phylogeny after Canale and colleagues (2009).

 

Discovery

The only skeleton (holotype MACN-CH 894) was unearthed in 1984 by an expedition led by Argentinian paleontologist José Bonaparte. This expedition also recovered the peculiar spiny sauropod Amargasaurus. It was the eighth expedition within the project named “Jurassic and Cretaceous Terrestrial Vertebrates of South America”, which started in 1976 and which was sponsored by the National Geographic Society. The skeleton is well-preserved and articulated (still connected together), with only the posterior two thirds of the tail, much of the lower leg, and the hind feet being destroyed by weathering. During fossilization, the skull and especially the muzzle were crushed laterally, while the premaxilla were pushed upwards onto the nasal bones. As a result, the upward curvature of the upper jaw is artificially exaggerated in the holotype. The skeleton belonged to an adult individual, as indicated by the fused sutures in the brain case. It was found lying on its right side, showing a typical death pose with the neck bent back over the torso. Unusually, it is preserved with extensive skin impressions. In view of the significance of these impressions, a second expedition was started to reinvestigate the original excavation site, leading to the recovery of several additional skin patches.

The skeleton was collected on a farm named “Pocho Sastre” near Bajada Moreno in the Telsen Department of Chubut Province, Argentina. Because it was embedded in a large hematite concretion, a very hard kind of rock, preparation was complicated and progressed slowly. In 1985, Bonaparte published a note presenting Carnotaurus sastrei as a new genus and species and briefly describing the skull and lower jaw. The generic name (Latin carno [carnis] – “flesh” and taurus – “bull”) refers to the bull-like horns, while the specific name sastrei honors Angel Sastre, the owner of the ranch where the skeleton was found. A comprehensive description of the whole skeleton followed in 1990. After AbelisaurusCarnotaurus was the second member of the family Abelisauridae that was discovered. For years, it was by far the best-understood member of its family, and also the best-understood theropod from the Southern Hemisphere. It was not until the 21st century that similar well-preserved abelisaurids were described, including AucasaurusMajungasaurus and Skorpiovenator, allowing scientists to re-evaluate certain aspects of the anatomy of Carnotaurus. The holotype skeleton is displayed in the Argentine Museum of Natural Sciences, Bernardino Rivadavia; replicas can be seen in this and other museums around the world. Sculptors Stephen and Sylvia Czerkas manufactured a life-sized sculpture of Carnotaurus that is now on display in the Natural History Museum of Los Angeles County. This sculpture, ordered by the museum during the mid-1980s, is probably the first life restoration of a theropod showing accurate skin.

Caudipteryx

Saturday, November 19, 2016

Caudipteryx

Caudipteryx (which means “tail feather”) is a genus of peacock-sized theropod dinosaurs that lived in the Aptian age of the early Cretaceous Period (about 124.6 million years ago). They were feathered and remarkably birdlike in their overall appearance. Two species have been described; C. zoui (the type species), in 1998, and C. dongi, in 2000.

Caudipteryx fossils were first discovered in the Yixian Formation of the Sihetun area of Liaoning Province, northeastern China in 1997.

Caudipteryx, like many other maniraptorans, has an interesting mix of reptile- and bird-like anatomical features.

Size comparison of Caudipteryx species to a human.

Caudipteryx had a short, boxy skull with a beak-like snout that retained only a few tapered teeth in the front of the upper jaw. It had a stout trunk, long legs and was probably a swift runner.

Caudipteryx has a short tail stiffened toward the tip, with few vertebrae, like in birds and other oviraptorosaurs. It has a primitive pelvis and shoulder, and primitive skull details in the quadratojugal, squamosal, quadrate, jugal, and mandibular fenestra (in the cheek, jaw, and jaw joint). It has a hand skeleton with a reduced third finger, like that of primitive birds and the oviraptorid Ingenia.

Feathers

The hands of Caudipteryx supported symmetrical, pennaceous feathers that had vanes and barbs, and that measured between 15–20 centimeters long (6–8 inches). These primary feathers were arranged in a wing-like fan along the second finger, just like primary feathers of birds and other maniraptorans. No fossil of Caudipteryx zoui preserves any secondary feathers attached to the forearms, as found in dromaeosaurids, Archaeopteryx and modern birds. Either these arm feathers are not preserved, or they were not present on Caudipteryx in life. An additional fan of feathers existed on its short tail. The shortness and symmetry of the feathers, and the shortness of the arms relative to the body size, indicate that Caudipteryxwas flightless.

The consensus view, based on several cladistic analyses, is that Caudipteryx is a basal (primitive) member of the Oviraptoridae, and the oviraptorids are nonavian theropod dinosaurs. Incisivosaurus is the only oviraptorid that is more primitive.

Centrosaurus

Saturday, November 19, 2016

Centrosaurus apertus specimen ROM 767. Exhibit in the Royal Ontario Museum, Toronto, Ontario, Canada.

The massive bodies of Centrosaurus were borne by stocky limbs, although at up to 6 m (19.7 ft) they were not particularly large dinosaurs. Like other centrosaurines, Centrosaurus bore single large horns over their noses. These horns curved forwards or backwards depending on the specimen. Skull ornamentation was reduced as animals aged.

Centrosaurus is distinguished by having two large hornlets which hook forwards over the frill. A pair of small upwards directed horns is also found over the eyes. The frills of Centrosaurus were moderately long, with fairly large fenestrae and small hornlets along the outer edges.

Centrosaurus, which moved on all fours, had powerful front limbs that would have enhanced the animal’s speed and agility. A ball-and-socket joint in the neck would also have been useful in defense. it allowed Centrosaurus to turn its head swiftly and bring its sharp horn into play against large predators, such as Tyrannosaurus, that attacked from the rear.

Complete skulls arranged in ontogenetic order. Complete skulls arranged in ontogenetic order. Complete Centrosaurus skulls in lateral view arranged in ontogenetic order from the relatively least mature to the relatively most mature specimens, based on the reduced multistate tree. Skulls are not to scale. (A) TMP 1992.082.0001; (B) ROM 767; (C) TMP 1994.182.0001, (D) AMNH FARB 5351, (E) CMN 348; (F) UALVP 11735; (G) USNM 8897; (H) TMP 1997.085.0001; (I) CMN 8795; (J) YPM 2015. Images of TMP 1994.182.0001, AMNH FARB 5351, and CMN 8795 are reversed (mirrored). AMNH FARB 5351 and TMP 1997.085.0001 are represented here by casts. Division of Vertebrate Paleontology, YPM 2015. Courtesy of the Peabody Museum of Natural History, Yale University, New Haven, Connecticut, USA. Joseph A. Frederickson​, Allison R. Tumarkin-Deratzian

 

Classification

The genus Centrosaurus gives its name to the Centrosaurinae subfamily. These were large North American horned dinosaurs characterized by their “prominent nasal horns, subordinate brow horns, short squamosals in a short frill, a tall, deep face relative to the chasmosaurines, and a projection into the rear of the nasal fenestra.” Its closest relatives appear to be Styracosaurus and Monoclonius. It so closely resembles the latter of these that some paleontologists have considered them to represent the same animal.

This cladogram follows the phylogenetic analysis performed by Ryan et al. (2016)

 

Ceratosaurus

Saturday, November 19, 2016

Ceratosaurus

Ceratosaurus (from Greek keras/keratos meaning “horn” and σαυρος/sauros meaning “lizard”), was a large predatory theropod dinosaur from the Late Jurassic Period (Kimmeridgian to Tithonian), found in the Morrison Formation of North America, and the Lourinhã Formation of Portugal (and possibly the Tendaguru Formation in Tanzania). It was characterized by large jaws with blade-like teeth, a large, blade-like horn on the snout and a pair of hornlets over the eyes. The forelimbs were powerfully built but very short. The bones of the sacrum were fused (synsacrum) and the pelvic bones were fused together and to this structure (i.e. similar to modern birds). A row of small osteoderms was present down the middle of the back.

Ceratosaurus, at first glance, looked like a fairly typical theropod, however its skull was quite large in proportion to the rest of its body, and large nasal and brow horns and possessed a prominent nose horn formed from protuberances of the nasal bones. In addition to the large nasal horn, Ceratosaurus possessed smaller hornlike ridges in front of each eye, similar to those of Allosaurus, these ridges were formed by enlargement of the lacrimal bones. Uniquely among theropods, Ceratosauruspossessed dermal armor, in the form of small osteoderms running down the middle of its back. Its tail comprised about half of the body’s total length and was thin and flexible with high vertebral spines.

The type specimen was an individual about 18 feet (5.5 m) long; it is not clear whether this animal was fully grown. David B. Norman (1985) estimated that the maximum length of Ceratosaurus was 20 ft (6.1 m), an assessment supported by a particularly large Ceratosaurus specimen from the Cleveland-Lloyd Dinosaur Quarry (UMNH 5728), discovered in the mid-1960s, which may have been 22 ft (6.7 m) long assuming similar proportions to the holotype.

Ceratosaurus is known from the Cleveland-Lloyd Dinosaur Quarry in central Utah and the Dry Mesa Quarry in Colorado. The type species, described by O. C. Marsh in 1884 and redescribed by Gilmore in 1920, is Ceratosaurus nasicornis. The first skeleton was excavated by rancher Marshall Parker Felch in 1883.

Artist's impression of C. nasicornis

Classification

Relatives of Ceratosaurus include GenyodectesElaphrosaurus, and the abelisaurs, such as Carnotaurus. The classification of Ceratosaurus and its immediate relatives has been under intense debate. Ceratosaurs are unique in their characters; they share some primitive traits with coelophysoids, but also share some derived traits with tetanuran theropods not found in coelophysians. Its closest relatives appear to be the abelisaurs.

In the past, Ceratosaurus, the abelisaurs, and the primitive coelophysoids were all grouped together and called Ceratosauria, defined as “theropods closer to Ceratosaurus than to Aves”. Recent evidence, however, has shown large distinctions between the later, larger and more advanced ceratosaurs and earlier forms like Coelophysis. While considered distant from birds among the theropods, Ceratosaurus and its kin were still very bird-like, and even had a more avian tarsus (ankle joint) than Allosaurus.

Chasmosaurus

Saturday, November 19, 2016

Chasmosaurus

Chasmosaurus is a genus of ceratopsid dinosaur from the Upper Cretaceous Period of North America. Its name means ‘opening lizard’, referring to the large openings (fenestrae) in its frill (Greek chasma meaning ‘opening’ or ‘hollow’ or ‘gulf’ and sauros meaning ‘lizard’). With a length of 4.3–4.8 metres (14.1–15.7 ft) and a weight of 1.5–2 tonnes (1.7–2.2 short tons), Chasmosaurus was a ceratopsian of average size. Like all ceratopsians, it was purely herbivorous. It was initially to be called Protorosaurus, but this name had been previously published for another animal. All specimens of Chasmosaurus were collected from the Dinosaur Park Formation of the Dinosaur Provincial Park of Alberta, Canada. C. russelli comes from the lower beds of the formation while C. belli comes from middle and upper beds.

Chasmosaurus was a medium-size ceratopsid. In 2010 G.S. Paul estimated the length of C. belli at 4.8 metres, its weight at two tonnes; C. russelli would have been 4.3 metres long and weighed 1.5 tonnes. The known differences between the two species mainly pertain to the horn and frill shape, as the postcrania of C. russelli are poorly known. Like many ceratopsians, Chasmosaurus had three main facial horns – one on the nose and two on the brow. In both species these horns are quite short, but with C. russelli they are somewhat longer, especially the brow horns, and more curved backwards. The frill of Chasmosaurus is very elongated and broader at the rear than at the front. It is hardly elevated from the plane of the snout. With C. belli the rear of the frill is V-shaped and its sides are straight. With C. russelli the rear edge is shaped as a shallow U, and the sides are more convex. The sides were adorned by six to nine smaller skin ossifications (called episquamosals) or osteoderms, which attached to the squamosal bone. The corner of the frill featured two larger osteoderms on the parietal bone. With C. russelli the outer one was the largest, with C. belli the inner one. The remainder of the rear edge lacked osteoderms. The parietal bones of the frill were pierced by very large openings, after which the genus was named: the parietal fenestrae. These were not oval in shape, as with most relatives, but triangular, with one point orientated towards the frill corner.

Depiction of the mega-herbivores in the Dinosaur Park Formation, C. belli on the left

Classification

Chasmosaurus was in 1915 by Lambe within the Ceratopsia assigned to the Chasmosaurinae. The Chasmosaurinae usually have long frills, like Chasmosaurus itself, whereas their sister-group the Centrosaurinae typically have shorter frills. Most cladistic analyses show that Chasmosaurus has a basal position in the Chasmosaurinae.

 

Permian Mass Extinction

Saturday, November 19, 2016

Permian Mass Extinction

The Permian–Triassic (P–Tr or P–Textinction event, colloquially known as the Great Dying, the End-Permian Extinction or the Great Permian Extinction, occurred about 252 Ma (million years) ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras. It is the Earth’s most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. It is the only known mass extinction of insects. Some 57% of all families and 83% of all genera became extinct. Because so much biodiversity was lost, the recovery of life on Earth took significantly longer than after any other extinction event, possibly up to 10 million years, although studies in Bear Lake County near the Idaho city of Paris showed a quick and dynamic rebound in a marine ecosystem, illustrating the remarkable resiliency of life.

Map of Pangaea showing where today’s continents were at the Permian–Triassic boundary

There is evidence for one to three distinct pulses, or phases, of extinction. Suggested mechanisms for the latter include one or more large meteor impact events, massive volcanism such as that of the Siberian Traps, and the ensuing coal or gas fires and explosions, and a runaway greenhouse effect triggered by sudden release of methane from the sea floor due to methane clathrate dissociation or methane-producing microbes known as methanogens; possible contributing gradual changes include sea-level change, increasing anoxia, increasing aridity, and a shift in ocean circulation driven by climate change.

 

Causes

Pinpointing the exact cause or causes of the Permian–Triassic extinction event is difficult, mostly because the catastrophe occurred over 250 million years ago, and since then much of the evidence that would have pointed to the cause has been destroyed by now or is concealed deep within the Earth under many layers of rock. The sea floor is also completely recycled every 200 million years by the ongoing process of plate tectonics and seafloor spreading, leaving no useful indications beneath the ocean.

Scientists have accumulated a fairly significant amount of evidence for causes, and several mechanisms have been proposed for the extinction event. The proposals include both catastrophic and gradual processes (similar to those theorized for the Cretaceous–Paleogene extinction event).

  • The catastrophic group includes one or more large bolide impact events, increased volcanism, and sudden release of methane from the sea floor, either due to dissociation of methane hydrate deposits or metabolism of organic carbon deposits by methanogenic microbes.
  • The gradual group includes sea level change, increasing anoxia, and increasing aridity.

Any hypothesis about the cause must explain the selectivity of the event, which affected organisms with calcium carbonate skeletons most severely; the long period (4 to 6 million years) before recovery started, and the minimal extent of biological mineralization (despite inorganic carbonates being deposited) once the recovery began.

The Permian-Triassic Extinction

End of the Permian period
Start of the Triassic period

Cretaceous – Paleogene Extinction Event

Saturday, November 19, 2016

The Cretaceous–Paleogene (K–Pgextinction event, also known as the Cretaceous–Tertiary (K–Textinction, was a mass extinction of some three-quarters of the plant and animal species on Earth that occurred over a geologically short period of time approximately 66 million years ago. With the exception of some ectothermic species like the leatherback sea turtle and crocodiles, no tetrapods weighing more than 25 kilograms (55 lb) survived. It marked the end of the Cretaceous period and with it, the entire Mesozoic Era, opening the Cenozoic Era that continues today.

In the geologic record, the K–Pg event is marked by a thin layer of sediment called the K–Pg boundary, which can be found throughout the world in marine and terrestrial rocks. The boundary clay shows high levels of the metal iridium, which is rare in the Earth’s crust but abundant in asteroids.

As originally proposed in 1980 by a team of scientists led by Luis Alvarez, it is now generally thought that the K–Pg extinction was caused by a massive comet or asteroid impact, estimated to be 10 km (6.2 mi) wide, 66 million years ago and its catastrophic effects on the global environment, including a lingering impact winter that made it impossible for plants and plankton to carry out photosynthesis. The impact hypothesis, also known as the Alvarez hypothesis, was bolstered by the discovery of the 180-kilometre-wide (112 mi) Chicxulub crater in the Gulf of Mexico in the early 1990s, which provided conclusive evidence that the K–Pg boundary clay represented debris from an asteroid impact. The fact that the extinctions occurred at the same time as the impact provides strong situational evidence that the K–Pg extinction was caused by the asteroid. It was possibly accelerated by the creation of the Deccan Traps. However, some scientists maintain the extinction was caused or exacerbated by other factors, such as volcanic eruptions, climate change, or sea level change, separately or together.

A wide range of species perished in the K–Pg extinction. The best-known victims are the non-avian dinosaurs. However, the extinction also destroyed a plethora of other terrestrial organisms, including certain mammals, pterosaurs, birds, lizards, insects, and plants. In the oceans, the K–Pg extinction killed off plesiosaurs and the giant marine lizards (Mosasauridae) and devastated fish, sharks, mollusks (especially ammonites, which became extinct) and many species of plankton. It is estimated that 75% or more of all species on Earth vanished. Yet the devastation caused by the extinction also provided evolutionary opportunities. In the wake of the extinction, many groups underwent remarkable adaptive radiations—a sudden and prolific divergence into new forms and species within the disrupted and emptied ecological niches resulting from the event. Mammals in particular diversified in the Paleogene, producing new forms such as horses, whales, bats, and primates. Birds, fish and perhaps lizards also radiated.

Extinction patterns

The K–Pg extinction event was severe, global, rapid, and selective. In terms of severity, the event eliminated a vast number of species. Based on marine fossils, it is estimated that 75% or more of all species were made extinct by the K–Pg extinction event.

The event appears to have affected all continents at the same time. Non-avian dinosaurs, for example, are known from the Maastrichtian of North America, Europe, Asia, Africa, South America and Antarctica, but are unknown from the Cenozoic anywhere in the world. Similarly, fossil pollen shows devastation of the plant communities in areas as far apart as New Mexico, Alaska, China, and New Zealand.

Even though the boundary event was severe, there was significant variability in the rate of extinction between and within different clades. Species that depended on photosynthesis declined or became extinct as atmospheric particles blocked sunlight and reduced the solar energy reaching the Earth’s surface. This plant extinction caused a major reshuffling of the dominant plant groups. Omnivores, insectivores and carrion-eaters survived the extinction event, perhaps because of the increased availability of their food sources. No purely herbivorous or carnivorous mammals seem to have survived. Rather, the surviving mammals and birds fed on insects, worms, and snails, which in turn fed on dead plant and animal matter. Scientists hypothesize that these organisms survived the collapse of plant-based food chains because they fed on detritus (non-living organic material).

In stream communities, few animal groups became extinct because such communities rely less directly on food from living plants and more on detritus that washes in from the land, buffering them from extinction. Similar, but more complex patterns have been found in the oceans. Extinction was more severe among animals living in the water column than among animals living on or in the sea floor. Animals in the water column are almost entirely dependent on primary production from living phytoplankton, while animals living on or in the ocean floor feed on detritus or can switch to detritus feeding. Coccolithophorids and mollusks (including ammonites, rudists, freshwater snails and mussels), and those organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example, it is thought that ammonites were the principal food of mosasaurs, a group of giant marine reptiles that became extinct at the boundary. The largest air-breathing survivors of the event, crocodyliforms and champsosaurs, were semi-aquatic and had access to detritus. Modern crocodilians can live as scavengers and can survive for months without food, and their young are small, grow slowly, and feed largely on invertebrates and dead organisms or fragments of organisms for their first few years. These characteristics have been linked to crocodilian survival at the end of the Cretaceous.

After the K–Pg extinction event, biodiversity required substantial time to recover, despite the existence of abundant vacant ecological niches.

Radiolaria have left a geological record since at least the Ordovician times, and their mineral fossil skeletons can be tracked across the K–Pg boundary. There is no evidence of mass extinction of these organisms, and there is support for high productivity of these species in southern high latitudes as a result of cooling temperatures in the early Paleocene. Approximately 46% of diatom species survived the transition from the Cretaceous to the Upper Paleocene. This suggests a significant turnover in species, but not a catastrophic extinction of diatoms, across the K–Pg boundary.

Marine invertebrates

There is significant variation in the fossil record as to the extinction rate of marine invertebrates across the K–Pg boundary. The apparent rate is influenced by the lack of fossil records rather than actual extinction.

Ostracods, a class of small crustaceans that were prevalent in the upper Maastrichtian, left fossil deposits in a variety of locations. A review of these fossils shows that ostracod diversity was lower in the Paleocene than any other time in the Cenozoic. However, current research cannot ascertain whether the extinctions occurred prior to or during the boundary interval itself.

Approximately 60% of late-Cretaceous Scleractinia coral genera failed to cross the K–Pg boundary into the Paleocene. Further analysis of the coral extinctions shows that approximately 98% of colonial species, ones that inhabit warm, shallow tropical waters, became extinct. The solitary corals, which generally do not form reefs and inhabit colder and deeper (below the photic zone) areas of the ocean were less impacted by the K–Pg boundary. Colonial coral species rely upon symbiosis with photosynthetic algae, which collapsed due to the events surrounding the K–Pg boundary. However, the use of data from coral fossils to support K–Pg extinction and subsequent Paleocene recovery must be weighed against the changes that occurred in coral ecosystems through the K–Pg boundary.

The numbers of cephalopod, echinoderm, and bivalve genera exhibited significant diminution after the K–Pg boundary. Most species of brachiopods, a small phylum of marine invertebrates, survived the K–Pg extinction event and diversified during the early Paleocene.

Fish

There are substantial fossil records of jawed fishes across the K–Pg boundary, which provides good evidence of extinction patterns of these classes of marine vertebrates. While the deep sea realm was able to remain seemingly unaffected, there was an equal loss between the open marine apex predators and the durophagous demersal feeders on the continental shelf.

Within cartilaginous fish, approximately 7 out of the 41 families of neoselachians (modern sharks, skates and rays) disappeared after this event and batoids (skates and rays) lost nearly all the identifiable species, while more than 90% of teleost fish (bony fish) families survived.

In the Maastrichtian age, 28 shark families and 13 batoid families thrived, of which 25 and 9 survived the K-T boundary event, respectively. Forty-seven of all neoselachian genera cross the K/T boundary, 85% being sharks. Batoids display with 15% a comparably low survival rate.

There is evidence of a mass kill of bony fishes at a fossil site immediately above the K–Pg boundary layer on Seymour Island near Antarctica, apparently precipitated by the K–Pg extinction event. However, the marine and freshwater environments of fishes mitigated environmental effects of the extinction event.

Terrestrial invertebrates

Insect damage to the fossilized leaves of flowering plants from fourteen sites in North America were used as a proxy for insect diversity across the K–Pg boundary and analyzed to determine the rate of extinction. Researchers found that Cretaceous sites, prior to the extinction event, had rich plant and insect-feeding diversity. However, during the early Paleocene, flora were relatively diverse with little predation from insects, even 1.7 million years after the extinction event.

Terrestrial plants

There is overwhelming evidence of global disruption of plant communities at the K–Pg boundary. Extinctions are seen both in studies of fossil pollen, and fossil leaves. In North America, the data suggests massive devastation and mass extinction of plants at the K–Pg boundary sections, although there were substantial megafloral changes before the boundary. In North America, approximately 57% of plant species became extinct. In high southern hemisphere latitudes, such as New Zealand and Antarctica, the mass die-off of flora caused no significant turnover in species, but dramatic and short-term changes in the relative abundance of plant groups. In some regions, the Paleocene recovery of plants began with recolonizations by fern species, represented as a fern spike in the geologic record; this same pattern of fern recolonization was observed after the 1980 Mount St. Helens eruption.

Due to the wholesale destruction of plants at the K–Pg boundary, there was a proliferation of saprotrophic organisms, such as fungi, that do not require photosynthesis and use nutrients from decaying vegetation. The dominance of fungal species lasted only a few years while the atmosphere cleared and there was plenty of organic matter to feed on. Once the atmosphere cleared, photosynthetic organisms, like ferns and other plants, returned. Polyploidy appears to have enhanced the ability of flowering plants to survive the extinction, probably because the additional copies of the genome such plants possessed allowed them to more readily adapt to the rapidly changing environmental conditions that followed the impact.

Amphibians

There is limited evidence for extinction of amphibians at the K–Pg boundary. A study of fossil vertebrates across the K–Pg boundary in Montana concluded that no species of amphibian became extinct. Yet there are several species of Maastrichtian amphibian, not included as part of this study, which are unknown from the Paleocene. These include the frog Theatonius lancensis and the albanerpetontid Albanerpeton galaktion; therefore some amphibians do seem to have become extinct at the boundary. The relatively low levels of extinction seen among amphibians probably reflect the low extinction rates seen in freshwater animals.

Non-archosaur reptiles

The two living non-archosaurian reptile taxa, testudines (turtles) and lepidosaurians (lizards and tuataras), along with choristoderes (semi-aquatic archosauromorphs that would die out in the early Miocene), survived across the K–Pg boundary. Over 80% of Cretaceous turtle species passed through the K–Pg boundary. Additionally, all six turtle families in existence at the end of the Cretaceous survived into the Paleogene and are represented by living species. Living lepidosaurs include the tuataras (the only living rhynchocephalians) and the squamates. The rhynchocephalians were a widespread and relatively successful group of lepidosaurians during the early Mesozoic, but began to decline by the mid-Cretaceous, though they were very successful in the Late Cretaceous of South America. They are represented today by a single genus located exclusively in New Zealand.The order Squamata, which is represented today by lizards, including snakes and amphisbaenians (worm lizards), radiated into various ecological niches during the Jurassic and was successful throughout the Cretaceous. They survived through the K–Pg boundary and are currently the most successful and diverse group of living reptiles with more than 6,000 extant species. Many families of terrestrial squamates became extinct at the boundary, such as monstersaurians and polyglyphanodonts, and fossil evidence indicates they suffered very heavy losses in the KT event, only recovering 10 million years after it. Giant non-archosaurian aquatic reptiles such as mosasaurs and plesiosaurs, which were the top marine predators of their time, became extinct by the end of the Cretaceous. The ichthyosaurs had already disappeared before the mass extinction occurred.

Archosaurs

The archosaur clade includes two surviving groups, crocodilians and birds, along with the various extinct groups of non-avian dinosaurs and pterosaurs.

Crocodyliforms

Ten families of crocodilians or their close relatives are represented in the Maastrichtian fossil records, of which five died out prior to the K–Pg boundary. Five families have both Maastrichtian and Paleocene fossil representatives. All of the surviving families of crocodyliforms inhabited freshwater and terrestrial environments—except for the Dyrosauridae, which lived in freshwater and marine locations. Approximately 50% of crocodyliform representatives survived across the K–Pg boundary, the only apparent trend being that no large crocodiles survived. Crocodyliform survivability across the boundary may have resulted from their aquatic niche and ability to burrow, which reduced susceptibility to negative environmental effects at the boundary. Jouve and colleagues suggested in 2008 that juvenile marine crocodyliforms lived in freshwater environments like modern marine crocodile juveniles, which would have helped them survive where other marine reptiles became extinct; freshwater environments were not as strongly affected by the K–Pg extinction event as marine environments.

Choristodera

The Choristodera, a generally crocodile-like group of uncertain phylogeny (possibly archosaurian) also survived the event, only to become extinct in the Miocene. Studies on Champsosaurus’ palatal teeth suggest that there were dietary changes among the various species across the KT event.

Pterosaurs

One family of pterosaurs, Azhdarchidae, was definitely present in the Maastrichtian, and it likely became extinct at the K–Pg boundary. These large pterosaurs were the last representatives of a declining group that contained 10 families during the mid-Cretaceous. Several other pterosaur lineages may have been present during the Maastrichtian, such as the ornithocheirids, pteranodontids and/or nyctosaurids, as well as a possible tapejarid, though they are represented by fragmentary remains that are difficult to assign to any given group. While this was occurring, modern birds were undergoing diversification; traditionally it was thought that they replaced archaic birds and pterosaur groups, possibly due to direct competition, or they simply filled empty niches, but there is no correlation between pterosaur and avian diversities that are conclusive to a competition hypothesis, and small pterosaurs were present in the Late Cretaceous.

Birds

Most paleontologists regard birds as the only surviving dinosaurs (see Origin of birds). It is thought that all non-avian theropods became extinct, including then-flourishing groups like enantiornithines and hesperornithiforms. Several analyses of bird fossils show divergence of species prior to the K–Pg boundary, and that duck, chicken and ratite bird relatives coexisted with non-avian dinosaurs. Large collections of bird fossils representing a range of different species provides definitive evidence for the persistence of archaic birds to within 300,000 years of the K–Pg boundary. The absence of these birds in the Paleogene is evidence that a mass extinction of archaic birds took place there. A small fraction of the Cretaceous bird species survived the impact, giving rise to today’s birds. The only bird group known for certain to have survived the K–Pg boundary is the Aves. Avians may have been able to survive the extinction as a result of their abilities to dive, swim, or seek shelter in water and marshlands. Many species of avians can build burrows, or nest in tree holes or termite nests, all of which provided shelter from the environmental effects at the K–Pg boundary. Long-term survival past the boundary was assured as a result of filling ecological niches left empty by extinction of non-avian dinosaurs.

Non-avian dinosaurs

Excluding a few controversial claims, scientists agree that all non-avian dinosaurs became extinct at the K–Pg boundary. The dinosaur fossil record has been interpreted to show both a decline in diversity and no decline in diversity during the last few million years of the Cretaceous, and it may be that the quality of the dinosaur fossil record is simply not good enough to permit researchers to distinguish between the options. There is no evidence that late Maastrichtian non-avian dinosaurs could burrow, swim or dive, which suggests they were unable to shelter themselves from the worst parts of any environmental stress that occurred at the K–Pg boundary. It is possible that small dinosaurs (other than birds) did survive, but they would have been deprived of food, as herbivorous dinosaurs would have found plant material scarce and carnivores would have quickly found prey in short supply.The growing consensus about the endothermy of dinosaurs (see dinosaur physiology) helps to understand their full extinction in contrast with their close relatives, the crocodilians. Ectothermic (“cold-blooded”) crocodiles have very limited needs for food (they can survive several months without eating) while endothermic (“warm-blooded”) animals of similar size need much more food to sustain their faster metabolism. Thus, under the circumstances of food chain disruption previously mentioned, non-avian dinosaurs died, while some crocodiles survived. In this context, the survival of other endothermic animals, such as some birds and mammals, could be due, among other reasons, to their smaller needs for food, related to their small size at the extinction epoch.

Whether the extinction occurred gradually or suddenly has been debated, as both views have support from the fossil record. A study of 29 fossil sites in Catalan Pyrenees of Europe in 2010 supports the view that dinosaurs there had great diversity until the asteroid impact, with over 100 living species. However, more recent research indicates that this figure is obscured by taphonomical biases and the sparsity of the continental fossil record. The results of this study, which were based on estimated real global biodiversity, showed that between 628 and 1078 non-avian dinosaur species were alive at the end of the Cretaceous and underwent sudden extinction after the Cretaceous–Paleogene extinction event. Alternatively, interpretation based on the fossil-bearing rocks along the Red Deer River in Alberta supports the gradual extinction of non-avian dinosaurs; during the last 10 million years of the Cretaceous layers there, the number of dinosaur species seems to have decreased from about 45 to about 12. Other scientists have pointed out the same.

Several researchers support the existence of Paleocene dinosaurs. Evidence of this existence is based on the discovery of dinosaur remains in the Hell Creek Formation up to 1.3 m (4.3 ft) above and 40 thousand years later than the K–Pg boundary. Pollen samples recovered near a fossilized hadrosaur femur recovered in the Ojo Alamo Sandstone at the San Juan River indicate that the animal lived during the Cenozoic, approximately 64.5 Ma (about 1 million years after the K–Pg extinction event). If their existence past the K–Pg boundary can be confirmed, these hadrosaurids would be considered a dead clade walking. Scientific consensus is that these fossils were eroded from their original locations and then re-buried in much later sediments (also known as reworked fossils).

Mammals

All major Cretaceous mammalian lineages, including monotremes (egg-laying mammals), multituberculates, metatherians, eutherians, dryolestoideans, and gondwanatheres survived the K–Pg extinction event, although they suffered losses. In particular, metatherians largely disappeared from North America, and the Asian deltatheroidans became extinct. In the Hell Creek beds of North America, at least half of the ten known multituberculate species and all eleven metatherians species are not found above the boundary. Multituberculates in Europe and North America survived relatively unscathed and quickly bounced back in the Palaeocene, but Asian forms were decimated, never again to represent a significant component on mammalian faunas.

Mammalian species began diversifying approximately 30 million years prior to the K–Pg boundary. Diversification of mammals stalled across the boundary. Current research indicates that mammals did not explosively diversify across the K–Pg boundary, despite the environment niches made available by the extinction of dinosaurs. Several mammalian orders have been interpreted as diversifying immediately after the K–Pg boundary, including Chiroptera (bats) and Cetartiodactyla (a diverse group that today includes whales and dolphins and even-toed ungulates), although recent research concludes that only marsupial orders diversified after the K–Pg boundary.

K–Pg boundary mammalian species were generally small, comparable in size to rats; this small size would have helped them find shelter in protected environments. In addition, it is postulated that some early monotremes, marsupials, and placentals were semiaquatic or burrowing, as there are multiple mammalian lineages with such habits today. Any burrowing or semiaquatic mammal would have had additional protection from K–Pg boundary environmental stresses.

Evidence

North American fossils

In North American terrestrial sequences, the extinction event is best represented by the marked discrepancy between the rich and relatively abundant late-Maastrichtian palynomorph record and the post-boundary fern spike.

At present the most informative sequence of dinosaur-bearing rocks in the world from the K–Pg boundary is found in western North America, particularly the late Maastrichtian-age Hell Creek Formation of Montana. This formation, when compared with the older (approximately 75 Ma) Judith River/Dinosaur Park Formations (from Montana and Alberta respectively) provides information on the changes in dinosaur populations over the last 10 million years of the Cretaceous. These fossil beds are geographically limited, covering only part of one continent.

The middle–late Campanian formations show a greater diversity of dinosaurs than any other single group of rocks. The late Maastrichtian rocks contain the largest members of several major clades: TyrannosaurusAnkylosaurusPachycephalosaurusTriceratops and Torosaurus, which suggests food was plentiful immediately prior to the extinction.

In addition to rich dinosaur fossils, there are also plant fossils that illustrate the reduction in plant species across the K–Pg boundary. In the sediments below the K–Pg boundary the dominant plant remains are angiosperm pollen grains, but the actual boundary layer contains little pollen and is dominated by fern spores. More usual pollen levels gradually resume above the boundary layer. This is reminiscent of areas blighted by modern volcanic eruptions, where the recovery is led by ferns, which are later replaced by larger angiosperm plants.

Marine fossils

The mass extinction of marine plankton appears to have been abrupt and right at the K–Pg boundary. Ammonite genera became extinct at or near the K–Pg boundary; however, there was a smaller and slower extinction of ammonite genera prior to the boundary that was associated with a late Cretaceous marine regression. The gradual extinction of most inoceramid bivalves began well before the K–Pg boundary, and a small, gradual reduction in ammonite diversity occurred throughout the very late Cretaceous. Further analysis shows that several processes were in progress in the late Cretaceous seas and partially overlapped in time, then ended with the abrupt mass extinction. The diversity of marine life decreased when the climate near the K-T boundary increased in temperature. The temperature increased about three to four degrees very rapidly between 65.4 and 65.2 million years ago, which is around the time of the extinction event. Not only did the climate temperature increase, but the water temperature decreased causing a drastic decrease in marine diversity.

Megatsunamis

The scientific consensus is that the asteroid impact at the K–Pg boundary left tsunami deposits and sediments around the area of the Caribbean Sea and Gulf of Mexico. These deposits have been identified in the La Popa basin in northeastern Mexico, platform carbonates in northeastern Brazil, and Atlantic deep-sea sediments. The megatsunami has been estimated to be over 100 metres (330 ft) tall, as the asteroid fell in an area of relatively shallow sea; in deep sea it would have been 4.6 kilometres (2.9 mi) tall.

Duration

The length of time taken for the extinction to occur is a controversial issue, because some theories about the extinction’s causes require a rapid extinction over a relatively short period (from a few years to a few thousand years) while others require longer periods. The issue is difficult to resolve because of the Signor–Lipps effect; that is, the fossil record is so incomplete that most extinct species probably died out long after the most recent fossil that has been found. Scientists have also found very few continuous beds of fossil-bearing rock which cover a time range from several million years before the K–Pg extinction to a few million years after it. The sedimentation rate and thickness of K-Pg clay from three sites suggest short duration of event, perhaps less than ten thousand years.

Chicxulub impact

Evidence for impact

In 1980, a team of researchers consisting of Nobel Prize–winning physicist Luis Alvarez, his son geologist Walter Alvarez, and chemists Frank Asaro and Helen Michel discovered that sedimentary layers found all over the world at the Cretaceous–Paleogene boundary contain a concentration of iridium many times greater than normal (30, 160 and 20 times in three sections originally studied). Iridium is extremely rare in Earth’s crust because it is a siderophile element, and therefore most of it traveled with the iron as it sank into Earth’s core during planetary differentiation. As iridium remains abundant in most asteroids and comets, the Alvarez team suggested that an asteroid struck the Earth at the time of the K–Pg boundary. There were earlier speculations on the possibility of an impact event, but this was the first hard evidence of an impact.

This shaded relief image of Mexico’s Yucatan Peninsula show a subtle, but unmistakable, indication of the Chicxulub impact crater. Most scientists now agree that this impact was the cause of the Cretatious-Tertiary Extinction, the event approximately 66 million years ago that marked the sudden extinction of the dinosaurs as well as the majority of life then on Earth. Author: NASA/JPL-Caltech

 

This hypothesis was viewed as radical when first proposed, but additional evidence soon emerged. The boundary clay was found to be full of minute spherules of rock, crystallized from droplets of molten rock formed by the impact. Shocked quartz and other minerals were also identified in the K–Pg boundary. Shocked minerals have their internal structure deformed, and are created by intense pressures such as those associated with nuclear blasts or meteorite impacts. The identification of giant tsunami beds along the Gulf Coast and the Caribbean also provided evidence for impact, and suggested that the impact may have occurred nearby—as did the discovery that the K–Pg boundary became thicker in the southern United States, with meter-thick beds of debris occurring in northern New Mexico.

Further research identified the giant Chicxulub crater, buried under Chicxulub on the coast of Yucatán, as the source of the K–Pg boundary clay. Identified in 1990 based on work by geophysicist Glen Penfield in 1978, the crater is oval, with an average diameter of roughly 180 kilometres (110 mi), about the size calculated by the Alvarez team. The discovery of the crater—a necessary prediction of the impact hypothesis—provided conclusive evidence for a K–Pg impact, and strengthened the hypothesis that the extinction was caused by an impact.

In 2007, a hypothesis was put forth that argued the impactor that killed the dinosaurs belonged to the Baptistina family of asteroids. Concerns have been raised regarding the reputed link, in part because very few solid observational constraints exist of the asteroid or family. Indeed, it was recently discovered that 298 Baptistina does not share the same chemical signature as the source of the K–Pg impact. Although this finding may make the link between the Baptistina family and K–Pg impactor more difficult to substantiate, it does not preclude the possibility. A 2011 WISE study of reflected light from the asteroids of the family estimated the break-up at 80 Ma, giving it insufficient time to shift orbits and impact the Earth by 66 Ma.

In a 2013 paper, Paul Renne of the Berkeley Geochronology Center reported that the date of the asteroid event is 66.043±0.011 million years ago, based on argon–argon dating. He further posits that the mass extinction occurred within 32,000 years of this date.

Effects of impact

In March 2010, an international panel of scientists endorsed the asteroid hypothesis, specifically the Chicxulub impact, as being the cause of the extinction. A team of 41 scientists reviewed 20 years of scientific literature and in so doing also ruled out other theories such as massive volcanism. They had determined that a 10-to-15-kilometre (6.2 to 9.3 mi) space rock hurtled into Earth at Chicxulub on Mexico’s Yucatán Peninsula. The collision would have released the same energy as 100 teratonnes of TNT (420 ZJ), over a billion times the energy of the atomic bombings of Hiroshima and Nagasaki.

The consequences of the Chicxulub impact were of global extent. Some of these phenomena were brief occurrences that immediately followed the impact, but there were also long-term geochemical and climatic disruptions that were catastrophic to the ecology.

The reentry of ejecta into Earth’s atmosphere would include a brief (hours long) but intense pulse of infrared radiation, killing exposed organisms. A paper in 2013 by a prominent nuclear winter modeler suggested that the global debris layer deposited by the impact contains enough soot to hint that the entire terrestrial biosphere burned, with an implication of this being that this would have caused a global soot-cloud blocking out the sun, creating the nuclear winter effect. However the suggestion that global firestorms occurred is debated, with opponents arguing that while ferocious fires probably did result locally, ferocious fires do not necessarily equal firestorms, and any such ferocious fires were instead limited to, the immediate American continent. This disagreement between researchers is termed the “Cretaceous-Palaeogene firestorm debate.”

This happened: 65 million years ago
End of the Cretaceous period
Start of the Palaeocene epoch

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