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Dinosaur paleobiogeography
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Dinosaur paleobiogeography
Dinosaur paleobiogeography is the study of dinosaur geographic distribution, based on evidence in the fossil record.
Thomas M. Lehman, in a study of Late Cretaceous dinosaur distribution, concluded that they were "remarkably provincial," with herbivorous dinosaurs exhibiting "persistent latitudinal and altitudinal zonation" in "[d]istinctive endemic associations." Provincialism was even present during the turbulent faunal turnover of the Maastrichtian stage, when the Laramide Orogeny triggered "the most dramatic event that affected Late Cretaceous dinosaur communities in North America prior to their extinction." This turnover event saw specialized and highly ornamented centrosaurines and lambeosaurines be replaced by more primitive upland dinosaurs in the south while northern biomes became dominated by Triceratops with a greatly reduced hadrosaur community.
Many dinosaur species in North America during the Late Cretaceous had "remarkably small geographic ranges" despite their large body size and high mobility. Large herbivores like ceratopsians and hadrosaurs exhibited the most obvious endemism, which strongly contrasts with modern mammalian faunas whose large herbivores' ranges "typical[ly] ... span much of a continent." Lehman observes that "it is often the most conspicuous and abundant species with the most restricted distributions." He notes that Corythosaurus and Centrosaurus haven't been discovered outside of southern Alberta even though they are the most abundant Judithian dinosaurs in the region. Another example is Pentaceratops, the only known Judithian ceratopsian from New Mexico. In modern North America if one was to sample hypothetical future sites in southwestern Texas, northern New Mexico and southern Alberta, 34 of the 41 large mammal species in the continent could be represented, with the remainder's geographic ranges not overlapping with the sites. Roughly 20 species would be located at each site, but contrasting with the provinciality of dinosaurs, 11-16 species out of twenty would be shared between all three sites. Only the rarer species among modern mammal communities would be able to distinguish different latitudinal zones, and some of these taxa are likely too rare to fossilize. This lack of provinciality is despite the strong temperature gradient. Restrictions in herbivorous dinosaur distribution may be due to foliage preferences, narrow tolerance for variation in climate or other environmental factors. The restrictions on herbivorous dinosaur distribution must have been due to ecological factors rather than physical barriers because carnivorous dinosaurs tended to have wider distributions, especially smaller ones.
As of Lehman's 2001 paper, restrictions in dinosaur occurrences based on distance from the paleo-shoreline had already been well documented. Vaguely distinguished inland-versus-coastal dinosaurs had been discussed previously in the scientific literature. Terrestrial sedimentary strata from the Judithian to the Lancian are generally regressive throughout the entire sequence the preserved changes in fossil communities represent not only phylogenetic changes but ecological zones from the submontane habitats to near-sea level coastal habitats. Modern life at high elevations in lower altitudes resembles life at low elevation in higher latitudes. There may be parallels to this phenomenon in Cretaceous ecosystems, for instance, Pachyrhinosaurus is found in both Alaska and upland environments in southern Alberta. Northern and Southern animal biomes approximately correspond respectively with the Aquillapollenites and Normapolles palynofloral provinces.
Dinosaur faunas of the Judithian age may represent the peak of dinosaur evolution in North America. Hadrosaurs were universally the dominant herbivore of the period and comprised more than half of "a typical assemblage." This was also the period of greatest generic diversity among large herbivorous dinosaurs. Just in Montana and Southern Alberta were ten genera of ceratopsians and ten genera of hadrosaurs. An association between Centrosaurus and Corythosaurus is characteristic of southern Alberta. Earlier research had found that lambeosaurines are less common in contemporary Montanan strata and with different centrosaurs as Monoclonius taking the place of Centrosaurus. Inland environments also differed, with the contemporary Two Medicine Formation preserving an inland fauna characterized by Maiasaura and the early pachyrhinosaur Einiosaurus. Farther south was characterized by lower taxonomic diversity in communities where lambeosaurines were less common and centrosaurs were completely lacking. There Kritosaurus, Parasaurolophus and Pentaceratops are the dominant fauna. The giant eusuchian Deinosuchus is also "conspicuous" in the southern biome. Farther south, in Texas, Kritosaurus predominates. The biomes of the Eastern US may have resembled those of Texas except completely lacking in ceratopsians. Parasaurolophus and Kritosaurus are also present in northern latitudes, so evidently exchange between them occurred, but both are uncommon outside of the southern biome.
In the south, little changes in the transition to the Edmontonian. However, in the northern biome a general trend in reduction of centrosaurines, with only pachyrhinosaurus surviving. Likewise among lambeosaurs, only the single genus Hypacrosaurus remains. Inland faunas are distinguished by a Saurolophus-Anchiceratops association while more coastal areas were characterized by Pachyrhinosaurus and Edmontosaurus. Pachyrhinosaurus occurred as far north as Alaska. "Archaic" elements such as hypilophodonts like Parksosaurus and the "(re)appearance" of basal neoceratopsians like Montanoceratops begin characterizing inland faunas. Lehman described Arrhinoceratops is a likely ancestor for Triceratops.
The Kirtlandian faunal age of the Cretaceous period follows the Judithian and is succeeded by the Edmontonian. It is Campanian in age and is characterized by the ceratopsian Pentaceratops sternbergii, which lived throughout the Kirtlandian. The geological formations found to date or persist from the Kirtlandian are the Bearpaw, the upper Kaiparowits Formation, the Kirtland, Fruitland, Williams Fork, Fort Crittenden, Ringbone, Corral de Enmedio, Packard, and El Gallo formations, and possibly the lower part of the Cerro del Pueblo Formation and upper region of the Aguja Formation. These formations are exposed in Alberta and Montana, Utah, New Mexico, New Mexico, Colorado, Arizona, New Mexico, Sonora, Baja California, Baja California, and possibly Coahuila, and Texas, respectively.
Two local faunas are known from the Kirtlandian faunal age. The Hunter Wash local fauna was defined as the vertebrates "obtained from the upper 40 feet of the Fruitland Formation and the lower 55 feet of the lower shale of the Kirtland Shale (now a formation) in Hunter Wash (member)." The Hunter Wash fauna therefore includes all taxa from the Bisti region of the Bisti/De-Na-Zin Wilderness, and the animals from the Fossil Forest and Ah-shi-sle-pah Wash. The Willow Wash fauna was named for all the fauna of the De-na-zin Member of the Kirtland Formation. The majority of the fauna from the Willow Wash were originally thought to belong to the Alamo Wash local fauna of the Ojo Alamo Formation, until it was found that the entire fauna was in fact from the older Kirtland Formation.
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Dinosaur paleobiogeography
Dinosaur paleobiogeography is the study of dinosaur geographic distribution, based on evidence in the fossil record.
Thomas M. Lehman, in a study of Late Cretaceous dinosaur distribution, concluded that they were "remarkably provincial," with herbivorous dinosaurs exhibiting "persistent latitudinal and altitudinal zonation" in "[d]istinctive endemic associations." Provincialism was even present during the turbulent faunal turnover of the Maastrichtian stage, when the Laramide Orogeny triggered "the most dramatic event that affected Late Cretaceous dinosaur communities in North America prior to their extinction." This turnover event saw specialized and highly ornamented centrosaurines and lambeosaurines be replaced by more primitive upland dinosaurs in the south while northern biomes became dominated by Triceratops with a greatly reduced hadrosaur community.
Many dinosaur species in North America during the Late Cretaceous had "remarkably small geographic ranges" despite their large body size and high mobility. Large herbivores like ceratopsians and hadrosaurs exhibited the most obvious endemism, which strongly contrasts with modern mammalian faunas whose large herbivores' ranges "typical[ly] ... span much of a continent." Lehman observes that "it is often the most conspicuous and abundant species with the most restricted distributions." He notes that Corythosaurus and Centrosaurus haven't been discovered outside of southern Alberta even though they are the most abundant Judithian dinosaurs in the region. Another example is Pentaceratops, the only known Judithian ceratopsian from New Mexico. In modern North America if one was to sample hypothetical future sites in southwestern Texas, northern New Mexico and southern Alberta, 34 of the 41 large mammal species in the continent could be represented, with the remainder's geographic ranges not overlapping with the sites. Roughly 20 species would be located at each site, but contrasting with the provinciality of dinosaurs, 11-16 species out of twenty would be shared between all three sites. Only the rarer species among modern mammal communities would be able to distinguish different latitudinal zones, and some of these taxa are likely too rare to fossilize. This lack of provinciality is despite the strong temperature gradient. Restrictions in herbivorous dinosaur distribution may be due to foliage preferences, narrow tolerance for variation in climate or other environmental factors. The restrictions on herbivorous dinosaur distribution must have been due to ecological factors rather than physical barriers because carnivorous dinosaurs tended to have wider distributions, especially smaller ones.
As of Lehman's 2001 paper, restrictions in dinosaur occurrences based on distance from the paleo-shoreline had already been well documented. Vaguely distinguished inland-versus-coastal dinosaurs had been discussed previously in the scientific literature. Terrestrial sedimentary strata from the Judithian to the Lancian are generally regressive throughout the entire sequence the preserved changes in fossil communities represent not only phylogenetic changes but ecological zones from the submontane habitats to near-sea level coastal habitats. Modern life at high elevations in lower altitudes resembles life at low elevation in higher latitudes. There may be parallels to this phenomenon in Cretaceous ecosystems, for instance, Pachyrhinosaurus is found in both Alaska and upland environments in southern Alberta. Northern and Southern animal biomes approximately correspond respectively with the Aquillapollenites and Normapolles palynofloral provinces.
Dinosaur faunas of the Judithian age may represent the peak of dinosaur evolution in North America. Hadrosaurs were universally the dominant herbivore of the period and comprised more than half of "a typical assemblage." This was also the period of greatest generic diversity among large herbivorous dinosaurs. Just in Montana and Southern Alberta were ten genera of ceratopsians and ten genera of hadrosaurs. An association between Centrosaurus and Corythosaurus is characteristic of southern Alberta. Earlier research had found that lambeosaurines are less common in contemporary Montanan strata and with different centrosaurs as Monoclonius taking the place of Centrosaurus. Inland environments also differed, with the contemporary Two Medicine Formation preserving an inland fauna characterized by Maiasaura and the early pachyrhinosaur Einiosaurus. Farther south was characterized by lower taxonomic diversity in communities where lambeosaurines were less common and centrosaurs were completely lacking. There Kritosaurus, Parasaurolophus and Pentaceratops are the dominant fauna. The giant eusuchian Deinosuchus is also "conspicuous" in the southern biome. Farther south, in Texas, Kritosaurus predominates. The biomes of the Eastern US may have resembled those of Texas except completely lacking in ceratopsians. Parasaurolophus and Kritosaurus are also present in northern latitudes, so evidently exchange between them occurred, but both are uncommon outside of the southern biome.
In the south, little changes in the transition to the Edmontonian. However, in the northern biome a general trend in reduction of centrosaurines, with only pachyrhinosaurus surviving. Likewise among lambeosaurs, only the single genus Hypacrosaurus remains. Inland faunas are distinguished by a Saurolophus-Anchiceratops association while more coastal areas were characterized by Pachyrhinosaurus and Edmontosaurus. Pachyrhinosaurus occurred as far north as Alaska. "Archaic" elements such as hypilophodonts like Parksosaurus and the "(re)appearance" of basal neoceratopsians like Montanoceratops begin characterizing inland faunas. Lehman described Arrhinoceratops is a likely ancestor for Triceratops.
The Kirtlandian faunal age of the Cretaceous period follows the Judithian and is succeeded by the Edmontonian. It is Campanian in age and is characterized by the ceratopsian Pentaceratops sternbergii, which lived throughout the Kirtlandian. The geological formations found to date or persist from the Kirtlandian are the Bearpaw, the upper Kaiparowits Formation, the Kirtland, Fruitland, Williams Fork, Fort Crittenden, Ringbone, Corral de Enmedio, Packard, and El Gallo formations, and possibly the lower part of the Cerro del Pueblo Formation and upper region of the Aguja Formation. These formations are exposed in Alberta and Montana, Utah, New Mexico, New Mexico, Colorado, Arizona, New Mexico, Sonora, Baja California, Baja California, and possibly Coahuila, and Texas, respectively.
Two local faunas are known from the Kirtlandian faunal age. The Hunter Wash local fauna was defined as the vertebrates "obtained from the upper 40 feet of the Fruitland Formation and the lower 55 feet of the lower shale of the Kirtland Shale (now a formation) in Hunter Wash (member)." The Hunter Wash fauna therefore includes all taxa from the Bisti region of the Bisti/De-Na-Zin Wilderness, and the animals from the Fossil Forest and Ah-shi-sle-pah Wash. The Willow Wash fauna was named for all the fauna of the De-na-zin Member of the Kirtland Formation. The majority of the fauna from the Willow Wash were originally thought to belong to the Alamo Wash local fauna of the Ojo Alamo Formation, until it was found that the entire fauna was in fact from the older Kirtland Formation.