Logo of the 2012 Annual Meeting of the Society of Vertebrate Paleontology
This was just flashed all over Facebook.... the logo for the 2012 meeting in Raleigh, North Carolina by Kristin Lamm. It features the phytosaur Rutiodon carolinensis . I have to get a few of the t-shirts.
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New Petrified Forest Superintendent Emphasizes Importance of Scientific Research in the Park Including Paleontology
From the Holbrook (Arizona) Tribune:
'Scientists have been unearthing something on the order of one to two new species a year at Petrified Forest National Park for the past few years, and that has new Park Superintendent Brad Traver excited and focused.
“In the last 10 years, we’ve added several new species to science through discoveries at this park. The things found here are influencing the field of study surrounding the Triassic Period. The work of the scientists here is significant,” said Traver, who officially took the helm at the Petrified Forest on May 22.I am excited to be part of this recent surge in our understanding of the park paleontology and geology and hope that we continue to accomplish this essential mission. From day one in the summer of 2001 one of our goals in properly managing the paleontological resources was to build a successful and professional scientific program at the park as well as encouraging and supporting research by partner institutions. Ten years later, park staff and colleagues have published more than 30 papers on the park geology and paleontology, and currently there are more than a dozen institutions conducting detailed paleontological and geological research in the park providing excellent data that allow us to have insight in the approximately 20 million years of history preserved in Chinle Formation strata in the park. In my opinion it is an important and extremely interesting story currently documented by over 800 papers, abstracts, and theses with many more on the way and the potential for hundreds more.
Traver, who has been with the National Park Service for 30 years, has taken up a permanent post at Petrified Forest, where he served as interim superintendent a few years ago.
“I’m thrilled to be headed back to Petrified Forest where I spent a few months in 2007 that were a highlight of my career,” said Traver.
Speaking of what he thinks is significant about the Petrified Forest, Traver said, “Each national park has its unique characteristics. The Petrified Forest has its own scenic beauty, but it isn’t highly scenic on the order of the Grand Canyon, or highly recreational like some of the other parks in the national system.
“This park has primarily a scientific story to tell, and our mission is to produce good science,” he said.
Traver said that mission is being accomplished through increased study of the park’s archeological and paleontological resources by on-site staff along with research partners from various universities and organizations.
Traver anticipates the park’s expansion to encompass the wider boundaries approved by Congress in 2004 will yield even more significant archeological and paleontological finds, including fossils of plants and animals dating 225 million years into the past.'
Possible Evidence for Archosaur Predation on Horseshoe Crabs
Diedrich, C. G. 2011. Middle Triassic horseshoe crab reproduction areas on intertidal flats of Europe with evidence of predation by archosaurs. Biological Journal of the Linnean Society103: 76–105. doi: 10.1111/j.1095-8312.2011.01635.x
Abstract - A systematically excavated track site in a 243.5 Myr old Middle Triassic (Karlstadt Formation, Pelsonian, middle Anisian) intertidal carbonate mud-flat palaeoenvironment at Bernburg (Saxony-Anhalt, central Germany) has revealed extensive horseshoe crab trackways attributable to the Kouphichnium Nopsca, 1923 ichnogenus. The exposed track bed of a Germanic Basin-wide spanned intertidal megatrack site is a mud-cracked biolaminate surface on which detailed tracks have been preserved because of rapid drying and cementation as a result of high temperatures, followed by rapid covering with a protective layer of arenitic storm or tsunami sediments. The different trackway types and their orientations have allowed a tidal sequence to be reconstructed, with the initial appearance of swimming horseshoe crabs followed by half-swimming/half-hopping limulids under the shallowest water conditions. The Bernburg trackways, which have mapped lengths of up to 40 m, were all produced by adult animals and exhibit a variety of shapes and patterns that reflect a range of subaquatic locomotion behaviour more typical of mating than of feeding activities. The closest match to the proportions and dimensions of the horseshoe crab tracks at Bernburg is provided by the largest known Middle Triassic limulid Tachypleus gadeai, which is known from the north-western Tethys in Spain. The horseshoe crab body fossils recognized in the German Mesozoic intertidal zones, instead, are from juveniles. The uniformly adult size indicated by the trackways therefore suggests that they may record the oldest intertidal reproductive zones of horseshoe crabs known from anywhere in the world, with the track-makers having possibly migrated thousands of kilometres from shallow marine areas of the north-western Tethys to reproduce in the intertidal palaeoenvironments of the Germanic Basin. Chirotherium trackways of large thecodont archosaurs also appeared on these flats where they appear to have fed on the limulids. With the tidal ebb, smaller reptiles such as Macrocnemus (Rhynchosauroides trackways) appeared on the dry intertidal flats, probably feeding on marine organisms and possibly also on horseshoe crab eggs.
Abstract - A systematically excavated track site in a 243.5 Myr old Middle Triassic (Karlstadt Formation, Pelsonian, middle Anisian) intertidal carbonate mud-flat palaeoenvironment at Bernburg (Saxony-Anhalt, central Germany) has revealed extensive horseshoe crab trackways attributable to the Kouphichnium Nopsca, 1923 ichnogenus. The exposed track bed of a Germanic Basin-wide spanned intertidal megatrack site is a mud-cracked biolaminate surface on which detailed tracks have been preserved because of rapid drying and cementation as a result of high temperatures, followed by rapid covering with a protective layer of arenitic storm or tsunami sediments. The different trackway types and their orientations have allowed a tidal sequence to be reconstructed, with the initial appearance of swimming horseshoe crabs followed by half-swimming/half-hopping limulids under the shallowest water conditions. The Bernburg trackways, which have mapped lengths of up to 40 m, were all produced by adult animals and exhibit a variety of shapes and patterns that reflect a range of subaquatic locomotion behaviour more typical of mating than of feeding activities. The closest match to the proportions and dimensions of the horseshoe crab tracks at Bernburg is provided by the largest known Middle Triassic limulid Tachypleus gadeai, which is known from the north-western Tethys in Spain. The horseshoe crab body fossils recognized in the German Mesozoic intertidal zones, instead, are from juveniles. The uniformly adult size indicated by the trackways therefore suggests that they may record the oldest intertidal reproductive zones of horseshoe crabs known from anywhere in the world, with the track-makers having possibly migrated thousands of kilometres from shallow marine areas of the north-western Tethys to reproduce in the intertidal palaeoenvironments of the Germanic Basin. Chirotherium trackways of large thecodont archosaurs also appeared on these flats where they appear to have fed on the limulids. With the tidal ebb, smaller reptiles such as Macrocnemus (Rhynchosauroides trackways) appeared on the dry intertidal flats, probably feeding on marine organisms and possibly also on horseshoe crab eggs.
1921 Excavation of the Holotype of the Phytosaur "Machaeroprosopus" adamanensis
Today (June 17th) is the 90th Anniversary of Charles Camp initiating collection of the holotype of the phytosaur "Machaeroprosopus" adamanensis (now Smilosuchus adamanensis) from the Blue Forest at Petrified Forest National Park (Arizona, USA). Susan Drymala, Rachel Guest (summer interns), and I took the opportunity to revisit the site. We started at the 1921 campsite (where Rachel found an old sardine can) and headed east, climbing the 50 foot ridge above the arroyo where the specimen was collected. We sat in the quarry for a bit, reminensed for a bit with old photos, thunked our rock hammers in a few times in honor of camp and departed on to other sites. It's always pretty cool to be able to literally follow in the footsteps of these older paleontologists, especailly one who was such a huge influence on Late Triassic vertebrate paleontology as Charles Lewis Camp. Below are Susan (left) and Rachel (right) at the site. Rachel is in the actual old diggings.
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Below, Rachel holds up an old slat of wood from the 1921 excavation.
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Charles L. Camp photo of the skeleton of "Machaeroprosopus" adamanensis as partially excavated in June 1921 (courtesy of the UCMP and PEFO).
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Below is the camp of Annie Alexander and Charles L. Camp in the Blue Forest in June of 1921 (courtesy of the UCMP and PEFO)
Footprint Evidence of Late Triassic Ornithischians from Central Europe?
This footprint assemblage from the Upper Triassic of Central Europe purportedly contains evidence for Triassic ornithischian dinosaurs (cf. Anomoepus isp.). As stated by the author this would be remarkable and offer new information regarding the early evolution of this group. However, the evidence is not conclusive as the taxonomic ID of the tracks is not certain, but I'd love to hear from someone who is more familiar with archosaur tracks what they think of this ID and discovery.
Niedźwiedzki, G. 2011. A Late Triassic dinosaur−dominated ichnofauna from the Tomanová Formation of the Tatra Mountains, Central Europe. Acta Palaeontologica Polonica 56: 291–300. doi:10.4202/app.2010.0027
Abstract - Osteological fossils of dinosaurs are relatively rare in the Late Triassic and Early Jurassic. Thus, ichnofossils are a critical source of information on Late Triassic terrestrial vertebrate communities. The outcrops of the Tomanová Formation (?late Norian–Rhaetian) in the Tatra Mountains of Poland and Slovakia have yielded a diverse ichnofauna. Seven more or less distinct morphotypes of dinosaur tracks have been recognized and are discussed. Most tracks are partly eroded or deformed, but are preserved well enough to be assigned to a range of trackmakers, including early ornithischians, small and large theropods (coelophysoids and/or possibly early tetanurans), and probably basal sauropodomorphs (“prosauropods”) or first true sauropods.
Niedźwiedzki, G. 2011. A Late Triassic dinosaur−dominated ichnofauna from the Tomanová Formation of the Tatra Mountains, Central Europe. Acta Palaeontologica Polonica 56: 291–300. doi:10.4202/app.2010.0027
Abstract - Osteological fossils of dinosaurs are relatively rare in the Late Triassic and Early Jurassic. Thus, ichnofossils are a critical source of information on Late Triassic terrestrial vertebrate communities. The outcrops of the Tomanová Formation (?late Norian–Rhaetian) in the Tatra Mountains of Poland and Slovakia have yielded a diverse ichnofauna. Seven more or less distinct morphotypes of dinosaur tracks have been recognized and are discussed. Most tracks are partly eroded or deformed, but are preserved well enough to be assigned to a range of trackmakers, including early ornithischians, small and large theropods (coelophysoids and/or possibly early tetanurans), and probably basal sauropodomorphs (“prosauropods”) or first true sauropods.
First Occurrence of a Middle Triassic Tetrapod Ichnofauna from Morocco
Klein, H., Voigt, S., Saber, H., Schneider, J. W., Hminna, A., Fischer, J., Lagnaoui, A., and A. Brosig. 2011. First occurrence of a Middle Triassic tetrapod ichnofauna from the Argana Basin (Western High Atlas, Morocco). Palaeogeography, Palaeoclimatology, Palaeoecology (advance online publication) doi:10.1016/j.palaeo.2011.05.021
Abstract - Footprints of Early Mesozoic terrestrial tetrapods are inadequately known from NW Africa despite pervasive and well-exposed continental strata of corresponding age in that region. Here, we report on the first occurrence of a tetrapod ichnofauna from the middle part of the Triassic Timezgadiouine Formation in the Argana Basin, Central Morocco that is outstanding in terms of abundance, preservation, and diversity. Footprints of this assemblage are assigned to the ichnotaxa Chirotherium barthii, Isochirotherium coureli, Synaptichnium isp., Atreipus–Grallator, Rotodactylus isp., Rhynchosauroides isp., and Procolophonichnium isp., suggesting early archosaurian, dinosauromorph, lepidosauromorph and possible therapsid or procolophonoid trackmakers. Based on comparison with the abundant record of Early Mesozoic vertebrate ichnofossils from Europe and North America, the ichnofauna presented here indicates a Middle Triassic (Anisian–Ladinian) age for the track-bearing horizon in the Argana Basin. Its clearly Euramerican affinity sheds light on the migration and dispersal of early archosaur-dominated tetrapod faunas considering that several ichnotaxa of the assemblage are documented from Africa for the first time.
As pointed out by Ben Creisler on the Dinosaur Mailing List, this paper is closely related to one published last year by many of the same authors.
Abstract - Footprints of Early Mesozoic terrestrial tetrapods are inadequately known from NW Africa despite pervasive and well-exposed continental strata of corresponding age in that region. Here, we report on the first occurrence of a tetrapod ichnofauna from the middle part of the Triassic Timezgadiouine Formation in the Argana Basin, Central Morocco that is outstanding in terms of abundance, preservation, and diversity. Footprints of this assemblage are assigned to the ichnotaxa Chirotherium barthii, Isochirotherium coureli, Synaptichnium isp., Atreipus–Grallator, Rotodactylus isp., Rhynchosauroides isp., and Procolophonichnium isp., suggesting early archosaurian, dinosauromorph, lepidosauromorph and possible therapsid or procolophonoid trackmakers. Based on comparison with the abundant record of Early Mesozoic vertebrate ichnofossils from Europe and North America, the ichnofauna presented here indicates a Middle Triassic (Anisian–Ladinian) age for the track-bearing horizon in the Argana Basin. Its clearly Euramerican affinity sheds light on the migration and dispersal of early archosaur-dominated tetrapod faunas considering that several ichnotaxa of the assemblage are documented from Africa for the first time.
As pointed out by Ben Creisler on the Dinosaur Mailing List, this paper is closely related to one published last year by many of the same authors.
90th Anniversary of Charles Camp Collecting at the Petrified Forest
Today marks the 90th anniversary of Charles Lewis Camp's first day collecting fossils in the Chinle Formation of what is now Petrified Forest National Park. Camp was a PhD student at Columbia University who was later hired by the Museum of Paleontology at the University of California at Berkeley, and on June 4th, 1921 he received a telegram from Museum Director Bruce Clark asking if he would be able to help out museum founder and benefactor Annie Alexander and her field partner Louise Kellogg who were collecting fossils in the Blue Forest east of Adamana, Arizona. The two woman had made significant finds of vertebrate fossils and needed assistance. Charles Camp arrived in Adamana on June 11 and joined the two women at their camp on June 12.
His field notebook for this day (which was also a Sunday) recounts his trip from New York to Adamana, notes that the exposures are of the Chinle Formation and Triassic in age, and visited the various fossil forests, including those in the Petrified Forest National Monument, which was then located to the south and much smaller than the present national park.
The paleontology of the park and the Chinle Formation owes much to the work of Charles Camp. It is an honor to follow in his footsteps and in the 90th anniversary of the his first work in the area, we will occassionally revisit his localities (many of which are known because of detailed notes and photos) on the same dates that he did. Paleontology is not done in a vacuum and we truly do see farther because we are "standing on the shoulder of giants". We should never forget the work our predecessors have done and honor it when we can.
His field notebook for this day (which was also a Sunday) recounts his trip from New York to Adamana, notes that the exposures are of the Chinle Formation and Triassic in age, and visited the various fossil forests, including those in the Petrified Forest National Monument, which was then located to the south and much smaller than the present national park.
The paleontology of the park and the Chinle Formation owes much to the work of Charles Camp. It is an honor to follow in his footsteps and in the 90th anniversary of the his first work in the area, we will occassionally revisit his localities (many of which are known because of detailed notes and photos) on the same dates that he did. Paleontology is not done in a vacuum and we truly do see farther because we are "standing on the shoulder of giants". We should never forget the work our predecessors have done and honor it when we can.
New Sauropodomorph from the Upper Triassic Los Colorados Formation of Argentina
Ezcurra, M. D., and C. Apaldetti. In Press. A robust sauropodomorph specimen from the Upper Triassic of Argentina and insights on the diversity of the Los Colorados Formation. Proceedings of the Geologists' Association. doi:10.1016/j.pgeola.2011.05.002
Abstract - The Late Triassic rocks document the first steps of the early dinosaur evolutionary radiation. Although the oldest dinosaurs were not abundant in their assemblages, sauropodomorphs achieved a wide taxonomic diversity and high abundance towards the Triassic–Jurassic boundary. In South America, this pattern is documented in the Ischigualasto-Villa Unión Basin of northwestern Argentina, in which dinosaurs achieved a numerical dominance over other tetrapods during the deposition of the upper levels of the late Norian-Rhaetian Los Colorados Formation. In this contribution we enrich the faunal list of this assemblage with the description of a new medium-sized basal sauropod specimen with a very robust tibial morphology. This new specimen differs from the other known sauropodomorphs described for the Los Colorados Formation and increases the alpha-diversity recorded for this group. A phylogenetic analysis recovered the new specimen at the base of Sauropoda and closely related to Lessemsaurus, Antetonitrus, and other basal sauropods. These results match with the high degree of robustness observed in the tibia of the specimen reported here, which closely approaches the morphology documented for other basal sauropods and departs from the morphospace occupied by non-sauropod sauropodomorphs. A two step pattern of tibia robustness increase is observed in the sauropodomorph phylogeny, a pattern that coincides and could be related with the acquisition of more habitual quadrupedal gait achieved by basal sauropods.
Abstract - The Late Triassic rocks document the first steps of the early dinosaur evolutionary radiation. Although the oldest dinosaurs were not abundant in their assemblages, sauropodomorphs achieved a wide taxonomic diversity and high abundance towards the Triassic–Jurassic boundary. In South America, this pattern is documented in the Ischigualasto-Villa Unión Basin of northwestern Argentina, in which dinosaurs achieved a numerical dominance over other tetrapods during the deposition of the upper levels of the late Norian-Rhaetian Los Colorados Formation. In this contribution we enrich the faunal list of this assemblage with the description of a new medium-sized basal sauropod specimen with a very robust tibial morphology. This new specimen differs from the other known sauropodomorphs described for the Los Colorados Formation and increases the alpha-diversity recorded for this group. A phylogenetic analysis recovered the new specimen at the base of Sauropoda and closely related to Lessemsaurus, Antetonitrus, and other basal sauropods. These results match with the high degree of robustness observed in the tibia of the specimen reported here, which closely approaches the morphology documented for other basal sauropods and departs from the morphospace occupied by non-sauropod sauropodomorphs. A two step pattern of tibia robustness increase is observed in the sauropodomorph phylogeny, a pattern that coincides and could be related with the acquisition of more habitual quadrupedal gait achieved by basal sauropods.
New Magnetostratigraphy Data for the Upper Triassic Chinle Formation and Dockum Group of New Mexico
Lots of good new data in this paper that are very useful. Unfortunately it is victimized by having to rely on previously published stratigraphic correlations that are generally erroneous and unsupported by radioisotopic dates, which especially affects some of the key conclusions of the paper. I also still get really confused with the "Chinle Group" terminology especially units like the "Petrified Forest Formation" which is not equivalent to the currently recognized Petrified Forest Member of the Chinle Formation. Trying to go back and forth between the two schemes (Chinle Formation vs. Chinle Group) is very confusing and often leads to mistakes unless one is extremely careful. Still I extend my congratulations to Kate Zeigler in getting this body of work out there. The correlations can be fine-tuned later.
Zeigler, K. E., and J. W. Geissman. 2011. Magnetostratigraphy of the Upper Triassic Chinle Group of New Mexico: Implications for regional and global correlations among Upper Triassic sequences. Geosphere 7:802–829; doi:10.1130/GES00628.1
Abstract - A magnetic polarity zonation for the Upper Triassic Chinle Group in the Chama Basin, north-central New Mexico (United States), supplemented by polarity data from eastern and west-central New Mexico (Mesa Redonda and Zuni Mountains, respectively), provides the most complete and continuous magnetic polarity chronology for the Late Triassic of the American Southwest yet available. Most of the Chinle Group sequence is composed of hematitic mudrocks that typically carry a well-defined, well-grouped magnetization (residing in both pigmentary and detrital hematite), with laboratory unblocking temperatures as high as 680 °C. Demagnetization experiments isolate magnetizations of south- or north-seeking declination and shallow inclination, which are interpreted as early acquired, Late Triassic magnetizations. Our proposed polarity correlations, coupled with biostratigraphic observations and recent U-Pb age determinations on detrital zircon–bearing strata in the Chinle Group in western New Mexico, West Texas, and Arizona, indicate that deposition of Chinle strata likely spanned a much shorter time span than previously considered. If this interpretation is correct, the Chinle Group can be correlated with only part of the Newark Supergroup or the Upper Triassic Tethyan sections. On a local scale, lower Chinle strata in the Chama Basin are significantly older than the Bluewater Creek Formation in western New Mexico, and the base of the Poleo Formation represents a disconformity of >13 m.y. duration. Magnetic polarity chronologies from upper Chinle strata in New Mexico and Utah suggest that strata considered to be part of the Rock Point Formation in north-central New Mexico are not time equivalent to type Rock Point strata in Utah or to the Redonda Formation of eastern New Mexico.
Zeigler, K. E., and J. W. Geissman. 2011. Magnetostratigraphy of the Upper Triassic Chinle Group of New Mexico: Implications for regional and global correlations among Upper Triassic sequences. Geosphere 7:802–829; doi:10.1130/GES00628.1
Abstract - A magnetic polarity zonation for the Upper Triassic Chinle Group in the Chama Basin, north-central New Mexico (United States), supplemented by polarity data from eastern and west-central New Mexico (Mesa Redonda and Zuni Mountains, respectively), provides the most complete and continuous magnetic polarity chronology for the Late Triassic of the American Southwest yet available. Most of the Chinle Group sequence is composed of hematitic mudrocks that typically carry a well-defined, well-grouped magnetization (residing in both pigmentary and detrital hematite), with laboratory unblocking temperatures as high as 680 °C. Demagnetization experiments isolate magnetizations of south- or north-seeking declination and shallow inclination, which are interpreted as early acquired, Late Triassic magnetizations. Our proposed polarity correlations, coupled with biostratigraphic observations and recent U-Pb age determinations on detrital zircon–bearing strata in the Chinle Group in western New Mexico, West Texas, and Arizona, indicate that deposition of Chinle strata likely spanned a much shorter time span than previously considered. If this interpretation is correct, the Chinle Group can be correlated with only part of the Newark Supergroup or the Upper Triassic Tethyan sections. On a local scale, lower Chinle strata in the Chama Basin are significantly older than the Bluewater Creek Formation in western New Mexico, and the base of the Poleo Formation represents a disconformity of >13 m.y. duration. Magnetic polarity chronologies from upper Chinle strata in New Mexico and Utah suggest that strata considered to be part of the Rock Point Formation in north-central New Mexico are not time equivalent to type Rock Point strata in Utah or to the Redonda Formation of eastern New Mexico.
More Dates for the Manicouagan Impact Structure
van Soest, M. C., K. V. Hodges, J.-A. Wartho, M. B. Biren, B. D. Monteleone, J. Ramezani, J. G. Spray, and L. M. Thompson. 2011. (U‐Th)/He dating of terrestrial impact structures: The Manicouagan example. Geochemistry, Geophysics, Geosystems 12, Q0AA16, doi:10.1029/2010GC003465
Abstract - The accurate dating of meteorite impact structures on Earth has proven to be challenging. Melt sheets are amenable to high‐precision dating by the U‐Pb and 40Ar/39Ar methods, but many impact events do not produce them, or they are not preserved. In cases where high‐temperature shock metamorphism of the target materials has occurred without widespread melting, these isotopic chronometers may be partially reset and yield dates that are difficult to interpret unambiguously as the age of impact. However, the (U‐Th)/He chronometer is sensitive to thermal resetting and can provide a powerful new tool for dating impactites. We report (U‐Th)/He dates for accessory minerals from the Manicouagan impact structure in Quebec, Canada. Nine zircons from a melt sheet sample yield a weighted mean age of 213.2 ± 5.4 Ma (2SE), indistinguishable from the published 214 ± 1 Ma (2s) U‐Pb zircon age for the impact. In contrast, five apatites from this sample yield dates between 205.9 ± 6.5 and 162.0 ± 5.3 Ma (2s), indicating variable postimpact helium loss due to low‐temperature thermal disturbance. Preimpact titanite crystals from a shocked meta‐anorthosite sample yield two dates consistent with the impact age, at 212 ± 27 and 214 ± 13 Ma (2s), and two younger dates of 189.6 ± 6.9 and 192.2 ± 9.8 Ma (2s), suggestive of postimpact helium loss. These results indicate that (U‐Th)/He chronometry is a suitable method for dating impact events, although interpretation of the results requires recognition of possible 4He loss related to reheating subsequent to impact.
Abstract - The accurate dating of meteorite impact structures on Earth has proven to be challenging. Melt sheets are amenable to high‐precision dating by the U‐Pb and 40Ar/39Ar methods, but many impact events do not produce them, or they are not preserved. In cases where high‐temperature shock metamorphism of the target materials has occurred without widespread melting, these isotopic chronometers may be partially reset and yield dates that are difficult to interpret unambiguously as the age of impact. However, the (U‐Th)/He chronometer is sensitive to thermal resetting and can provide a powerful new tool for dating impactites. We report (U‐Th)/He dates for accessory minerals from the Manicouagan impact structure in Quebec, Canada. Nine zircons from a melt sheet sample yield a weighted mean age of 213.2 ± 5.4 Ma (2SE), indistinguishable from the published 214 ± 1 Ma (2s) U‐Pb zircon age for the impact. In contrast, five apatites from this sample yield dates between 205.9 ± 6.5 and 162.0 ± 5.3 Ma (2s), indicating variable postimpact helium loss due to low‐temperature thermal disturbance. Preimpact titanite crystals from a shocked meta‐anorthosite sample yield two dates consistent with the impact age, at 212 ± 27 and 214 ± 13 Ma (2s), and two younger dates of 189.6 ± 6.9 and 192.2 ± 9.8 Ma (2s), suggestive of postimpact helium loss. These results indicate that (U‐Th)/He chronometry is a suitable method for dating impact events, although interpretation of the results requires recognition of possible 4He loss related to reheating subsequent to impact.
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