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Second Triassic Extinction Constrained in North America

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Second Triassic Extinction Constrained in North America South American Reptiles Found in Virginia: Second Triassic Extinction Constrained in North America Debate on the causes of mass extinctions is now Gondwana fauna replaced focusing on resolving the precise pattern of tax0 Based on the new data from Virginia the Carnian nomic turnover and correlated physical events at episode of faunal change is characterized by the each of the thirteen extinction episodes which replacement of a Gondwana like assemblage of punctuate the last 600 million years of the history terrestrial vertebrates with another of typical North of life The marine record shows three apparent American aspect dominated by the crocodile like mass extinctions in the Triassic period, with the phytosaurs and other archosaurian reptiles Al- largest of all extinctions, the Permian-Triassic though still poorly constrained in time, this transi- event (245 Ma), ushering in the period, and an- tion must have taken place over less than 5 million other great event, the Triassic Jurassic mass ex years and involved not only the extinction of many tinction (201 Ma) closing it (Fig 1) In between is groups of reptiles, but also the origination of many the poorly known Carnian-Norian event (218 Ma) others According to Cornet, the plant record The latter two events are recorded in spectacular shows much the same pattern with the replace- detail in the continental fill of the early Mesozoic rift ment of a diverse spore-rich assemblage by a basins of the Atlantic passive margin called the diverse oollen-rich one Newark Supergrouo (Fg 2 3) currently under Terrestrial vs. marine record mult~d~scolnary study by a team headed by Paul The marine record snows no obvious anomalous Olsen at Lamont increase in faunal turnover within the Carman but Newark's temporal control an interval of high faunal turnover is reported at Time control is vital to the understanding of the the marine Carnian-Norian boundary However, mechanisms behind mass-extinctions, but it is no such turnover is apparent in the fauna or flora usually very difficult to establish at the appropriate from Newark Supergroup strata at that point In scale in ancient rocks For example an error in fact as shown by extraordinarily abundant and dating of  1 000 000 years for rocks well preserved footprints (Fig 5) and less abun- 200 000 000 years old is considered quite good dant bones no terrestrial reptile extinctions at all using radiometric techniques such as K Ar are known through this boundary, and the domi- 7g. 2: Newark Supergroup ' Ar/ 'Ar or Rb Sr but unsuitable for trying to nant forms continue through Clearly, either the resolve whether faunal or floral change is gradual Newark Carnian eoisode is misdated or the ma- CALIBRATION OF NEWARK AND RICHMOND BASINS or unusually abrupt In the continental rocks of the rine and terrestrial transitions are not synchronous. Newark Supergroup however, Olsen found that In addition. Jack Seokoski (Universitv of Chicaao) MILLONS AGES nc- NEWARK very fine temporal control is provided by sedimen- has suggested that the apparent marine ~arnian; YEARS BASIN tary cycles produced by the rise and fall of large Norian mass extinction could be an artifact, pro- SECTION lakes, controlled by orbitally induced (Milan- duced by very high ammonite evolution rates. 199-1 kovitch type) climate change, with a period of Thus the evidence from the Newark Supergroup HETTANGIAN -201 around 21,000 years (Lamont Newsletter 13, suggests rather slow change through the Triassic. TRIASSIC 1986) These lake level cycles, called Van Houten Contrast with Triassic-Jurassic event cycles, allow time resolution at the less than Faunal and floral chanae in the Newark soannina 21,000 year level Correlation with areas outside of the Triassic-Jurassic boundary shows a very differ- the Newark Supergroup is provided by a high- ent pattern A large number of at least local verte- resolution magnetostratigraphy being developed brate and palynomorph extinctions are concen- by Dennis Kent William K Wine (Lamont) along trated around the boundary, with survivors with Paul Olsen and palynologically-basedbiostra constituting the earliest Jurassic assemblages, ap- tigraphy being refined by Bruce Cornet and Sarah parently without replacement by the introduction J Fowell (Lamont) Such correlation is essential in of new taxa This is especially well seen in the determining whether the extinctions seen in New- Newark and Fundy basins, where new finds of NORIAN ark strata are synchronous with the mass extinc vertebrate bones and footprints by the Lamont tions seen in the marine record group are filling in the gap between the well Rich Carnian assemblage known typically Late Triassic age assemblages Because of a lack of fossils the Carnian episode and those of the Early Jurassic, allowing tight has been very poorly known, but recently Olsen constraints on the duration of the period of extinc- Hans-Dieter Sues (Smithsonian) and Noel Boaz tions (Virginia Museum of Science) discovered a spec- tacularly rich reptile assemblage near Richmond Virginia which should shed considerable light on its faunal transition Unlike all other North Ameri EXTINCTION RATE THROUGH TIME can Triassic age assemblages, this assemblage is dominated by mammal like reptiles and is closely comparable to those known for many years from Southern Africa Brazil and Argentina Most com- 0 mon is an advanced traversodont mammal-like reptile almost identical to Massetognathus (Fig EARLY 4A) otherwise known from Argentina Most of the other reptiles present are unknown from North KEY: A. New Richmond basin fossil assemblage America but resemble southern hemisphere B. Triassic-Jurassic boundary C. Position of youngest diverse "Triassic-aspectm forms while others appear to be completely new footprint assemblage A major discovery is excellent jaw and skull D. Position of trackway shown in Fig. 5 material of what appears to be one of the closest Fig. 3 reptilian forms closest to mammals (Fig 4B) For _J Fig. 3. Use of lake level cycles has allowed a fine-scale time the first time it is clear that the differences between I I I I I calibration of the stratigraphic sections in the Newarkand early Late Triassic assemblages of Laurasia and 40 300 200 100 0 Richmond basins. Ages are the paleontologically defined time divi- Gondwana were due to differences in age rather tritllons 01 years sions of the Early Mesozoic. Time in millions of years is based on lake than geographic isolation level cycles (400,000 year cycle) shown as horizontal bars in the Newark basin section. 4 Olsen, P. E., 1989, South American reptiles found in Virginia: second Triassic extinction constrained in North America, Lamont Newsletter, 19, 4-5. WINTER 1 1989 Fig. 6: A newly completed excavation for Norian age reptile footprints from the Newark basin supervised by ShayMaria Silvestri (Rutgers, Newark) produced a very well preserved foot- print assemblage showing that there were virtually no extinctions of ecologically dominant reptile groups through the preceding 17 million years. This figure be- low shows how the footprints from this locality have been tectonically deformed and how the deformation is graphically removed prior to taxonomic analysis. PRESENT tectonically circle Fig 4 Mammal-like reptiles from the Rich- Fig 5 Extraordinarily preserved trackway of Rhyncho- mond basin. Reconstruction of Massetognathus sauroides hyperbates from Late Carnian age strata of the (A) a herbivore with a crushing and chopping Newark basin A Complete sinuous trackway with tree dentition (8) is a preliminary sketch of the newly stJmp (T) B Enlargement of single left hand impression from discovered form which appears to be very close to trackway C, Reconstruction of lizard like trackmaker Slab the ancestry of mammals assembly and drawing by SM Silvestn Faunal extinction constrained diversity Triassic pollen assemblage with the survi- and the associated extinctions by about 60,000 5 Thus far the Lamont group has been able to show vors making up a Jurassic assemblage of very low 20,000 years, which is close in time but hard tc that the major Triassic-Jurassic faunal shift took diversity overwhelmingly dominated by Corollina. understand as a causative agent (Fig. 3). place in less than 800,000 years. Strata about The extinctions include a large number of species shocked quartz horizon found 600,000 years older than the boundary have pro- of pollen resembling the pollen of flowering plants, To olsen and his colleagues, the most plausible duced bones of the ecologically dominant Triassic and, based on lake level cycles, the transition took cause of the Triassic-Jurassic extinctions remain; reptiles, the phytosaurs and procolophonids. place over an interval of less than 40,000 Years. the great bolide impact which produced the Mani, From beds about 100,000 to 200,000 years youn- Within the Newark, the palynoflora never recov- couagan structure of Quebec (Fig. 2). This hypoth, ger than the boundary have come the rich osse- ered its previous levels of diversity. The correlation esis is by the discovery by D,D, Badju ous assemblages from the McCoy Brook Forma- using pollen and spores strongly corroborates the kov, M.A. Nazarov, and others (Moscow State tion of Nova Scotia discovered two years ago hypothesis that the continental vertebrate exhlc- University) of a shocked quartz horizon in the (Lamont Newsletter 12, 1986) by Olsen and Neil ti0ns Were synchronous with the massive Triassic- marine Trlassic.Jurassic boundary in Austria (Fig Shubin (U.C. Berkeley); these are characteristi- Jurassic marine extinctions. The hont group 3) However, the best available dates from Mani cally Early Jurassic in aspect and completely lack hypothesize an extremely rapid, perhaps, cats- couagan range from 206 +6 to 215 ± millior the dominant "Late Triassic forms." In addition, strophic, taxonomic turnover at the Triassic- years, compared to 201 ± million years for the reptile footprint assemblages of "typical" Late Jurassic boundary.
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