Extreme Ecosystem Instability Suppressed Tropical Dinosaur Dominance for 30 Million Years

Extreme ecosystem instability suppressed tropical dinosaur dominance for 30 million years

Jessica H. Whitesidea,1, Sofie Lindströmb, Randall B. Irmisc,d, Ian J. Glasspoole,f, Morgan F. Schallerg, Maria Dunlaveyh, Sterling J. Nesbitti, Nathan D. Smithj,2, and Alan H. Turnerk

aOcean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton SO14 3ZH, United Kingdom; bDepartment of Stratigraphy, Geological Survey of Denmark and Greenland, DK-1350 Copenhagen K, Denmark; cNatural History Museum of Utah, Salt Lake City, UT 84108-1214; dDepartment of Geology & Geophysics, University of Utah, Salt Lake City, UT 84112-0102; eDepartment of Geology, Colby College, Waterville, ME 04901-8858; fScience and Education, Field Museum of Natural History, Chicago, IL 60605; gDepartment of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180; hDepartment of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912; iDepartment of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061; jDepartment of Biology, Howard University, Washington, DC 20059; and kDepartment of Anatomical Sciences, Stony Brook University, Stony Brook, NY 11794

Edited by Paul E. Olsen, Columbia University, Palisades, NY, and approved May 15, 2015 (received for review March 25, 2015) A major unresolved aspect of the rise of dinosaurs is why early a result of extreme environmental fluctuations in the tropics dinosaurs and their relatives were rare and species-poor at low enhanced by high atmospheric CO2, which suppressed large- paleolatitudes throughout the Late Triassic Period, a pattern persist- bodied herbivorous dinosaurs until after the end-Triassic ing 30 million years after their origin and 10–15 million years after mass extinction. they became abundant and speciose at higher latitudes. New paly- We present, to our knowledge, the first high-resolution p nological, wildfire, organic carbon isotope, and atmospheric CO2 paleoenvironmental multiproxy record from the same sedi- data from early dinosaur-bearing strata of low paleolatitudes in west- mentary sequences that produce abundant early dinosaur and ern North America show that large, high-frequency, tightly correlated other vertebrate fossils (6, 8, 10). Specifically, we sampled δ13 variations in Corg and palynomorph ecotypes occurred within a fluvial and overbank sediments of the Upper Triassic Chinle p context of elevated and increasing CO2 and pervasive wildfires. Formation of the Chama Basin in north central New Mexico Whereas pseudosuchian archosaur-dominated communities were (13, 14). This nonmarine succession from low-paleolatitude able to persist in these same regions under rapidly fluctuating ex- Pangaea moved from ∼10°N to 14°N during the late Norian and treme climatic conditions until the end-Triassic, large-bodied, fast- Rhaetian (15), suggesting that this area would have experi- growing tachymetabolic dinosaurian herbivores requiring greater encedasemiaridclimatethroughtheentiresequence(11)(Fig. resources were unable to adapt to unstable high CO environmental 2 1).Theformationinthisregioncontainsexceptionallydiverse conditions of the Late Triassic. and abundant vertebrate assemblages, which allows the early evolution of dinosaurs, their contemporaneous flora, and their Early Mesozoic | carbon cycling | atmospheric CO | terrestrial ecosystems | 2 paleoenvironment to be examined through time. Furthermore, wildfires tight age control is provided by a recent U–Pb radioisotopic age EARTH, ATMOSPHERIC,

of 211.9 ± 0.7 Ma from the Hayden Quarry (HQ) in the lower AND PLANETARY SCIENCES ne of the major predictions of models of elevated atmospheric portion of the Petrified Forest Member of the Chama Basin OCO2 is the increased frequency and magnitude of events (7), and magnetostratigraphic data (16) that are consistent with comprising very high temperatures, an enhanced hydrological cycle, a late Norian to Rhaetian age for the sequence. and increased precipitation extremes (1, 2). Because such envi- ronmental extremes act as limitations on organisms, past time in- tervals of elevated CO and associated climate extremes might be Significance

2 EVOLUTION expected to profoundly influence biogeographic patterns, especially on land, which is relatively unbuffered climatically compared with This is, to our knowledge, the first multiproxy study of climate and associated faunal change for an early Mesozoic terrestrial the oceans. One such time of elevated CO2 was the Triassic Period, during which both dinosaurs and mammals first appeared. In par- ecosystem containing an extensive vertebrate fossil record, in- ticular,ithasremainedanopenquestionwhytheglobalecological cluding early dinosaurs. Our detailed and coupled high-resolution dominance of dinosaurs was delayed in the tropics for at least 30 records allow us to sensitively examine the interplay between million years after their first appearance and diversification into the climate change and ecosystem evolution at low paleolatitudes ’ three major clades Sauropodomorpha, Theropoda, and Ornithi- during this critical interval of Earth s history when modern ter- schia (3, 4). Hypotheses proposed to explain this lag have focused restrial ecosystems first evolved against a backdrop of high CO2 in largely on competition (or lack thereof) with nondinosaurian ar- a hothouse world. We demonstrate that these terrestrial ecosys- chosaurs, principally those on the line to crocodylians (pseudo- tems evolved within a generally arid but strongly fluctuating suchians), but none provide a clear explanation for this unusual and paleoclimate that was subject to pervasive wildfires, and that persistent biogeographic pattern. these environmental conditions in the early Mesozoic prevented The rise of dinosaurs to ecological dominance was a dia- large active warm-blooded herbivorous dinosaurs from becoming chronous evolutionary event (5–8). Small carnivorous early established in subtropical low latitudes until later in the Mesozoic. theropod dinosaurs were widespread at low paleolatitudes, Author contributions: J.H.W. and R.B.I. designed research; J.H.W., S.L., R.B.I., I.J.G., M.F.S., whereas evidence for Triassic herbivorous dinosaurs (i.e., and M.D. performed research; J.H.W., S.L., R.B.I., I.J.G., M.F.S., S.J.N., N.D.S., and A.H.T. sauropodomorphs and ornithischians) in the tropics is com- analyzed data; and J.H.W., S.L., R.B.I., I.J.G., S.J.N., N.D.S., and A.H.T. wrote the paper. pletely absent (6, 7, 9, 10) (Fig. 1). In addition, tropical North The authors declare no conflict of interest. American theropod dinosaurs were rare and species-poor (5, This article is a PNAS Direct Submission. 7, 10) compared with higher-latitude assemblages. These 1To whom correspondence should be addressed. Email: [email protected]. patterns have been hypothesized to track largely zonal cli- 2Present address: The Dinosaur Institute, Natural History Museum of Los Angeles County, matic conditions across Pangaea (6, 8, 11, 12) (Fig. 1), but Los Angeles, CA 90007. detailed paleoclimatic data and mechanistic explanations have This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. been lacking. Here, we argue these biogeographic patterns are 1073/pnas.1505252112/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1505252112 PNAS | June 30, 2015 | vol. 112 | no. 26 | 7909–7913 Downloaded by guest on September 23, 2021 middle Norian-Rhaetian siltstone member. These fluctuations suggest rapid changes in local ~225-202 Ma floral assemblages. Carbon Isotopes. The carbon isotopic composition of bulk organic Humid 13 matter (δ Corg) covaries with palynomorph relative abundance, Arid ? with the former displaying multiple positive/negative fluctuations (1–2‰) in the Petrified Forest and siltstone members (Fig. 2). Humid The upper portion of the section is less densely sampled be- cause of a rarity of organic material in these red, oxidized sediments, but still displays continued variability of similar magnitude, particularly at the Coelophysis Quarry (Fig. 2). These ? redbed samples have total organic concentrations (TOC) com- parable to many samples from reduced layers lower in section (SI Appendix, Table S1), and there is no statistically significant re- 13 lationship between TOC and δ Corg values, so it is unlikely these late Carnian-early Norian fluctuations represent variable TOC. Although the Poleo Sand- ~232-223 Ma stone at the base of the stratigraphic section is not densely sampled because of its predominantly coarse grain size, data from organic-rich siltstone lenses yield more-constant isotopic values. Similar variation in carbon values through the section are also exhibited in wood and charcoal isotopic records, indicating a consistent signal that cannot be explained simply by changing source composition or taphonomy. Furthermore, each negative or positive shift of this magnitude occurs in a stratigraphic in- terval of 1 m or less within conformable sequences. 13 Comparison of this δ Corg record with environmentally diagnostic 13 palynomorphs suggests that δ Corg is likely influenced directly or indirectly by environmental, plausibly climatic, changes. Both Alis- porites and the Patinasporites/Enzonalasporites pollen complex are generally considered indicators of relatively arid Triassic environ- Ornithischia Sauropodomorpha Theropoda ments (17). However, within this larger climatic context, Alisporites is Patinasporites Common more abundant in more humid environments and the / Enzonalasporites vesicate pollen complex is more abundant in drier Rare environments, based on lacustrine strata of similar age in eastern North America (18). The relative abundance of both Camerosporites Fig. 1. Late Triassic Pangean map showing latitudinal climate zones (11, 12) (R2 = 0.68) and Alisporites (R2 = 0.63) negatively correlates with and the distribution of major dinosaur clades. See SI Appendix for occurrence 13 δ Corg values, whereas the Patinasporites/Enzonalasporites pollen data. Question marks indicate geochronologic uncertainty for the Thailand 2 13 complex (R = 0.75) correlates positively with δ Corg values, sug- sauropodomorph and Argentine Laguna Colorada heterodontosaurid occur- 13 rences (i.e., they may be Early Jurassic in age). Each dinosaur symbol in most gesting that humidity-dependent plant C discrimination may 13 cases represents a region with multiple fossiliferous localities containing the be responsible for the isotopic shifts, with more C-depleted values illustrated clades. being associated with distinctly more humid climates (e.g., ref. 19). 13 Both the palynomorphs and covarying δ Corg values therefore reflect strong environmental fluctuations, all potentially related to Results high atmospheric pCO2 (Fig. 2), and thus unusually strong envi- Palynomorphs. We recovered abundant woody charcoal, coalified ronmental pressures associated with climatic change. wood, palynomorphs, and other organic material from samples within greenish gray-colored reduced intervals, particularly those Paleowildfire Temperatures. The extensive charcoal record in the preserved within paleochannels (Fig. 2). Palynomorph data show Petrified Forest Member provides additional evidence of paleo- a major change from a seed fern-dominated (Alisporites) assem- environmental variability and aridity. Abundant charred wood < > blage with accessory gymnosperms (Patinasporites/Enzonalasporites, (ranging from 1mmto 8 cm in maximum diameter) occurs in a Camerosporites,andMonosulcites) in the Poleo Sandstone to one minimum of four stratigraphically different paleochannels (Fig. 2), dominated by conifers and seed ferns (Patinasporites/Enzonalas- suggesting a pervasive influence of paleowildfire on the landscape. porites, Alisporites, and/or Froelichsporites) in the lower portion of Reflected light microscope data from the lower Petrified Forest Member in the HQ paleochannels (Fig. 2) indicate significant the Petrified Forest Member (Fig. 2). No palynomorphs were re- differences in the intensity of these fires. Woody charcoal from the covered from the upper portion of the Petrified Forest and lower HQ 2 paleochannel formed at elevated temperatures of at least portion of the siltstone members, but assemblages from the upper 680 °C, whereas in the slightly stratigraphically higher HQ 3 portion of the siltstone member exhibit a lower diversity, with Camerosporites Froelichsporites paleochannel, the sampled charcoal was exclusively semifusinitic the loss of and , and much rarer (only partially charred) and generated by low temperature Protodiploxypinus (Fig. 2). The relative abundance of these paly- (320 °C) combustion (see SI Appendix, SI Text and Table S3). Camerosporites Alisporites Froelichsporites nomorph taxa ( , , ,and These values bracket those previously reported (350–450 °C) from Protodiploxypinus Patinasporites/Enzo- ) varies inversely with that of the nearby Snyder Quarry (20). Widely variable paleowildfire burn nalasporites (Fig. 2). The abundance of taxa also changes rapidly temperatures reflect an arid but fluctuating environment that within short stratigraphic intervals; Alisporites, Froelichsporites,and allowed variable accumulation of combustible organic matter. Patinasporites/Enzonalasporites vary by an order of magnitude Wildfire activity likely stimulated plant community change. The through stratigraphically adjacent samples within the HQ interval. abundance of large macroscopic woody charcoal indicates these Fern spores are locally abundant in isolated samples from the lower were probably surface fires (21), which have greater potential to portion of the Petrified Forest Member and upper portion of the enhance both vegetation mortality and postfire runoff and erosion

7910 | www.pnas.org/cgi/doi/10.1073/pnas.1505252112 Whiteside et al. Downloaded by guest on September 23, 2021 TOP NOT MEASURED meters M. JUR. ENTRADA SANDSTONE 120 13 pCO (ppm) Key Reptile % Relative Abundance δ C 2 Groups 110 -32 -28 -24 -20 -16 0 1000 2000 3000 4000 0 10 40 6010 40 20 Lineages Continue 100 Into Jurassic

90 CoQ 80

"SILTSTONE" MEMBER "SILTSTONE" 70

60 Temps Alisporites Monosulcites

(°C) Patinasporites/ Froelichsporites Camerosporites Enzonalasporites 50 Protodiploxypinus from CHINLE FORMATION Charcoal 40 CaQ 350-

UPPER TRIASSIC (Norian-Rhaetian) SQ 30 450° H3 320° 211.9 ± 20 H2 0.7 Ma 680° H4

PETRIFIED FOREST MEMBER 10 A BC DEFG HI J K SS

POLEO 0 basal basal theropod 13 dinosaurs mudstonesandstone δ Corg archosaurs 13 CO2 Chama conglomerate δ Ccharcoal crocodile-line 13 δ Cwood CO Hartford dinosauromorphs

2 archosauriforms Drepanosauridae

13 Fig. 2. Organic carbon isotope (δ Corg), palynomorph, and vertebrate fossil records from the Upper Triassic Chinle Formation of the Chama Basin, New Mexico. Stratigraphic section modified from ref. 6; see SI Appendix for detailed HQ sections and vertebrate biostratigraphic data. Radioisotopic age from δ13 EARTH, ATMOSPHERIC, ref. 7. Blue Corg data are carbon isotopic composition of bulk organic matter, red are charcoal, and green are vitrinized wood. The pCO2 was calculated AND PLANETARY SCIENCES following the methodology of Schaller et al. (27, 35, 40); error bars represent S(z) = 3,000 ± 1,000 ppm (see SI Appendix, SI Text, for details). Samples from the Hartford Basin (purple squares) were aligned using U–Pb dates from pedogenic carbonates (29) and the Newark age model [further correlation to Newark

pCO2 data (35) will require additional geochronologic constraints for the Ghost Ranch section]. Vertebrate fossil localities: CaQ, Canjilon Quarry; CoQ, Coelophysis Quarry; H2, H3, and H4, HQ paleochannels; SQ, Snyder Quarry. Vertebrate taxa: A, Drepanosauridae (nonarchosauriform archosauromorph); B, Vancleavea campi (nonarchosaur archosauriform); C, Phytosauria; D, Aetosauria; E, Shuvosauridae; F, noncrocodylomorph Loricata; G, Crocodylomorpha; H, Dromomeron romeri (lagerpetid dinosauromorph); I, Silesauridae (nondinosaurian dinosauriform); J, nonneotheropod Theropoda; K, Neotheropoda. EVOLUTION

(21). Based on these conditions, we hypothesize a scenario where the taxonomic composition of plant communities, which would environmental changes led to impaired ecosystem function, result- lead to community-induced changes in carbon isotope value, be- ing in fuel buildup due to increased floral mortality, and hotter fires, cause both Triassic and modern C3 plants display taxonomic dif- as seen at HQ 2. A hot fire would have enhanced vegetation death, ferences in carbon isotope discrimination (23–25). Although low damaging soils and increasing erosion, and causing positive feed- organic contents in some samples resulted in larger relative un- back effects that accelerated floral change. Alternatively, increased certainties in a few instances, the large isotopic excursions exceed aridity may have accomplished the same by reducing fuel moisture these uncertainties, and many of the shifts are recorded by sam- content. This would have also promoted fire ignition and facilitated ples with precise values (see SI Appendix, Table S1 and Fig. S1). extensive burns at high temperatures. Similarly, although we cannot exclude the possibility that the ex- cursions in the bulk organic samples partly reflect differential Discussion concentration of charcoal and coalified wood versus other organic These wildfire data are consistent with the linkage between xe- matter because of relative enrichment during diagenesis (26), rophytic palynomorph abundance and bulk organic carbon iso- when we restrict consideration to just charcoal and coalified wood, tope values (Figs. 2 and 3), which we hypothesize represents a large excursions in isotopic composition are still evident, indicating rapidly and dramatically changing climate signal that is likely an that they are not a taphonomic effect of differential preservation. indirect indicator of fluctuations in aridity. Changes in isotopic Furthermore, the observed excursions are larger than differences values and palynomorph composition can reflect direct effects of caused by variable burning temperatures(cf.ref.26),indicatingthat climate (temperature, precipitation, atmospheric gas inventories), wildfires cannot be the sole source of the variations. more indirect effects through plant community change or plant The observed relationship between δ13C and floral change, physiology, or a combination of these factors. Specifically, organic wildfires, and overall climate variability is set against the backdrop carbon isotopic values can directly reflect changes in aridity by way of unusually high atmospheric CO2 over the interval we sampled of decreased primary productivity, as observed in modern vascular (27). We calculated atmospheric pCO2 from multiple stable carbon plants (22). Alternatively, climate shifts almost certainly changed isotope analyses of soil carbonate and preserved organic matter,

Whiteside et al. PNAS | June 30, 2015 | vol. 112 | no. 26 | 7911 Downloaded by guest on September 23, 2021 Camerosporites Hartford Basin of eastern North America (27) (see ref. 29 for 65 y = - 1.4663x -30.361

Palynomorph Relative Abundance (%) geologic age), providing strong evidence for the global relevance of R² = 0.681 60 these results. The relatively small offsets between these data are 55 likely the result of unconstrained differences in paleoaltitude at 50 these two widely separated sites, assumptions in the S(z) parameter, SI Ap- 45 or slight taxonomic differences in plant fractionation (see pendix SI Text p 40 , ). The observed secular CO2 increase toward the Rhaetian could be a result of increased midocean ridge degassing 35 or, more plausibly, a decrease in the rate of continental weathering, 30 driven by the establishment of weathering-limited environments in 25 Pangaea’sinterior(30);theδ13C record of marine carbonates in- 20 dicates no decrease in the rate of organic carbon burial during this 15 time (31). Regardless of the source of this change, the overall very p 10 high CO2 levels are expected to be associated with a higher fre- quency of seasonal extremes, consistent with the evidence presented 5 here for large-magnitude climate fluctuations. In contrast, the 0 05.32- -23.00 -22.50 -22.00 -21.50 -21.00 -20.50 00.02- overall evidence of increasing aridity from the Petrified Forest δ 13 C Member into the siltstone member is most simply attributed to the org northward drift of central Pangaea from the humid tropics into the more arid subtropics (12). Alisporites Our data are consistent with previous estimates of precipitation 65 y = -7.473x -145.3 ’ Palynomorph Relative Abundance (%) and temperature (32, 33), and suggest Pangaea s low-paleolatitude R² = 0.6332 60 continental interior experienced strong environmental fluctuations 55 in the Late Triassic. These conditions, accentuated by frequent 50 wildfire activity, brought about vegetational change over both long- 45 term and potentially seasonal timescales (34). Despite (or perhaps 40 because of) these major and repeated environmental fluctuations, 35 vertebrate assemblages changed little through the succession. Members of the same major clades are preserved through the 30 section, with only species-level turnover apparent at some intervals 25 (Fig. 2). This suggests that the vertebrate biota were resistant to the 20 fluctuating environment and the changing plant communities as- 15 sociated with it. Even with a well-sampled fossil record, it is clear 10 that dinosaurs and their close relatives remained rare components 5 of the fauna, comprising less than 15% of specimens except for the taphonomically aberrant Coelophysis Quarry (5, 7), and were 0 -05.32 -23.00 -22.50 -22.00 -21.50 -21.00 -20.50 00.02- dominated in diversity, abundance, and body size by pseudosuchian 13 δ Corg archosaurs (7). Furthermore, large-bodied sauropodomorphs are completely absent from not only the Chinle Formation but from all low-latitude localities in Triassic Pangaea (Fig. 1). Patinasporites/Enzonalasporites 65 Although our paleoenvironmental results are, to our knowledge,

Palynomorph Relative Abundance (%) the first quantitative multiproxy record from sites containing a rich 60 early dinosaur fossil record, they support cyclostratigraphic and 55 qualitative sedimentological evidence of hyperseasonality and ex- 50 tremely high CO2 during the Late Triassic throughout the tropics 45 (e.g., refs. 11, 14, and 32–35). Dinosaurs are generally rare in these 40 areas, and there is no evidence for the large herbivorous forms 35 found at higher latitudes (5, 7, 8, 36). In contrast, climate mod- 30 eling, sedimentological evidence (e.g., coals and paleosols), and stable isotope data provide evidence for cooler and more humid 25 conditions in the temperate high latitudes of both Laurasia and 20 Gondwana (37–39), where large herbivorous dinosaurs are com- 15 mon. Thus, all available data suggest that the climate extremes we 10 observe in the western Pangaea record and their ecological con- y = 21.512x + 496.69 5 sequences were a feature of the Late Triassic tropics in general. R² = 0.749 0 Our data demonstrate that a generally stable vertebrate com- -05.32 -23.00 -22.50 -22.00 -21.50 -21.00 -20.50 00.02- munity with a rarity of dinosaurs (especially large-bodied forms) 13 δ Corg coexisted with dramatically fluctuating plant communities, the latter reflecting highly variable environmental conditions enabled Fig. 3. Linear regression of δ13C values versus the relative abundance of org by high atmospheric pCO2. We propose that this unstable envi- select palynomorph taxa. ronment, characterized by recurring arid/humid extremes, pre- vented the establishment of the types of dinosaur-dominated faunas that are observed in coeval but much higher-latitude re- using a standard diffusion model (28) (see SI Appendix, SI Text and cords from South America, Europe, and southern Africa (5), Table S2), as ∼1,200 ± 400 ppm at the base of the section, in- where aridity and temperatures were less extreme. These faunal ∼ ± p creasing to 2,400 800 ppm near the top. The minimum CO2 contrasts support the idea that early dinosaurs were latitudinally levels at ∼212 Ma are recorded in both the Chinle Formation data sorted (6, 8). Our results suggest that fluctuating aridity in tropical presented here and studies of contemporaneous exposures in the and subtropical Pangea may have been an important driver in this

7912 | www.pnas.org/cgi/doi/10.1073/pnas.1505252112 Whiteside et al. Downloaded by guest on September 23, 2021 sorting, resulting in resource-limited conditions that could not Appendix, SI Text). These sections were correlated to each other using both the support a diverse community of fast-growing tachymetabolic large top of the Poleo Sandstone and base of the Entrada Sandstone as a datum. dinosaurs, which required a particularly verdant and stable envi- Stratigraphic placement of vertebrate fossil localities followed the same method. ronment to thrive. This could explain why Triassic dinosaur faunas Isotopic samples were prepared using standard methods (SI Appendix, SI Text). at low latitudes are restricted to small, slower-growing carnivorous Charcoal samples were embedded in epoxy resin, polished, and analyzed under forms, whereas large-bodied herbivores, including sauropodomorph a reflected light microscope with an oil immersion objective; paleowildfire dinosaurs, are absent at low paleolatitudes during the Late Triassic temperatures were calculated based on reflectance values from published “hothouse.” The unpredictable availability and composition of plant experimental data (see SI Appendix, SI Text). The carbon isotopic composition food and water resources in this environment would have been a of bulk organic matter of sediment and wood was analyzed by mass spec- challenge to the high metabolic requirement of large-bodied her- trometry (see SI Appendix, SI Text). Organic and inorganic carbon isotope bivorous dinosaurs, whereas carnivorous small theropods and other measurements from paleosols were used to estimate the concentration of small-bodied dinosauromorphs were not so dependent. Likewise, atmospheric CO2, according to the soil diffusion model of Cerling (see SI Appendix, SI Text). diverse and abundant smaller-bodied herbivorous, omnivorous, and carnivorous pseudosuchians, with their lower resource requirements ACKNOWLEDGMENTS. We thank A. Downs, A. Kasprak, D. Musher, and (41, 42), could withstand the dramatic climatic fluctuations. R. Price-Waldman for assistance with fieldwork and C. Johnson for 13 Thus, we provide, to our knowledge, the first mechanistic ex- assistance with δ Corg analysis. Fieldwork and research were funded planation for the diachronous rise of dinosaurs during the Late by the US National Science Foundation (EAR 0801138 to J.H.W. and EAR 1349650, 1349554, 1349667, and 1349654 to R.B.I., J.H.W., N.D.S., Triassic Period. Furthermore, these data demonstrate the long- S.J.N., and A.H.T.), Richard Salomon Foundation (J.H.W.), National Geo- term ecological consequences of high CO2 and climate extremes graphic Society Research & Exploration Grant 8014-06 (to K. Padian, R.B.I., over geological timescales and provide context for predictions of S.J.N., N.D.S., and A.H.T.), University of California Museum of Paleontology anthropogenic climate change. (R.B.I.), University of Utah (R.B.I.), the Grainger Foundation (I.J.G.), the Dyson Foundation (M.F.S.), Field Museum of Natural History Women’s Methods Board (N.D.S.), and Geocenter Denmark (S.L.). S.L. publishes with the per- mission of the director of the Geological Survey of Denmark and Greenland. All samples (isotopic, palynological, and charcoal) were taken from fresh, un- Fieldwork was conducted with the permission and support of Ghost Ranch weathered rock and placed in precise measured stratigraphic sections (see SI Conference Center.

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