Our recent study on the nature of the ascent of the dinosaurs (1) argued three main points: (i) that a major terrestrial tetrapod mass extinction is concentrated at the palynologically identified Triassic-Jurassic boundary in eastern North America (based on footprints and bones); (ii) that truly large predatory dinosaurs appear immediately after the boundary (based on footprints); and (iii) that both the boundary and the mass extinction level are associated with a modest Ir anomaly and fern spike, plausibly of asteroid or comet impact origin. Although the comment of Thulborn (2) addresses a number of paleontological issues in (1), his only point salient to the conclusions of our paper is his extraordinary claim that very large theropod dinosaurs were already present in the Carnian, some 20 million years or so before the Triassic-Jurassic boundary [in contrast to our assertion in point (ii), above]

Fowell, Sarah J.

Hartline, Brian W.

Huber, Heinz

Kent, Dennis V.

Koeberl, Christian

Montanari, Alessandro

Olsen, Paul E.

Rainforth, Emma C.

Sues, Hans-Dieter

Szajna, Michael J.

Analysis of tetrapod footprints and skeletal material from more than 70 localities in eastern North America shows that large theropod dinosaurs appeared less than 10,000 years after the Triassic-Jurassic boundary and less than 30,000 years after the last Triassic taxa, synchronous with a terrestrial mass extinction. This extraordinary turnover is associated with an iridium anomaly (up to 285 parts per trillion, with an average maximum of 141 parts per trillion) and a fern spore spike, suggesting that a bolide impact was the cause. Eastern North American dinosaurian diversity reached a stable maximum less than 100,000 years after the boundary, marking the establishment of dinosaur-dominated communities that prevailed for the next 135 million years

Columbia University Academic Commons

Ascent of Dinosaurs Linked to an Iridium Anomaly at the Triassic-Jurassic Boundary

Response to Comment on“Ascent of Dinosaurs Linked toan Iridium Anomaly at theTriassic-Jurassic Boundary”Our recent study on the nature of the ascent ofthe dinosaurs (1) argued three main points: (i)that a major terrestrial tetrapod mass extinctionis concentrated at the palynologically identifiedTriassic-Jurassic boundary in eastern NorthAmerica (based on footprints and bones); (ii)that truly large predatory dinosaurs appear im-mediately after the boundary (based on foot-prints); and (iii) that both the boundary and themass extinction level are associated with amodest Ir anomaly and fern spike, plausibly ofasteroid or comet impact origin.Although the comment of Thulborn (2)addresses a number of paleontological is-sues in (1), his only point salient to theconclusions of our paper is his extraordi-nary claim that very large theropod dino-saurs were already present in the Carnian,some 20 million years or so before theTriassic-Jurassic boundary [in contrast toour assertion in point (ii), above]. His claimis extraordinary because the evidence citedby Thulborn is an isolated and probablyincomplete plaster cast of a footprint fromstrata dated by floral data as Carnian—yetthere are no other osteological or footprintdata anywhere in the world of large Carniandinosaurs, let alone very large theropodfootprints. Here, we will not address theage of the track-bearing strata, which wetake at face value as Triassic (3). Instead,we concentrate on the claim that the sup-posed track maker was a theropod dinosaur.Superficially, the photograph of a plastermold provided by Thulborn [previously pub-lished in (4)] would seem to represent a verylarge (43 cm) tridactyl footprint that, apartfrom its age, would normally be assumed tobe dinosaurian, probably theropod, in origin.However, assessment of the footprint is ham-pered by the truncation of the rear of thetrack, a lack of pad impressions, and thegeneral blotchy coloration of the artificialmold, as well as the very superficial docu-mentation of the material. The track is cer-tainly not, contrary to the assertion of Thul-born (2), “identical to . . . [the] ichnogenusEubrontes,” the type material of which wasredescribed by one of us (5). Indeed, visiblein the photograph [figure 1 in (2)] are adepression lateral to digit II that could beinterpreted as an impression of a large digitI,and an apparent metatarso-phalangeal im-pression behind the phalangeal pads. If this-interpretation is correct, this minimally tetra-dactyl track would unquestionably not belongto the ichnogenus Eubrontes, and wouldprobably not be interpreted as theropod inorigin. Pentadactyl footprints of a size com-parable to this Australian ichnite have beendescribed from strata rather more securelydated as Carnian in North Carolina, USA (6)(Fig. 1). Were the rear of these tracks to betruncated artificially, as is the track in thefigure produced by Thulborn, the resultwould look superficially Eubrontes-like, al-though not directly assignable to that ichno-genus. Indeed, many other putatively tridac-tyl early dinosaur footprints have upon rigor-ous examination proved to be pentadactyl andnondinosaurian (7). In any case, until theAustralian material is properly described andillustrated, it will be difficult to assess itsrelevance to theropod dinosaur evolution. Weregard the North Carolina material as beingmost similar to Parachirotherium as de-scribed by Haubold (8), albeit without manusimpressions, and suggest a similar assign-ment is possible for the material cited byThulborn (2). These types of tracks have beeninterpreted as dinosauroid (8), if not dinosau-rian, but given our current state of knowl-edge, they could just as well be crurotarsan inorigin. The line drawing of the trackway pro-vided in (9) provides no additional informa-tion pertinent to this discussion, because it isfar too diagrammatic to be reliable.Our study (1) proffered two scenarios thatmight explain the increase in size in theropoddinosaurs across the Triassic-Jurassic boundaryin eastern North America. (i) The appearance ofthe much larger theropods could represent adispersal event from some unknown location.(ii) It could represent an evolutionary eventtriggered by the elimination of competitors. Wefavor the second hypothesis, but if the tracksThuborn cites prove to be both of theropodorigin and Carnian in age, we would be morefavorably inclined to our first scenario. Indeed,we are well aware that the terrestrial tetrapodrecords of Triassic extra-tropical regions areextremely poorly known and surely will havemany surprises, and we look forward to the fulldocumentation and description of track assem-blages from the higher latitudes from bothhemispheres.On the other hand, Thulborn argues that“[t]he Carnian biota of Gondwana was fun-damentally different from the contempo-rary biota of Laurasia,” an assessment thatFig. 1. (A) Medium-sized and (B) very large examples of cf. Parachirotherium sp. from Carnian agestrata of the Peking Formation of the Deep River basin, North Carolina [images from (6); reprintedwith permission of Southeastern Geology]. Middle digits are superﬁcially similar to theropodfootprints. (A) shows impression of left pes; (B) shows natural cast of right pes, with much smallersuperimposed trackway of Apatopus lineatus.TECHNICAL COMMENTwww.sciencemag.org SCIENCE VOL 301 11 JULY 2003 169cmay be correct ( pending better correlationtools) but that is not strongly supported bythe current faunal evidence. What we cansay is that the Carnian of eastern NorthAmerica does not have “a flora dominatedby conifers and an insect fauna depauperatein beetles and cockroaches, but rich inflies,” as stated by Thulborn. These easternNorth American Carnian floras are domi-nated by ferns and cycadeoids (10, 11) aswell as by conifers, and only one localityhas produced flies. Even at the latter La-gersta¨tte, flies are third in abundance aftertrue bugs and beetles (11). All other Tria-ssic insect localities in eastern North Amer-ica are dominated by beetles and roachesand lack flies (12).Contrary to Thulborn’s assertion that “itis not particularly surprising that largetheropod dinosaurs should appear in onefaunal assemblage but not in the other,” weargue that it is surprising, indeed startling,because large, highly mobile animals suchas theropods could easily have walked fromBrisbane to New York during a single life-time in Triassic Pangea. If they did not doso, something must have prevented them.Even if climatic or ecological processesmaintained a strong terrestrial provincialityduring the Late Triassic, as we have main-tained elsewhere (13), perhaps excludinglarge theropods in regions of Pangea, theabrupt breakdown of that provinciality atthe Triassic-Jurassic boundary would stillsignal a global biological event of immenseimportance, rather than a “local” event.P. E. OlsenLamont-Doherty Earth Observatoryof Columbia UniversityPalisades, NY 10964–1000, USAH.-D. SuesCarnegie Museum of Natural History4400 Forbes AvenuePittsburgh, PA 15213–4080, USAE. C. RainforthLamont-Doherty Earth Observatoryof Columbia UniversityD. V. KentLamont-Doherty Earth Observatoryof Columbia Universityand Department of Geological SciencesRutgers UniversityPiscataway, NJ 08854–8066, USAC. KoeberlH. HuberDepartment of Geological Sciences(Geochemistry)University of ViennaAlthanstrasse 14A-1090 Vienna, AustriaA. MontanariOsservatorio Geologio do Coldigiocom1-62020 Frontale di Aprio, ItalyS. J. FowellDepartment of Geology and GeophysicsUniversity of Alaska, FairbanksFairbanks, AK 99775–5780, USAM. J. SzajnaB. W. HartlineReading Public Museum500 Museum RoadReading, PA 19611, USAReferences and Notes1. P. E. Olsen et al., Science 296, 1305 (2002).2. T. Thulborn, Science 301, 169 (2003); www.sciencemag.org/cgi/content/full/301/5630/169b.3. We are not nearly as conﬁdent about its supposedCarnian age, however, because recent paleomagneticwork has shown that much of the supposedly Carnianstrata in the Northern Hemisphere actually correlatesto the marine Alpine Norian (14, 15), and Gond-wanan correlations to the European type areas forTriassic stages are hardly more secure.4. D. Hill, G. Playford, J. T. Woods, Triassic Fossils ofQueensland (Queensland Palaeontogr. Soc., Brisbane,Australia, 1965).5. P. E. Olsen, J. B. Smith, N. G. McDonald, J. Vertebr.Paleontol., 18, 586 (1998).6. P. E. Olsen, P. Huber, Southeast. Geol. 38 (no. 2), 77(1996).7. M. J. King, M. J. Benton, Palaeogeogr. Palaeoclimatol.Palaeoecol. 122, 213 (1996).8. H. Haubold, H. Klein, Hallesches Jb. Geowiss. B 22, 59(2000).9. H. R. E. Staines, J. T. Woods, Aust. J. Sci. 27, 55 (1964).10. R. C. Hope, O. F. Patterson III, North Carolina Div.Min. Res. Spec. Pub. 2 (1969).11. N. C. Fraser, D. A. Grimaldi, P. E. Olsen, B. Axsmith,Nature 380, 615 (1996).12. P. E. Olsen, R. W. Schlische, P. J. W. Gore, Internat.Geol. Congr. Guidebooks Field Trips T351 (1989).13. P. E. Olsen, V, Schneider, H.-D. Sues, K. M. Peyer, J. G.Carter, Geol. Soc. Am. Abst. Prog. 33 (no. 2), A-27(2001).14. L. Krystyn, Y. Gallet, J. Besse, J. Marcoux, Earth Planet.Sci. Lett. 203, 343 (2002).15. J. E. T. Channell, H. W. Kozur, T. Sievers, R. Mock, R.Aubrecht, M. Sykora, Palaeogeogr. Palaeoclimatol.Palaeoecol. 191, 65 (2002).26 February 2003; accepted 17 April 2003T E C H N I C A L C O M M E N T11 JULY 2003 VOL 301 SCIENCE www.sciencemag.org169c

Response to Comment on "Ascent of Dinosaurs Linked to an Iridium Anomaly at the Triassic-Jurassic Boundary"

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Response to Comment on "Ascent of Dinosaurs Linked to an Iridium Anomaly at the Triassic-Jurassic Boundary"

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