Research Paper GEOSPHERE Paleoenvironments, taphonomy, and stable isotopic content of the terrestrial, fossil-vertebrate–bearing sequence of the El Disecado GEOSPHERE, v. 16, no. 4 Member, El Gallo Formation, Upper Cretaceous, Baja California, México https://doi.org/10.1130/GES02207.1 David E. Fastovsky1, Marisol Montellano-Ballesteros2, Henry C. Fricke3, Jahandar Ramezani4, Kaori Tsukui4, Gregory P. Wilson5, Paul Hall6, Rene Hernandez-Rivera2, and Geraldo Alvarez2 18 figures; 3 tables; 1 supplemental file 1Department of Geosciences, University of Rhode Island, 9 East Alumni Avenue, Kingston, Rhode Island 02881, USA 2Universidad Autónoma Nacional de México, Instituto de Geología, Ciudad Universitaria, Coyahuacán, 04510, D.F. México CORRESPONDENCE: [email protected] 3Colorado College, Colorado Springs, Colorado 80903, USA 4Massachusetts Institute of Technology Earth, Atmospheric and Planetary Sciences, 77 Massachusetts Avenue, Bldg. 54-1116, Cambridge, Massachusetts 02139, USA 5 CITATION: Fastovsky, D.E., Montellano-Ballesteros, University of Washington, Department of Biology, Box 351800, Seattle, Washington 98195-1800, USA 6 M., Fricke, H.C., Ramezani, J., Tsukui, K., Wilson, Center for Computation and Visualization, 180 George Street, Brown University, Providence, Rhode Island 02912, USA G.P., Hall, P., Hernandez-Rivera, R., and Alvarez, G., 2020, Paleoenvironments, taphonomy, and stable iso- topic content of the terrestrial, fossil-vertebrate bear- ing sequence of the El Disecado Member, El Gallo ABSTRACT perhaps refecting an absence of permanent stand- macro vertebrados, troncos silicifcados, restos Formation, Upper Cretaceous, Baja California, México: ing water in this depositional setting. macroscópicos de plantas, y polen, la cual fue Geosphere, v. 16, no. 4, p. 991– 1011, https://doi .org The Late Campanian (Late Cretaceous), upper Here we report a high-precision U-Pb date depositada posiblemente en la parte distal de un /10.1130 /GES02207.1. part of the El Disecado Member, El Gallo Forma- of 74.706 + 0.028 Ma (2σ internal uncertainty), abanico subaéreo. La unidad fue depositada de tion, Baja California, México, preserves a rich fossil obtained from zircons in an airfall tuff. The tuff is forma episódica y bajo una alta energía, siendo Science Editor: Andrea Hampel Associate Editor: Cathy Busby assemblage of microvertebrates and macroverte- located low within the sequence studied; therefore, sus rasgos más característicos canales de ríos brates, silicifed logs, macroscopic plant remains, most of the sedimentology and fossils reported trenzados y depósitos de fujos de escombros. Received 22 October 2019 and pollen that was likely deposited as the dis- here are slightly younger. This date, which improves La estabilidad del paisaje está indicada por la pre- Revision received 30 March 2020 tal part of a subaerial fan. The unit was episodic upon previously published 40Ar/39Ar geochronology, sencia de horizontes compuestos de paleosuelos Accepted 19 May 2020 and high energy, with its salient features deriving ultimately allows for comparison of these El Gallo que contienen horizontes B con un moteado de Published online 30 June 2020 from active river channels and sheet, debris-fow faunas and environments with coeval ones globally. Fe2O3, cutans, y concreciones de carbonato de cal- deposits. Landscape stability is indicated by the Primary stable isotopic nodules associated cio. Todos estos rasgos indican una cicli cidad de presence of compound paleosol horizons, con- with roots in the paleosols of the terrestrial por- humedad/aridez en horizontes debajo de la super- taining Fe2O3 mottling in B horizons, cutans, and tion of the El Disecado Member are compared with fcie atribuibles, probablemente, a la ciclicidad en calcium carbonate concretions. All of these fea- ratios from similar sources from coeval northern el clima. El drenaje estaba orientado principal- tures indicate wet/dry cyclicity in subsurface and eastern localities in North America. Distinctive mente hacia el norte y en menor grado al oeste; horizons, likely attributable to such cyclicity in the latitudinal gradients are observed in both δ13C and sin embargo, se conservan algunas corrientes de climate. Drainage was largely to the north and to δ18O, refecting the unique southern and western, fujo dirigidas hacia el sur y este, las cuáles junto a lesser extent, the west; however, some current coastal geographic position of this locality. These con la ubicación proximal de los depósitos mari- fow to the south and east is preserved which, in differences are best explained by differences in nos marginales, sugieren la infuencia de mareas conjunction with the proximal location of marginal the foras that populated the northern and east- en este ambiente. marine deposits, suggest the infuence of tides in ern localities, relative to the southern and western Los vertebrados fósiles preservados en esta this setting. foras reported here. parte del Miembro El Disecado son casi exclu- The fossil vertebrates preserved in this part of sivamente alóctonos, se conservan como clastos the El Disecado Member are almost exclusively aislados desarticulados en equivalente hidraúlico allochthonous, preserved as disarticulated isolated RESUMEN “hydraulic equivalence” en el sistema fluvial clasts in hydraulic equivalence in the braided fu- trenzado. Una asociación de microvertebrados re la- vial system. A relatively diverse micro vertebrate La parte superior del Miembro El Disecado tia v mente diversa se conserva; los componentes assemblage is preserved, the largest components de la formación El Gallo, Baja California, México, más grandes son, en primer lugar, los dinosaurios This paper is published under the terms of the of which are frst, dinosaurs, and second, turtles. Campaniano Tardío (Cretácico Tardío), conserva y en segundo, las tortugas. Fósiles no tetrápodos CC-BY-NC license. Non-tetrapod fossils are relatively uncommon, una asociación fósil rica en microvertebrados y son relativamente raros, refejando probablemente © 2020 The Authors GEOSPHERE | Volume 16 | Number 4 Fastovsky et al. | Vertebrate-bearing paleoenvironments of the El Gallo Formation, Late Cretaceous, Baja California, México Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/16/4/991/5144984/991.pdf 991 by MIT Libraries user on 27 January 2021 Research Paper la ausencia de agua permanente en este ambiente These Late Cretaceous fossil localities from the sedimentary rocks composed of three members de depósito. west coast of North America contain the southern- (in ascending order): La Escarpa, El Disecado, and Aquí reportamos una edad U-Pb de alta pre- most and westernmost fossil vertebrates known from El Castillo, the last of which is encompassed within cisión de 74.706 + 0.028 Ma (2σ de incertidumbre the Campanian of North America, and thus provide the El Disecado Member and restricted to the east- interna), obtenida de zircones de una toba de caída a biogeographically unique datum for comparison ern side of the basin (Figs. 1 and 2). “airfall.” La toba está localizada en la parte inferior with better known, contemporaneous North Amer- Fulford and Busby (1993, p. 302) character- de la secuencia estudiada; por lo tanto, casi toda ican Campanian terrestrial biotas to the north and ized the El Gallo Formation as a “retrogradational la sedimentología y fósiles registrados aquí son east. Here, we situate this fossil assemblage in its sequence of alluvial plain through floodplain ligeramente más jóvenes. Esta fecha perfecciona paleoenvironmental, taphonomic, and stratigraphic deposits capped by tidal deposits” within a forearc la geocronología 40Ar/39Ar publicada previamente, contexts. We present evidence that this portion of the basin. They recognized four “successions,” of which y por último permite la comparación de la fauna y El Disecado Member, here termed the Fossiliferous Successions 2–4 constitute the El Disecado Mem- el ambiente de El Gallo con otros contemporáneos. Terrestrial Sedimentary Sequence (FTS), represents ber of Kilmer (1963). The FTS is correlative with Proporciones primarias de isótopos estables a physically active and biologically productive distal the uppermost part of Succession 2 and most of (δ13C; δ18O) obtenidas de dientes aislados de ha dro- terrestrial fan deposit, built by a sandy low-sinuosity Succession 3. It constitutes ~130 m, running from saurio (dinosaurio) y de nódulos de carbonato fuvial system forming in a cyclical wet/dry climatic local sea level upsection to the stratigraphic level in asociados con raíces en los paleosuelos de la parte setting. Our new high-precision U-Pb zircon age from which the El Disecado Member begins to produce continental del Miembro El Disecado fueron com- this member improves upon its legacy 40Ar/39Ar geo- marine fossils. Previous 40Ar/39Ar geochronology paradas con proporciones de fuentes similares de chronology and helps place its rich fossil fauna in its (Renne et al., 1991) determined a Campanian (Late localidades norteñas y orientales contemporáneas regional biostratigraphic context. Cretaceous) age for the El Gallo Formation and its en América del Norte. Gradientes latitudinales To enrich our understanding of the context of vertebrate fauna (Fig. 2). distintivos fueron observados en δ13C y δ18O, refe- the vertebrates, we analyzed stable isotopic ratios jando la singular posición geográfca costera al sur (18O and 13C) retrieved from both the teeth of had- y occidente de esta localidad. Estas diferencias se rosaurs (dinosaurs)