Paleontology has undergone a renaissance in the past 50 years, expanding from an empirical field focused on stratigraphic context to the theoretically grounded discipline of paleobiology. This transformation has been propelled by conceptual advances in two broadly construed areas, evolution and paleoecology. Phylogenetic systematics has revised our understanding of the evolutionary relationships among organisms. New understanding of tempo and mode in evolution, evolutionary hierarchies, the role of mass extinctions and recoveries, and developmental evolution has led to unexpected insights on evolutionary processes. Within paleoecology, taphonomy has led to greater understanding of the nature of the fossil record. Evolutionary paleoecologists have unearthed temporal and spatial patterns, at various scales, in diversity and community organization and have investigated the processes responsible for them. Other advances in paleoecology involve trace fossils; paleobiogeography; novel uses of fossils in understanding the environment; and the new discipline of conservation paleobiology. New concepts have been furthered by incorporating tools from other disciplines, including quantitative analytical methods, biostratigraphic innovations, geochemical and molecular tools, and advanced microscopy techniques. Fueling these advances are fossil discoveries revealing previously unknown Archean-Proterozoic worlds, detailed accounts of the explosion of life in the Cambrian, and floras and faunas yielding surprising and unexpected insights into the origins and evolution of important plant and animal groups.

Huizachal Canyon, one of a series of generally east-west trending canyons in Tamaulipas, northeastern Mexico, exposes a pre–Late Jurassic sequence of fossiliferous pyroclastic and epiclastic rocks, the Huizachal Group. Heretofore considered a sedimentary package associated with either metamorphic or intrusive rocks, structural relationships and petrographic studies presented here indicate that the Huizachal Group was in fact deposited unconformably upon an older, undescribed, sequence of pyroclastic rocks. Four igneous units are recognized in the steeply dipping older volcaniclastic sequence (respectively, units A–D): a complex suite of pyroclastic flows, accretionary lapilli tuff(s), and lava flows (unit A); a homogeneous, finegrained felsitic rock (unit B); a sequence of conglomerates (unit C); and a mixed assemblage of rocks including mafic-to-intermediate composition lava flows and intercalated conglomerate and tuff (unit D). Most of these rocks have undergone extensive late-stage or postdepositional silicification, but relatively immobile trace elements demonstrate that these rocks range from subalkaline basalt to rhyolite. The younger, relatively flat-lying Huizachal Group overlies these rocks in angular unconformity. The fossil assemblage comes from a <10-m-thick sequence in the lower part of the Huizachal Group, which also is the result of pyroclastic volcanic deposition. Some of the organisms entrained within these tuff(s) were reworked by volcanic processes; others appear to have been actively trapped in a manner analogous to Pompeii. New U-Pb isotopic data from zircon in a volcaniclastic rock from the lowest part of the Huizachal Group (La Boca Formation) yields an age of 189 ± 0.2 Ma (analytical error). The sedimentary rocks immediately above this unit contain fossils considered to be Early Jurassic in age. Thus, the zircon isotopic age agrees with, and is supportive of, the age estimates based upon fossil vertebrates. Trace element geochemistry of the volcanic units is strongly suggestive of subalkalic ocean-continent Andean volcanism. Thus, the volcanic and sedimentary rocks of Huizachal Canyon were most likely deposited in a convergent plate margin setting instead of an extensional rift system as previously proposed. El Cañón del Huizachal, es uno de una serie de cañones con orientación este-oeste en Tamaulipas, en el Noreste de México; ahí aflora el Grupo Huizachal, una secuencia de rocas fosilíferas piroclásticas y epiclásticas pre-jurásicas tardías. Hasta ahora considerado como un paquete asociado con rocas ígneas intrusivas o metamórficas, las relaciones estructurales y los estudios petrográficos presentados aquí sugieren que el Grupo Huizachal estuvo, de hecho, depositado de manera discordante por encima de una secuencia de rocas volcánicas piroclásticas más vieja, no descrita. Se reconocen cuatro unidades ígneas en la secuencia vulcanoclástica más antigua (respectivamente unidades A–D): un conjunto complejo de flujos piroclásticos, tobas de lapili acrecional, y flujos de lava (unidad A); una roca homogénea félsica de grano fino (unidad B); una secuencia de conglomerados (unidad C); y un conjunto mixto de rocas que incluye flujos de lava de composición máfica a intermedia, así como una toba y un conglom-erado intercalados (unidad D). Casi todas estas rocas han sufrido una silicificación extensiva en su fase tardía, o post-deposicional, pero elementos traza relativamente inmóviles demuestran que estas rocas comprenden desde basalto subalcalino a riolita. El conjunto de fósiles proviene de una secuencia de <10 m de espesor de la parte inferior del Grupo Huizachal. Esta secuencia es también el resultado de un depósito volcánico piroclástico. Algunos de los organismos arrastrados dentro de estas tobas fueron retraba-jados por procesos volcánicos; otros parecen haber sido atrapados enérgicamente de una manera análoga a la de Pompeya. Nuevos datos de U-Pb isotópicos de zircón en una roca vulcanoclástica de la parte más inferior del grupo Huizachal (Formación La Boca) dio una edad de 189 ± 0.2 Ma (error analítico). Las rocas sedimentarias inmediatamente arriba de esta unidad contienen los fósiles considera-dos como de edad jurásica temprana. De este modo, la edad isotópica del zircón concuerda y apoya la edad estimada basada en los vertebrados fósiles. La geoquímica de elementos traza de las unidades volcánicas sugiere fuertemente un vulcanismo andino subalcalino océano-continente. Así, las rocas volcánicas y sedimentarias del Cañón Huizachal fueron depositadas probablemente en un ambiente de margen de placa convergente en vez de un sistema de rift extensional como se había propuesto anteriormente.

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