Triassic Life on Land

The Great Transition

Hans-Dieter Sues, Nicholas Fraser

• Publisher: Columbia University Press
• Year: 2010
• ISBN:
  9780231509411
• Topic: Scienze fisiche
• Subtopic: Geologia e scienze della terra

The Triassic period is generally viewed as the beginning of the Age of Dinosaurs. For paleontologists, however, it also marks the rise of the world's first modern land ecosystems.

Over the past three decades, extensive, worldwide fieldwork has led to the discovery of many new species of Triassic animals and plants, suggesting that faunal and floral changes already began in the Middle Triassic and were more protracted than previously thought. The Late Triassic is a pivotal time in the evolution of life on land, with many of the major groups of present-day vertebrates and insects first appearing in the fossil record. This book provides the first detailed overview of life on land during the Triassic period for advanced students and researchers. Noted vertebrate paleontologists Hans-Dieter Sues and Nicholas C. Fraser also review the biotic changes of this period and their possible causes.

CHAPTER 1

Introduction

The Triassic Period marks one of the great transitions in the history of life (figure 1.1). At the end of the preceding period, the Permian, about 250 million years ago, ecosystems on land and especially in the sea had been devastated during the greatest biotic crisis of the Phanerozoic (Raup and Sepkoski 1982; Benton 2006; Erwin 2006). The Triassic was a time for new beginnings. During some 50 million years, it witnessed not only the recovery of ecosystems from the end-Permian extinctions but also the emergence and diversification of many of the principal groups of extant terrestrial and marine animals. Although the Triassic ended with another mass extinction on land and in the sea about 200 million years ago, at that point the structure and dynamics of ecosystems did not substantially differ from those of the present day.

As the first of the three periods constituting the Mesozoic Era, the Triassic is often referred to as the “Dawn of the Age of Dinosaurs.” While this term is not inappropriate, the Triassic encompassed much more than this designation would imply. The first dinosaurs did appear during the Late Triassic, as did the first mammaliaforms, lepidosaurs, and turtles. Beetles proliferated, the first flies buzzed through the air, and the first water bugs sculled across lakes and ponds, possibly satisfying the appetites of some of the earliest frogs and salamanders. Among land plants, the first representatives of a number of extant groups of conifers and ferns appeared during the Triassic. These changes in terrestrial ecosystems parallel those in the marine biosphere, where calcareous nannoplankton (Erba 2006) and important extant groups of animals, including scleractinian corals (Stanley 2003) and teleost fishes or their closest relatives (Arratia 2001; Hurley et al. 2007), made their first appearances. Thus, a more appropriate designation for the Triassic would be “Dawn of Modern Ecosystems.”

Ward (2006:160) observed that “the Triassic was a huge experiment in animal design.” Numerous unusual land animals flourished during the Triassic and then apparently vanished near or at the end of this period. Among insects, the Titanoptera, best known from the Middle Triassic of Australia and the Middle or Late Triassic of Kyrgyzstan, deserve special mention. Close relatives of grasshoppers and their kin (Orthoptera), they could attain a wingspan of at least 40 centimeters (Sharov 1968; Grimaldi and Engel 2005). The forewings of many titanopterans bear prominent stridulatory structures, similar to those in present-day ensiferan orthopterans, and the sounds produced by these giant insects must have filled the forests of their time.

Figure 1.1. Subdivision of Phanerozoic Eon based on the timescale by Gradstein, Ogg, and Smith (2004), with the date for the Triassic-Jurassic boundary modified based on Schaltegger et al. (2008).

Among reptiles, Tanystropheus, from the Middle and Late Triassic of Europe, has an enormously elongated neck that is longer than its trunk and tail combined (Wild 1973; Nosotti 2007). Longisquama, from the Middle or Late Triassic of Kyrgyzstan, sports a row of greatly elongated, hockey-stick-shaped integumentary structures on its back (Sharov 1970; Voigt et al. 2009). Drepanosaurus, from the Late Triassic of Italy, has a greatly enlarged ungual phalanx on the second digit of each hand and a peculiar, spikelike bone at the end of its apparently prehensile tail (Pinna 1980, 1984).

As its name indicates, the Triassic is divided into three parts. In 1834, the German salt-mining expert Friedrich August von Alberti proposed the name “Trias” (derived from the Greek word treis [three]) for a succession of lithostratigraphic units long recognized in southern Germany, which (from oldest to youngest) are the Buntsandstein (colored sandstone), Muschelkalk (clam limestone), and Keuper (derived from a word for the characteristic marls of this unit; figure 1.2). Of these units, the Buntsandstein and Keuper each comprise predominantly continental siliciclastic strata, whereas the intervening Muschelkalk is made up of marine carbonates and evaporites. Alberti noted that similar deposits were widely distributed across Europe and already suspected their presence in India and North America. The threefold rock succession established by Alberti corresponds roughly to the standard division of the Triassic into Lower, Middle, and Upper Triassic series, or, in units of geological time, Early, Middle, and Late Triassic epochs. Later, Alberti (1864) and other researchers employed fossils of marine invertebrates to correlate parts of the Alpenkalk, an old term referring to various carbonate units exposed along the northern and southern flanks of the Europe an Alps, with the Triassic strata in the Germanic basin. During the late nineteenth century, geologists, mostly working in the European Alps, established what would become the standard marine stage–level division for the Triassic (from oldest to youngest): Scythian, Anisian, Ladinian, Carnian, Norian, and Rhaetian (figure 1.3). The Scythian was subsequently further divided into the Induan and Olenekian stages (Kiparisova and Popov 1956). Although this division has been formally adopted by the Subcommission on Triassic Stratigraphy, some researchers still prefer a four-part subdivision of the Early Triassic (from oldest to youngest: Griesbachian, Dienerian, Smithian, and Spathian). During the second half of the nineteenth century, geologists began to identify and map marine strata of Triassic age in southeastern Europe, Turkey, the Himalayas, the western United States, and British Columbia, Canada. Since then, sedimentary rocks of Triassic age, mostly shallow-water marine carbonates and continental red beds, have been discovered on all continents and on the islands of Greenland, Madagascar, and Svalbard. Gregor (1970) estimated the combined maximum thickness of Triassic-age strata on Earth to be about 9 kilometers and the total volume of Triassic sedimentary rocks to be about 45 million cubic kilometers—substantially less than for either the Jurassic or Cretaceous periods, both of which, as a result, have historically attracted much more paleontological interest than the Triassic (Sheehan 1977).

Figure 1.2. Distribution of strata of the classical Germanic Triassic—Buntsandstein, Muschelkalk, and Keuper—in the southern German state of Baden-Württemberg. (Modified from Schoch 2006b)

STRATIGRAPHIC CORRELATION

Published absolute dates for the Triassic Period and its stages have varied considerably over the years (figure 1.3). More recently, however, refined methods of radiometric dating have led to increasingly better resolution; still, much work remains to be done. Repeated uranium-lead (U-Pb) dating of zircon crystals from an ash bed at the Permian-Triassic boundary at Meishan in Zhejiang Province, China, has generated average ages ranging from 250 million to 251 million years (Ma) (Renne et al. 1995; Bowring et al. 1998). Recent revised processing has yielded a date of 252.6 ± 0.2 Ma for the Permian-Triassic boundary (Mundil et al. 2004). At the top of the period, zircons extracted from a tuff below the Triassic-Jurassic boundary in British Columbia generated a U-Pb date of 199.6 ± 0.3 Ma (Pálfy et al. 2000), but the validity of this result was later questioned (Pálfy and Mundil 2006). Most recently, Schaltegger et al. (2008) obtained new U-Pb dates from chemically abraded zircons from the Pucara Group in northern Peru, which place the Triassic-Jurassic boundary at 201.58 ± 0.17/0.28 Ma. Based on new magnetostratigraphic correlations using an astrochronological polarity timescale based on the Late Triassic formations of the Newark Supergroup in eastern North America, Muttoni et al. (2004) redated the boundary between the Carnian and Norian stages at about 228 Ma. This yields a very long Norian stage, with a duration exceeding 20 Ma (Furin et al. 2006).

Figure 1.3. Comparison of recent timescales for Triassic Period. Left to right: time-scale by Gradstein, Ogg, and Smith (2004), timescale based on Muttoni et al. (2004), with “long Carnian” (LC) and (preferred) “long Norian” (LN) options, and timescale from Brack et al. (2005). (Modified from a diagram by Dickinson and Gehrels [http://gsa.confex.com/gsa/responses/2008CD/283.ppt])

Subdivided into Early, Middle, and Late Triassic, the ages for the stages of the Triassic, based on the latest (2009) Geologic Time Scale of the Geological Society of America, result in three very unequal time intervals, with durations of 6, 10, and 33.4 Ma, respectively.

The first problem encountered when trying to get a global overview of the Triassic is the correlation of the suite of sedimentary rocks in the Germanic basin with possibly coeval strata elsewhere. This proves particularly challenging due to the inherent difficulties of correlating continental and marine strata. For example, it has proven difficult to identify terrestrial equivalents of the Middle Triassic marine Muschelkalk. Consequently, it is still impossible to correlate many Triassic continental strata confidently at a level of resolution below that of the stage (Olsen and Sues 1986).

METHODS FOR STRATIGRAPHIC CORRELATION

Biostratigraphy

Although the use of fossils and fossil assemblages has a venerable tradition in the stratigraphic correlation of sedimentary rocks, this process often proves challenging because of the incompleteness of the fossil record. Not only must potential index fossils be common and relatively widespread, and have a short temporal range (in terms of geological time), but they should also include at least a few forms that provide tie-ins with the standard marine sequences.

Pollen and spores have proven particularly useful for the biostratigraphic correlation of nonmarine strata for much of the Phanerozoic. Produced by plants in vast quantities and easily dispersed over great distances by wind or water, they are very abundant in many strata and frequently even find their way into marginal marine environments. Moreover, there are many characteristic forms of pollen and spores whose plant producers were apparently shortlived (in terms of geological time) as well as widely distributed and thus are well suited as index fossils. Perhaps the greatest downside is that pollen and spores are susceptible to destruction under oxidizing conditions, a feature of many terrestrial depositional environments. Nevertheless, palynological zonation and correlation of Triassic continental strata have been widely employed with great success (e.g., Cornet 1993; Visscher and van der Zwan 1980; Litwin, Ash, and Traverse 1991; Heunisch 1999; Deutsche Stratigraphische Kommission 2005).

Dating back at least to the work of Huxley (1869), researchers have used tetrapod fossils for intercontinental correlation of Triassic continental sequences. In recent years, the principal advocates for a tetrapod-based zonation of Triassic continental strata have been Spencer Lucas and his former students Andrew Heckert and Adrian Hunt. Because of the inherent difficulties in correlating continental and marine formations, Lucas proposed a biochronological scheme for continental deposits in de pen dent from the Standard Global Chronostratigraphic Scale, which is based on marine strata. He and his associates have published many papers advocating the use of various tetrapod groups, especially phytosaurs and aetosaurs, for regional and even global correlation of continental sequences (e.g., Lucas 1993, 1998, 1999; Lucas and Huber 2003). Lucas (1998) proposed and defined eight successive land-vertebrate faunachrons (LVFs) during the Triassic Period. Each LVF was characterized by the first appearance datum (FAD) in the fossil record of a particular tetrapod taxon. For example, the first (oldest) LVF, the Lootsbergian, was defined on the FAD of the dicynodont therapsid Lystrosaurus. Lucas then augmented the definition of each LVF by the occurrence of additional characteristic taxa. Thus, additional characteristic tetrapods for the Lootsbergian are the procolophonid parareptile Procolophon, the cynodont therapsid Thrinaxodon, and the archosauriform reptile Proterosuchus (Lucas 1998).

However, Rayfield et al. (2005, 2009; see also Lucas et al. 2007) and other authors (e.g., Lehman and Chatterjee 2005; Parker 2005, 2007) have shown that several of Lucas’s LVFs are problematical because their purported index fossils have longer stratigraphic ranges than originally assumed or have more restricted geographic distribution, or their taxonomic status is uncertain. New radiometric data also underscore the need for calibration of any biostratigraphic zonation scheme against a chronostratigraphic standard (e.g., Mundil and Irmis 2008).

Magnetostratigraphy

Earth has a magnetic field, which is driven by circulation in the planet’s molten outer core and flows from pole to pole. At the present day, the north magnetic pole is located close to the north rotational pole (normal polarity). For reasons that are still not fully understood, the magnetic field reverses at irregular intervals, with the north magnetic pole moving close to the south rotational pole (reversed polarity). Such reversals have occurred many times during the Phanerozoic. Certain minerals, such as the iron oxide magnetite, are readily magnetized. Magnetite is common in a variety of rocks including basalts. When basalt cools from molten lava it passes through a threshold termed the Curie point, at which magnetite and other magnetic minerals take up and lock in magnetization from Earth’s field at that time. In sedimentary rocks, minute particles of magnetic minerals orient themselves with Earth’s magnetic field at the time of deposition of the sediments. Sophisticated instrumentation now allows researchers to measure this “fossilized” magnetization and put together the complex history of episodic reversals of Earth’s magnetic field in deep time, establishing what is called the Global Magnetic Polarity Time Scale. Using radiometric dating of rocks for precise calibration, a regional magnetostratigraphic succession can then be correlated with this global scale (e.g., Kent, Olsen,...

Table of contents

• Cover 
• Series Page
• Title Page
• Copyright
• Contents 
• Preface
• 1. Introduction
• 2. Early and Early Middle Triassic in Gondwana
• 3. Early and Early Middle Triassic in Laurasia
• 4. Late Middle and Late Triassic of Gondwana
• 5. Late Middle and Late Triassic of Europe
• 6. Late Triassic of Great Britain
• 7. Triassic of the Central Atlantic Margin System
• 8. Late Triassic of the Western United States
• 9. Two Extraordinary Windows into Triassic Life
• 10. Biotic Changes During the Triassic Period
• 11. The End of the Triassic: Out with a Bang or a Whimper?
• Glossary
• References
• Index