Transylvanian Dinosaurs
Page 9
The uplands of the island are often cloudy and humid, although where there is fresh lava, the gray rock is barren and sunbaked. Yet even these highlands have stands of woodland, though not nearly as dense or as common as in the lowlands. Instead, shrubs, weedy plants, and other ground cover dominate the landscape.
NOPCSA, DWARFS, AND ISLANDS
Lecturing at the 27 November 1912 meeting of the Zoological and Botanical Society in Vienna, Nopcsa summarized what was then known about the dinosaurs from the Upper Cretaceous of Transylvania.2 He then described their evolutionary relationships within Dinosauria, the conditions under which their remains were found, and their paleoecology, which he regarded as swamp dwelling. In doing so, he emphasized that, compared with the dinosaurs of the same age elsewhere in the world, all of the dinosaurs of Transylvania were strikingly smaller:
During the time of the Upper Cretaceous [Haţeg] formed an island and the smallness of the Transylvanian Cretaceous dinosaurs can be examined as a consequence of insular life [italics in original], which in other groups, as for example in the fossil elephants of the Mediterranean islands, likewise led to a reduction in body size in comparison with the continental forms.3
Figure 4.1. Othenio Abel (1875–1946)
Audience members at this lecture were impressed with Nopcsa’s inference that the members of the Haţeg fauna were dwarfs, but they were somewhat less than sanguine about whether the dinosaurs were dwarfs because of their island habitation. Pointing out that this was the first time that the Transylvanian dinosaurs had been clearly identified as a dwarfed fauna, Othenio Abel (figure 4.1), the session’s convener, objected to Nopcsa’s insular hypothesis, predicating his critique on three arguments.4 First, Abel asserted that the basis for dwarfing could not be directly attributed to island life per se, as was found in the dwarf elephants of Malta and the dwarf hippos of the Mediterranean islands. Instead, he argued that their diminution in size was due to the inevitable inbreeding that comes with insular isolation, as well as through other geographical interventions, such as isolation caused by mountain uplift and river dynamics. As a second objection, Abel noted that insular habitation does not always lead to dwarfing, pointing to examples in which certain reptiles and birds have tended toward gigantism through isolation on islands (e.g., crocodiles in Madagascar, Komodo dragons in Indonesia, turtles from the Galápagos Islands, moas in New Zealand). Third, Abel worried about Nopcsa’s use of the word “island” in reconstructing the Haţeg paleoenvironment, questioning whether the region that supported the fauna was actually small enough to be considered an island, or instead was sufficiently large to be considered a mainland.
Abel’s comments stayed with Nopcsa as he assembled his case for dwarfed Transylvanian dinosaurs. In 1915, he provided further documentation of body-size differences between the animals he had recovered from the Haţeg Basin and those elsewhere, in order to answer Abel’s criticisms.5 While suggesting that dwarfing either was due to the Transylvanian dinosaurs retaining a primitive stage of evolution or was a symptom of evolutionary degeneration, Nopcsa still appears to have been uncertain about the importance of either. As a good neo-Lamarckian, he explicitly advocated degeneration produced by insular isolation, opining that growth in the bones from Transylvania may have been pathologically limited through illnesses (caused perhaps by malnutrition or starvation).6 He called these changes in body size—from dwarfing in the Transylvanian dinosaurs to gigantism among dinosaurs in general—arrostic (literally, “arresting”) alterations.7 For Nopcsa, arrostic changes were nothing more than the heritable acquisition of endocrine disease: hyper-function of the pituitary gland in the evolution of gigantism, hypofunction of the thyroid gland in the evolution of dwarfing. We will return to these biological explanations in chapter 5.
Nopcsa used local geology to explore the paleogeography of the region that would have spurred the evolution of increasingly smaller descendants from larger ancestors. To do so, he took to the rocks, returning to the research he conducted previously while a graduate student at the University of Vienna under the supervision of Eduard Suess.8 Nopcsa’s dissertation research had taken him back home to Transylvania, in the heart of the Southern Carpathians, to conduct a comprehensive and detailed study of the geologic history of the hills, mountains, and rolling plains that he knew so well as a child living in Săcel. In the valleys of the Sibişel, Cerna, and Jiu rivers, the slopes of the Retezat, Sebeş, and Poiana-Ruscă mountain ranges, and the villages of Zeicani, Fărcădin, and Pui, he occupied himself with the real grunt work of a field geologist—walking, sample collecting, note taking, and mapping—as well as the marveling, wondering, and questioning that comes from these physical labors. Nopcsa not only looked at the rocks from oldest to youngest—from the granites, schists, and gneisses dating back to the Precambrian through the overlying sediments dating from the Early Jurassic to the last Ice Age—he also interpreted the ancient environment of depositions and their significance with respect to Earth’s major upheavals.
From Nopcsa’s detailed dissertation research, published in 1905 as a nearly 200-page monograph, we learn that the Haţeg region 150 million years ago was generally a warm, marine-dominated environment.9 The widespread limestones and chalk marls that dominate the Lower Jurassic and Lower Cretaceous were precipitated in the warm Neotethyan Ocean. Nopcsa thought that the central part of the Retezat Mountains, whose uplift he regarded as beginning no later than the Early Jurassic, may already have risen above the sea. Had the Haţeg region been an island in the Late Jurassic and Early Cretaceous? Nopcsa, thus far, remained uncertain. Sedimentary evidence of beaches that would have ringed the island—key to his insular hypothesis—is absent on the flanks of these mountains (though Nopcsa attributed this lack of evidence to erosion occurring since the Jurassic). However, following its emergence during the mid-Cretaceous, the Haţeg region was again covered by a warm sea, its waters abounding with predatory ammonites and the strange, reef-building bivalves called rudists, as well as oysters, clams, snails, and single-celled foraminifera. Some evidence of this remains in the region. Sea level fluctuated greatly through this interval, peaking some 80 million years ago, during the Campanian.
Terrestrial conditions at the very end of the Cretaceous are well preserved in the Transylvanian region. Outcrops of these Maastrichtian rocks are known in the Haţeg Basin itself, as well as in the southern parts of the Poiana-Ruscă Mountains and in the Apuseni Mountains (Plates VII and VIII). Nopcsa interpreted these rocks (a thick sequence of conglomerates, sandstones, and mudstones) as a system of nearshore, fluvial, and lacustrine environments, finding support for his interpretation of terrestrial conditions in the unionid clams, turtles, crocodilians, and dinosaurs often found in abundance at various Transylvanian locations.10 This emergent habitat endured until the early Eocene, approximately 55 million years ago. Sea level rose again in the middle Eocene, as the shales interbedded with coarse limestones overlying the terrestrial deposits testify, only to recede again in the Oligocene.
Nopcsa’s last word on the paleogeography of the Haţeg Basin came in 1923.11 Using the paleogeographic maps available to him at that time, he described the area as a “mere archipelago” of five major islands, distinctly separated from each other and the mainland since the beginning of the Late Cretaceous. Extensive overthrusting, which began some 80 million years ago, produced the present-day Carpathian ranges and the main outlines of current European topography. As Earth’s crust buckled, the seas withdrew, uncovering what were to become the Southern Carpathian Mountains. Here, the Transylvanian dinosaurs and other members of the fauna were entombed in the region’s riverbeds, freshwater lakes, and formidable bogs.
PLATE TECTONICS AND NEOTETHYAN PALEOGEOGRAPHY
Nopcsa’s schooling at the hands of Eduard Suess introduced him to the possibility that the global positions of the continents may not have been fixed over geologic time. In his nineteenth-century attempts to explain the formation of folded mountains ranges like the Alps, Suess suggested tha
t such major geologic upheavals were due to the gradual contraction of the planet.12 With this shrinking, due to Earth’s cooling, its outer crust was forced to wrinkle, fold, and subside. In so doing, large regions of the crust created depressions into which the seas drained, thereby exposing regions of dry land. As Suess’s student, Nopcsa came to see Earth as a changeable, mobile matrix of continents and oceanic basins formed through shrinking, rather than as a permanently fixed geography.
Nevertheless, Suess’s contractionist theory didn’t survive long into the twentieth century before it collapsed—there were just too many improbable estimates of cooling and contraction rates, a recognition that continental and oceanic crusts were different in their composition and density, and the discovery that radioactivity in the crust created a stable heat balance across geologic time. Yet Suess had opened the door widely for the possibility of large-scale crustal mobility.
Although he was not the first to propose major lateral displacements of the continents, one person has been rightly credited as the principal developer of the ideas of continental drift, Alfred L. Wegner.13 In 1912, Wegener proposed that, in the distant past, all the continents had been united, had later broken apart, and thereafter drifted through the ocean floor to their current locations. Much of his evidence came from the jigsaw fit between the margins of the continents, biogeography, paleoclimatology, and evidence that the present-day continents are actually, though very slowly, moving. Nopcsa himself contributed to Wegener’s theory by incorporating aspects not only of paleontology and biogeography, but also of geochemistry and theoretical geophysics.14 For their time, Nopcsa’s arguments for continental mobility—involving the relationship of different kinds of volcanism that came from the movement of continental plates over the top of zones of subducting oceanic crust—were quite innovative, agreeing with the present-day view of magma origin.15 Nevertheless, many felt that the evidence in support of Wegener’s multi-faceted theory, called continental drift, was ambiguous, and it failed to attract a powerful following—that is, until the late 1950s and the 1960s.
In the 1950s, geophysicists initiated studies of Earth’s past magnetic field, as recorded in geologic deposits, and this work later established considerable variation in the position of Magnetic North through time.16 In particular, the pole position on one continental landmass at any given time in the past was not the same as the pole positions on the others; instead, there were many pole positions. One explanation for this apparent polar wandering was continental drift. By moving the continents across the face of the globe, the pole positions could all become superimposed. Drift received a spark of renewed interest.
In addition, thanks in part to research spawned by the events of World War II, the oceanic basins were investigated as never before. The discovery of oceanic ridges, trenches, volcanoes, and mountains came from mapping the ocean floor using radar, which imparted considerably more topography than had ever been expected before. Abundant shallow-earthquake activity and volcanism were associated with the ridges, whereas deep earthquakes were linked with the trenches. Soon it was discovered that the age of the oceanic crust was not uniform. By mapping, sampling, and radiometrically dating the oceanic crust, it was revealed that the youngest oceanic floor was at the midoceanic ridges, and the oldest was in proximity to the continents. New oceanic crust was formed at the ridges and spread away laterally, indicating that the continents must have been closer to each other in the past. In other words, they had to have moved to their present positions.
As these new data flooded into laboratories during the 1960s, Wegener’s theory of continental drift was to receive a vigorous dusting off. Wegener’s early evidence and all of the more recent discoveries could be causally related in what is now known as plate tectonics. Developed in further detail by geologists, geochemists, geophysicists, and paleontologists, this unified theory links all of the dynamic processes of the globe—how mountains are formed, why volcanoes are distributed in the Ring of Fire around the Pacific margin, how the sea floor is formed, and why certain zones are earthquake prone—with the relative motions of a mosaic of large, rigid, lithospheric plates that constitute Earth’s outermost shell. A variety of motions can occur at the boundaries between these plates. Lithospheric plates moving toward each other produce a zone of convergence, while in moving away from each other they form zones of divergence. Plates can also slide past each other to produce transformational movement, such as that along the San Andreas fault system in California. New plate material is produced at divergent boundaries (e.g., along the crests of midoceanic ridges), while old material is destroyed and recycled by subduction at convergent boundaries, as is the case along the deepsea trenches off the Pacific coast of Japan. Given the available evidence, geologists think that each lithospheric plate floats on Earth’s upper mantle, a 660 km thick, plastic layer of dense, semimolten, magnesium- and iron-rich silicates. Plate motion is driven by large-scale convection currents in this upper mantle. It is this motion, imparted to the continents, which produced the global patterns that originally caught Wegener’s eye.
With the acceptance of plate tectonics in the late 1960s, scientists clamored to reconstruct Earth’s geography in bygone times, and in doing so they produced a flurry of paleogeographic maps. At first, these were done by hand, but with the coincident increase in computer access, computer modeling of plate motion and continental repositioning became the working tool of choice. The global models provided in 1970 by Robert S. Dietz and John C. Holden were based on the best fit of the continental positions of present-day coastlines, the margins of the continental slope, and patterns of fracture zones in the ocean floor.17 Later computer simulations, such as those by Alan G. Smith and his collaborators from England, followed suit, but they added further constraints (figure 4.2).18 First, based on their present coastlines and shelf margins, the continents were reassembled into their positions prior to the opening of the Atlantic, Indian, and other oceans, much like the jigsaw-puzzle approach used by Dietz and Holden. Second, a paleogeographic latitude–longitude grid was superimposed on the reassembled continents, and successive stages of breakup—with reference to the positions of the paleomagnetic poles for each interval of time (from those apparent polar-wandering curves)—were determined.19 It is through such paleogeographic reconstructions that we came to know of the existence of the late Paleozoic supercontinent called Pangea, with its huge eastward-opening Tethyan Ocean that provided Pangea with a northern Laurasian half and a southern Gondwanan half. This kind of research has also been used to graphically document the opening of the North Atlantic in the Late Triassic–Early Jurassic, and then the South Atlantic in the mid-Cretaceous.
Figure 4.2. Global paleogeographic reconstructions for the Triassic, approximately 220 million years ago. (Dietz and Holden 1970 reconstruction [above]; A. Smith et al. 1994 reconstruction [below])
As these computer models of plate tectonics became available, other scientists began investigating more fine-grained changes in global conditions. Most important have been new models of paleoclimatologic change, including variations in oceanic circulation patterns, fluctuations in the surface temperature of the oceans and the atmospheric temperature, changes in patterns of atmospheric pressure and circulation, and alterations in weather patterns.20 Data taken from depositional environments (based on surface outcrops and boreholes), as well as information on fossil plants thought to indicate particular environments, can also be plotted on paleogeographic maps to characterize important aspects of regional paleoenvironments.
With both these new global and more regional paleogeographic models, we have come to appreciate that the history of the Tethyan region was much more complicated than previously thought. Instead of a single oceanic basin, it was formed by two different, spreading ridge systems, and therefore had two successive basins—the Paleotethys and the Neotethys. By the Late Jurassic, the Paleotethyan Ocean, the great incisure into Pangea that had formed in the Devonian, was on its way out as the Neotethyan Ocean incr
eased at its expense throughout the rest of the Mesozoic. The Paleotethys formed in the Devonian, when a stretch of European terranes (which were to become northern Europe) separated northward from the northern margin of what is now known as Africa. This opening ocean, created by the Paleotethys midoceanic spreading ridge, gave the world’s geography a huge incisure that opened to the east and partially separated Pangea into Laurasia and Gondwana for much of the remaining Paleozoic. The Neotethyan Ocean began to close toward the east as the African plate rotated counterclockwise and the northern margin of the Neotethyan Basin began undergoing subduction. This produced a number of island arcs in what is now the region of the Middle East and the Caucasus.21 In addition, subduction of the Neotethyan oceanic crust under the more southerly Arabian portion of the African continental plate formed an island chain across present-day Iran.22
Figure 4.3. A generalized tectonic map of the Eastern European Alpine system. A: Apuseni Mountains; BM: Balkan Mountains; EC: Eastern Carpathian Mountains; M: Moesian Platform; RM: Rhodopian Mountains; SC: Southern Carpathian Mountains; TB: Transylvanian Basin. (After Burchfiel 1980)