To try and unravel this problem, researchers with an interest in past climates have studied preserved woody charcoal, pollen remains, and the geochemistry of river sediments in New Mexico that have also preserved a variety of early dinosaur fossils. The results demonstrate that although the world was still generally warm and dry during the Late Triassic, it was increasingly affected by frequent, rapid, and extreme shifts in atmospheric CO2 that led to sudden temperature spikes, increasing fluctuations in rainfall, and, most significantly, persistent wildfires that scorched wide areas of ground. The resulting climatic unpredictability in the tropics that we touched upon in Chapter 2 may have placed particularly strong stressors on animal communities living at the equator. As a result, these regions remained under the rule of other giant reptiles (pseudosuchian archosaur communities that bear greater resemblance to crocodiles and alligators), with some theropod dinosaurs eventually arriving and going about their business capturing prey, fishing, or foraging. Meanwhile, the increasingly large-bodied, fast-growing sauropodomorph herbivores, requiring much more reliable food sources, could not break into lower latitudes until conditions stabilized during the Jurassic. The state of tropical environments thus shaped even the ferocious dinosaurs from their earliest appearance and evolution. These habitats at once locked out some of the newer, more experimental forms while also providing a productive home to some of the earliest types of carnivorous dinosaur that picked their way through the bleak world of the Triassic.6
IN 2009 ISABEL Valdivia Berry and her husband, Erico Otilio Berry, were exploring the locality of Los Molles, Neuquén Province, Argentina. Many tourists come to Los Molles for its spectacular mountain views, hiking, and hot springs. It is also exploited by multinational companies as one of the largest shale and gas reserves in South America. However, Isabel was more interested in the other treasures that the surrounding rocks are known to offer, at least to those who know where to look. The Los Molles Formation dates to the Early to Middle Jurassic (201.4 million to 161.5 million years ago), and its rocky outcrops provide vivid snapshots of the marine and river delta ecosystems that were forming on the coasts of the Gondwanan landmass, a considerable portion of which, including much of South America, was primarily located within the tropics during this period. Ichthyosaurs, which resemble a cross between a dolphin and a crocodile and whose name aptly means “fish lizard,” ruled the Early Jurassic seas and are found in abundance at Los Molles. However, the Berrys, on their winter expedition to the region, happened upon a very different type of fossil. Poking out of the rock face was a near-complete skull and significant portion of the body of a dinosaur, which, as they would discover, had never been seen or described before.
After Isabel brought her startling find to Museo Provincial de Ciencias Naturales “Professor Dr. Juan A. Olsacher” of Zapala, a team of paleontologists, headed by Dr. Leonardo Salgado of the National University of the Río Negro–Conicet region, set off to perform detailed excavation at the find site, recovering further fragmentary material. Following years of painstaking research, Leonardo and his team eventually published the find as Isaberrysaura mollensis in 2017. More recently, this dinosaur has even been recognized as one of the earliest stegosaurs, large, heavily built herbivores clad with rows of plates and bearing spike-tipped tails. Beyond its evolutionary position, however, Isaberrysaura is significant for two main reasons. First, it represents overdue recognition of the remarkable women, from the famous nineteenth-century paleontologist Mary Anning onward, who have made important contributions to paleontology, with “Isaberry” dedicating the find to Isabel herself. Second, when the paleontologists analyzed the specimen, they realized that within the skeleton, where the stomach should have been, there was, astonishingly, a mass of mineralized food. As Leonardo puts it, “We always ask ourselves what dinosaurs ate. Usually we have to rely on assumptions based on the shape of teeth. However, here was our chance to go one better.”7
The team’s findings provide a new, more intimate picture of increasingly large dinosaurs and their increasingly green surroundings in the tropical latitudes of the Jurassic. The gut of Isaberrysaura contained two types of seed. One set could not be identified, but the larger seeds were clearly members of the cycad group. The seeds were almost entirely intact, suggesting that they were gobbled down, in a manner similar to that used by modern reptiles to eat their food, instead of being chewed. Furthermore, as cycads are toxic to most animals, if Isaberrysaura survived this culinary experience, then the microbes inside its stomach must have been well adapted to digesting these plants. This one-in-a-million find—while providing a vanishingly brief single-meal snapshot of the dietary behaviors of dinosaurs—gives us a new appreciation of the close relationship that diversifying dinosaurs had with novel emerging plant life in the Late Triassic and Early Jurassic. Not only were herbivores consuming gymnosperms, the new green arrivals on Earth, but, Leonardo argues, they may also have played a role in their increasing success. As mentioned, he found that the seeds left in the stomach were almost complete. This means that when they passed out of the Isaberrysaura, they could go on to be fertilized and grow. In essence, these early stegosaurs were potentially some of the first wide-ranging seed dispersers, perhaps even contributing to the health of these rapidly expanding gymnosperm plant communities.
At the end of the Triassic 201.4 million years ago, there was yet another mass extinction event on land. Global drying, an increasing instability and seasonality of climate, and the deadly accompaniment of volcanic eruptions killed many large amphibians and all reptiles with the exception of the dinosaurs and the ancestors of those that are alive today, including crocodiles, turtles, and lizards. It also caused a major turnover in seed-producing plants. With the rise of the Jurassic, however, the Earth’s climate stabilized. The arid conditions of the Triassic gave way to wetter and warmer (approximately 3.5°C warmer than today) conditions that encouraged the expansion of lush vegetation, including tropical forests. While conifers continued to dominate, as they had done from the Middle-Late Triassic, these new conditions enabled the diversification of gymnosperm plants. Study of the fossil pollen found from the same geological formation in which Isaberrysaura was found, under a microscope, shows that it belonged to a mix of conifers (such as the family Araucariaceae that now extended across the entire planet), cycads, and non-seed-producing ferns. Together these plants would have provided a combination of rich open and forest environments for increasingly diverse dinosaur species, not just in the tropics but also beyond. Ginkgo trees began to dominate at warmer temperate latitudes, with pine-based boreal forests extending into the cooler environments. Although atmospheric CO2 levels stabilized in the Jurassic, as was seen for the end of the Triassic, generally high CO2 could still lead to frequent, regionally variable high-temperature events and rainfall variability. Nevertheless, overall increasing stability and a lush gymnosperm vegetation relied upon by ever-growing herbivorous dinosaurs provided some semblance of steadiness, and dinosaurs rapidly filled every single ecosystem and niche in an increasingly mixed landscape that was full of vibrant green life. The Jurassic was thus a time when not just dinosaurs but also their gymnosperm food sources came to rule the Earth.8
The relationship between dinosaurs and gymnosperms may even have been so intimate as to represent an example of “coevolution.” The sauropods—dinosaurs with massive necks, huge tails, and an overall tendency toward gigantism—flourished by the end of the Jurassic. Studies of the shape and size of their teeth, as well as reconstructions of their jaw movements and skull dimensions, show that they were herbivores. The vast size of these animals and their rapid growth rate, when compared to known modern large herbivores, shows that they must have been bulk feeders, with a paraphernalia of gut bacteria that helped them to digest their leafy meals through fermentation. Like large stegosaurs (later relatives of Isaberrysaura), they did not chew their food; instead, they swallowed it whole. Study of fossil plant remains found in the same contexts as sauropods has provided so
me insights into the kind of plants that were likely on the menu of these giant vegans. When all of this information is combined, it seems clear that these proliferating long-necked dinosaurs fed in bulk on gymnosperms, including conifers such as Araucaria and Ginkgo trees, and ferns in a variety of different settings, ranging from humid forest edges to more open areas. Subtle variations in the shape of their teeth suggest that they diversified so that while some may have focused on treetops, others swept more mobile necks around ground level to feed. This was not just a one-way street, however, and like the large herbivores of the modern tropics, the sauropods likely “gave back” to their ecological settings. Their wide-ranging, crashing movements would have benefited gymnosperms as their seeds were dispersed without damage, and plants such as cycads may have attracted color-seeing dinosaurs with their brightly colored seeds. It has even been suggested that the cones on conifer trees, particularly the so-called monkey puzzle Araucariaceae, evolved their shape as a way to survive and benefit from dinosaur herbivory! Although the strength of this relationship has been hotly debated and an exact link between gymnosperm success and dinosaur abundance seems unlikely, the varied, enormous herbivorous dinosaurs of the Jurassic, as well as their predators, undeniably benefited from an expansion and diversification of lush gymnosperm vegetation. It is also intriguing that, in North America at least, sauropod diversity eventually decreased, while stegosaurs all but disappeared, at a time of gymnosperm decline in the Early-middle Cretaceous (143.1 million to c. 100 million years ago). In fact, it is around this time that an even more dramatic link between dinosaurian green “fingers” and plant evolution has been proposed—this time centering particularly on the tropics.9
PROFESSOR PAUL BARRETT has the job many of us would have wanted as children (and, let’s face it, probably still want). His office in the Natural History Museum in London is just down the hall from one of the largest and most famous collections of dinosaurs anywhere in the world. Even better, every day he gets paid to walk down the hall to study these bones, which include the first skeleton of an Iguanodon known to science, used as part of some of the first academic descriptions of dinosaurs as a biological group, and a part of the first Tyrannosaurus rex skeleton ever discovered. Nevertheless, like the other researchers in this chapter, Paul is not just interested in dinosaurs for dinosaurs’ sake. He wants to know how they influenced life on Earth and how they were a part of broad processes that have shaped the environmental and evolutionary history of our planet with a lasting legacy to this day. As many of us have only ever seen a dinosaur as a collection of bones, it can be hard to think about them as living, active beings, particularly when we often imagine them as ultimately unsuccessful and consigned to the static fossil record by an asteroid. Furthermore, in museum collections that include dinosaurs with machete-like teeth, the gentler herbivorous beasts of this earlier time are not often thought of as major players. Yet Paul’s work over the last twenty years has looked into some of the most intriguing hypotheses for how populations of herbivorous dinosaurs impacted changing plant evolution and how, in turn, changing climates, environments, and plant communities affected the dinosaurs.
In the 1970s, the American paleontologist Robert Bakker presented the intriguing idea, in the world-leading scientific journal Nature, that “dinosaurs invented flowers.” Looking at broad changes in the fossil record, he noticed an apparent link between a significant change in dinosaur feeding behavior and the origin and spread of the flowering angiosperm plants, at the expense of gymnosperms, during the Cretaceous (143.1 million to 66.0 million years ago). In the Jurassic, a dominance of long-necked sauropods feeding high in the canopy allowed slow-growing gymnosperms to proliferate from the floor up. By the Early Cretaceous, ornithischian browsers that preferred to remain lower down for their meals, such as the tank-like anklyosaurs, which had wrecking balls for tails, and the Iguanodon, one of the most numerous dinosaurs across Europe, massively increased pressure on plants growing on the ground floor of vegetation communities. The resulting high disturbance was hugely problematic for the lethargic gymnosperms, leaving the fast-growing angiosperms to radiate out from the tropics and dominate the plant world. Whether due to a change in behavior, to new, sophisticated jaw types developed by certain herbivorous dinosaurs to specialize in angiosperm consumption, or simply to the scale of disturbance caused by hulking Cretaceous dinosaur communities dominated by animals larger than one ton, the global spread of angiosperms, and flowers, from the equator to the poles may, remarkably, have been a dinosaurian achievement.10
“While it is a wonderful idea, I will unfortunately have to stop your readers there,” says Paul. He tells me that when we review fossils, dinosaur feeding behavior, and plant evolution in detail, it becomes clear that the dinosaurs did not “invent” the angiosperms. Nor can they be given sole credit for their global expansion. First, when we take into account preservation and geographical biases in the dinosaur fossil record and limited understandings as to how different dinosaur fossils relate to each other, we see that the sauropods did not decline everywhere in the Cretaceous. In fact, they even flourished in areas such as tropical Gondwana, where the giant, sixty-ton, long-necked and long-tailed Patagotitan roamed the forests of Argentina late into this period. Nor did the ornithischians and their “low-browsing” tendency succeed everywhere, remaining largely absent from Gondwana and the Southeast Asian portion of Laurasia. Second, there is actually little difference in the spatial distribution of major herbivore types between the Jurassic and Early Cretaceous. Third, as noted above, not all sauropods browsed in the canopy, and alongside stegosaurs and other herbivores, many long-necked giants would have actually already placed major pressures on vegetation growing under one meter tall in the Jurassic, prior to the advent of angiosperms. Finally, the major expansion of dinosaurs with special jaws, such as the iguanodonts, occurred a considerable time after the origins of flowering plants.11
Nevertheless, all is perhaps not lost in the search for dinosaur “gardening,” and there may be some indications that a more “diffuse” coevolution, as Paul puts it, existed. The disturbance of ground vegetation by increasingly large dinosaur ecosystems, or “dinoturbation,” evidently increased across America, Africa, and Asia from the Early Jurassic, reaching its maximum during the Cretaceous. This undoubtedly provided some assistance to fast-growing angiosperms and potentially drove their diversification by the Late Cretaceous. It also remains possible that one major group of dinosaurs, the hadrosaurs, or “duck-billed” dinosaurs, which originated in the Early Cretaceous and exploded in number in the Late Cretaceous, both helped and depended on the planetary colonization of angiosperms. In particular, novel innovations in hadrosaur teeth may have differentiated them markedly from other reptiles, giving them a tooth complexity rivaling that of modern herbivorous mammals and allowing them to eat a diversity of plant matter. Perhaps the starkest evidence of a direct relationship between dinosaurs and angiosperms exists in the preserved last meal of a Kunbarrasaurus individual, an ankylosaur from the Early Cretaceous of Australia, which included a variety of angiosperm fruits, meaning that we know dinosaurs did eat these plants. Indeed, it has been suggested that the lack of mechanical defenses against herbivores, such as thorns and spines, among the earliest angiosperms shows that these plants actually wanted to be eaten and dispersed by bulk-feeding dinosaurs.12
In fact, our record of dinosaur fossils and angiosperm origins and diversification is simply not yet complete enough to rule out a close relationship between dinosaurs and angiosperms. The origins and expansion of this new type of plant certainly benefited from increased levels of Cretaceous CO2 as well as the expansion of insect pollinators. It would not be surprising if dinosaurs also added their considerable weight to this process, especially given evidence for the role of large animals in seed dispersal and habitat modifications across the tropics and beyond today. There also seems no doubt that dinosaurs benefited from the varied ecosystems that angiosperms were to dominate by the end
of the Cretaceous. Spreading out from tropical forests, angiosperms accompanied dinosaurs all over the planet, into more arid, riverine environments and even as far north as the Arctic Circle in Russia, potentially providing dinosaurs with more varied ecosystems in which to lay their eggs, reproduce, and feed. Larger and larger food chains that supported the giant carnivorous tyrannosaurs by the end of the Cretaceous suggest that the rise of the angiosperms and the loss of dominance of gymnosperms did not hinder the dominant land animals of the time but rather drove them to bigger, wider, and greater success. Popular imagery and even conservation presentations tend to link dinosaurs to “ancient” Gondwanan cycads and conifer trees. Yet, the angiosperms, which diversified rapidly in Cretaceous megathermal forests and went on to dominate the majority of modern environments, clearly grew alongside dinosaurs and also sustained them right up until the end. Then about 66 million years ago an asteroid hit Mexico and brought their green relationship to an end. Or did it?13
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