Glossary

Biogeography is the scientific study of the distribution of species and ecosystems across geographic space and through geological time. It examines the spatial patterns of biological diversity and seeks to explain these patterns through the interplay of abiotic factors—such as plate tectonics, sea-level fluctuations, and climatic regimes—and biotic factors, including physiology, ecology, dispersal capacity, and evolutionary history. The discipline is conventionally divided into two complementary branches: ecological biogeography, which investigates present-day environmental controls on species ranges and community composition, and historical biogeography, which reconstructs how past geological and evolutionary events have shaped the distributions observed today. When applied to the fossil record, the field is often termed paleobiogeography. Two fundamental mechanisms are central to historical biogeography: vicariance, in which a once-continuous population is divided into geographically isolated segments by the formation of a physical barrier (such as an ocean basin or mountain range), and dispersal, in which organisms actively or passively cross pre-existing barriers to colonize new areas. A third process, geodispersal, occurs when the removal of a barrier (e.g., by sea-level regression forming a land bridge) allows previously separated biotas to intermingle. Biogeography has been pivotal for understanding the evolutionary history of dinosaurs and other Mesozoic vertebrates. The fragmentation of the supercontinent Pangaea from the Middle Jurassic onward produced repeated cycles of vicariance and geodispersal that created a complex, reticulate biogeographic history for dinosaurs, explaining why Late Cretaceous faunas show pronounced continental endemism—for instance, the dominance of ceratopsids and hadrosaurids in Laramidia versus titanosaurs and abelisaurids in Gondwana. The discipline thus provides an essential framework for interpreting why certain lineages are found on particular continents and how tectonic, climatic, and ecological factors interact to control organismal distributions across deep time.

Continental drift is the hypothesis, formally introduced by German meteorologist and geophysicist Alfred Lothar Wegener in 1912 and elaborated in his 1915 book *Die Entstehung der Kontinente und Ozeane* (*The Origin of Continents and Oceans*), that Earth's continents were once assembled into a single supercontinent called Pangaea and have since moved laterally across the planet's surface over geological time to attain their present positions. The proposition rests on multiple convergent lines of evidence: the geometric fit of opposing coastlines (especially the South American and African margins), the distribution of identical fossil organisms (*Mesosaurus*, *Lystrosaurus*, *Glossopteris*) across ocean basins too wide to have been crossed by their bearers, matching stratigraphy and mountain belts on now-separated landmasses, and anomalous paleoclimatic signatures such as glacial deposits in present-day tropical Africa and coal from tropical plants in Antarctica. Although Wegener marshaled compelling observational evidence, his hypothesis was widely rejected in his lifetime because he could not identify a credible physical mechanism capable of driving continental masses through oceanic crust. The missing mechanism—seafloor spreading driven by mantle convection—was supplied in the late 1950s and 1960s by Harry Hess and others, transforming continental drift into the broader theory of plate tectonics, which is now the unifying framework of the Earth sciences.

Continental drift is the hypothesis, first comprehensively articulated in 1912, that Earth's continents were once joined in a single supercontinent (Pangaea) and have since moved apart across the globe. Plate tectonics is the broader, now well-established scientific theory—formalized in the late 1960s—stating that Earth's outermost rigid layer (the lithosphere) is fragmented into a dozen or more large and small plates that move relative to one another atop the hotter, more mobile asthenosphere beneath. The plates interact at three types of boundaries: divergent boundaries, where plates move apart and new crust forms at mid-ocean ridges; convergent boundaries, where plates collide, producing subduction zones, deep-sea trenches, and mountain ranges; and transform boundaries, where plates slide laterally past each other. The driving force is primarily mantle convection—heat generated by radioactive decay deep within Earth circulates the semi-fluid asthenosphere, dragging or pushing the overlying plates. For paleontology and the study of dinosaurs in particular, plate tectonics is the key framework for understanding how organisms that evolved on a single landmass came to be found as fossils on widely separated modern continents. During the Triassic Period (approximately 252–201 Ma), Pangaea began to rift apart, first splitting into the northern landmass Laurasia and the southern landmass Gondwana, then further fragmenting through the Jurassic and Cretaceous periods. This progressive continental separation drove vicariance (the division of a once-continuous population into isolated groups), shaped dispersal corridors and barriers, and produced the complex, reticulate biogeographic patterns observed in the dinosaur fossil record worldwide.

**Gondwana** is the ancient large landmass—variously termed a supercontinent or superterrane—that incorporated present-day South America, Africa, Arabia, Madagascar, India, Australia, Antarctica, and the micro-continent of Zealandia. It was fully assembled by the late Neoproterozoic to early Cambrian (approximately 600–500 Ma) through a series of continental collisions collectively known as the Pan-African orogenies, during which multiple Precambrian cratons were welded together along extensive suture belts. In the late Paleozoic, Gondwana joined with the northern landmass Laurasia to form the supercontinent Pangaea, constituting its southern half. Gondwana's breakup commenced in the Early Jurassic (approximately 180 Ma), triggered in part by the eruption of the Karoo-Ferrar Large Igneous Province, and proceeded in stages through the Cretaceous and into the Cenozoic, progressively yielding the modern southern continents and the Indian subcontinent. The existence and subsequent fragmentation of Gondwana are supported by multiple independent lines of evidence, including shared fossil assemblages (notably the *Glossopteris* flora), Permo-Carboniferous glacial deposits (tillites), matching geological structures across now-separated continents, paleomagnetic data, and marine magnetic anomaly records from the southern ocean floors. Gondwana's dispersal fundamentally shaped global ocean circulation, climate patterns, and the biogeographic evolution of southern hemisphere biota.

**Laurasia** is the northern landmass that formed part of the Pangaea supercontinent from approximately 335 million years ago (Early Carboniferous) and separated from the southern landmass Gondwana around 175 million years ago (Middle Jurassic) during Pangaea's breakup. It comprised the continental crust that now constitutes North America, Europe, Scandinavia, Siberia, Kazakhstan, and China. The **Tethys Sea** lay between Laurasia and Gondwana, acting as a major oceanic barrier that drove independent evolutionary trajectories on either side. Laurasia itself did not remain a unified landmass: internal fragmentation progressed through the Late Cretaceous and Paleogene, culminating in the opening of the Norwegian Sea around 56 million years ago, which finally separated North America–Greenland from Eurasia. In paleontology, Laurasia served as the primary arena for the diversification of iconic Late Cretaceous dinosaur groups—including tyrannosaurids, ceratopsids, dromaeosaurids, and hadrosaurids—whose distributions were shaped by intermittent land connections such as the Bering Strait land bridge linking Asia and North America. The concept of Laurasia was proposed by South African geologist Alexander du Toit in 1937, modifying Alfred Wegener's single-supercontinent hypothesis by envisioning two primordial landmasses separated by the Tethys.

A mid-ocean ridge (MOR) is a continuous underwater mountain range system formed at divergent tectonic plate boundaries, where two oceanic plates spread apart and new oceanic crust is continuously generated through volcanic upwelling of mantle-derived basaltic magma. The global mid-ocean ridge system stretches approximately 65,000 kilometers—making it the longest and largest single volcanic feature on Earth—and lies at an average water depth of about 2,500 meters below sea level, with ridge crests rising roughly 2,000–4,500 meters above the surrounding ocean floor. As tectonic plates diverge along the ridge axis, decompressional melting of the ascending asthenosphere produces basaltic magma that erupts at or near the seafloor, constructing new oceanic crust and driving the process known as seafloor spreading. This continuous crustal recycling, combined with subduction of old oceanic lithosphere at convergent margins, maintains a dynamic equilibrium in Earth's surface area and constitutes one of the primary mechanisms driving plate tectonics, continental drift, and long-term paleogeographic reorganization across geological time.

Pangaea was a supercontinent that incorporated nearly all of Earth's landmasses into a single continuous body of land. It existed as a fully assembled supercontinent for approximately 160 million years, from its coalescence around 335 million years ago (Ma) during the Early Carboniferous to the onset of its fragmentation around 175 Ma in the Middle Jurassic. Pangaea formed through the progressive collision and suturing of three major pre-existing continental units—Gondwana, Euramerica (Laurussia), and Siberia—during the late Paleozoic, culminating in its maximum packing by approximately 250 Ma in the Late Permian. The supercontinent was surrounded by a single global ocean known as Panthalassa, while a large embayment called the Tethys Sea separated the eastern portions of its northern and southern landmasses. Because of Pangaea's immense size and the resulting distance of interior regions from moderating oceanic influences, its climate was characterized by extreme continentality: vast arid deserts dominated the interior, seasonal temperature swings were severe, and climate models indicate the establishment of a powerful "megamonsoonal" circulation pattern that drove intense wet-dry cycles along coastal margins. Pangaea's existence had profound consequences for the evolution and distribution of life on Earth. During the Triassic, terrestrial vertebrates—including early dinosaurs—could disperse across nearly the entire globe over continuous land without oceanic barriers, producing cosmopolitan faunas. The supercontinent's subsequent breakup, initially splitting into northern Laurasia and southern Gondwana during the Jurassic, progressively isolated populations on diverging landmasses and drove the independent evolutionary radiations that generated much of the biodiversity observed in the later Mesozoic and Cenozoic eras.