On the Uneven Distribution of Triassic Marine Reptiles in Time and Space: Geological and Biotic Explanations of Three Megabiases

The Triassic Period is pivotal for the origin of the modern world, shaping both plate tectonics and vertebrate evolution and diversity to the present day. In the oceans, the Early Triassic saw the full expression of a major evolutionary trend: the return of terrestrial vertebrates (Amniota) to the marine environment. This secondary adaptation persists today, exemplified by whales among mammals and sea turtles among reptiles. Approximately two million years after the beginning of the Triassic, the two principal groups of marine reptiles—Sauropterygia and ichthyosaurs (Ichthyosauriformes)—first appear in the fossil record. Despite the relatively simple geography of the Triassic world, with two major oceanic realms (Panthalassa and Tethys), their spatial distribution is already markedly uneven. Ichthyosaurs are recorded from Early Triassic localities in both Tethyan and Panthalassan settings, albeit only in the Northern Hemisphere. In contrast, sauropterygians are largely restricted to the Tethys, with rare exceptions. Only the lineage leading to plesiosaurs dispersed beyond the Tethys early in its history. A second, global-scale bias reinforces this pattern: Triassic marine reptile Lagerstätten are overwhelmingly confined to the Northern Hemisphere, constituting a major megabias in the record. These spatial biases appear stable throughout the remaining ~48 million years of the Triassic. However, recent radical revisions of the Triassic timescale reveal a third, temporal megabias: a near-complete absence of marine reptile Lagerstätten during the subsequent ~25 million years. Globally, this interval is represented by essentially a single well-documented unit, the Pardonet Formation of British Columbia, Canada. The magnitude of these megabiases is unparalleled in the remainder of the Mesozoic and Cenozoic and demands explanation. If the Northern Hemisphere record is broadly representative, the scarcity of sauropterygians in Panthalassa may reflect limited dispersal capabilities linked to comparatively low metabolic rates; pistosauroids form an exception, showing evidence of more advanced marine adaptation. In contrast, even the earliest ichthyosaurs exhibit open-ocean adaptations, likely enabling rapid dispersal but obscuring their geographic origin. A minor clade, Thalattosauria, mirrors these patterns: plesiomorphic askeptosauroids are restricted to the Tethys, whereas derived forms dispersed across the Northern Hemisphere. The origins of the second and third megabiases are less clear. The absence of a Southern Hemisphere record correlates with the scarcity of Triassic marine deposits on Gondwanan continents, although the causes of this pattern remain unresolved. Likewise, the temporal gap in Lagerstätten suggests a geological control unless attributed to stochastic processes. Any investigation of Mesozoic marine reptile evolution must account for these three megabiases, bearing in mind that absence of evidence is not evidence of absence—particularly when assessing hypotheses of lineage decline in the Late Triassic.