The Mesozoic Marine Revolution (MMR) describes the increase in shell-crushing and boring predation among shallow-sea (neritic) and some deep-sea marine animals during the Mesozoic era (251 million years ago to 66 million years ago). It also includes activities like moving sediment and reshaping the bottom areas of the ocean by benthic marine creatures. The term was first introduced by Geerat J. Vermeij, who used research by Steven M. Stanley as a basis. Initially, the MMR was thought to begin in the Cretaceous period (145 million years ago to 66 million years ago). However, recent studies suggest it started earlier, during the Triassic period, specifically in the Anisian or Aalenian stages. This event happened at the same time as the rise of large marine reptiles, such as ichthyosaurs, sauropterygians (like plesiosaurs, nothosaurs, and placodonts), thalattosaurs, and thalattosuchians. It also involved the spread of shell-destroying predators, such as teleosts, stomatopods, and decapods. Additionally, there was a trend toward more animals living within the ocean floor, and many sessile organisms that could not reattach after being moved faced extinction or habitat loss.

The MMR marks an important change in marine life, shifting from the Paleozoic fauna to a more modern-looking type of marine life that developed throughout the Mesozoic. However, it was not the first time predatory pressure increased. This happened earlier, near the end of the Ordovician period. Some evidence shows that certain Paleozoic creatures, like crinoids, had already started adapting to shell-crushing behaviors.

Causes

The Mesozoic marine revolution was caused by the development of shell-crushing behavior among marine predators, especially marine reptiles, with this technique becoming most advanced during the Late Cretaceous. This led to shelled marine invertebrates evolving defenses against such predation or facing extinction. Evidence of these changes can still be seen in many invertebrates today. These predators likely included: Triassic placodonts, Triassic ichthyosaurs, Triassic omphalosaurids, Triassic plesiosaurs, Jurassic pliosaurs, Late Cretaceous mosasaurs, and Cretaceous ptychodontoid sharks. Many gastropods also evolved to eat prey with shells. However, since most shell-crushing predators were generalists, their impact on the development of anti-predator shell features has been debated, with some scientists believing the changes were less widespread than others suggest. Still, a trend of specialist predators evolving during the Mesozoic marine revolution has been observed alongside the broader trend of generalist predators.

The splitting of Pangaea and the creation of new oceans during the Mesozoic brought together marine communities that had previously been separated, forcing them to compete and adapt. Increased shelf space from rising sea levels and a very warm climate provided more opportunities for evolution, leading to greater biodiversity.

The rapid growth of flowering plants during the Cretaceous improved water movement, increasing the speed of weathering and nutrient flow into the oceans. This has been suggested as a possible cause of the Mesozoic marine revolution.

Another idea is the evolution of hermit crabs. These animals use the shells of dead gastropods, effectively making the shells last twice as long. This gives shell-crushing predators nearly twice as many prey to target, creating a new opportunity for these predators to thrive.

Effects

The Mesozoic marine revolution led to a shift in marine life from the stationary, attached-to-the-seafloor lifestyle of ancient Paleozoic creatures to the modern lifestyles of creatures that live buried in the seafloor or drift in the water. Creatures that could not reattach to their habitat, like brachiopods, were easier for predators to catch. Those that could hide or move quickly had better chances to survive and pass on their traits. The average energy use per individual among shallow-water marine snails increased by about 150% from the Late Triassic to the Late Cretaceous.

Three major changes happened during this time:

• A decrease in the number of creatures that filtered food from the water while attached to the seafloor.
• A rise in the number of creatures that lived buried in the seafloor.
• A middle stage of creatures that could move along the seafloor.

Major groups affected by the Mesozoic marine revolution included stationary crinoids, snails, brachiopods, and creatures that lived attached to the seafloor, such as certain bivalves.

Affected taxa

The Mesozoic Marine Revolution greatly changed crinoids, causing many of their forms to become extinct. Crinoids, which are usually attached to the seafloor, were easy targets for predators that eat hard-shelled animals, especially during the Triassic. Surviving crinoids, like comatulids, could move, hide at night, or shed limbs to escape danger.

During the late Mesozoic, sessile stalked crinoids moved from shallow water areas to deeper offshore habitats. This shift likely happened because predators in shallow water increased, forcing crinoids to find safer, deeper environments where predation was less common. This migration did not happen at the same time globally. In the Southern Hemisphere, it occurred much later—starting in the Late Eocene in Australia and Antarctica, and in the Early Miocene in Zealandia.

Echinoids faced little major predation during the Mesozoic Marine Revolution, except for general burrowing. However, fossilized "vomit" shows that cidaroids were eaten by predators. Echinoids later expanded into new roles, such as coral grazers, by the Late Cretaceous. Cidaroids may have also contributed to the decline of crinoids. Predation by echinoids continued into the Cenozoic era.

Brachiopods, which were the most common bottom-dwelling organisms during the Paleozoic, suffered greatly during the Mesozoic Marine Revolution. Their attached lifestyle made them easy prey for durophagous predators. If attacked, brachiopods could not reattach to surfaces, reducing their survival chances. Unlike bivalves, brachiopods (except for lingulids) never adapted to live buried in sediment, leaving them vulnerable throughout the Mesozoic Marine Revolution. Due to increased predation and competition with bivalves, brachiopods became rare in marine ecosystems by the Cenozoic, despite their earlier diversity and abundance.

Bivalves adapted more easily to the changes caused by the Mesozoic Marine Revolution. Many burrowed into sediment, using siphons to gather nutrients while staying hidden. Corbulids developed layers of a strong protein in their shells to resist predation. Others, like Pecten, evolved the ability to jump short distances by closing their shells rapidly.

Epifaunal bivalves, like mussels and oysters, were heavily preyed upon. Their ability to attach tightly to surfaces made them harder for small predators to eat. This predation was especially intense before the Norian period, but extinction rates later decreased.

Benthic gastropods were heavily targeted throughout the Mesozoic Marine Revolution. Weaker-shelled species were forced into isolated habitats. Paleozoic archaeogastropods were replaced by neritaceans, mesogastropods, and neogastropods. Neritaceans and mesogastropods typically have symmetrical, weaker shells with an umbilicus, while neogastropods lack an umbilicus and developed shell sculptures for defense.

Some Muricidae evolved the ability to drill through shells to consume prey. These marks, though rare, were found on sessile invertebrates, suggesting they impacted Paleozoic-type ecosystems during the Mesozoic Marine Revolution.

Bryozoans showed no major anti-predatory adaptations during the Jurassic, indicating they were not significantly affected by the Mesozoic Marine Revolution during this time.