Mesozoic restructuring of marine ecosystems

The skull of the mid-Triassic placodont Placodus gigas , on which large flattened teeth are visible, allowing it to split the shells and shells of its prey

The Mesozoic restructuring of marine ecosystems was caused by the fact that a number of marine predators developed adaptations that allowed them to feed on invertebrates living in durable shells and shells; it reached its peak in the Late Cretaceous. Faced with a new threat, marine invertebrates were forced to adapt or die out. The consequences of these processes occurring in the Mesozoic can be observed today on the example of many marine invertebrates. Among the predators posing a threat to them are: placodonts , ichthyosaurs , plesiosaurs (Triassic), pliosaurs (Jurassic), mosasaurs (Late Cretaceous), and also some chalk sharks. ^[one]

There is also an opinion that the breakup of Pangea during the Mesozoic contributed to the unification of previously isolated marine communities and forced the animals that inhabited them to adapt to new conditions. The expansion of shelf zones caused by rising sea levels, as well as climate warming, led to the rapid evolution and adaptive radiation of marine organisms. ^[one]

One example is the evolution of hermit crabs . They use empty shells of snails as protection, and protection from vertebrate predators that feed on armored animals. ^[one]

The main result of the Mesozoic restructuring of marine ecosystems was a decrease in the number of sedentary bottom animals characteristic of the Paleozoic (that is, attached to or lying on the bottom surface, such as bivalves or brachopods ) and the transition of many invertebrates to a burrowing or planktonic way of life. ^[4] Sedentary animals, such as brachopods, especially those who could not re-attach to the substrate, once torn off from it, became easy prey for newly appeared predators, while they were able to hide in burrows and crevices, and also able to quickly swim - got an evolutionary advantage. ^[one]

As a result, we observe three main trends during this period: 1) a decrease in the number of sedentary invertebrate-filtrators that live on the surface of the seabed; 2) an increase in the number of invertebrates burrowing into the ground; 3) the stability of the number of mobile - crawling and hiding - inhabitants of the seabed, its surface. ^[8]

The main victims of the Mesozoic restructuring of marine ecosystems can be considered: sessile sea ​​lilies , slowly crawling snails , brachopods and not knowing how to bury in the ground of bivalves .

Gastropods

Benthic gastropods became an object of active hunting from the side of predators that appeared in the Mesozoic; snails with fragile shells were eventually crowded out into isolated and less accessible predators. The archaic forms that arose in the Paleozoic were successively replaced by newer ones. ^[1] Snails with insufficiently strong shells died out. Survivors of time showed ribs of strength on the sinks. ^[one]

Another surviving group - the needlefish - developed the ability to predate , these animals began to drill holes in the shells of other invertebrates and eat their soft bodies. Holes drilled with needles appear on the shells of sedentary marine invertebrates in the Mesozoic. ^[9]

Sea Lilies

Sea lilies are severely affected by the Mesozoic restructuring of marine ecosystems, many forms of sea lilies became extinct at this time. The attached way of life made them easy prey for sea predators that appeared from the Triassic and were able to split shells and shells. ^[5] Surviving species (in particular stalkless ones ) acquired the ability to swim and / or crawl, became nocturnal animals or learned to throw tentacles captured by a predator . ^[7]

The fact that modern sea lilies live only at a depth of 200 m suggests that they could not survive in the shallow waters captured by predators and therefore survived only in deeper waters. ^[10]

Shoulderfoot

Shoulders were dominant benthic organisms throughout the Paleozoic, but were severely damaged during the Mesozoic. Their attached lifestyle made them easy prey for predators to appear. ^[1] The inability of the shoulders to reattach to the substrate if the predator was able to tear off the animal but could not split its shell, further reduced their chances of survival. Unlike bivalves, brachopods (except for lingulata ) could not learn to dig into the ground and therefore remained vulnerable throughout the Mesozoic.

Bivalves

Bivalves adapted to changes faster than the brachopods. Most bivalves learned to dig into the ground, sticking out only their siphons above the surface - to breathe and eat (due to filtration), but at the same time remain safe. ^{[1] [4]} Others, for example, pectenes known from the Early Cretaceous, reduced the size of their wings and, due to this, became able to “jump” along the bottom, fleeing predators.

Like the brachopods, those bivalves that lived on the surface of the bottom encountered strong pressure from predators. These animals (such as mussels and oysters ) were able to firmly attach to the substrate, which protected them from small predators. Bivalves living on the surface of the seabed were subjected to strong pressure from predators until the Norian age (Late Triassic ), but after this time the rate of their extinction decreased. ^[8]

Sea urchins

Sea urchins suffered less than others (primarily because they learned to dig into the ground), however, the remaining petrified semi-digested remains from them show that some species of irregular sea urchins often became prey of predators. ^[11] Sea urchins evolved, including niches of predators; perhaps, and by the late Cretaceous learned to eat coral. ^[one]

Irregular sea urchins are also believed to have contributed to the decline of sea lilies. ^{[5] [ clarify ]}