The Mesozoic or Mesozoic Era is one of the eras in the history of our planet that has aroused the most interest in the scientific community, but it has also captured the imagination of the entire world. This era spanned approximately 185 million years , started with one mass extinction, ended with another, and even had one in its course, after its first third.
During the Mesozoic, life flourished in the seas, as evidenced by the rocks of these ages and the fossils of marine animals that appear in them. In these seas there were reefs inhabited by very diverse fauna, such as cephalopods, bivalves, crustaceans, brachiopods, corals, echinoderms, fish and marine reptiles . And no, there were no marine dinosaurs, that we know of. And it is that all the marine reptiles that dominated the food chains during the Mesozoic can be classified as belonging to different lineages, not that of the dinosaurs.
marine invertebrates
Cephalopods are the group of molluscs that includes today’s squid, octopus, and cuttlefish. During the Mesozoic, three groups of cephalopods were very abundant: ammonites, belemnites, and nautiloids.
Ammonite or ammonoid fossils are so abundant that they have become an icon of Paleontology. Actually, this group originated in the Paleozoic, about 410 million years ago, but they were so abundant and diverse during the Mesozoic that they are even used in biostratigraphy , that is, they are used as guide fossils for local and regional dating. Like the non-avian dinosaurs, they disappeared 66 million years ago.
The most characteristic feature of ammonites is their outer shell or shell, which is spirally coiled . But unlike snails or gastropods, this coiling is planispiral and, moreover, its internal cavity was divided into chambers . These internal cavities were separated by septa and connected by a siphon, so that different chambers could be filled or emptied of air to modify the flotation. The animal’s soft body was housed in the last outermost chamber, called the habitation chamber.
The ammonites showed a great variety of shapes and sizes, some reaching up to two meters in diameter. Externally, the shells could present ornamentation in the form of ribs, tubercles or spines, on some occasions very ornate. Also, some of them unrolled their shells, reaching very extravagant shapes. These uncoiled-shell ammonites are known as heteromorphic ammonites .
The belemnites were possibly similar to the modern squid in their external appearance. Its most abundant fossils correspond to its internal shell, equivalent to the feather of squid and cuttlefish.
Regarding the nautiloids , they are the only ones of these three groups of cephalopods that have survived to the present day. Like the ammonites, they also have a spirally wound outer shell. The main way to tell them apart is in the septa between the chambers: in nautiloids, these septa are simple and curved; while in ammonoids they had more complicated shapes , becoming fractals .
Other very abundant invertebrates during the Mesozoic, although its golden age was the Paleozoic, are the brachiopods. They managed to survive several great mass extinctions and, in fact, have survived to this day. Although today their representation in marine ecosystems is very minor compared to what they had in the past. Externally they can remind us of bivalve molluscs , in reality they are not molluscs, but a different group of invertebrates that independently acquired an external shell formed by two valves. In Mesozoic marine rocks we very frequently find large concentrations of brachiopods.
Echinoderms are the group to which starfish, sea urchins, and crinoids, among others, belong. They have an internal skeleton that forms a kind of shell that protects the viscera of the animal. They were also very abundant during the Paleozoic and Mesozoic periods . Especially the crinoids , the current sea lilies. Its body is shaped like a cup or calyx, from which 5 arms usually start, which are sometimes subdivided. Mesozoic marine sedimentary rocks are often abundant in plates and stem fragments of crinoids, which became very abundant in Jurassic seas, and came in many sizes and shapes.
marine vertebrates
Regarding marine vertebrates, there was a great diversity of fish, both bony (which we call osteichthyes) and cartilaginous (which we call chondrichthyans, and they are the group to which sharks and rays belong). But also a great variety of reptiles adapted to aquatic life inhabited the seas, such as turtles, crocodiles, plesiosaurs, ichthyosaurs and mosasaurs. Of all of them, the ichthyosaurs were the ones that developed the greatest adaptation to aquatic life. Some Mesozoic marine reptiles became as gigantic as the dinosaurs, which is why they are often confused.
All this Mesozoic marine fauna would give to talk for hours and hours. And it is that, although popular interest is always biased in favor of terrestrial vertebrates, life in the sea is possibly the most diverse. And the Mesozoic seas were a good example of this.
References:
Seilacher, A. 1993. Ammonite aptychi; how to transform a jaw into an operculum. American Journal of Science. 293: 20–32.
Martinez-Chacon, ML; Rivas, P. (eds.) 2009. Invertebrate Paleontology. 2009, Spanish Society of Paleontology, Geological and Mining Institute of Spain, University of Oviedo, University of Granada, 524 p.
Belinchón, M. et al. 2009. Chronicles of Fossils: the paleontological collections of the Valencia Museum of Natural Sciences . City Hall of Valencia.
