Mesozoic Era
Mesozoic Era Mesozoic - 2

The Mesozoic - 2

Phanerozoic Eon
   Paleozoic Era
   Mesozoic Era
      Triassic Period
      Jurassic Period
      Cretaceous Period
   Cenozoic Era

The Mesozoic
   Marine Reptiles

Marine Life in General

Mesozoic oceans were populated by a rich and diverse fauna of fish, reptiles, and a variety of cephalopod mollusks including the ammonites and the belemnites (nautiloids were also present but less common). Both these molluscan groups were adapted for speed and mobility. Fish were mostly slow moving heavy scaled ("ganoid") types, which were probably not as agile teleosts only become predominant towards the later Cretaceous).

From the Jurassic onwards Plankton increased in diversity, with phytoplankton such as coccoliths, diatoms and silicoflagellates together with a zooplankton dominated by foraminifera and radiolarians. Arthropods such as the amphipod, decapod and isopod crustaceans together with nudibranch mollusks and the annelid and polychaete worms were also probably part of the plankton. MAK020428



The fossil record of Mesozoic annelids, like the fossil record of all annelids, is poor. We can only make a few, general remarks.

The end-Permian extinction more or less destroyed the entire Paleozoic benthic fauna. The Mesozoic benthic communities, developed an entirely new style, possibly (i.e., this is complete speculation) based on the very few anoxia-tolerant detritivores who would have flourished in the benthic carnage of the end-Permian. Whatever their origin, Mesozoic and Cenozoic benthic communities are dominated by infaunal (burrowing) deposit-feeders, rather than epifaunal suspension feeders. This was surely good for the annelids who are quite handy with low-oxygen, burrowing ways of making a living. Oligochaetes probably evolved in the Late Jurassic. However, they were unable to employ the usual annelid skills on land until the Late Cretaceous, when angiosperms began creating large quantities of humus, permitting the evolution of the oligochaete earthworms.


Brachiopods suffered greatly during the end-Permian extinction. They were able to make a considerable come-back during the Late Triassic, but ultimately declined and were ecologically replaced by bivalves. Their fate may have been tied to substrate. The brachiopods of the Late Triassic resurgence were strongly associated with carbonate shelves, the classic reef environment of the Late Paleozoic and Early Mesozoic. The rise in sea levels during the Jurassic and Early Cretaceous drowned these platforms on a global basis. That is, the residents of the carbonate platforms gradually found themselves too deep in the water column for sunlight to sustain photosynthesis, and the shelf ecosystems collapsed. This permitted the bivalves to "mussel" their way in, as they were better adapted to the soft and unstable sand & mud sea bottoms within the new photic zone. In fact, with the evolution of the rudists, the bivalves were able to make their own quick and sloppy reefs on even the softest substrate.

As a consequence, the surviving Mesozoic brachiopods became off-shore specialists, occupying deeper-water and more cryptic environments in crevices and on submarine cliffs below the photic zone. Some developed poisonous tissues. The more robust and globose terebratulides such as Terebratella and probably some species of Tichosina were free on the substrate. A few of these developed semi-infaunal strategies.

Mesozoic brachiopods, like many other invertebrates, show considerable differentiation between Tethyan (tropical) and Boreal (subtropical and temperate) types in the Late Triassic and Jurassic. Also like many other invertebrates, these distinctions broke down in the Cretaceous, as rising sea levels and flattened climate zonation homogenized most marine fauna.


Early Mesozoic bryozoans were largely cheilostomes and cyclostomes. During the Early Cretaceous, however, the cyclostomes declined while cheilostomes diversified. The reasons for this replacement are unclear. Both suffered massive extinctions in Maastrichtian time, possibly coinciding with the more general KT extinctions. The cheilostomes rebounded during the Cenozoic. The cyclostomes generally did not. McKinney & Taylor (2001).


CunnolitesMesozoic cnidarians are mostly known from their greatest success story, the scleractinian corals. Several groups of scleractinians developed tight symbiotic relationships with photosynthetic zooxanthellae with a resulting huge boost to their productivity. The scleractinians suffered considerably from the drowning of the carbonate platforms on which their reefs were based during the Late Jurassic and Cretaceous. However, they recovered quickly after the KT extinctions.


The End-Permian extinction at the end of the Paleozoic Era took a heavy toll on the stemmed echinoderms. The blastoids became extinct at that time and the crinoids suffered heavy losses. In general, Paleozoic echinoderms were epifaunal suspension and detritus feeders. Like so many high school students, their strategy was to sit more or less stationary on the sea bottom with their mouths open and wait for food to come to them. In the Mesozoic and Cenozoic, the echinoderms became more like undergraduates -- still bottom-feeders, but now willing to dig for it (infaunal detritus feeders) or, if sufficiently pressed, to go and hunt for it (armored herbivores and carnivores).

RoveacrinusThis use of rather heavy armor runs counter to a general trend among Mesozoic life forms to shed heavy plates and to depend more on speed, or on other behavioral adaptations for survival. However, behavioral strategies depend on having the neural equipment to select a response and adapt it to local conditions. Echinoderms are poorly adapted for this sort of thing because they are attractive, but brainless. So as time went on, echinoderms, like other attractive but brainless organisms, were increasingly forced to rely on heavy make-up, intimidating ornament, and a thick skin. The surviving crinoids, for example, were articulates, with rounded, closely fitting armor plates, usually bearing elaborate ornamentation. Some also gave up sessile life, left their stems behind, and became motile. These swimming crinoids, the Rovecrinidae, are discussed briefly elsewhere.

However, for the most part, the old crinoid fauna simply died out. The future of the Echinodermata lay with the Echinoidea and Asteroidea. Echinoids are rare in Paleozoic faunas, but radiated extensively during the Mesozoic and Paleogene. Paleozoic, and even Triassic, urchins have no compound plates, and the interambulacral plates are constructed in many columns [1]. These earliest sea urchins are generally small and lack strong spine development -- characters which developed over the course of the Mesozoic. 


Sponges as a whole did well and slowly diversified until the very end of the Mesozoic. However, this general trend is made up of varying fates of different groups of sponges. Demosponges and calcisponges recovered from the end-Permian extinction and dominated the reef fauna once more in many locations during the Late Triassic. However, they were gradually replaced by scleractinian corals. Hexactinnelids and some stromatoporoids continued as important frame builders for the coral reefs of Jurassic Europe. Demosponges and hyalosponges became more common in the Cretaceous. As sea levels rose, these sponges were sometimes able to thrive in regions which had become too deep for the corals. Mesozoic stromatoporoids (demosponges probably not related to the Paleozoic forms) were significant reef-builders in the Cretaceous. All types of reef-building sponges virtually disappeared at the KT boundary and never recovered.    ATW040905


Mesozoic Tetrapods

Mesozoic TetrapodsThe Mesozoic era was an extremely long period of time, which saw the rise and fall of successive "dynasties" of life. At least half a dozen succesive evolutionary communities or empires of land vertebrates (tetrapods) can be distinguished. Identifying them by characteristic large herbivores, these can be called the lystrosaur (Earliest Triassic [Induan]), kannemeyeriid- traversodontid (primarily Gondwanan, though this may be sampling bias) (Early [Olenekian] to Late (Carnian) Triassic ], plateosaur-vulcanodontid (Late Triassic [Norian] - Early Jurassic), sauropod- stegosaur (Middle to Late Jurassic), iguanodont- nodosaur (Early to Mid Cretaceous), and ceratopsian-hadrosaur Late Cretaceous - Laurasia only, Gondwana was predominantly Titanosaurid, with Abelisaurid carnivores) communities or "empires". In the sea one finds what could be perhaps termed the mixosaur- nothosaur (Mid Triassic), shastasaur (Late Triassic), ichthyosaur- plesiosaurid- rhomaleosaurid Latest Triassic [Rhaetian] - Early Jurassic), ophthalmosaur- pliosaurid- metriorhynchid (Middle Jurassic-Early Cretaceous), and protostegid- elasmosaurid- mosasaur communities (Mid to Late Cretaceous).

The following diagram is from Fig. 3. of Dr Robert T. Bakker's 1977 paper "Tetrapod Mass Extinctions - A model of the regulation of speciation rates and immigration by cycles of topographic diversity" in A. Hallam, ed. Patterns of Evolution as illustrated by the Fossil Record, Elsevier Scientific Publishing Company, Amsterdam, Oxford, New York, pp.439-68

Although subsequent research has modified some of the family rankings and stratigraphic correlations, the basic pattern remains.

Diversity of tetrapods, Early Triassic to Cretaceous. Standard marine stages are indicated by initials in boxes at top. Narrow extensions of bars indicate that the family is present but very rare. Families known from only one formation are omitted. Roman numerals at top show the successive "dynasties". Biomass D for large herbivores for Triassic taken from Fig. 2; D calculated for a few large Jurassic and Cretaceous samples from the following formations: 1, Tendaguru (Kimmeridgian); 2, Morrison (Kimmeridgian/Tithonian); 3, Old Man (Campanian); 4, Lower Edmonton A (Campanian/Maastrichtian); 5, Lower Edmonton B (Maastrichtian); 6, Lance-Hell Creek-Frenchman (latest Maastrichtian).
Family abbreviations:
Marine Aquatics: A = henodontids; B = pachypleurosaurids; C = mixosaurids; D= placocheliids; E = nothosaurids; F = shastasaurids; 0 = pliosaurids; H = metriorhynchids; I= rhomaleosaurids; J = ichthyosaurids; K = plesiosaurids and cryptoclidids; L = stenopterygiids; M = teleosaurids; N = rhamphorhynchids; O = protostegids; P = mosasaurids; Q = cheloniids; R = toxocheliids; S = ichthyornids; T = hesperornids; U = elasmosaurids; V = polycotylids; W = ornithocheirids pteranodontids); X = ornithodesmids.
Large Fresh-Water Aquatics: A =amphisbaenids; B = varanids; C = pelomedusids; D = dermatemyids; E = crocodylids; F = trionychids; G = pholidosaurids; H = goniopholids; I = glyptopids; J = emyids; K = champsosaurids.
The sampling of non-marine tetrapods during the first eight stages of the Jurassic is so poor that the records are not worth plotting on this compilation.
Large Terrestrial Herbivores: A = camarasaurids; B = diplodocids; C = stegosaurids; D = brachiosaurids; E = cetiosaurids; F = camptosaurids; G = hypsilophodontids; H = panoplosaurids; f = ceratopsids; J = iguanodontids; K = hadrosaurids; L = protoceratopsids; M = titanosaurids; N = pachycephalosaurids; 0 = euoplocephalids.

Marine Reptiles

illustration © Walking with Dinosaurs © 1999 ABC, BBC

During the Mesozoic era there were a number of lineages of marine reptiles. In comparison with the mammals, many more types of reptiles, having attained an existence on land, returned to the seas. Aquatic adaptation (whether freshwater, estuarine, or marine) is a common phenomenon among reptiles, due to their low metabolic rate, tolerance of anoxia and of low body temperatures, and easy ability to make use of fermentative metabolism for muscle activity. Moreover it does not require great structural or physiological changes, as indicated by the modern marine iguana. Reptiles move with a naturally sinuous motion, which is easily adaptable to swimming (as it comes originally from the swimming motion of the fish). In marine iguanas aquatic locomotion requires only one quarter the metabolic activity of terrestrial locomotion. When looked at this way, it is not surprising that reptiles have returned to the water, like their tetrapod and fish ancestors, whenever conditions were favourable.

The following is a list of aquatic reptiles, with some basic data. Note: Many of these pages are under construction or very incomplete. So here they are: the marvelous Mesozoic marine reptiles:

Picture name time-span habitat location approx size food
Plesiochelyidae Plesiochelyidae late Jurassic [Kimmeridgian] to early Cretaceous estuarine, near shore marine Central to East Laurasia shell c.75 cm??? invertebrates, fish, plant material?
Protostegidae Desmatochelyidae + Protostegidae Cretaceous
Desmatochelyidae - Albanian to Maastrichtian
Protostegidae - Turonian to Maastrichtian
open ocean Desmatochelyidae - cosmopolitan
Protostegidae - west Laurasia ("inland sea") only?
Desmatochelyidae - av. 1.2 meters (shell)
Protostegidae - up to 4 meters long (Archelon)
jellyfish, other invertebrates, fish?
Toxochelyidae Toxochelyidae late Cretaceous (Coniacian to Maastrichtian) open ocean west Laurasia ("inland sea" to west Atlantic) shell 60-120 cm jellyfish, other invertebrates, fish?
Cheloniidae latest Cretaceous [Maastrichtian] to Recent open ocean Worldwide 75 cm to over 1 meter long jellyfish, other invertebrates, fish?
Ichthyosaurus Ichthyosauria Triassic to Jurassic,
a few stragglers into the Cretaceous
open ocean world wide 1 to 23 meters mostly fish, also cephalopods, smaller reptiles
Placodus Placodontia Triassic near shore marine Tethys Sea 1 to 2.5 meters probably shellfish (Bivalve mollusks)
Pachypleurosauridae Pachypleurosauridae Middle Triassic [Early Anisian to Ladinian] estuarine, near shore marine world wide 20 cm to 1 meter fish, crustacea, etc
Nothosaurus Nothosauridae Middle to early late Triassic [Anisian to Carnian] near shore marine world-wide 2 to 8 meters mostly fish
Plesiosaurus Plesiosauria rare in the Triassic
common in the Jurassic and Cretaceous
open ocean
( a few estuarine species)
world wide 2 to 14 meters fish, cephalopods, other reptiles
Plotosaurus Mosasauroidea Aigialosauridae Late Jurassic to Early late Cretaceous (Tithonian to Turonian)
Mosasauridae: late Cretaceous (Turonian to Maastrichtian)
open ocean Aigialosauridae: Europe
Mosasauridae: world wide
Aigialosauridae: av. 1 meter
Mosasauridae: 2.5 to 17  meters
fish, cephalopods, other reptiles
Thalattosauria Triassic near shore marine Tethys Sea, also west Laurasia about 1.5 meters fish, crustacea etc
Nanchangosaurus Nanchangosaurus earliest Middle Triassic (Early Anisian) near-shore and estuarine China no info (~1 meter?) probably fish, crustacea, etc
Teleosaurus Teleosauridae Jurassic estuarine, near shore marine world wide 2 to 6 meters mostly fish
Metriorhynchus Metriorhynchidae Jurassic to Early Cretaceous open ocean world wide 2 to 6 meters mostly fish

MAK020428, modified MAK030915

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page uploaded on Kheper Site on 28 May 1998, 
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last modified ATW040905
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text content by M. Alan Kazlev 1998-2003, ATW 2003-04