Mesozoic Marine Reptiles Progress Report (Patreon)

Hello dear patrons, Happy New Year for 2022 and apologies as usual for my silence. My workload remains CRUSHING and I truly despair on the getting of things done...

As some of you know, I was supposed to finish my Mesozoic marine reptiles book by the end of 2021. Buuut that hasn't happened; I'm killing myself working on it now (in 'spare time') and still have some way to go. The good news is that several chapters are done and some have been sent off for checking and reviewing. On that note, I thought you might enjoy seeing some of the text as it current stands, so here we are with an extract from one of the Introductory chapters... Let me know what you think!

The World of Mesozoic Marine Reptiles

The Mesozoic was such a long stretch of time that making generalisations about the world during this time is difficult. The shapes and arrangements of the continents changed, as did the locations and sizes of seas and oceans. Thanks to a combination of factors, Earth during the Mesozoic was in a ‘hothouse’ phase. These factors include the partly unified form of the continents (which led to hot, dry continental interiors and small coastlines), the absence of continental landmasses over the poles (which reflect sunlight and thus cool the planet), and high atmospheric CO2(which traps heat).

Hothouse worlds have little ice, and a consequence of little ice is high sea levels. Sea levels during the Mesozoic were the highest in history, and low-lying areas were flooded. Vast continental and continental shelf regions were covered by warm, shallow seas mostly less than 60 m deep, and with surface temperatures of almost (or over) 30° C (today’s average is 18° C). The seafloor in such places was within the reach of sunlight, so algae could grow and complex communities could thrive. The Mesozoic was more volcanically active than today, and with constant wildfire activity. Both factors resulted in greater chemical runoff into the seas, and thus more fertile oceans. The closeness of the Mesozoic’s warm, shallow continental seas to land meant that they benefitted from the runoff of iron, nitrogen and other chemical elements.

This almost perfect combination of factors – warmth, shallowness and chemical richness – meant that Mesozoic seas provided ideal environments for the evolution of marine reptiles. The fossil record shows that these animals exploded in diversity from the Triassic onwards. The high temperatures and abundance of resources encouraged rapid growth and rapid evolution, and the complexity and richness of these environments encouraged specialisation and diversification.

However, times were not always good. On several occasions during the Mesozoic, volcanic events released large quantities of carbon into the oceans, the result being acidification, a reduction in oxygen content, and mass die-offs of micro-organisms and a collapse of food chains. Mass extinctions were the result, a topic we explore later.

The Shape of Mesozoic Seas

During the Triassic (between 251 and 201 million years ago), the continents were united as the supercontinent Pangaea. Areas that today form China jutted eastwards and partially enclosed the Tethys, which was bordered by the eastern margin of Pangaea to its west and south. The rest of the globe (around 70% of it) was covered by the ocean Panthalassa. Open ocean was thus a major feature of the Triassic world. The Tethyan shallows are more important from the point of view of the fossil record and extended from eastern North America and northern Africa all the way to southern China and Australasia.

During the Jurassic (between 201 and 145 million years ago), continental fragmentation and high sea levels (sometimes more than 100 metres higher than those of today) resulted in the creation of extensive shallow seas. Most of Europe was flooded, only the higher areas remaining emergent as islands. Continental shallows surrounded the north, east and south of the southern continents, all of which were massed together as Gondwana.

On North America’s west coast, a tectonically active zone resulted in island arcs, marine trenches and sea volcanoes. This created fertile, complex environments, present here right to the end of the Cretaceous. An inlet of Panthalassa flooded North America and formed an intercontinental sea – the Sundance Sea – that extended from British Columbia to Wyoming. Ichthyosaurs and plesiosaurs related to European forms flourished here.

As Gondwana moved south and the northern continents (grouped together as Laurasia) moved north, a seaway emerged, connecting Europe with eastern Panthalassa. This is the Hispanic Corridor or Caribbean Seaway and it probably explains why Europe, Mexico and the Caribbean shared similar Jurassic marine reptiles. Another seaway – the Mozambique Corridor or Trans-Erythraean Seaway – allowed Tethyan animals to move south-west and into a region between Antarctica and the conjoined Africa and South America. Metriorhynchoids, parvipelvians and plesiosaurs related to European forms existed in western Argentina during the Late Jurassic. Perhaps they used the Hispanic or Mozambique corridors to get there.

Meanwhile, another seaway – the Viking Corridor – connected the Western Tethys with a semi-enclosed sea in the north, the Boreal Ocean (also called the Boreal Sea or Arctic Ocean). Despite its Arctic Circle location (and low temperatures and seasonal darkness), this was inhabited by ichthyosaurs and plesiosaurs, a fact which tells us much about the biology of these animals.

Our knowledge of Jurassic marine reptiles is dominated by information from Europe, and it’s mostly for this reason that we’ve tended to imagine Europe as the area where these groups originated. This is a consequence of human history and the use of Jurassic rocks in industry and building, but it’s also because European Jurassic sediments are on the land surface today and have survived erosion and subduction. The complexity of the Jurassic world demonstrates that numerous other regions were important in marine reptile history and may have been the true origination sites of these groups, it’s just that the fossil record of those places is not as well understood or well sampled.

The main theme of the Cretaceous from an oceanic point of view is the fragmentation of Gondwana, the southern movement of Antarctica, and the inundation of North America. India became an island and moved north but modern India is only part of ‘Greater India’, the northern edge of which was more than 2500 km north of the region’s modern margin. This was an enormous submerged continental area. Further east, another submerged continental region – Zealandia or Greater New Zealand – existed in the south-west Pacific. Some of the animals known as fossils here occurred around Antarctica as well as southern and western South America. The environment and ecosystems present across this area formed a biogeographic region known as the Weddellian Province. Weddellian animals must have been cold tolerant since they lived in seas where ice formed during winter.

A key Cretaceous event was the opening of the South Atlantic between South America and Africa. This produced more coastline but also provided another route for north-south movement. Mosasaurs known from Europe and North America were living around the coasts of Angola and Brazil by around 75 million years ago. Much of northern Africa and western South American was inundated during the Late Cretaceous (between around 94 and 70 million years ago) due to a rise in sea level.

Throughout the Cretaceous, the westward movement of North America (caused by its tectonically active western coast) pulled the continent downwards into the crust. Consequently, an arm of the Boreal Ocean moved south, forming the Mowry Sea during the first half of the Cretaceous. An arm of the North Atlantic then moved north from the Gulf of Mexico, and ultimately met the Mowry Sea to form the Western Interior Sea, also called the WIS, Western Interior Seaway, Niobraran Sea or North American Inland Sea. North America was divided in two. During the middle of the Cretaceous, an arm of the WIS called the Hudson Seaway connected to the Labrador Seaway west of Greenland, dividing North American into three. The WIS was a key feature of the Late Cretaceous world and was important for mosasaurs and plesiosaurs. Continental uplift ultimately caused the WIS to shrink, but it persisted until the Paleocene, 60 million years ago.

Mesozoic climates and temperatures

Until recently, the assumption about the Mesozoic world was that hot conditions prevailed, that the seas were always warm, and that the poles were too warm for ice or snow. Because modern reptiles (especially big ones) are animals of subtropical and tropical places, it made sense that the marine reptiles of the Mesozoic had evolved in such a hothouse world.

Most evidence (including the distribution of fossil organisms and the temperate-sensitive oxygen isotopes preserved within them) shows that the Mesozoic was warm overall, that the world cooled during the Late Jurassic and Cretaceous, and that Late Cretaceous warming made things hotter again. During the warmest parts of the Mesozoic (the Late Triassic, the end of the Early Cretaceous and the early part of the Late Cretaceous), global temperatures were more than 5°C warmer than those of today, sometimes 10°C higher. The oceans at these times had surface waters over 10°C higher than the modern average (which is 18°C), perhaps almost 20°C higher in the tropics.

It is not true, however, that the Mesozoic was perpetually warm. Geological data including ice-damaged rock surfaces and stones dropped by floating ice, climatic modelling and isotopes from fossils show that things were more complex. Cold events occurred during the early part of the Jurassic, in the Late Jurassic, and at least twice during the Early Cretaceous.

Some researchers argue that sea and continental ice formed during the Early and Middle Jurassic, and that Western Tethyan sea surface temperatures during these times were close to freezing. Other experts argue that conditions were warmer, perhaps by 10-15°C. This still means that many seas during this time were cool, and that animals living here swam in temperate seas, not tropical ones. Sea surface temperatures of 10°C and less have also been reported for the Early Cretaceous North Atlantic, again meaning temperate seas for the time. The seas were also close to – sometimes below – freezing in the far north and south during the Late Cretaceous, yet plesiosaurs and mosasaurs were here at these times.

How did Mesozoic marine reptiles cope with these conditions? It seems that some, many or most Mesozoic marine reptiles were endothermic. That is, ‘warm blooded’, and able to generate heat, retain it, and maintain a high internal temperature. Several lines of evidence support this. The internal texture of their bones shows that they grew quickly (something known to correlate with a high metabolic rate), exceptional fossils show how their bodies were insulated by blubber (meaning that they were retaining internally produced heat), and the oxygen isotopes in the phosphate of their teeth are linked to high body temperatures. Endothermy was likely present in ichthyosaurs, plesiosaurs and related sauropterygians, mosasaurs and metriorhynchoids, which means that it evolved multiple times independently.

This explains why Mesozoic marine reptiles of several groups – parvipelvians and certain plesiosaur and mosasaur clades among them – thrived in cold regions like the Jurassic Boreal Ocean and Cretaceous waters around Antarctica.