Ancient SeaReptiles: Plesiosaurs, Ichthyosaurs, Mosasaurs, and More
by Darren
Naish
Smithsonian
Books, 2023. 192 pages
My go-to book
on marine reptiles used to be Richard Ellis’s Sea Dragons: Predators of the
Prehistoric Oceans (2003), which is highly readable but long since obsolete
thanks to a raft of new fossils and analytical techniques. Ancient Sea
Reptiles, which reflects the latest information in text and diagrams while
remaining readable, is my new one.
An excellent Introduction sets up our voyage into the Mesozoic. Dr. Naish explains land masses, climate, temperatures (until recently no one was sure whether marine reptiles braved cold seas), and a capsule history of discoveries by naturalists and paleontologists. The first ichthyosaurs, mosasaurs, and plesiosaurs came out of Europe in 18th and 19th centuries. Speaking of Europe, Naish zaps the myth Many Anning was ever obscure or forgotten, even if she didn’t always get proper credit. More discoveries came out of North America, although Edward Drinker Cope in 1869 delayed proper study of his stunning Elasmosaurus by mistaking the neck for the tail and putting the skull on the wrong end. More mosasaurs and plesiosaurs also came out of North America, supplemented in modern times by marine reptile finds all over the world: from Australia, Morocco, China, and many other places.
Diving into evolution,
Naish straightens out the convoluted mess of theories, family trees, and
cladograms. These lead to the predominant modern hypothesis, that all the
marine reptiles form a superclade descended from a common ancestor. That
ancestor may resemble Womengosaurus, 255 million years old. The evolution
within the clade was complex. With nearly 200 million years of changing
conditions and evolutionary pressures, bodies responded in all kinds of
different ways. Not only did the same body plans appear (and reappear) from
different reptilian lineages, but similar body plans were shared among
creatures as different as ichthyosaurs, cetaceans, and fishes.
Each of the
major groups gets a chapter, but the “and More” in the title is very important.
Most readers will have at least a general idea of the three largest groups,
even if their relationships are very complex.
Naish shows us in Chapter 4 the marine reptiles were much more diverse
than is generally known, not to mention weirder. Mesosaurs, a bit crocodilian to our eyes,
prowled the shallows and ventured on land. Placodonts looked like bony, husky,
broad-bodied marine lizards. The platyochelids looked like bizarre turtles with
shells of heavy scales: I was remined of a swimming waffle iron. Nothosaurs had
long, shallow skulls, a bit alligatorish. Then there’s Tanystropheus, with a
neck as long as the body and tail put together. It appears to have been an
amphibious shoreline ambusher that picked off fish in the shallows. There are
many more groups. Above the Mesozoic oceans soared pterosaurs and, eventually,
seabirds. There were sea snakes, too, some with tiny hind limbs.
The
ichthyosaurs looked the most like modern fishers or cetaceans. They were around
more than 100 million years from the 1-meter (m) types of the early Triassic to
the amazing shastosaurs, which reached 21 m and probably longer. They split
into many groups and evolved countless variations. The Suevoleviathan had unusually
large front fins and a gigantic tuna-like tail. Some had enormous eyes
indicating they, like some modern cetaceans, didn’t let the need for oxygen
keep them from diving deep to hunt fish and squid.
The
plesiosaurs might be the most famous group of all. They are classically
described as looking like “a snake threaded through the body of a turtle.”
Naish notes the media stars are the elasmosaurs, with their extremely long
necks, but necks and skulls came in all lengths and thicknesses. (He also notes
they did NOT produce the alleged Loch Ness monster.) For 130 million years, the plesiosaurs evolved,
differentiated, and even produced the pliosaurids, which had massive heads and short
(sometimes almost absent) necks. There was also the giant Liopleurodon, once
estimated at 25 m but really well under half that (still a giant!) Kronosaurus was another large and relatively
famous species (among the types resurrected, with gills in the novels of Max
Hawthorne), and up to 11 m long. Leptocleidids were smaller types inhabiting
estuaries and lakes, filling niches many modern seals occupy: indeed, some look
considerably like four-flippered seals.
Naish spends
some time on the interesting and still disputed topic of just how these
creatures swam. Were they underwater flyers, like penguins? Rowers? It now
looks more complex, with precisely synchronized fore and hind paddle movements
for top efficiency.
The thalattosuchians
were the ocean-going crocodylomorphs, though unrelated to modern crocodiles. The
teleosaurids came first, starting with predators of the shallows and moving
into the oceans, while the later-developing metriorhynchids were pure
ocean-going animals with smooth skins.
The mosasaurs
were unique in being, literally, huge seagoing lizards. Naish says they can be
thought of as “whale-lizards,” albeit scaly-skinned, driven by their shark-like
tails. While the discovery of a soft-shelled egg 29cm long, which made headlines
in 2020, led to speculations mosasaurs laid eggs, the evidence is strong that
they bore live young (exactly what laid that egg is still a mystery). One
branch, the tylosaurines, produced giants 14 meters long. Here again underwater
flight has been suggested, at least for the long-limbed and deep-chested Plioplatecarpus.
In this case, too, the idea has been largely dismissed. Mosasaurus itself might
have grown as long as 18 m, although the Jurassic Park films make it the
size of a small U-boat.
Finally, we
have the sea turtles. On group, the protostegids, which may not have been
turtles at all, is extinct. This is unfortunate, since it produced the spectacular
Archelon, from North America, 4.6 m long and with a sharp parrot-like beak and a
cover of skin and/or scales over a full ribcage, unlike modern turtles where
ribs and carapace are fused. The others
are the hard-shelled turtles, relatives of those still with us today, and the
leatherbacks, which swam pretty much unconcernedly through the K-Pg event and
everything since. The only real enemies of the jellyfish-loving adults,
decimating their ranks today, are plastic bags.
The
illustrations are superb throughout. The book offers a plethora of photographed
fossils, artwork, and line drawings which connect us to the creatures being
discussed and to the technical topics like the importance of salt glands. The
diagrams of evolutionary relationships are equally helpful.
It’s not a
perfect book. While
Naish gives many sources in text, there are no footnotes, endnotes, or other citations
and only a token bibliography. This Smithsonian series doesn’t have citations
in general, and Nasih himself doesn’t consider them critical for a popular
book, but I’m a fan of them: I love the way books by people like Ellis and
Susan Casey (and, for that matter, me) give us many pages of things to look up
as curiosity dictates. Finally, the book just ends. There are two lines on the
future of the oceans at the end of the turtle chapter, and it just stops. Naish
had more material he could not incorporate, but even a short summary of this
broad topic we’ve just covered would make it feel more complete.
The marine reptiles, then, were a group of astonishing numbers, variations, and sizes. Naish has given us the best guide in print to these creatures and their world. An exciting aspect, threaded throughout the book, is that discoveries, theories, and analysis of these animals is progressing faster than ever before. Naish may have to revise this superb book in ten years or so.
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