Evolution of Marine Reptiles

Amphibia-Amniota: Basic anatomical and reproductive differences

Reptiles

Skin covered by scales.
Eggs are laid in ground, developing embryonary membranes and (soft) or calcareous (hard) shells. Some groups can develop embryo retention and .
Larval stages .

Key terms

Reptiles: all members of the amniote clade
that includes living turtles, lizards, crocs

: reptiles with two skull openings
behind orbit: the lizard-croc clade

Neodiapsid: diapsid clade that includes
+ Sauria

Lepidosaurs: tuatara and kin
(rhynchocephalians) and squamates

: snakes, lizards, amphisbaenians
( = ‘worm-lizards’)

Archosauromorphs: archosaurs and close
relatives (like protosaurs & )

Archosaurs: and kin, pterosaurs,
dinosaurs and kin (including birds)

Earliest reptiles have an skull
(= where there are no post-orbital openings)

The diapsid skull evolved from
an anapsid ancestor…

The diapsid skull (or a modified version
of it) is present in lizards, snakes, crocs, birds

Turtles have an anapsid skull and hence have
conventionally been placed outside of .

However, molecular data places them deep
within Diapsida. If correct, this means that they
evolved their .

Archosauromorphs: archosaurs and their close relatives

Non-archosaurian archosauromorphs include , trilophosaurs and rhynchosaurs. Mostly terrestrial animals; include small, lizard-like predators and omnivores, long-necked amphibious forms, specialised herbivores.

: lizards, snakes, tuatara and their relatives

Mostly small diapsids with flexible skulls: key events in evolution include appearance of different in skull. Post-Triassic group, many key divergences in

(Sphenodon): two species of chunky, lizard-like diapsids, endemic to New Zealand. Endangered, now restricted to offshore
Islands.
Enlarged teeth and marginal teeth all . Propaliny
present; halves of lower jaw
rotate about symphysis.
motion
used to break up prey.
Complete LTB long thought to be a ‘primitive’ feature;
contributed to the idea that Sphenodon is an archaic relict.

Squamates: over species
‘lizards’ to amphisbaenians
and snakes.

Reptile physiology and anatomy makes the transition to marine life relatively easy. However, to take to marine life a reptile still has to…
--- move efficiently in the water
--- feed effectively in the water
---
….. oh, and cope with , reproduction and heat loss too…

The evolution of locomotion in water, examples from the marine reptile fossil record
: Where propulsion is only generated during the ,
e.g., paddling, rowing (bears, paddling seabirds, swimming rodents, diving ducks)

Continuous locomotion: there’s , powered by body and/or tail. Axial locomotors practising continuous locomotion can be axial swimmers (using wave-like sweeps of tail) or axial swimmers (using swivelling of propulsive structure, like tail fin). Another category within continuous locomotion: (using lift- or drag-based limbs).

Trend seen frequently in evolutionary history of marine reptiles. Transition from axial swimmers to axial swimmers. Seen in ichthyosaurs, , mosasaurs.

Trend seen rarely in evolutionary history of marine reptiles. Transition from axial undulatory swimmers to paraxial swimmers. Seen in .

Salt and sea-water
Seawater contains about 35g of salt per liter. Marine mammals void unwanted salt via that produce concentrated . Reptiles don’t have such efficient kidneys: instead they use salt-excreting skull glands.

Marine iguana salt glands
are in the

Crocodile salt glands
are on the tongue

Sea snake salt glands
are

Sea turtle salt glands
are between the eyes

Seabird salt glands are set in bony
depressions above the eyes

Sauropterygians
Major clade of Mesozoic marine reptiles, initially mostly but worldwide by Jurassic.
, pachypleurosaurs, , pistosaurs and plesiosaurs.

Key characters: condition, retracted external nostrils, closed palate extending to braincase, absence of
several skull bones (lacrimal, tabular, postparietal, supratemporal), large retroarticular process, scapula superficial to
clavicle, small ilium etc.

Placodonts
Triassic sauropterygians from Europe, Middle East and China, famous for their
dentition. Evolved an elaborate, turtle-like armour composed of interlocking .

Small amphibious Triassic sauropterygians (to 2m) known
from 100s of specimens. Some suction-feeding features.
Embryos show that present already.

Viviparity evolved early on within Sauropterygia – before these animals evolved and pelagic habit

Nothosauroids
The best known nothosauroid: Nothosaurus from Middle and Upper Triassic Europe, Middle East and China. Several species ranging from 1.25 to 3.5 m long. Fantastic !

Pistosaurs: ancestors of plesiosaurs
Several Late Triassic taxa appear to be ‘’. Resemble plesiosaurs in having
nostrils, parietal crest, a simplified humerus shape and other characters.
Enlarged, wing-shaped hands suggest (presumably underwater ‘flight’).

Examples of non-plesiosaurian sauropterygians

Plesiosaurs
Carnivorous Mesozoic marine reptiles
Approx. years ago
Plesiosaurs are derived sauropterygians
Sauropterygia – including placodonts, pachypleurosaurs, nothosaurs, pistosaurs

Quick history of plesiosaurs

Plesiosaurs evolved during the
Late from .

Several lineages evolved giant
size (10m+, 5tons+) during
Jurassic & Cretaceous.

‘Pliosauromorphs’ ( heads,
necks) evolved several
times from ‘plesiosauromorphs’
( heads, necks). Most
‘pliosauromorphs’ belong within
Pliosauroidea.

Plesiosauroids include very long-
necked elasmosaurids as well
as long-snouted polycotylids.

Plesiosaurs mostly pelagic ocean-
going animals, but there were
lineages
within Leptocleidia.

Mass extinction end of Cretaceous.

The skull and dentition
Modified diapsid (‘euryapsid’) condition:
Position of the intenal and external nares
Single large temporal fenestra

Polycotylid debate
Traditional Classifications treat them as
But apart from the , most of the characters are typically plesiosauroid.
So, are unreliable
It now makes more sense to refer to plesiosaurs as either pliosauromorph or plesiosauromorph.

Mesozoic plesiosaur ‘pregnant female’ with a

Ichthyosaurs
Carnivorous Mesozoic marine reptiles
Approx. years ago (before )
Exact origin of ichthyosaurs unknown
Teeth set in (not sockets)
Ichthyopterygia – increasingly through time
Huge

The big question about all the Mesozoic marine reptile lineages we’ve looked at
(thalattosaurs, hupehsuchians, sauropterygians, also ichthyosaurs) is: where do their affinities lie within Reptilia?

Difficult to resolve because all have a strongly modified morphology: in the skull, diapsid condition has been modified
to (where there’s a supratemporal fenestra, but no obvious
laterotemporal fenestra).

Seems that all of these lineages are within
. Sauropterygians have
sometimes been argued to be close to
lepidosaurs; other studies suggest that all
of these lineages are archosauromorphs.

Cretaceous lizards invade the seas
During the ‘mid’ Cretaceous (from c ), several lineages of anguimorph lizards (related to ) took to the seas as mid-sized (c1m), long-bodied, shallow-water foragers on reefs etc.
From shallow marine sediments of Europe, Middle East, N America. Dolichosaurs and aigialosaurs.
was present,
even in these mid-sized
amphibious lineages.
Carsosaurus with 4
embryos in body cavity.

Mosasaurs
Large to gigantic aquatic anguimorphs (mostly marine).
Flexible zone in , evidence for feeding. Evolutionary trends: longer snout, retracted
Nostrils, longer and wider paddle-like limbs, more
paddle-like tail, with some evolving tail.