Ichthyosaurs and plesiosaurs were marine reptiles that lived during the Mesozoic era, between 250-90 million years ago. Ichthyosaurs had streamlined bodies and crescent-shaped fins adapted for speed, while plesiosaurs had long necks and flippers but lacked a hydrodynamic shape, indicating slower movement. Both groups breathed air and gave birth to live young rather than laying eggs. While ichthyosaurs resembled fish, plesiosaurs retained lizard-like traits in their small heads and broad bodies.

1. Ichthyosaurs
(Extinct)

2. Ichthyosaurs

3. Ichthyosaurs
• “Ichthyosaur” means “fish lizard”
• predatory marine reptiles that swam the
world’s oceans while dinosaurs walked
the land
• Appeared slightly earlier than dinosaurs
(250 million years ago)

4. Ichthyosaurs
• These animals rapidly diversified from
being lizards with fins to developing a
much more streamlined, fish-like form
built for speed

5. Ichthyosaurs
• Triassic period to middle Jurassic Period
• Streamlined shape
• four, strong, crescent-shaped fins
• Homocercal caudal fins
• They breathed air with lungs through
nostrils.

6. A specimen of the Jurassic icthyosaur Ichthyosaurus intermedius
More advanced ichthyosaurs — like the one shown above, on
display at the Senckenberg Museum in Frankfurt, Germany — had
compact, very fishlike bodies with crescent-shaped tails.

7. There was a
general
transition
from lizard-
shaped body
plan to fish-
shaped body
plan.

8. THREE STRANGE FEATURES
1. Limb skeletons that look like corncobs
2. Gigantic eye bones
3. Vertebrae resembling ashtrays or
hockey pucks

9. 1. Limb skeletons that look like corncobs
• The limb skeletons went through a
drastic modification as Ichthyosaur
a. The lower arm bones became shorter and
shorter
b. Finger bones also became shorter and
shorter, and eventually became disk-shaped
c. The number of finger bone increased early
in the evolution
d. Thumb disappeared at one point

11. 2. Gigantic eye bones
• Ichthyosaurs had
exceptionally well-
developed sclerotic
rings
• Sclerotic rings
maintain eye shape
during locomotion

12. 3. Vertebrae resembling ashtrays or
hockey pucks
• The only thing that is unusual about the
backbone of lizard-shaped ichthyosaurs
is that there are too many vertebrae
compared to average reptiles

14. Viviparity
• most ichthyosaurs are believed to have given birth to
live young, rather than laying eggs like contemporary
land-bound reptiles thus, they did not have to come
ashore to reproduce
The proof of this lies in the
remains of some
ichthyosaurs, such
as Temnodontosaurus, that
contain fossilized fetuses

16. Ichthyosaurs
• Despite their fish-like appearance, their
anatomy shows they were once land-lubbers
• two pairs of limbs, with digit-like bones rather
than rays or spines in their flippers
• a shoulder girdle connected to the skull. The
roof of the skull had a pair of openings called
fenestra: a hallmark of reptiles (diapsids).
• lack of gills

18. Plesiosaurs
(Extinct)

21. Plesiosaurs
• “near lizard”
• Small head like a head of a lizard
• broad body
• short tail
• Their limbs evolved into (four) limb
flippers
• 2m to 20m long
• Long neck
• Lacked streamlined, hydrodynamic
body shape

23. Plesiosaurs
• It is thought that they swam slowly
below the surface of the water, and
then used their long necks to position to
snap up fish or cephalopods.
• flippers gave them ultimate
maneuverability which helped them
rotate their body quickly to catch
escaping prey.

24. Ichthyosaurs Plesiosaurs
“fishlizard”  "near lizard"
crescent-shaped fins long necks
Pointed tails
Streamlined body Limbs evolved to
shape flippers
Fast swimmers Lacked streamlined
body shape
Slow swimmers

25. SIMILARITIES
• Marine reptiles
• Extinct
• Lack gills
• viviparous