Not all reptiles of the Mesozoic were dinosaurs.

Many reptiles (and other amniotes) have returned to the sea:

• Abundant food
• Equitable temperatures
• Easy migrations

However, aquatic amniotes have to deal with:

• Breathing (remain air-breathers)
• Feeding (small fish, large fish & amniotes, shellfish, vegetation, etc.)
• Locomotion (flippers, fins, etc.)
• Reproduction (come out of water to lay eggs or some form of internalized reproduction)

First reptiles (or maybe non-reptilian sauropsids, the sister group to Reptilia) were mesosaurs:

• Early Permian of Gondwana
• Long needle-like teeth for catching small fish
• Big (webbed?) hands and feet for paddling, tall deep tail for swimming
• Could probably crawl on land, and probably laid eggs on land
• Probably did not travel far from shore

Most primitive relatives of Mesozoic marine reptiles were similar in general form (long needle-like teeth, webbed hands and feet, deep tail, some terrestrial ability, probably shore-dwelling or fresh-water) to mesosaurs, but later forms become more specialized for life in the sea.

Many different clades of Mesozoic marine reptiles, from almost every clade:

• Testudines (marine turtles)
• Lepidosauria, both Rhynchocephalia (pleurosaurs) and Squamata (mosasauroids)
• Pseudosuchia (various marine crocs)
• Even Dinosauria (hesperornithiform birds)

We will cover the most diverse and highly specialized forms: euryapsids (esp. ichthyosaurs and plesiosaurs), mosasaurs, and marine turtles.

Euryapsida: the sister group to archosaurs and their relatives (Archosauriformes). Euryapsida + Archosauriformes together are called Archosauromorpha.

• First appear in Early Triassic
• Most primitive members have webbed fingers and could likely crawl around on land
• Early euryapsids (and most later ones) seem to have been fish eaters, with needle- or cone-shaped teeth.
• A variety of smallish Triassic forms: thallatosaurs, huphesuchians, pachypleurosaurs, nothosaurs, and placodonts (the latter with big crushing teeth, probably mollusk eaters)

Ichthyopterygia (ichthyosaurs):

• First appear in Early Triassic; die out in the earliest part of Late Cretaceous
• Range from 1.5 to 15 m long
• Most of long snouts and cone-shaped teeth: fish or squid eaters
• Fore- and hindlimbs turned into flippers (for steering)
• Carbonized impressions show big tail fin and dorsal fin: very dolphin- or tuna-like
• VERY large eyes in some species: probably dove to deep water hunting squid
• HIGHLY transformed anatomy: probably very fast swimmers but incapable of surviving on land
• Remains show that they retained young inside body until birth

Plesiosauria (plesiosaurs):

• First appear in latest Triassic; die out at the end of the Cretaceous
• Fore- and hindlimbs turned into large flippers for swimming; tail generally short
• Two general body types:
  • Small head, needle-like teeth, long neck: small fish or squid eaters
  • Large head, big cone-shaped teeth, shorter neck: large fish or marine reptile eater
  • The large-headed forms appear to evolve a number of times from the long-necks
• Include the largest known marine reptiles of all (bigger than all theropods, and in fact bigger than all dinosaurs other than sauropods!)
• Might have been able to crawl up onto land to lay eggs, like sea turtles

Mosasauridae (mosasaurs):

• Late Cretaceous only; survived until very end of Cretaceous
• True lizards
  • Mosasauroidea includes Mosasauridae and some non-oceanic semi-aquatic forms
  • Close relatives of monitor lizards and sister group to snakes
• Forelimbs powerful webbed flippers; hindlimbs reduced
• Tail powerful and deep
• Most mosasaurs with large cone-shaped teeth: fish, squid, ammonoid, and marine reptile eaters; a few have crushing teeth for feeding on shellfish
• Remains show that they retained young inside the body until hatching

Marine turtles:

• A couple of different closely-related families of marine turtles
• First groups are Late Cretaceous; some died out in Early Cenozoic, others survive to today
• Fore- and hindlimbs turned to flippers, forelimbs provide must of the thrust
• Feed on shellfish, fish, jellyfish, vegetation, etc.
• Shells are typically thinner than land-living turtles
• Crawl onto beach to lay eggs, but otherwise fully marine

Just as several different groups of reptiles went back to the sea, some others took to the air.

A number of Permian & Triassic gliding reptiles, but only two powered flying groups:

• Avialae (birds, a type of theropod dinosaur)
• Pterosauria (pterosaurs, possibly the sister group to Dinosauriformes within Ornithodira, but may be more primitive archosauriforms)

Pterosaurs:

• First appear in Late Triassic; died out at very end of Cretaceous
• Had long hindlimbs and S-shaped neck.
• Flew using extended manual digit IV (ring finger)
• Skin stretched between finger and body, between legs, and from arm to neck
• Wings reinforced by special internal fibres
• Body covered with hair-like structures
• Some think they walked bipedally, but most evidence suggests quadrupedal (almost gorilla-like)
• Active powered fliers with large muscle attachments
• Possibly warm-blooded
• Ranged from 15 cm to 12-14 m wingspan!
• Earlier forms relatively small, short metacarpi, and long tails; later forms ( Pterodactyloidea, the true pterodactyls) could be very large, had long metacarpi, and short tails
• Variety of diets: insectivores, fish eaters, fruit eaters, etc.

No evidence of direct competition between birds and pterosaurs.

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