• 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 to return to an aquatic life were mesosaurs:

• Early Permian of Gondwana
• Members of the primitive reptile group Parareptilia
• 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, although probably not too well
• Now known to have retained eggs (or young) in the body until they were developed enough to swim
• Probably did not travel far from shore

Most primitive relatives of Mesozoic marine sauropsids 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 sauropsids, from almost every clade:

• Testudines (marine turtles)
• Lepidosauria, both Rhynchocephalia (pleurosaurs) and Squamata (mosasauroids)
• Pseudosuchia (a small radiation of Middle Triassic marine poposauroids and two major types of marine crocodyliforms: the Jurassic-Early Cretaceous thallatosuchians and the Late Cretaceous-Paleogene dyrosaurs)
• Even Dinosauria (hesperornithiform birds)

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

A reminder of amniote phylogeny:
:

Euryapsida: more closely related to Archosauria than to Lepidosauria, so part of the larger clade of Archosauromorpha. Euryapsids:

• First appear in Early Triassic
• They MIGHT be basal members of the lineage that contains turtles, if turtles turn out to be archosauromorphs
• Most primitive members have webbed fingers and may have crawled around on shore
• 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, hupehsuchians, superficially turtle- or placodont-like saurosphargids, filter-feeding Atopodentatus, pachypleurosaurs, nothosaurs, and placodonts (the latter with big crushing teeth, probably mollusk eaters)
• Recent discovery shows that even
• Remains show that even primitive euryapsids retained young inside body until birth, thus allowing them to live their entire life in the water. This allowed for the extreme specializations of the two highly derived pelagic (open seas) euryapsid groups: ichthyosaurs and plesiosaurs

Ichthyopterygia (ichthyosaurs):

• First appear in Early Triassic; die out in the earliest part of Late Cretaceous
• Range from 0.4 to 15 m long
• Most of long snouts and cone-shaped teeth: fish or squid-eaters for the most part. However, some early forms had short snouts and no teeth, and may have been suction feeders
• Newly-discovered oldest and smallest form could still crawl on land
• Otherwise, fore- and hindlimbs turned into flippers (for steering)
• Carbonized impressions show big tail fin and dorsal fin in derived forms: 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

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!)
• A newly-described fossil shows that at least some plesiosaurs gave birth live to a single large-bodied baby (more like whales than the many small babies found in ichthyosaurs, mosasaurs, and sea snakes). This may in fact suggest extended parental care for at least the derived plesiosaurs.

Mosasauridae (mosasaurs):

• Late Cretaceous only; survived until very end of Cretaceous
• True lizards
  • Mosasauroidea includes the pelagic 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 (Chelonioidea):

• A couple of different closely-related families of marine turtles
• First groups are late Early 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.
  • A newly discovered form actually used suction feeding
• Shells are typically thinner than land-living turtles
• Crawl onto beach to lay eggs, but otherwise fully marine

Recent work shows that the pelagic ichthyosaurs and plesiosaurs (and possibly the mosasaurs) had an elevated metabolism, more like endotherms than ectotherms.

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Last modified: 6 July 2017