We now switch to the sauropsid side of the amniote tree. Sauropsida is, for all intents and purposes, the same is Reptilia used in a phylogenetic sense.
Synapomorphies of Sauropsida mostly involve soft tissue. One example will suffice here:
The living diversity of Sauropsida breaks down into:
- Testudines (turtles) (Triassic - Recent) Strange in so many ways that we can't even start to describe
- Sauria (Permian - Recent) Including living:
- Lepidosauria (Permian - Recent) Lizards and Sphenodon, the New Zealand tuatara.
- Archosauria (Permian - Recent) Crocs and birds.
Sauria is characterized by Two pairs of temporal fenestrae:
- Infratemproal fenestrae (acquired independently from those of synapsids)
- Supratemporal fenestrae
We began our survey with an interesting diversion: Reptiles of the Sea.
Organizing the information
I. Reptiles have successfully invaded the oceans at least seven times (ten if you count birds). At least four of those invasions led to long-term radiations of marine saurians encompassing a bewildering range of ecolgies. For them not to seem confusing and random we need organizational schemes.
- Phylogeny: Position in the evolutionary Tree of Life.
- Biostratigraphy: Position in the Geologic Time Scale.
- Locomotor grade: Style of locomotion revealed by body form
- Feeding guild: Diet and method of acquiring food revealed by feeding structures.
"Marine reptiles" are not monophyletic. There have been several successful invasions of the shallow oceans and of the open oceans.
Some groups (E. G. squamates, crocodylians, and birds) have done it more than once.
Odd considering that amniotes had spent the last half of the Paleozoic evolving adaptations for life on land, foremost among them, the amniotic egg that would drown if laid in water.
Here there are interesting patterns:
- Modern marine reptiles consist of reef-dwelling sea snakes and the Galapagos marine iguana - more of a shoreline grazer. Apparantly the Cenozoic oceans with their advanced sharks and marine mammals are hostile to marine reptiles.
- Although there were fresh-water aquatic reptiles in the late Paleozoic, there were no marine reptiles. Apparantly the great Permo-Triassic extinction event opened the door to the invasion of the oceans, perhaps by extinguishing some key competition.
- Major turnover pulses seem to be linked to other extinction events:
- The Late Triassic event snuffs the shallow marine members of several lineages. For thalattosaurs, protorosaurians, and placodonts, that was the end. Only the open ocean ichthyosaurs and eosaurpoterygians survived. Afterward, rhynchocephalians and crocs joined the marine mix.
- The Cretaceous-Tertiary extinction snuffed mosasaurs, eosauropterygians, and hesperornithiform birds, all of whom had been doing nicely.
- A turnover occurs at the beginning of the Late Cretaceous, with ichthyosaurs disappearing and mosasaurs and hesperornithiform birds taking to the water.
A Rogue's Gallery of Marine Reptiles
Major groups taken roughly in their order of appearance in the fossil record:
The euryapsids encompass a great range of morphological diversity in three major groups:
- Ichthyosauria - eel - swordfish shaped marine predators.
- Eosauropterygia - plesiosaurs and their kin
- Placodontia - non-charismatic bottom-walkers that could crush anything
Eosauropterygia: (Middle Triassic - end of Cretaceous) The plesiosaurs of your childhood prehistoric animal books are best known examples. Marine predators.
- Limb-propelled swimmers
- Long necks
- Trend toward heads with longish snouts and long temporal regions.
Greatest range of morphological disparity in the Late Triassic.
- The "nothosaurs:" Paraphyletic.
- Shallow/near-shore marine.
- Sizes ranged from less than 30 cm. (as in Keichousaurus above) to crocodile-sized Paranothosaurus
- In many we see sexual dimorphism in which one gender has robust forelimbs and other has slender ones.
- The plesiosaurs were pelagic.
- Earliest known members of the plesiosaur lineage were nothosaur-like in general form, but by the Late Triassic, forms like Pistosaurus were invading the open oceans.
- Neck elongation achieved by increase in number of vertebrae.
- Seal-like flippers retain distinct elbow, wrist, and fingers (enclosed in mitten of flesh). Hard to see how it could have come onto the beach.
- Raises question of how eggs were laid. One way around this is ovovivipary - retaining eggs until hatching then "giving live birth". This is done by some squamates and we have fossils of small nothosaurs with large embryos.
- By the beginning of the Jurassic, forms like Plesiosaurus showed the major plesiosaurian derived features:
Thus, fully dependent on flippers for locomotion.
Plesiosaurs were successful and diverse for the rest of the Mesozoic. Although ancestrally long-necked, short-necked, large-headed forms (often called "pliosaurs") evolved at least twice (in Jurassic and Cretaceous repsectively). Ecologically these were top predators. indeed, the Jurassic group contained animals like Liopleurodon the size of a sperm whale.
Other plesiosaurs took the long-necked morph to extremes. Two possible functions:
- Allowed biting parts to approach prey closely.
- Facilitated feeding off of the bottom.
Placodontia: (Middle - Late Triassic) including Placodus pictured here was restricted to shallow marine environments of the middle and late Triassic. They probably swam clumsily or walked on the bottom in the manner of a snapping turtle. In contrast to their locomotor apparatus, their skulls were intensely modified for withstanding terrific occlusal forces.
As this palatal view shows, their teeth were transformed into a dental pavement with which they presumably crushed hard-bodied invertebrates. Some placodonts evolved extensive armor that superficially resembled that of turtles.
Ichthyosauria: (Early Triassic - beginning of Late Cretaceous)
Included many ecologically shark or dolphin-like pelagic predators. In fact, ichthyosaurs were the first marine reptiles to invade the open oceans.
Ichthyosaurs appear in the fossil record highly modified for marine life, with:
- limbs modified as flippers
- a distinct tail bend supporting a caudal fin
- greatly enlarged eyes
- no obvious means of coming ashore to lay eggs. In derived forms, ichthyosaurs clearly gave live birth.
The first ichthyosaurs were rather different from the familiar forms in being:
- Eel-like in shape.
- With sharp teeth in front of snout and blunt teeth in back - possible shellfish probers?
By the Late Triassic, there were whale-sized ichthyosaurs like Cymbospondylus. See also.
The early Jurassic was the peak of ichthyosaur diversity and the time in which they assumed their familiar form. Most preyed on small fish and cephalopods, but some were orca-sized macropredators.
Indeed, the eel-like near shore forms became extinct in the Late Triassic. Pelagic ichthyosaurs diminished through the Late Jurassic and Cretaceous, becoming extinct at the beginning of the Late Cretaceous.
Protorosauria: (Late Permian - Late Triassic)
Small to moderately large saurians close to the ancestry of archosaurs.
- The earliest, like Protorosaurus were fully terrestrial and ecologically similar to living varanid lizards,
- but during the Middle Triassic we pick up fresh-water aquatic or fully marine. The best known example is Tanystropheus, but others are being discovered.
- Along with their transition to marine life came an elongation of the neck by the evolution of a small number of very long vertebrae.
- The neck was also stiffened by long, slender cervical ribs. Thus, some reconstruction in the popular press are simply impossible.
- Indeed, the most derived forms are a real biomechanical puzzle.
Thalattosuchia: (Early Jurassic - Early Cretaceous) Sea-crocs!
We will take these up in detail in a later lecture. For now, be aware that during the Mesozoic and Cenozoic, crocs have invaded the oceans several times.
Chelonioidea: (Late Jurassic - Recent) Sea turtles
The first turtles are land or freshwater animals from the Triassic. Sea turtles were diverse and common by Late Cretaceous. Living sea turtles and their fossil relatives like Toxochelys, Protostega, and Archelon are monophyletic with respect to other turtles. Derived characters they share include
Living sea turtles primarily eat jellyfish and seaweed and so, presumably did ancient ones. Unlike other marine reptiles, these creatures sailed through the turnover of the mid-Cretaceous and the Cretaceous-Tertiary extinction event suffering little damage.
- Long fingers enclosed in a mitten of flesh to form paddles
- Reduction of the skeleton of the shell
Mosasauroidea: (Late Cretaceous) Giant sea lizards
Mosasaurs are scleroglossan lizards closely related to monitors, snakes, Gila monsters, and anguids. As squamates their skulls are highly kinetic.
This flexibility enabled mosasaurs to swallow very large prey items whole.
Indeed, some researchers posit a very close relationship with snakes, with whom they shared derived characters like.
Mosasaurs were Komodo dragon-sized to sperm whale-sized predators. This, coupled with their ability to engulf large prey made them the top marine predators of their age. Mosasaur stomach contents typically include other marine reptiles.
Probably incapable of emerging onto land. One Plotosaurus has been found with embryos inside.
Worldwide distributions, so probably at home in open oceans.
And yet as far as locomation is concerned, they were big eel-like ambush predators, like Late Triassic ichthyosaurs.
The notion that mosasaurs and snakes are sister-taxa is strengthened by a series of recent discoveries of fossil snakes of Late Cretaceous age with:
- hind limbs
- fossils in marine setting
Consider the distribution of ecological types through time.
Some odd patterns:
- A profusion of shell crunchers among the sudden profusion of marine reptiles in the Early to Middle Triassic
- A shortage of pursuit predators in the Cretaceous.
The first pattern might reflect the effects fo the Permo-Triassic extinction. Maybe so many lineages of marine reptiles became established in the Early Triassic because there was suddenly
- Little competition
- Stuff to eat that couldn't run away.
In the Cretaceous, reptiles mostly remained ambush predators. This may reflect the changing roster of fishy predators. Starting in the Late Jurassic, ichthyosaurs began to wane as modern sharks and large predatory bony fish proliferated. By the Late Cretaceous, modern lamniform sharks like Cretoxyrhina were common.