Lecture Notes for: Amniote Origins and the Diversification of Reptiles
Classification of "Reptiles"
Class Amphibia
Class Reptilia
Subclass Anapsida
Order Testudines (turtles)
Subclass Diapsida
Infraclass Lepidosauromorpha
Superorder Lepidosauria
Order Squamata (lizards and snakes)
Order Sphenodonta (the tuatara)
Superorder Sauropterygia
Order Plesiosauria Infraclass Archosauromorpha
Superorder Archosauria
Order Pterosauria (pterodactyls)
Order Crocodilia (crocodiles)
Order Saurischia (dinosaurs, in part)
Order Ornithischia (dinosaurs, in part)
Infraclass Ichthyopterygia (ichthyosaurs)
Subclass Synapsida (mammal-like reptiles)
Class Aves
Class Mammalia
Amphibian Deficiencies:
Amphibians were 1st on land, but not truly terrestrial since
1. Species still returned to water to reproduce
- external fertilization - eggs require moisture
2. Restricted to moist habitats b/c soft, breathable skin is subject to dessication
Reptilian Innovations:
4 major advances made for an improved terrestrial vertebrate
1. Amniotic egg with a porous shell + 3 membranes = terrestrial reproduction
a. amnion = fluid filled cavity in which embryo floats
b. allantois = respiratory surface/gas x-change + storage chamber for nitrogenous wastes
c. chorion = a protective membrane, oxygen and carbon dioxide freely pass
2. Scaly, keratinized skin + improved kidneys (high densities of renal tubules) = less water loss from body.
3. Lateral compression of skull + larger, longer jaw muscles = greater gripping/crushing force.
4. Leg bones elongated + strengthened wrist and ankle bones = better body support with greater mobility, speed, and even upright posture.
Reptiles: The first amniotes
1. Reptiles were the 1st true land animals
2. There are currently 7000 species of living reptiles
3. Occupy nearly all terrestrial and aquatic habitats (except antarctica).
4. Represent some of the smallest and largest animals that have ever roamed the planet.
"Reptilia" is a paraphyletic clade
1. Birds have traditionally not been included in this group despite being obvious decendents of a reptile ancestor. This is b/c evolutionary taxonomists have argued that birds have so many novel adaptations that they deserve their own clade. In a strict cladistic sense, however, reptiles = anapsid + diapsid animals (including all birds).
3 major reptilian groups divided by skull type:
Anapsida, Diapsida, Synapsida.
Openings in the temporal region of the skull seem to be a reliable indicator of evolutionary lineages. The reasons for initial evolution of skull openings is unclear, but there may be 3 factors:
1) differential concentration of mechanical stress in the skull; temporal openings reduce the # of suture lines in the skull & thus reduce the possibility of skull fracture,
2) provide areas for concentrated muscle attachment; muscles are more strongly attached to ridges and edges than to smooth surfaces,
3) lighter skulls.
Note: The euryapsid skull type is thought to be derived from the diapsid skull type via loss of the lower temporal opening. The two marine reptile groups that have the euryapsid skull type (ichthyosaurs and plesiosaurs) are only distantly related to each other, and thus, are though to have convergently evolved this condition.
Anapsid Reptiles: Order Testudines
Origin: Turtles 1st appear in the fossil record in the late Triasic period (approx. 200 mybp) and have changed very little in appearance since then.
Morphology: Turtles are enclosed in shells consisting of a carapace (made from fusion of bony plates to the thoracic vertebrae and ribs, and a plastron (made from fusion of dermal plates with several ventral bones), and can retract into their shells for protection. Fig 3.28 (Kardong)
Respiration: The turtle shell creates some problems for breathing because the turtle cannot expand its chest to draw air into the lungs. Instead, turtles must employ certain abdominal and pectoral muscles as a diaphram.
Feeding: Turtles lack teeth but use a jaw equipped with tough, horny plates for gripping food.
Sensory: Most turtles are mute, but compensate for this by having keen sense of smell and excellent color vision.
Reproduction: Turtles are oviparous (eggs released by female w/ development outside the maternal body) but have internal fertlization. All turtles, even marine forms, bury their shelled, amniotic eggs into the ground. Interestingly, the sex of turtle hatchling is determined by temperature of the nest. Low nest temp = males, high nest temp = females.
Diapsid Reptiles: Superorder Lepidosauria (snakes, lizards)
Ecology: Comprise 95% of all living reptiles (nearly 6000 species) inhabit a huge range of terrestrial & aquatic habitats. Big size range, cm à several meters (Komodo dragon of Indonesia)
Origins: Lizards were the earliest advanced reptile groups to appear in the fossil record (late Permian 225 mybp). Snakes do not appear until the late Jurassic.
Morphology: Both groups retain the primitive lateral undulation of vertebral column. Lizards have movable eyelids, snakes eyes are permanently covered with a transparent cap. Lizards have external ears, snakes do not. Snakes are entirely limbless lacking both a pelvic and pectoral girdle.
Sensory:
Primary mode of sensing in lizards is vision. Lizards have retinas rich in both cones and rods = keen daylight vision. Lizards also have an external ear but hearing is underdeveloped and not used as a primary mode of sense.
In constrast, snakes have no ear and are totally deaf, but they can sense low frequency vibrations conducted through the ground. Most snakes employ chemical senses instead. In addition to the usual olfactory areas in the nose (which are not well developed), snakes have a pair of pit-like Jacobson's organs in the roof of the mouth. The tongue is flicked out into the air to sample molecules in the air, and then delivers these molecules to the richly inervated Jacobson's organs to determine scents. Pit vipers (which include the majority of venomous snakes) have very specialized heat sensitive pit organs which can detect temperature differences as small as 0.003° C. They sense these temperature differences and use them to aim strikes at warm blooded prey.
Feeding: A very important innovation of this group is the kinetic skull with movable joints. The specialized mobility of the skull
1) enables lizards and snakes to better seize and manipulate prey,
2) substantially increases the closing force of the jaw, and
3) allows them to swallow prey several times their own diameter.
Skull mobility is considered a major factor influencing the diversification of snakes and lizards.
Reproduction: Most lizards and snakes are oviparous (eggs released by female & develop outside maternal body). Some are oviviparous (eggs develop inside the female but no nourishment from female) and some are viviparous (eggs develop within female w/ additional nutritional aid, offspring born as juveniles).
Diapsid Reptiles: Superorder Archosauria (crocodiles, dinosaurs)
Several improvements to the basic reptile body plan
1) lighter, narrower skull
2) socketed teeth à easier to replace w/ less trauma to tissues
3) tendency towards bipedal locomotion
- reduced forelimbs & pelvic girdle,
- hindlimbs under body supported by strengthened pelvic girdle.
- no lateral flexure of the vertebral column.
Diapsid Reptiles: Superorder Archosauria
Order Crocodilia
Origin: Arose sometime in the Paleozoic era (approx 225 mybp) and are the sole survivors of the archosaurian lineage that gave rise to the great Mesozoic radiation of dinosaurs and their kin
Morphology: Have changed very little structural since their beginnings.
Respiration: Well developed lungs + a second palate that allows crocodiles to breath when mouth is filled with water or food.
Reproduction: Alligators and crocodiles are oviparous (eggs released by female and then develop outside the body). As with turtles and some lizards, incubation temperature of the nest often determines offspring sex; low temp = females, high temp = males.
Orders Saurichia & Ornithischia (the dinosaurs)
Most famous of the archosaurs
Several additional innovations made these reptiles extremely successful:
1. erect, digitgrade foot posture = faster, more efficient movement
2. lighter, stronger pelvic girdle = larger, stronger leg muscles
3. strenthened ankles = withstand strain of high speed locomotion
4. rigid tail = counterbalance during high speed maneuvering
Saurichia had two independent lines of evolution
1. Theropods were most carnivorous. ranged between 3-40 ft. Included some of the best known dinosaurs Tyrannosaurus and Allosaurus
2. Sauropodomorphs were mostly herbivorous (averaged around 80 ft). Secondarily evolved quadrapedalism. Including taxa like Brontosaurus and Brachiosaurus
Ornithischia
were exclusively beaked, herbivorous inlcuding stegosaurs, ceratopsians (triceratops), etc.
The End of the "Age of Reptiles"
The C-T extinction event:
38% of all known plant & animal genera die
70% of all marine plant & animal species gone
Land-dwelling reptiles were hit particularly hard!
19 families of dinosaurs vanish
3 families of crocodiles disappear
Several possible explanations
1. supernova explosion: irradiation of gonads à massive deleterious deletions. But many other groups of animals did not suffer massive extinctions during this period.
2. fatal constipation: radiation of hard to digest angiosperms (in the Cenozoic) led to few poor food sources for herbivorous dinosaurs, carnivores eventually starved. But angiosperms were around for approx. 30my before dinosaurs went extinct.
3. mammals ate dinosaur eggs. But mammals were around for 100my before dinosaur extinction.
4. prolonged global cooling:
- dinosaurs evolved and diversified during a period with a relatively stable climate
- Late Cretaceous marked the beginning of 5 my years of cooling
- most dinosaurs extinct 1-2 my before end of Cretaceous
5. meteor impact:
- large amounts of irridium at K-T boundary indicate extra-terrestrial impact
- could have caused massive tidal waves, fires, and atmospheric dust that blocked the sun leading to global cooling.
- 2 known sites of meteor impact date to approximately same period.
Are Dinosaurs really extinct?
Technically, NO! Many scientists believe that birds are dinosauria