Natural History of Vertebrates
Chapter 11 - The Lepidosaurs:
These notes are provided to help direct your study from the textbook. They are not designed to explain all aspects of the material in great detail; they are a supplement to the discussion in class and the textbook. If you were to study only these notes, you would not learn enough to do well in the course. These notes are also linked with the notes from Vertebrate Structure and Development (ZO 515).
List of Terms
Lepidosauromorpha, second lineage of Diapsida (figures 14.1 and 14.2)
- short cervical vertebrae
- single-headed ribs
- articulating surface on the midline of the rib
- articulating surface of bone cap the long bones, producing a cartilaginous plate between the center and the end of the bone; the cartilaginous plate is called the epiphyseal disk. when the plate is ossified the bone stops growing, thus lepidosaurs have determinate growth (same as in mammals)
- transverse cloacal slit
several marine forms (figure 14-21)
placodonts (Triassic) had broad flat teeth used to crush molluscs, some looked very similar to turtles though the shell was formed in a different manner
plesiosaurs from the Triassic/Jurassic through the Cretaceous,
rowed through the water with flippers, 1 group increased the number of cervical vertebrae up 76 and had a very small skull
ichthyosaurs, Triassic through Cretaceous, though highpoint in the Jurassic,
resemble dolphins (another example of convergent evolution), had a dorsal and caudal fin, gave birth to live young
Spenodontia - tuatara, sister taxon to the Squamata
lizards - increased flexibility of the skull, loss of temporal bar and the quadratojugal (figure 11-1)
snakes - further increased flexibility by loss of the second temporal bar, postorbital to squamosal connection
amphisbaenian - small, legless, burrowing, skull is heavy with rigid joint, head is used as a ram to push through the soil (figure 11-2)
Mosaurs - marine lizards, from the Late Cretaceous, body resemble a crocodile with flippers, more than 10 meters in length,
elongate skull, numerous sharp pointed teeth
Late Cretaceous Extinction
was extremely pervasive
not only did dinosaurs die out, but so did many other lineages of diapsids (pterosaurs, pleisosaurs) as well as a number of mammalian lineages and marine forms of invertebrates.
world-wide about 65% of all species/genera died out at this time.
probably due to a meteor that formed the Chicxulub Crater off the Yucatan Coast at 64.4 ±0.5 mybp
the extinction could have also resulted from decrease in global temperature and decreasing sea level which led to an decreased rate of speciation while leaving the extinction rate the same, but this is not widely believed.
Sphenodontia - Tuatara
found only on small islands off the coast of New Zealand
adults about 60 cm
active at dusk and dawn (crepuscular) which is somewhat unusual when compared to snakes and lizards, perhaps because it is fairly cold
feed mainly on invertebrates (insects, crabs) and smaller vertebrates (frogs, lizards, birds) and these are active at dawn and dusk
modern sphenodonts have acrodont teeth which are fused tightly to the bone. teeth and jaws produce a shearing effect that can cut the head off a seabird.
live in burrows that they share with seabirds. seabirds provide food both directly and indirectly
Squamata - lizards, snakes, amphibaenians
lizards were first seen in the Early Triassic, most modern groups are seen by the Late Jurassic, snakes were first seen from the Late Cretaceous, and amphisbaenian were not seen until the Paleocene.
3 cm to 3 meters, largest ever was 5.5 m.
most lizards are under 20 g
also have determinate growth
most lizards are insectivorous, some are herbivorous by eating fruits
several large lizards eat larger prey, prey size is related to body size.
most lizards simply catch prey with their jaws, some have a sticky tongue that can be shot forward to catch prey (chameleons)
there are two basic life styles for a predator. this differences applies to many groups of vertebrates, not just lizards, however lizards present some very well defined examples of these two strategies.
sit and wait - sits where prey are likely to come by a grab them in a quick rush
active foraging - actively searches for prey and either stalks or corners the prey
In some cases, a given species may be placed somewhere between the two extreme strategies. Some lizards (monitor lizards) can switch between the two strategies depending on the available prey. They are sit and wait when prey are active and moving about, but switch to active foraging when the prey are inactive.
much less active, expend less energy but also take in less energy
tend to have high sprint speed, but not much stamina
higher anaerobic capacity
tend to be cryptically colored (well hidden)
tend to have stout, wide bodies
their predators are active foragers
an example are horned lizards (Phrynosoma) and fence lizards (Sceloporus)
much more active, expend more energy, but also take in more energy
tend to have a slower sprint speed, but have greatly increased stamina
tend to be more brightly colored with lines that run down the sides to confuse predators
higher aerobic capacity
tend to be long and thin
their predators are sit and wait predators
an example would be race runners (Cnemidophorus)
most lizards avoid predators by running away or hiding, most do not have any specialized weapons that would be useful for defense
they can confuse predators by automizing their tails. Basically, they self amputate their tail. By constricting muscles in the tail, they can break through the caudal vertebrae, sever muscles and ligaments connecting the vertebrae, constrict arteries and veins to stop blood loss. The tail then breaks and a piece falls off at the slights pull. Reflexes in the tail cause the lost piece to wriggle and attract the attention of the predator.
Tail regenerates, though the vertebrae do not. A cartilaginous rod develops to support the regenerated tail. A portion of the tail with the cartilaginous rod can not be automized a second time. A subsequent automy must be above the first break, thus there are a limited number of times that a lizards can use this strategy during a lifetime.
There is a cost to this. Must expend energy to grow a new tail. Also status is often dependent on having a nice tail, and the loss of the tail can result in loss of status and thus access to mates or territory.
because lizards are out during the day and active they are much easier for humans to observe and study than many other vertebrates. Many lizards are extremely visually oriented and vision is also something that humans are good at.
Many lizards use a variety of visual signaling devices to:
warn intruders that may enter their territory
signal to a potential mate.
Anole have been studied extensively in this regard. Anoles have a thin fleshy appendages below the chin called a dewlap. Many anoles have a large and brightly colored dewlap which can be displayed to either males or females.
If two males meet, a variety of posturing occurs such as, raising a dorsal or nuchal crest, head bobbing, push-up behavior, appearance of darkened eyespots.
If the intruder is a receptive female, then courtship begins which consists of more head bobbing, but no display of spots or crests.
other species of lizards, especially more secretive ones such as skinks, use pheromones which are secreted from the cloaca. The Jacobson's organ is used for the detection.
most lizards have some form of courtship, which serve as pre-mating isolating mechanism.
In males the copulatory organ is a hemipenes, which are paired on either side in the cloaca, though only one will be inserted in the cloaca of the female at one time.
Hemipenes often have numerous spines which seem to hold the hemipenis in the cloaca of the female during sperm transfer.
oviparity - eggs are released from the body and development is supported by the yolk.
ovoviparity - eggs re retained in the body of the female and development is supported by the yolk.
viviparity - eggs retained in the oviduct and development is supported by nutrient transfer from the mother to the fetus, may have a well-developed chorioallantoic placenta. Matretrophy -nutrients supplied by the mother.
Viviparity has evolved at least 45 times in lizards (35 times in snakes).
Viviparity is probably better suited for cold climates as the female can better control the temperature of incubation for the eggs (development of the young). The down side is that it reduces her to a single clutch and she is more likely to be preyed upon when pregnant. Thus viviparity is less common in the tropics, where temperature regulation is not a important.
Several species of lizards are parthenogenetic in that females can produce fertile eggs without males. Many parthenogenetic species arose from hybrids between two bisexual species. Parthenogenesis may take several forms, but in general, the lizards goes through meiosis and the polar body then reunites with the egg nucleus to produce a zygote, similar to self-fertilization in Mendel's garden peas.
Several parthenogenetic species must copulate with a male to initiate egg development, however the sperm are simply discarded. The parthenogenetic female tricks the male of a bisexual species into what has been termed a parasitic copulation. All of the extant parthenogenetic species are short-lived, as species, which illustrate the long-term value of sex.
Parental care is known for some species, which basically consist of guarding the nest from predators. There is essentially no interaction with the young after hatching.
range in size: smallest at 0.1 meters to largest at 10 meters
Snakes are legless with extremely elongate bodies. There are a number of morphological features found in snakes that go along with an elongate body.
some have only one carotid artery
kidneys are usually placed one on top of the other.
extremely kinetic skull that allows the swallowing of prey larger than the head.
Loss of legs is probably associated with a habitat of dense vegetation, such as a grassland or as an adapatation to exploit small openings in rocks.
Four types of locomotion
curvilinear (lateral undulation) - irregular curves that approximate a sine wave that press backward on the substrate pushing the smake forward.
rectilinear – lift body, move forward, lower, and push straight back. The snake moves in a straight line but it is pretty slow. Good mode of locomotion for stalking prey.
concertina – used in narrowe spaces, such as a burrow. anchors the posterior part o fthe body, pushes anterios part forward, then anchor the front of the body and pulls the posterior part along.
sidewiding – used by small snakes on unstable substrates, such as sand. Move by throwing loops of body forward and then riding forward where the loop touches the ground.
The skull is highly modified for swallowing large prey. If a snake is eating dangerous prey it will need to immobilize or kill it first to protect itself. Can do this either by constriction or poison.
Three categories of venomous snakes
Opistoglyph – enlarged teeth at the rear of the mxilla, some snakes have solid fangs, other species have a groove in the fangs. Holds prey in its mouth until it stops struggling. Usually eat lizards or birds. An example would be a boomslang.
Proteroglyph – hollow fangs on the front of the maxilla, other teeth on the maxilla behind the fangs. Fangs are short and fixed. Some smaller teeth in skull on palatines, ectopterygoid, and pterygoid. Examples would be cobras and sea snakes.
Solenoglyph – hollow fangs on the maxilla, only teeth on the maxilla which rotates to erect the fangs or fold to close the mouth. Large fangs. Other teeth on the pterygoid. Examples would be rattlesnakes or copperheads.
Poison ranges from neurotoxic (for example, cobras) which interfer with the central nervous system to hemotoxic which are proteins that begin the digestive process (for example rattlesnakes).
Last updated on 13 April 2003
Provide comments to Dwight Moore at email@example.com
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