Sarcopterygii - Lobe-Finned Fishes and Tetrapods
The lobe-finned fishes include such "living fossils" as
These are the closest living relatives of the four-legged vertebrates, tetrapods.
Tetrapods are the four-legged, primarily terrestrial vertebrates, derived from a specialized group of
shallow-water fishes that probably resembled living (extant) lobe-finned fishes.
- Amphibia (salamanders, frogs, caecilians)
- Synapsida (mammals)
- Anapsida (turtles)
- Diapsida (reptiles, including birds)
Let's meet the non-avian Diapsids: The Reptiles
Recall that the study of birds is known as Ornithology.
The study of reptiles (and amphibians) is known as Herpetology from the Greek herpeton, meaning "crawling".
Typical reptile characteristics include...
- keratinized epidermal scales
- in some species, bony dermal plates (scutes)
- undifferentiated, conical teeth
- powerful jaws made for crushing and tearing
- paired front and back limbs, usually with five toes each
- legs directed away from the body, not underneath and supporting it
- respiration by means of lungs
- entirely terrestrial
- amniotic egg
- ectothermic poikilotherms (most species)
- nitrogenous waste excreted primarily as uric acid
- fecal and nitrogenous waste and reproductive products exit body via a cloaca
- internal fertilization
- many adaptations for conserving water
- complex brain and nervous system
- cone-dominated retina with excellent color vision
Crocodilia - First Cousins of Birds
Crocodilians include the crocodiles, alligators, caimans and the gharial.
There are 23 extant species of crocodilians.
Evolutionary relationships are still not fully clear. Most data suggest that they are most closely related to birds. However, a molecular study published in 1999 suggested that crocodilians may be more closely related to turtles than birds.
The jury is still out.
There are three Crocodilian families:
- Gavialidae (one species, Gavialis gangeticus, the Indian Gharial)
- Crocodilidae (14 species of true crocodiles)
- Alligatoridae (8 species in 4 genera of alligators and caimans)
- Alligator mississippiensis and Alligator sinensis
- The rest are caimans.
- Here's a comparison of general appearance of the three families:
- are top carnivores in the ecosystems in which they live (Don't try this at home, kids.)
- can be very long-lived. The record age was 115 for a captive crocodile.
- are often keystone species. In the Everglades, alligators dig water hole refuges that many other species rely upon for survival in the dry season.
- Crocodylus porosus (Saltwater Crocodile) is the largest extant reptile, and the largest terrestrial predator.
- But even the "saltie" is dwarfed by the recently discovered, (fortunately extinct, "super croc", Crocodylus thorbjarnarsoni.
- are riparian (meaning their habitat is along the banks of a river), though some crocodiles may swim relatively far into the ocean.
- Alligators are strictly fresh water species.
- are powerful swimmers, moving with a side-to-side motion and propelled by the huge, muscular tail.
- are excellent, fast runners, though with poor stamina. They are lunging sprinters that rely on surprise to capture their prey.
- Sex is determined, not only by sex chromosomes, but by incubation temperature of eggs.
- have excellent vision in both day and night.
- have very acute hearing. Mama hears her young calling inside the eggs, and this triggers her to gently mouth the eggs to help them hatch.
- mothers are attentive to their young, guarding them until they are 1-2 feet long. Babies will utter distress calls if in danger, and LOOK OUT. Here comes mama!
- have integumentary sense organs (ISO) that may sense changes in pressure, water salinity, and prey movements under water.
- are quite vocal. Bulls communicate during mating season with loud, deep bellows to mark territory.
- fight during mating season, though usually not to the death.
- have been heavily exploited for meat and skin by humans. Some species have come to the brink of extinction.
- The American Crocodile is among the most endangered, though it is a rather docile animal.
Chelonia - Turtles and their relatives (Anapsida)
Members of Order Chelonia are of uncertain evolutionary affinity. It is possible that these shelled reptiles comprise a polyphyletic group (i.e., they have more than one ancestral origin).
How did such an odd body form evolve? Here is one hypothetical scenario:
- external bony shell
consisting of a dorsal carapace
and ventral plastron
- The shell is part of the skeleton, and--in a living turtle--is covered with skin.
- In many species, you can distinguish male from female turtles by the shape of the plastron: males' are indented. (Why do you suppose this is?)
- Turtles lack teeth, but have sharp cutting edges to the maxilla (upper
jaw) and mandible (lower jaw).
- Some marine turtles have scary mouth adaptations to help them keep hold of slippery prey.
- Turtles may be marine, fresh water, or terrestrial.
- Among the most beloved are the sea turtles, palladins of the turtle world.
- Some are carnivorous, others herbivorous, some omnivorous
- Here's a
gallery of a few of the many turtle species.
The only extant species of Tuatara (Sphenodon punctatus) lives in New Zealand, and nowhere else. Although it resembles a typical lizard, it belongs to a distinct and very ancient lineage, Rhynchocephalia.
(The basic "lizard shape" was very common among early amniotes. But not all of those early amniotes gave rise to what we now know as "lizards". The earliest known fossil reptile, Hylonomus, was shaped like a lizard. Therefore, lizard shape can be considered a basal amniote characteristic.)
Rhynchocephalia can serve as an outgroup for the Squamata, the order containing lizards and snakes.
The word tuatara means "peaks on the back" in the Maori language. Let's let the Kiwis tell you about tuataras themselves.
Primitive characteristics of Tuataras:
- skull has unmodified diapsid form (unlike other modern diapsids)
- teeth are projections of jaw bone, and are not replaced (as in other reptiles)
- single row of bottom teeth fits precisely between the double row of upper teeth
- well-developed parietal eye, a derived version of which is found in many other amniotes.
- very primitive ears (no external ears)
- abdominal ribs (gastralia)
- vertebrae are hourglass shaped, similar to vertebrae of fish and amphibians
Squamata: Lizards and Snakes
The name Squamata comes from the Latin squama, meaning "scale". These are the original scaley-skinned animals, though all mammals and birds also have skin components homologous to reptilian scales.
With over 9000 species, this is the most diverse order of reptiles.
Our understanding of squamate evolution is still in a state of flux, but here is one recent phylogeny.
(For the intrepid: click on the image of the phylogenetic tree for a complete overview of recent research in this area.)
Who are these Squamates?
To warm up, let's watch some in action.
Lacertilia: The Lizards
Lizards are among the most diverse reptiles, with more than 5000 species. Various species may be
- terrestrial, aboveground dwellers
- burrowing (fossorial)
- arboreal (living in trees)
- skin equipped with chromatophores,
Some of the more familiar lizards include...
- mostly nocturnal
- lack movable eyelids
- clear scale covers and protects the eye
- amazing, adhesive toe pads allow them to climb almost anything, whether flat, upside down, or vertical.
Monitor Lizards (Varanidae)
- New World lizards
- often brightly colored and ornamented
- very diverse in form, color, ecology, behavior, diet
- Very active, said to be the most intelligent lizards
- may be predatory or frugivorous
- includes the world's largest lizard, the Komodo Dragon
...and more than we have time to meet today.
- about 160 species
- famous for their ability to change color in response to environmental situations
- some are extremely colorful
- zygodactylous feet: two toes forward, two toes back
- long, sticky tongue is rapidly extruded to capture prey
- eyes can rotated independently of each other, and provide almost spherical field of view.
- popular as pets, but very difficult to keep healthy and happy
- distinctive appearance, movement, and behavior
Serpentes: The Snakes
Snakes are highly derived lizards that lack movable eyelids and external ears. (Which is how you can distinguish them from legless lizards.)
- many species have jaws that can disarticulate to allow the snake to swallow prey much larger than its head
- found on every continent except Antarctica
- all are predatory; they are ecologically important carnivores
- many snakes are well known for killing prey with venom, which has different components and mechanisms of action, depending on the snake species and its evolutionary history.
- recent research suggests all snakes are derived from a common venomous ancestor
- non-venomous snakes have vestiges of venom glands and sometimes mildly toxic venom
- evolutionary relationships within the snakes are still in flux. This cladogram is handy:
Boas and Pythons
- large, muscular snakes kill prey by constriction
- non-venomous, though descended from a venomous ancestor
- can sense the heat (long infrared radiation) emitted by prey via heat-sensitive pit organs between or underneath the scales around the edge of the mouth
- heat stimuli are sent to the visual cortex of the brain by the pit organs' nerves, so the snake processes this information as vision: they "see" the heat signature of their prey.
- Boas, pythons, anacondas, etc. are all in this group
- Burmese pythons released in the Everglades are causing one of the worst environmental disasters in this ecosystem's history.
Colubrids: Your Every-day, Non-venomous Snake
- deliver venom via hollow fangs that fold back (solenoglyphous fangs) when mouth is closed
- venoms have various mechanisms of action in different species, but many are cytotoxic, causing cell destruction and tissue necrosis/death.
- Most species are heavy-bodied and not particularly fast moving (except when striking)
- may not always deliver venom when biting, as in defense biting
- Like pythons and boas, some vipers (New World pit vipers) can sense infrared
- their pit organs are paired, and located on either side of the snout
- Infrared stimuli are processed as visual signals, as in pythons and boas.
There are 304 genera with almost 2000 species, making this the most diverse snake family.
- The vast majority of colubrids are non-venomous
- Like all snakes, they evolved from a venomous ancestor
- Some colubrids have "mild" venom...enough to weaken or kill small prey, but not enough to harm any but the most sensitive human
- a few species (e.g., Boomslang) are venomous enough to kill a human
- Rule of Thumb: If you're not sure, then don't approach or handle it!
- Here's a Gallery of Handsome Colubrid Snakes
Elapids: Cobras, Mambas, Coral Snakes, Kraits, Sea Snakes
Some of the most venomous snakes in the world belong to this family, the closest sister group to which is the harmless Colubridae (most of the other common snakes you'll ever see).
- deliver venom via hollow, fixed fangs that do not fold into the mouth (proteroglyphous fangs)
- venoms are primarily neurotoxic: they act on the nervous system to cause paralysis in prey
- long, slender, fast-moving snakes with excellent vision
- resemble the colubrids, to which they are closely related
Britton, Adam. 2002. Crocodilian Biology Database. . 3/27/02