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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.

Tetrapoda includes

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...

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:

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:


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:

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
  • skin equipped with chromatophores,

    Some of the more familiar lizards include...



    Monitor Lizards (Varanidae)

    Chamaeleons ...and more than we have time to meet today.

    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:

    Boidae: Boas and Pythons
    • These is the most primitive snake family.
    • They have vestigial remnants of hind limbs and pelvis deep inside the body.
    • keratin spurs project from these remnants, but do not contain bone.

    • 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 family
    • Burmese pythons released in the Everglades are causing one of the worst environmental disasters in this ecosystem's history.

    Viperidae: The Vipers
    • venomous
    • deliver venom via hollow fangs that fold back (solenoglyphous fangs) when mouth is closed. Hollow fangs deliver venom as if by hypodermic needle.
    • 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 with pit organs
    • 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.

    Colubridae: The Most Diverse Snake Family

    There are 304 genera with almost 2000 species, making this the most diverse snake family.

    • Like all snakes, colubrids evolved from a venomous ancestor
    • The vast majority of modern colubrids are non-venomous
    • Some colubrids are mildly venomous...enough to weaken or kill small prey, but not enough to harm any but the most sensitive human
    • Venomous colubrids are often rear-fanged, and must open their mouths very wide to deliver a fang bite.

    • rear-fanged snakes must hang on and chew to deliver venom: the fangs are not hollow; venom often runs down the sides of the fangs and into the wound during the bite.
    • 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

    Elapidae: Cobras, Mambas, Coral Snakes, Kraits, Sea Snakes

    Sister group to the Colubridae, this family includes some of the most venomous snakes in the world.

    • 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