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VERTEBRATES: Evolution and Diversity

The earliest ancestors of vertebrates were plate-skinned fishes known as OSTRACODERMS. (from the Greek ostraco meaning "shell" and derm meaning "skin"). This was a diverse array of armored fishes that lacked a lower jaw (mandible) or paired fins (remember the lancelets! Do they have paired fins? What is the advantage of paired appendages?).

One group of these fish (OSTEOSTRACANS) evolved paired pectoral fins just behind the head. A separate group (ANASPIDS) closely resembled modern-day lampreys.

Vertebrates, as we already have discussed, are either

Only the latter group is monophyletic, as jawed vertebrates all show similar embryogeny with respect to jaw formation. (The jaw bones develop from portions of the cartilaginous arches that once formed the support of the anterior gill slits.) The jawless vertebrates, however, showing the primitive condition, may have had several different ancestors.

The Devonian period saw the rise of the first jawed fishes, the PLACODERMS, all of which are now extinct. Another group, the ACANTHODIANS which lived at the same time as the placoderms, may have included the ancestors of all modern bony fishes.

Let's meet the Vertebrates, Class by Class

SUPERCLASS AGNATHA - The Jawless Fishes

SUPERCLASS GNATHOSTOMATA - The Jawed Vertebrates

  • Actinopterygii - The Ray-finned Fishes Fun Fish Facts

  • Skeleton composed primarily of bone
  • ectothermal poikilotherms
  • Tail is primitively homocercal (see what happens if you don't come to class?)
  • Skin covered with dermal scales that shed readily.
  • Skin copiously supplied with mucous glands
  • Fins may be either medial and unpaired, or lateral and paired
  • Primitively, fins have rays made of cartilage or bone
  • Toothy, terminal (i.e., at the end of the body) mouth
  • Gill arches covered by a bony operculum
  • Swim bladder, derived from the digestive tract, may or may not be open to the pharynx
  • Heart is two chambered (one ventricle (caudal) and one atrium (cranial)
  • Brain differentiated, with small olfactory lobes and cerebrum; large optic lobes and cerebellum (what does this tell you about what fish do best, and what they might not be quite as good at?)
  • Most species dioecious, a few hermaphroditic; some change sex during the lifespan, depending on environmental circumstances

    The Amazing Swim Bladder

  • Many pelagic fish (but not all) have this gas-filled sac found dorsal to the digestive tract
  • In some species, it is connected by the pneumatic duct to the esophagus or pharynx. (These fish gulp or burp air to control the amount of gas in the swim bladder.)
  • In other species, the swim bladder is not connected to the outside world. This can be a problem! What happens to the bladder when the fish goes deeper (HINT: higher pressure)? What happens when the fish goes more shallow?

    II. Tetrapods

  • These are the four-legged vertebrates, derived from a specialized group of shallow-water fishes.

  • A monophyletic lineage of tetrapods must include these ancestral, extinct relatives.

    Tetrapoda: Class Amphibia

    The amphibians made first landfall during the Devonian (400 mya). The first tetrapod had come ashore, with those defining characteristics: There are three main Orders: Modern Caudates (salamanders and newts) most closely resemble the ancestral amphibians, in that they retain a tail. But all three extant orders of amphbians are highly derived in their own ways, with each species showing amazing specializations in both morphology and behavior.

    Fun Amphibian Facts

  • ectothermal poikilotherms
  • Primitively, larvae are aquatic, with gills
  • Skin is scaleless, and serves a major respiratory function: must remain moist
  • Skin is well supplied with glands, including poison glands. Some species far more poisonous than others.
  • large mouth with very small teeth
  • Mostly insectivorous/carnivorous (ambush predators)
  • Heart is three-chambered: One ventricle, and two atria. Oxygenated blood enters via one atrium, and unoxygenated through the other: both are mixed in the ventricle, which then pumps the blood to the rest of the body. This means that the oxygenation system isn't perfect!
  • Separate sexes

    Tetrapoda: Superclass Amniota

    The Amniota includes
  • The main homology linking the amniotes together is the Amazing Amniotic Egg

  • All amniotes share this character, though it is highly derived in some groups (e.g., mammals).
  • There is some argument as to the proper classification of amniotes, but they are generally divided into three main groups on the basis of skull morphology: Today's living remnants of these three lineages:

    Of greatest note here is the reorganization of what we used to call "Class Reptilia." This has traditionally included the animals that are now known to be Anapsids (turtles) and Diapsids (dinosaurs, birds, crocodilians, snakes, lizards, and tuataras), so "Class" Reptilia as it is usually constructed is polyphyletic, and is now in the process of being reorganized.

  • Anapsida - Turtles and their relatives

  • Diapsida - Dinosaurs, Birds, Crocodilians, Tuataras, Snakes, and Lizards

  • amphisbaenids (Order Squamata, Suborder Amphisbaenia) "worm lizards" - found extensively in South America and tropical Africa, these are burrowing legless lizards that superficially seem to resemble earthworms due to convergence)

  • crocodilians (Order Crocodilia)- sister taxon to birds and dinosaurs

    "Traditional" Reptile characteristics:

    Birds are Reptiles?

  • Birds are perhaps the most derived of all Diapsids
  • Birds began as feathered reptiles derived from a group of small, carnivorous dinosaurs known as therapods
  • The most famous of these is Archaeopteryx.
  • Feathers are homologous to scales/scutes of crocodilians.
  • Some secondarily flightless birds (ratites) have evolved where selective pressures were appropriate.
    Diagnostic Characteristics of Birds
    Birds are a diverse group comprised of 28 orders.

    Vast diversity of form and behavior, with some of the most complex mating behaviors known in the animal kindgom. (A word about LEKs)

    Many species are MIGRATORY. (See pp 371-373 of your text)