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CRANIATA: Animals with Skulls

Craniates are all united by the presence of at least a small skull, or cranium, encasing the brain. And we all arose from a set of interesting ancestral fishlike creatures.

Ancestral Craniates

The earliest ancestors of craniates were plate-skinned fishes known as ostracoderms (like the little yellow fish in this DRAMATIC video). The name comes from the Greek ostraco meaning "shell" and derm meaning "skin".


One offshoot of this lineage, the osteostracans, evolved paired fins just behind the head. (What's the advantage of paired fins?)

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.

One placoderm, Dunkleosteus, bears the distinction of having had the strongest bite of any known animal. Watch the cheesy, fakey video!

Craniate Relationships

Craniates can be divided into:

  • Jawed vertebrates all show similar ontogeny with respect to jaw formation, and so are believed to be monophyletic.
  • The jaw bones develop from portions of the cartilaginous arches that once formed gill supports in the earliest chordates.
  • The condition of being "jawless" is primitive, and is not a good diagnostic character to use for constructing phylogenies.

    Let's Meet the Vertebrates

    I. Hyperotreti, the Hagfishes II. Vertebrata, the Vertebrates
  • A. Hyperoartia - The Lampreys

  • B. Gnathostomata - The Jawed Vertebrates

    Gnathostomes - The Jawed Vertebrates

    Chondrichthyes - The Cartilaginous Fishes

    Sharks, skates and rays

    Chondrichthyan Features and Facts

  • all carnivorous/predatory
  • skeleton composed entirely of cartilage (bone has been secondarily lost)
  • many pelagic (free-swimming) sharks constantly swim to keep water moving over the gills, lest they suffocate
  • placoid dermal scales cover the skin and form replaceable teeth
  • Sharks hunt via keen sense of smell, and localize prey with the lateral line system, which detects movement in the water.

  • Chondrichthyans may be

    Actinopterygii - The Ray-finned Fishes

  • The most diverse (~30,000 species) and numerous of vertebrates

  • Vast diversity in form, function and natural history

    Fun Fish Facts

  • Skeleton composed primarily of bone
  • ectothermal poikilotherms
  • Skin covered with dermal scales that protrude through the epidermis and shed readily.
  • Skin copiously supplied with mucous glands (science terminology note: mucous is an adjective. mucus is a noun.)
  • Fins may be either medial and unpaired, or lateral and paired

  • Fin rays are composed 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, and provides buoyancy.
  • 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 where they might not excel?)

    Sarcopterygii - Lobe-Finned Fishes and Tetrapods

    The lobe-finned fishes include such "living fossils" as the Coelacanths and the lungfishes. (More cool lungfish information here.)
    These are the closest living relatives to the four-legged vertebrates, or tetrapods.


    These are the four-legged, primarily terrestrial vertebrates, derived from a specialized group of shallow-water fishes that probably resembled extant lobe-finned fishes.

    Tetrapoda includes


    The amphibians made first landfall during the Devonian (400 mya). The first tetrapod had come ashore, with these defining characteristics: There are three main Orders: Modern Caudates (salamanders and newts) most closely resemble the ancestral amphibians, in that they retain a tail as well as the five-digits on each limb. 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 (behaviorally thermoregulate)
  • Primitively, larvae are aquatic, with gills and lateral lines
  • Skin is scaleless, and serves a major respiratory function: must remain moist
  • Skin is well supplied with glands, including poison glands. Some species are far more poisonous than others!
  • large mouth with very small teeth
  • Mostly insectivorous/carnivorous (they are sit-and wait/ambush predators)
  • Heart is three-chambered
  • internal fertilization in caecilians and salamanders
  • external fertilization in most frog species

    Reptiliomorpha (Amniota): An Egg to Beat them All

  • The amniotic egg is a synapomorphy with respect to all the extant Reptiliomorpha.

  • Check out this link to see what's what in the amniotic egg
  • Three main lineages of Reptiliomorphs, classified on the basis of skull morphology: To which group do we belong? Have a look and see if you can figure it out.

    Today's living remnants of these three lineages:

    Anapsida - Turtles and their relatives

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

    Diapsid Synapomorphies


  • highly derived diapsids - great diversity in both form and behavior
  • complex behaviors - courtship, migration, etc.
  • evolved from feathered carnivorous ancestors known as therapods
  • famous therapod: Archaeopteryx.
  • feathers--epidermal scale derivatives--are homologous to scales/scutes of other diapsids (check out those leg scales)

    Avian Synapomorphies

    There have been rare instances of avian gynandromorphs. At this time, the genetics and development of these anomalous birds is not fully understood.

    Mammalia: Our Friends, the Mammals

  • Synapsida: The Mammals

  • Monotremes - Egg-Laying Mammals (Platypus and Echidna)

  • Marsupials - Pouched Mammals

  • Eutherians - Placental Mammals

    The Synapsid vertebrates are probably the most familiar to all of us, as we happen to be one of the many species of synapsids. What's so special about the synapsids?

  • They were the first vertebrates to radiate into a wide diversity of terrestrial habitats

    Mammalian characteristics:

    The Importance of Teeth

    The earliest true mammals showed up in the late Triassic, and were mouse-sized carnivores.

    They had diphyodont dentition, meaning they were unable to constantly replace lost teeth (as their amniote ancestors could), but rather replaced them only once over the span of a lifetime.

    Humans and all other mammals show this pattern today.

    Another dental inovation: differentiated teeth (we are HETERODONTS, as opposed to HOMODONTS as all other vertebrates are.)

    The number of each of these types of teeth varies with species, and the "dental formula" is one diagnostic character of mammal species.

    In humans, the adult dental formula is 2:1:2:3/2:1:2:3 - This is the number of teeth in half of the upper jaw (2 incisors, one canine, 2 premolars, and 3 molars) and in half of the lower jaw (two incisors, 1 canine, 2 premolars and 3 molars).

    Note that a few mammals have secondarily reverted to the homodont condition, but this is derived with respect to other mammals.

    Some mammals are hypsodonts. This means that their teeth grow continually throughout their lives, and must grind against each other to remain a normal, manageable shape. Some examples: rodents, lagomorphs (hares, rabbits, and pikas), horses, elephants and many other herbivores. What do you suppose is the advantage of having continually growing teeth? What could be some disadvantages?

    What makes us mammals?

  • Hair (a derivative of the integument)
  • Integumentary glands


    Mammals may be
  • insectivores - feed on insects
  • carnivores -feed primarily on herbivores
  • omnivores - feed on a variety of plant and animal matter


  • Egg-laying: Monotremes
  • Live birth of altricial young which are then raised in a pouch: Marsupials
  • Internal brooding of young via a placenta - Eutherians (placental mammals) Most mammals have defined mating seasons, with females accepting male advances only during her estrous cycle. When not in "heat" (estrus), the female is unreceptive to male sexual advances.

    Family Ties: The Primates

    The first primate was This lineage gave rise to the How has our evolutionary history made us who we are today? The earliest hominids (human-like apes) were not only social, but cooperative, and evolved into the most primitive human social group, the hunter-gatherer:

  • male - hunter of meat
  • female - gatherer of vegetable matter What do these two types of foraging strategies require, and what genetically-based behaviors might confer a selective advantage to each sex?

    Our Favorite Ape: Homo sapiens

    Paleoanthropology is the study of human origins and evolution. Because humans and chimpanzees have existed as separate species for only a few million years, this branch of science examines only a very small, recent portion of the fossil record.

    Terms to know:

    Humans, apes, and monkeys all diverged from a common anthropoid ancestor that shared all the characteristics common to all primates. Natural selection has driven the specialization of each primate species to be what it is today, and that includes our own species.

    The earliest fossil members of our genus (Homo) range in age from about 2.5 to 1.6 million years, and are currently classified as Homo habilis:

    Homo erectus shared common ancestry with H. habilis, and shows up in the fossil record from about 1.8 million to 0.5 million years ago:

    Homo sapiens neanderthalensis , named for the Neander Valley in Germany where its fossils were first found, may have arisen from an H. erectus-like ancestor, as H. erectus is known to have migrated into both Europe and Asia (timing uncertain).

    There's a good deal of argument about how anatomically modern Homo sapiens sapiens came onto the scene, but it's generally agreed that we originated in Africa, where all the oldest hominid fossils are found. Here's a nice overview of our hominid relatives, for those who would like to delve a little deeper.

    What makes humans different from the other great apes?

    EXTRA CREDIT OPPORTUNITY! View "Walking with Beasts" video, and be prepared to answer Bonus Questions on the video on the final exam. Here are the links. Each episode is a little under 30 minutes long.