CRANIATA: Animals with Skulls The earliest ancestors of craniates 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.

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.

Craniates can be divided thusly:

  • 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 the support of the anterior gill slits.
  • 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

    Let's meet the Gnathostomes The extant Jawed Vertebrates are presently divided thusly...

    Chondrichthyes - The Cartilaginous Fishes Sharks...

    Skates...

    ...and Rays

    Chondrichthyan Features and Facts

  • The chondrichthyan skeleton is composed entirely of cartilage.

  • The cartilaginous skeleton of sharks and their relatives is believed to have been secondarily derived, with a bony skeleton representing the primitive vertebrate condition.

  • In most vertebrates, the skeleton is first cartilaginous during embryonic development, and then is ossified (made bony) later. In cartilaginous fishes, some major genetic event apparently interfered with that process, resulting in a permanently cartilaginous skeleton.

  • all chondrichthyans are carnivorous/predatory (though skates, rays and some sharks may prey primarily on molluscs, crustaceans, and other invertebrates, and some sharks feed on plankton)

  • many pelagic (free-swimming) sharks must remain in constant motion to keep fresh water moving over their gills, or they will die from lack of oxygen.

  • skin is covered with placoid scales (derived from the dermis, just underneath the epidermis), which form the famous replaceable teeth at the edges of the mouth.

  • Sharks hunt primarily via their keen sense of smell, and localize prey with the lateral line system, which can detect movement in the water.

  • Chondrichthyans may be

    Actinopterygii - The Ray-finned Fishes

  • These are the most numerous of vertebrates, both in species (about 30,000) and individuals.

  • 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

  • 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, 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 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

    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.

    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.

  • Tetrapods include the Amphibia 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
  • 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
  • dioecious

    Amniota: An Egg to Beat them All

  • 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, in which it has given rise to the placenta and other structures).
  • 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 "Reptilia" as it usually has been 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 Reptile Features and Fun
    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. Diagnostic Characteristics of Birds

    For you to get all the details we discussed in class, it is ESSENTIAL that you do your text readings and read at least the first page of any link above. Additional links you must read and know were mentioned in class, so be sure to note and read those, too.
    VERTEBRATES: Evolution and Diversity 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

  • They diversified into herbivorous, carnivorous forms known as PELYCOSAURS (they looked somewhat like lizards, but were NOT! Lizards are diapsids.) You've probably seen pictures of one of the most famous pelycosaurs, Dimetrodon.

  • One group of carnivorous pelycosaurs gave rise to the THERAPSIDS, the only synapsids to survive beyond the Paleozoic great extinction (about 245 mya).

  • Therapsids had limbs positioned directly beneath the body, allowing for a more energy-efficient gait and movement (Think of the sideways limb attachment of crocodilians and other reptiles for comparison.)

  • Most of the therapsids vanished during the great extinction at the end of the Permian (245 mya)

  • The last surviving therapsids, the CYNODONTS, were the dominant mammal ancestors in the Mesozoic

    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

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

    MAMMAL REPRODUCTION
  • 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.

    Let's meet some interesting mammalian orders.

    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?