Bilateria(also referred to as "Bilateralia" in older literature): Monophyletic group containing most animals.

Synapomorphies: Origin of an antero-posterior axis (over an oral-aboral axis) Presence of a flow-through gut with distinct mouth and anus. Triboblastic: I.e. a third germ layer, mesoderm is present. Consider the basic steps by which we get a proper embryo with a top, bottom, front, and back from a zygote: Polarity: In all ova yolky matrial tends to concentrate at one end, yeilding: "Animal" pole, the non-yolky side "Vegetal" pole, the yolky part. Cleavage Phase of rapid cell division with little overall growth. The zygote transforms into a hollow sphere of cells, the blastula. The space in the middle is the blastocoel The blastula has three cell types: Smaller cells nearer the "animal" pole. Larger, yolky cells nearer the "vegetal" pole. A third type of cell girdling the large cells of the "vegetal" pole.

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Gastrulation: Cells of the "vegetal" hemisphere form a flat plate then invaginate inward, yielding a two layered hemisphere. The rim of this concavity qickly closes into a small opening, the blastopore. While this is occurring, cells from the third cell type stream in through the blastopore and move forward along the inner surface. These become mesoderm. Depending on the taxon, blastopore will become either mouth or the anus. The embryo is now a gastrula. It possesses three basic germ layers: Endoderm: Destined to give rise to the gut tube and associated structures. Ectoderm: Destined to give rise to the skin and, in vertebrates, central nervous system. Mesoderm: Destined to give rise to a large range of internal organs (depending on the taxon.)

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The Coelom: Presence of a coelom, or internal body cavity, surrounded by mesoderm in which the internal organs are suspended. Allows for: increased gut expansion, hydrostatic structure against which muscles may act, facilitating purposeful locomotion use of coelomic fluid in gas and nutrient transport, breaking dependence on simple diffusion and allowing larger body size. Developed as: Pseudocoelomate: In which the coelom develops between the endoderm and mesoderm as a remnant of the blastocoel and is properly called the pseudocoel. Eucoelomate: In which a proper coelom develops within the mesoderm. (See above illustration.) Acoelomate: In some critters, including platyhelminth flatworms, the coelom is drastically reduced or absent. Coelom formation in eucoelomates can occur in two fashions: Schizocoely (typical of "protostomes") in which coelom forms in cavities that develop in existing mesoderm Enterocoely (typical of "deuterostomes") where the coelom forms by the outpouching of mesoderm during gastrulation. Specialized organs The presence of coelom and mesoderm seems to provide the developmental flexibility for the development of specialized organs. In bilateralians we usually see organs dedicated to: Circulation Excretion of nitrogenous wastes Nerve ganglia for coordination of behavior

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Bilaterian Phylogeny

Eek. A bucket of worms in more than one sense of the term. No consensus, but among the many bilaterian groups, three larger clades are recognized by many authors:

• Deuterostomia: Includes vertebrates and echinoderms plus minor groups. Distinct deuterostomous pattern of development.
  • Blastopore becomes anus; mouth comes from secondary opening
  • Enterocoelomate
  • Indeterminate development: i.e. the fate of a given cell is fulid and based on inductive relationships with neighboring cells and tissues.
  • Radial cleavage of embryonic cells (ability to twin). I
  • Nerve chord dorsal to digestive tract in adults.
  • In contrast, typical bilateralians have protostomous development in which:
    • Blastopore becomes mouth
    • Schizocoelomate
    • Determinate development, in which cell fates are absolute.
    • Spiral cleavage (all mesoderm from 4d cell)
    • Nerve chord ventral to digestive tract

• Ecdysozoa: Includes arthropods, nematodes, onychophorans, and other critters that regularly shed an external cuticle.

• Lophotrochozoa: United by molecular characters. Further divided into:
  • Trochozoa
    • Trochophore larva
    • Annelida and Mollusca, only members with substantial fossil record
  • "Lophophorata"
    • A Formerly thought to be monophyletic on the basis of morphology, but more likely polyphyletic group of lophophore-bearing lophotrochozoans.

Note: Whether Ecdysozoa and Lophotrochozoa form a monophyletic "Protostomia" is up in the air.

"Lophophorates" are morphologically similar in sharing:

• Lophophores
  • Hollow extensions of second (of three) coeloms
  • Surround mouth, anus outside
  • Cilliated
  • Function in collection of food and in gas exchange
• Essentially headless
• U-shaped guts
• Two major groups:
  • Bryozoa
  • Phoronida + Brachiopoda

Brachiopoda & Phoronida: Larger bodied. DNA strongly suggests two form a clade.

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Phoronida: (No record) Represent what we might imagine to be the ideal ancestral lophophorate: Always solitary Macroscopic Infaunal benthic marine Possesses Nephridia for waste excretion Closed circulatory system with blood cells and hemoglobin (no distinct heart) Why do we,as paleontologists, care about critters with no fossil record? Because they illuminate the relationships of critters with good records. Consider Simon Conway-Morris' notion of a halkieriid origin of brachiopods. Sounded reasonable only as long as you didn't know about phoronids. But the weird part: Recent molecular work suggests that perhaps phoronids are a valveless form of brachiopod. Stay tuned.

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Brachiopoda: (Cambrian - Rec.) Always solitary Always encased in shell (phoronids notwithstanding) Always benthic marine Possesses Nephridia for waste excretion Closed circulatory system with a distinct heart but which doesn't seem to be involved in gas transport. Coelomic fluid with the oxygen-binding protein hemerethryn. "Mantle tissue" that secretes the valves. (Not homologous to mollusc mantle.) Refer to lab for details of valve morphology.

Major clades:

• Lingulata (sometimes called "Linguliformes" or "Linguliformea"): (Cambrian - Rec.)
  • Chitinophosphatic valves, lacking hinge articulation
  • Muscular pedicle
  • Burrowing infaunal habit
• Inarticulata proper (sometimes called "Craniiformea" or "Craniata" (not to be confused with the TRUE Craniata) (Cambrian - Rec)
  • Calcareous valves, lacking hinge articulation
  • Pedicle reduced or absent
  • Epifaunal
• Articulata (sometimes "Rhynchonelliformea") (Late Cambrian - Rec.)
  • Greatest diversity of brachiopods, living or extinct
  • Calcareous valves with complex hinge articulation
  • In many groups, the lophophore is supported by a brachidium.
  • Pedicle variously developed, but dead horny tissue
  • Anus lacking. Gut tube is blind ended, with waste preiodically being expelled through mouth. (In stark contrast to lingulids, which have a proper gut tube.)
  • Epifaunal, with a wide variety of habits

• Systematics: Six classically recognized groups:
  • Strophomenida: (Ord. Jur) Strophic with D-shaped profile. Often lacking pedicle and colonizing soft substrates.
  • "Orthida": (Cam. Per.) Morphologically the most ancestral of articulates. Possibly paraphyletic.
  • Pentamerida: (Cam. Dev.) Astrophic with biconvex valves. Large cruralium and spondylium divided mantle cavity into five chambers.
  • Rhynchonellida: (Ord - Rec.) Astrophic with pointed beak and biconvex valves. Strongly plicate with deep fold and sulcus.
  • Spiriferida: (Ord - Jur.) Paraphyletic? Brachidium developed into complex spiralium.
  • Terbratulida: (Dev. - Rec.) Astrophic, Smooth valves, looping brachidium.

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• Ecology:
  • Shallow marine benthic
  • Lingulids can tolerate brackish water.
  • Typical articulates with pedicle required hard substrate. Those without (E.G. concavo-convex Strophomena) lay on soft substrate. Some were cemented to the substrate.

  The utility of such strategies depended on the energy of the environment.

  • Moderate energy: E.G. a reef environment below low tide line. A direct pedicle or spiny attachment to a rigid substrate or host.
  • Low energy only: E.G. a sandy bottom below wave base. Rafting on soft sediment. (Even moderate energy here would bury the rafter.)
  • High energy: E.G. Intertidal zone. Here, sessile creatures must be firmly attached to a hard substrate, as in craniiform brachiopod cemented to strophomenid brachiopod.

    Brachiopods retain strong bilateral symmetry, even in their feeding organs. In phoronids, their soft-bodied relatives, the feeding organ, the lophophore is exposed. It is covered with cilia. Food particles intercepted by the lophophore tentacles are moved to the mouth by ciliary action. In brachiopods, the lophophore is enclosed between upper and lower valves. These control the flow of the ciliary current across the lophophore, eliminating the need to be able to capture food coming from any direction.

    Plicae: In many bivalved organisms, most notably brachiopods, the commisure at which the valves meet forms a series of zig-zag plicae. Functions include: Feeding: The area of the gape is increased but not the width. A greater volume of water cna be processed while large particles are excluded. Additionally, many brachiopods have a median fold and sulcus separating incurrent from excurrent water. Strength: It is much more difficult for the valves of a plicate shell to be twisted apart.

  • Positioning and attachment: Position has two crucial aspects:
    • Location
    • Orientation
    Typically, sessile organisms have to attach to rigid substrates. This limits their feeding to within a few cm. of the bottom, and excludes them from soft (sandy or muddy) substrates. E.G.: brachiopods typically attach to the substrate using a very short fleshy stalk, the pedicle.
    Orientation: Using the pedicle, it can adjust its orientation to optimize suspension feeding. Its feeding, however, is limited to within 2 - 3 cm of the substrate, and the substrate must be rigid.
    • Location: Some brachiopods overcame this limitation by attaching to larger suspension-feeding organisms like stalked echinoderms, either with their pedicle (in which case they could still control their orientation somewhat) or using spines on their valves (in which case they depended on the substrate-host to orient them properly).
    • Others pioneered soft substrates, rafting on them by means of deep globular morphologies or the support of elongate spines.

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  • Evolutionary trends:
    • Strophic, D-shaped types seem to predominate in Early Paleozoic.
    • In Late Paleozoic, proliferation of odd-ball strophomenids with specializations for sementation to hard substrate (E.g. Prorichthofenia, Leptodus). Productids developed morphology for support in soft sediment.
    • General trends toward increasing complexity of hinge articulations and brachidia.

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  Bryozoa: You've probably seen them without knowing it. General characteristics: Exclusively colonial Non-living skeleton (zooecium) Each "individual" is a zooid Colony (zoarium) begins with an initial zooid, the ancestrula, which buds off clones to form the remainder of the colony. (But a soft tissue connection persists.) Individual zooids connected by funiculus and stolon through which nutrients are shared. Lophophore is protracted when it "inflated" by coelomic fluid when circumferential muscles of body wall contract. It is retracted by retractor muscles. Small enough to handle gas-exchange by simple diffusion. Lack nephridia (organs for excretion of insoluble wastes) These, thus, accumulate in tissues. Great powers of regeneration. Occasionally, all cells of the zooid except for the body wall degenerate into a brown body that is ultimately ejected from the zooecium. The zooid then regenerates from the body wall. Possible functions: Resting stage during stressful intervals A radical means of ridding the body of insoluble wastes. Reproduction of both zooids and colonies is often by budding. Individuals reproduce sexually, as well. Most zooids are hermaphroditic, although ovaries and testes are usually not active simultaneously. Ova are retained in the zooecium free-swimming sperm are snared by the lophophore and conveyed to the ovum. The zygote escapes as it transforms into a planktonic larva.

Phylactolaemata (schematic): (No record) Chitinous (i.e.soft) zoarium All zooids similar Zooecial walls incomplete, allowing zooids freely to share coelomic fluid. Freshwater Can propagate through statoblasts - little capsules of cells enclosed in calcareous capsules that form along the funiculus. In times of stress, when the zooid dies, these disperse. When proper conditions return, they open and a new ancestrula regenerates.

• Gymnolaemata: Ordovican onward, probably paraphyletic, box-like zooids. Characters:
  • Orifice protected by operculum
  • Zooecia box-like with nearly complete walls perforated only by pore plates thus...
  • Lophophore protraction requires deformation of zooecium. Achieved differently in different taxa.
  • Colonies include specialized non-feeding zooids, including
    • Vibracia
    • Avicularia.

  • Two major groups:

    Ctenostomata: (Ord. - Rec.) excellent fossil record Membranous zooecia. Lophophore protraction similar to that in phylactolaemates.

    Cheilostomata: (Jurassic - Rec.) (Schematic) excellent fossil record encrusters with largely calcareous zoaria. Lophophore protraction through action of protractor muscles deforming: flexible frontal wall